Pete Lowry

Thomas Givnish

Jean-Yves Dubuisson

Yohan Pillon D'avoir

Je Remercie

Brigitte Meyer-Berthaud

Valérie Sarramegna

Gildas Gâteblé

Pierre-Éric Lauri

Jérémie, Philippe, Gildas, Hervé, Grégoire Laure William

Dimitri

Thomas

Santiago

Merci Beaucoup Léna

Eva Aurore

Marie-Louis Tom

Je Remercie Profondément

Christine Heinz

Yves Caraglio

Nick Rowe

Claude Edelin

François

Létocart

Dominique

Hélène Guillaume Rémy

La Bande ! Malik #forestcharacterisation

Alex # Plancabane

Andréa # Originalgirl

Pupuce #YC Ricky # Chercherbison

Joe # Singlecelled

Teva # Tchinkonk

Flo # Bestcuisto

Kenji # Tchâa

Soané # Maloténiaouli

Tom # Chinois

Jason # Piatiap

Baptiste #monbon Riddick # Haïti

Nelly # Bestcoiffeuse

Léna # Mhd

Sarah # Bichedemer

Cesar # Maitredestage

Georgy # Monike

Rachelle # Iaaiseralibre

Kiam # Childhood

Kim # Marteauburin

Akhio # Futurtatoo

Jean-Éric # Lachipo

A-Tena # Capitalekompa

Simmon #l'ancien

Chloé #bestvoisine Alrick # Tombeatonton

Romain # Lamatraque

Léo # Larguéleshort

Stéphanie # Teatime

Fred Joseph William

Marcel Jean

Miguel Nicaise

Jean-Paul Sam

Soraya Félix

D Bruy

L Barrabé

J Munzinger

S Isnard

Eva Gril Gril

Eva Gril

David Gril

Gril Gril D D Bruy

Bruy Bruy

Christine Bruy Bruy E E Heinz

Christine Sandrine

Heinz Hei Z Stine E Hei Rine Rine Rine Isnard Rd Rd

convergence: correlated evolution and environmental contingencies of monocauly in the flora of New Caledonia

Keywords: Architecture des plantes, Convergence évolutive, Forme de croissance, Îles, Phylogénie, Traits fonctionnels Convergent evolution, Functional traits, Growth habit, Islands, Phylogeny, Plant architecture Acalyphoideae, plant architecture, taxonomy Chapter

L'évolution convergente des formes de croissance est un phénomène fondamental reliant l'écologie et l'évolution des plantes. Remarquablement illustré dans plusieurs systèmes insulaires, ce phénomène n'a jamais été clairement identifié en Nouvelle-Calédonie, pourtant connue pour la richesse et l'originalité de sa flore. Par une approche combinant architecture des plantes, traits fonctionnels, taxonomie, phylogénie et données environnementales, cette thèse analyse l'histoire évolutive de la monocaulie, une forme de croissance mal connue, en Nouvelle-Calédonie. Les monocaules sont des plantes autoportantes ligneuses dont les fonctions majeures sont assurées par une seule tige apparente. En Nouvelle-Calédonie, elles sont représentées par 182 espèces dicotylédones appartenant à 41 genres et 30 familles et sont souvent menacées d'extinction. L'évolution répétée de la monocaulie en Nouvelle-Calédonie, issue d'au moins 31 événements d'apparition, est l'un des cas les plus remarquables de convergence en milieu insulaire. Dans le genre Atractocarpus, la monocaulie est apparue récemment deux à trois fois via diverses réductions des branches en inflorescences, montrant l'importance des processus hétérochroniques dans l'évolution des formes de croissance. La monocaulie est fortement corrélée à plusieurs traits démontrant des contraintes majeures dans la coordination fonctionnelle. L'évolution de la monocaulie est fortement associée aux forêts denses humides et au substrat ultramafique, et pourrait avoir contribué à la diversification des lignées par des phénomènes de partitionnement de niche. La remarquable convergence de la monocaulie en Nouvelle-Calédonie peut s'expliquer par quatre hypothèses majeures liées (i) à la structure particulière des forêts denses humides (en lien avec les cyclones) favorisant l'exploration unidirectionnelle de l'espace, (ii) aux contraintes édaphiques liées aux substrats ultramafiques induisant une paupérisation architecturale, (iii) à l'absence historique de grands brouteurs, auxquels les monocaules sont particulièrement sensibles, et (iv) à la persistance des forêts denses humides lors des épisodes glaciaires (servant de refuges pour ces espèces sensibles) et leur expansion post-glaciaire (fournissant de nombreuses opportunités écologiques).

Avant-propos :

Cette thèse de doctorat est le fruit de trois années de travail menées à l'Unité Mixte de Recherche botAnique et Modélisation de l'Architecture des Plantes et des végétations (UMR AMAP) au sein du Centre IRD de Nouméa qui accueille l'Herbier de Nouvelle-Calédonie (NOU).

Cette thèse a été financée par une bourse du Ministère Français de l'Enseignement supérieur, de la Recherche et de l'Innovation, administrée par l'Université de Montpellier.

Certains des chapitres présentés ici ont fait ou vont faire l'objet de publications scientifiques. Ces publications doivent être citées préférentiellement au présent document, notamment pour les actes taxonomiques.

Le document est rédigé en anglais pour le rendre accessible au plus grands nombre. Un résumé étendu en français est disponible à la fin du document.

Foreword:

This PhD Thesis is the result of three years of research conducted within the Joint Research Unit "botAnique et Modélisation de l'Architecture des Plantes et des végétations" (UMR AMAP) at the French National Research Institute for Sustainable Development (IRD) from Nouméa, which hosts the Herbarium of New Caledonia (NOU).

This PhD project was funded by a grant from the French Ministry of Higher Education, Research and Innovation, administered by the University of Montpellier.

Several chapters of this PhD have been or will be published in scientific journals. These publications should be cited rather than this PhD document, especially for taxonomic acts.

Table of Contents

Chapter 1 -General Introduction

Islands as models in ecology and evolution

Most of major ecological and evolutionary theories have been influenced by works on island biotas [START_REF] Darwin | On the Origin of Species by Means of Natural Selection[END_REF][START_REF] Wallace | Island life: or, the Phenomenon and Causes of Insular Faunas and Floras, Including a Revision and Attempted Solution of the Problem of Geological Climates[END_REF][START_REF] Macarthur | The theory of island biogeography[END_REF][START_REF] Warren | Islands as model systems in ecology and evolution: prospects fifty years after MacArthur-Wilson[END_REF][START_REF] Whittaker | Island biogeography: Taking the long view of nature's laboratories[END_REF]. Given their isolation and particular environmental conditions and history, oceanic islands are exceptionally suited for the investigation of convergent evolution and adaptive radiation [START_REF] Elmer | Adaptation in the age of ecological genomics: insights from parallelism and convergence[END_REF][START_REF] Whittaker | Island biogeography: Taking the long view of nature's laboratories[END_REF]. One of the most famous and remarkable adaptations of insular floras, involved in both convergent evolution [START_REF] Givnish | Giant lobelias exemplify convergent evolution[END_REF][START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF] and adaptive radiation [START_REF] Bramwell | Some morphological aspects of the adaptive radiation of Canary Islands Echium species[END_REF][START_REF] Jorgensen | Adaptive radiation of island plants: evidence from Aeonium (Crassulaceae) of the Canary Islands[END_REF], is the so-called "secondary woodiness" (i.e. the evolution of woodiness in herbaceous lineages). This notable pattern was formerly described by [START_REF] Darwin | On the Origin of Species by Means of Natural Selection[END_REF], extensively studied by [START_REF] Carlquist | Island Biology[END_REF], and remains a hot topic in plant science (e.g. Lens et al., 2012a;[START_REF] Rowe | Herbs and secondary woodiness -keeping up the cambial habit[END_REF]. Though first described from islands, the phenomenon also occurs in continental floras, leading to a generalization of concepts (Lens et al., 2012b;[START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF][START_REF] Kidner | First steps in studying the origins of secondary woodiness in Begonia (Begoniaceae): combining anatomy, phylogenetics, and stem transcriptomics[END_REF]Whittaker et al., 2017).

Islands, as natural laboratories for the study of ecology and evolution, continue to provide opportunities for understanding general patterns and processes of plant evolution [START_REF] Whittaker | Island biogeography: Taking the long view of nature's laboratories[END_REF]. In particular, information on functional traits and phylogenetic relations of island species represent gaps that need to be filled [START_REF] Santos | New directions in island biogeography[END_REF][START_REF] Keppel | Towards an eco-evolutionary understanding of endemism hotspots and refugia[END_REF].

Among islands, New Caledonia has been recognized as a particularly promising model for ecological and evolutionary studies [START_REF] Grandcolas | New Caledonia: a very old Darwinian island?[END_REF][START_REF] Murienne | Testing biodiversity hypotheses in New Caledonia using phylogenetics[END_REF][START_REF] Jaffré | The metal hyperaccumulators from New Caledonia can broaden our understanding of nickel accumulation in plants[END_REF][START_REF] Grandcolas | Ten false ideas about New Caledonia biogeography[END_REF]. The long-term isolation and particular history of New Caledonia have resulted in a flora that harbors many peculiarities, including the outstanding richness in endemic and microendemic species [START_REF] Lowry | Diversity, endemism, and extinction in the flora of new Caledonia: a review[END_REF][START_REF] Grandcolas | New Caledonia: a very old Darwinian island?[END_REF][START_REF] Morat | The taxonomic database « FLORICAL » and characteristics of the indigenous flora of New Caledonia[END_REF][START_REF] Wulff | Conservation priorities in a biodiversity hotspot: analysis of narrow endemic plant species in New Caledonia[END_REF]Ibanez et al., 2017b), a high incidence of relictual lineages, which remains puzzling [START_REF] Pillon | Time and tempo of diversification in the flora of New Caledonia[END_REF][START_REF] Pillon | How many genera of vascular plants are endemic to New Caledonia? A critical review based on phylogenetic evidence[END_REF], and an exceptional diversity of some globally rare lineages [START_REF] Jaffré | Distribution and Ecology of the Conifers of New Caledonia[END_REF][START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF][START_REF] Pouteau | New Caledonia: a Pleistocene refugium for rain forest lineages of relict angiosperms[END_REF]. Recent biogeographic and phylogenetic studies failed to find much evidence for old in situ radiations and rather indicates that the island's richness might result from a combination of relictual lineages, repeated colonization events, and recent speciation [START_REF] Grandcolas | New Caledonia: a very old Darwinian island?[END_REF]. This led some authors to consider New Caledonia as an "old Darwinian island" [START_REF] Grandcolas | New Caledonia: a very old Darwinian island?[END_REF][START_REF] Swenson | Sapotaceae biogeography supports New Caledonia being an old Darwinian island[END_REF] and one of the world's main biogeographical regions [START_REF] Guillaumin | Les régions florales du Pacifique[END_REF][START_REF] Good | The geography of the flowering plants[END_REF][START_REF] Takhtajan | Flowering Plants, origin and dispersal[END_REF][START_REF] Van Balgooy | Plant geography of the Pacific[END_REF][START_REF] Morat | Richesse et affinités floristiques de la Nouvelle-Calédonie: conséquences directes de son histoire géologique[END_REF]. Despite these peculiarities, virtually no example of remarkable convergence has been demonstrated to date, in contrast with striking cases of convergent evolution and adaptive radiation found in other islands [START_REF] Elmer | Adaptation in the age of ecological genomics: insights from parallelism and convergence[END_REF][START_REF] Whittaker | Island biogeography: Taking the long view of nature's laboratories[END_REF].

Convergence and adaptive value of traits

An important feature of insular biota is their taxonomic and functional disharmony [START_REF] Carlquist | Island Biology[END_REF][START_REF] Cody | Short-term evolution of reduced dispersal in island plant populations[END_REF][START_REF] Gillespie | Oceanic Islands: Models of Diversity[END_REF][START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF][START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF][START_REF] Schlessman | Incidence, Correlates, and Origins of Dioecy in the Island Flora of New Caledonia[END_REF]. Taxonomic disharmony, i.e. the uneven representation of some lineages, is widely regarded as a strong evidence of long-distance dispersal and has been largely documented (e.g. [START_REF] Carlquist | Island Biology[END_REF][START_REF] Loope | Islands: Biological Diversity and Ecosystem Function[END_REF][START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF]. Functional disharmony, i.e. the uneven representation of functional trait attributes, has rather been studied through the concept of "island syndromes" [START_REF] Whittaker | Island biogeography: Taking the long view of nature's laboratories[END_REF]. Among frequently cited trait attributes, those over-represented on islands are woodiness [START_REF] Carlquist | Island Biology[END_REF][START_REF] Shmida | Growth Form Diversity on the Canary Islands[END_REF][START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF], sexual dimorphism [START_REF] Sakai | Gender and sexual dimorphism in flowering plants: a review of terminology, biogeographic patterns, ecological correlates, and phylogenetic approaches[END_REF][START_REF] Jorgensen | Adaptive radiation of island plants: evidence from Aeonium (Crassulaceae) of the Canary Islands[END_REF][START_REF] Schlessman | Incidence, Correlates, and Origins of Dioecy in the Island Flora of New Caledonia[END_REF], heterophylly [START_REF] Cockayne | New Zealand plants and their story[END_REF][START_REF] Friedmann | Observations sur l'hétérophyllie dans les îles Mascareignes[END_REF][START_REF] Givnish | Thorn-like prickles and heterophylly in Cyanea: adaptations to extinct avian browsers on Hawaii[END_REF], monocarpy [START_REF] Jorgensen | Adaptive radiation of island plants: evidence from Aeonium (Crassulaceae) of the Canary Islands[END_REF][START_REF] Read | Mass flowering and parental death in the regeneration of Cerberiopsis candelabra (Apocynaceae), a long-lived monocarpic tree in New Caledonia1[END_REF], and loss of dispersability [START_REF] Carlquist | Island Biology[END_REF][START_REF] Cody | Spacing patterns in Mojave Desert (USA) plant communities: Nearest-neighbor analyses[END_REF][START_REF] Fresnillo | Variation in dispersability among mainland and island populations of three wind dispersed plant species[END_REF]. The high frequency of a trait attribute in islands could result from (i) preferential establishment of species presenting the attribute, (ii) repeated in situ independent evolution of the attribute, and/or (iii) in situ diversification of lineages presenting the attribute (Figure 1.1). Identifying which mechanisms account for the high incidence of a given trait attribute is thus crucial for understanding which ecological and evolutionary processes have driven the development of the current taxonomic and functional diversity.

Among the important phenomena of evolutionary theory stands convergent evolution, i.e. the appearance of a similar trait attribute under similar selection pressures but not inherited from a common ancestor [START_REF] Mcghee | Convergent Evolution: Limited Forms Most Beautiful[END_REF][START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. While the term "convergence" has been expanded in community ecology, where it concerns plant association rather than plant evolution (e.g. [START_REF] Cody | Character Convergence[END_REF][START_REF] Lamont | High leaf mass per area of related species assemblages may reflect low rainfall and carbon isotope discrimination rather than low phosphorus and nitrogen concentrations[END_REF][START_REF] Díaz | The plant traits that drive ecosystems: Evidence from three continents[END_REF][START_REF] Fukami | Species divergence and trait convergence in experimental plant community assembly[END_REF][START_REF] Grime | Trait convergence and trait divergence in herbaceous plant communities: mechanisms and consequences[END_REF], a phylogenetical approach is needed to demonstrate evolutionary convergences. Convergence provides the most powerful line of evidence for adaptive evolution, i.e. the selective advantage of traits in a given environment [START_REF] Endler | Natural selection in the wild[END_REF][START_REF] Grime | Trait convergence and trait divergence in herbaceous plant communities: mechanisms and consequences[END_REF][START_REF] Wake | Homoplasy: From Detecting Pattern to Determining Process and Mechanism of Evolution[END_REF][START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF].

Convergent evolution is particularly striking when environmental conditions are constraining, as in arid (e.g. [START_REF] Arakaki | Contemporaneous and recent radiations of the world's major succulent plant lineages[END_REF], frost-prone (e.g. [START_REF] Givnish | Giant lobelias exemplify convergent evolution[END_REF][START_REF] Boucher | Evolution and biogeography of the cushion life form in angiosperms[END_REF] or nutrient poor environments (e.g. [START_REF] Albert | Carnivorous plants: phylogeny and structural evolution[END_REF][START_REF] Ellison | Evolutionary ecology of carnivorous plants[END_REF]. By contrast, when the environment is less constraining (or at smaller phylogenetic scale), biotic factors (i.e. competition) prevail, leading to divergent selection among close relatives [START_REF] Weiher | Assembly Rules, Null Models, and Trait Dispersion: New Questions from Old Patterns[END_REF][START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. Divergent selection, sometimes leading to adaptive radiation (i.e. the rise of a diversity of ecological roles, [START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF] results from different exploitation of resources among closely related species [START_REF] Schluter | The ecology of adaptive radiation[END_REF][START_REF] Grime | Trait convergence and trait divergence in herbaceous plant communities: mechanisms and consequences[END_REF]. However, while convergent and divergent evolution are opposite evolutionary phenomena, divergent selection, when occurring repeatedly in distinct lineages, can also lead to convergence [START_REF] James | Explaining Evolutionary Shifts between Bee and Hummingbird Pollination: Convergence, Divergence, and Directionality[END_REF]. Some authors consider that convergence is related to the concept of evolutionary constraint, i.e. that evolutionary pathways are constrained by a limited set of functional and developmental possibilities [START_REF] James | Explaining Evolutionary Shifts between Bee and Hummingbird Pollination: Convergence, Divergence, and Directionality[END_REF][START_REF] Mcghee | Convergent Evolution: Limited Forms Most Beautiful[END_REF]. Convergence has often been used as a key argument against stochasticity in evolutionary theories [START_REF] Patterson | Homology in classical and molecular biology[END_REF][START_REF] Db | Homoplasy: The Result of Natural Selection, or Evidence of Design Limitations?[END_REF][START_REF] Kreitman | Molecular evidence for natural selection[END_REF][START_REF] Schluter | Parallel speciation by natural selection[END_REF]. Physical laws and phylogenetic background are though to impose functional constraint, while environment provides an overall standard value [START_REF] Mcghee | Convergent Evolution: Limited Forms Most Beautiful[END_REF][START_REF] Díaz | The global spectrum of plant form and function[END_REF][START_REF] Olson | Plant height and hydraulic vulnerability to drought and cold[END_REF]. Convergence thus provides a framework for elucidating evolutionary constraints and prerequisites of some trait associations. This might especially pertain to functional traits that are widely represented in global plant spectra [START_REF] Westoby | Plant Ecological Strategies: Some Leading Dimensions of Variation Between Species[END_REF][START_REF] Reich | The Evolution of Plant Functional Variation: Traits, Spectra, and Strategies[END_REF][START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF][START_REF] Wright | The worldwide leaf economics spectrum[END_REF][START_REF] Wright | Relationships Among Ecologically Important Dimensions of Plant Trait Variation in Seven Neotropical Forests[END_REF][START_REF] Díaz | The global spectrum of plant form and function[END_REF] and whose correlated evolution leads to more integrative plant traits such as growth habit.

Convergence in growth habit

Most of the striking examples of convergence in the plant world concern growth habit, illustrated for instance by rosette-trees in high-elevation tropical regions [START_REF] Givnish | Giant lobelias exemplify convergent evolution[END_REF][START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF], cactoid trees in deserts [START_REF] Arakaki | Contemporaneous and recent radiations of the world's major succulent plant lineages[END_REF][START_REF] Hernàndez-Hernàndez | Phylogenetic relationships and evolution of growth from in Cactaceae (Caryophyllales, Eudicotyledoneae)[END_REF], cushion plants in high-alpine mountains [START_REF] Boucher | Reconstructing the origins of high-alpine niches and cushion life form in the genus Androsace S.L. (Primulaceae)[END_REF][START_REF] Boucher | Evolution and biogeography of the cushion life form in angiosperms[END_REF], and lianas in tropical (seasonal) rainforests [START_REF] Gianoli | Evolutionary implications of the climbing habit in plants[END_REF][START_REF] Isnard | The evolution of angiosperm lianescence: a perspective from xylem structure-function[END_REF].

Growth habit, the general physiognomical appearance of a plant, results from the combination of morphological and anatomical traits, with a particular importance of structural traits (see [START_REF] Rietz | Life-forms of terrestrial flowering plants, 1[END_REF][START_REF] Lacza | A survey of the plant life-form systems and the respective research approaches, I[END_REF]Millan, 2016 for reviews). The term is used in this study as a synonym for other widely used terms with similar definitions (e.g. vegetative form, life form, growth form… [START_REF] Warming | Oecology of plants : an introduction to the study of plant-communities[END_REF]. Despite different terminologies, the notion of growth habit reflects specific strategies of resource acquisition and use, and consequently involves strong relations with the environment. Such integrative plant traits are of ecological and evolutionary interest [START_REF] Eriksson | Pollination Systems, Dispersal Modes, Life Forms, and Diversification Rates in Angiosperm Families[END_REF][START_REF] Dodd | Phylogenetic analysis of trait evolution and species diversity variation among Angiosperm families[END_REF][START_REF] Gianoli | Evolution of a climbing habit promotes diversification in flowering plants[END_REF][START_REF] Bonser | Growth form evolution and shifting habitat specialization in annual plants[END_REF] because they involve a set of underlying traits, potentially reinforcing the effect on fitness [START_REF] Arakaki | Contemporaneous and recent radiations of the world's major succulent plant lineages[END_REF]. For instance, convergence toward a liana habit implies correlated evolution of a suite of traits at the morphological scale (e.g. climbing organs, [START_REF] Isnard | Moving with climbing plants from Charles Darwin's time into the 21st century[END_REF][START_REF] Couvreur | Global diversification of a tropical plant growth form: environmental correlates and historical contingencies in climbing palms[END_REF][START_REF] Sousa-Baena | Convergent Evolution and the Diverse Ontogenetic Origins of Tendrils in Angiosperms[END_REF], anatomical scale (e.g. heteroxylly, [START_REF] Angyalossy | Liana anatomy: a broad perspective on structural evolution of the vascular system[END_REF][START_REF] Isnard | The evolution of angiosperm lianescence: a perspective from xylem structure-function[END_REF] and physiological scale (e.g. high specific hydraulic conductivity, [START_REF] Ewers | Water transport in the liana Bauhinia fassoglensis (Fabaceae)[END_REF][START_REF] Zhu | Hydraulic properties and photosynthetic rates in co-occurring lianas and trees in a seasonal tropical rainforest in southwestern China[END_REF][START_REF] Santiago | Physiological implications of the liana growth form[END_REF]. Hence, the liana growth habit is present in at least 130 plant families [START_REF] Gentry | The distribution and evolution of climbing plants[END_REF][START_REF] Isnard | The evolution of angiosperm lianescence: a perspective from xylem structure-function[END_REF] and can account for up to 40% of species and stems in some tropical forests [START_REF] Pérez-Salicrup | Lianas and trees in a liana forest of Amazonian Bolivia[END_REF][START_REF] Schnitzer | A mechanistic explanation for global patterns of liana abundance and distribution[END_REF][START_REF] Schnitzer | The past, present, and potential future of liana ecology[END_REF]. The evolution of the liana growth habit was shown to increase clade diversification rates [START_REF] Gianoli | Evolution of a climbing habit promotes diversification in flowering plants[END_REF][START_REF] Couvreur | Global diversification of a tropical plant growth form: environmental correlates and historical contingencies in climbing palms[END_REF][START_REF] Gianoli | Evolutionary implications of the climbing habit in plants[END_REF], as found for other growth habits (e.g. [START_REF] Roquet | Replicated radiations of the alpine genus Androsace (Primulaceae) driven by range expansion and convergent key innovations[END_REF][START_REF] Frenzke | Evolution of Epiphytism and Fruit Traits Act Unevenly on the Diversification of the Species-Rich Genus Peperomia (Piperaceae)[END_REF].

Convergence toward a particular growth habit often preferentially occurs in some clades [START_REF] Arakaki | Contemporaneous and recent radiations of the world's major succulent plant lineages[END_REF][START_REF] Boucher | Evolution and biogeography of the cushion life form in angiosperms[END_REF], showing the importance of prerequisites and evolutionary constraints for growth habit evolution [START_REF] Rowe | Plant growth forms: an ecological and evolutionary perspective[END_REF]. For instance, some anatomical constructions and morphological attributes facilitate the evolution of a climbing habit [START_REF] Isnard | Mechanical architecture and development in different growth forms of Clematis: implications for canalised evolution of growth forms[END_REF][START_REF] Isnard | The mechanical role of the leaf sheath in rattans[END_REF]. By contrast, anatomical characteristics associated with the evolution of a climbing habit can constrain evolution toward self-supporting shrubs and trees [START_REF] Rowe | Plant growth forms: an ecological and evolutionary perspective[END_REF][START_REF] Wagner | Evolution of shrubs and lianas within Aristolochia subgenus Isotremacombining molecular phylogeny with biomechanical and anatomical approaches[END_REF][START_REF] Wagner | Escaping the lianoid habit: Evolution of shrub-like growth forms in Aristolochia subgenus Isotrema (Aristolochiaceae)[END_REF]. As we move forward in developing our understanding of plant habit evolution (e.g. [START_REF] Dubuisson | Ecological diversity and adaptive tendencies in the tropical fern Trichomanes L. (Hymenophyllaceae) with special reference to climbing and epiphytic habits[END_REF][START_REF] Verboom | Testing the adaptive nature of radiation: growth form and life history divergence in the African grass genus Ehrharta (Poaceae: Ehrhartoideae)[END_REF][START_REF] Bonser | Growth form evolution and shifting habitat specialization in annual plants[END_REF][START_REF] Rowe | Plant growth forms: an ecological and evolutionary perspective[END_REF][START_REF] Mort | Physiological Evolution and Association between Physiology and Growth Form in Aeonium (Crassulaceae)[END_REF][START_REF] Wagner | Escaping the lianoid habit: Evolution of shrub-like growth forms in Aristolochia subgenus Isotrema (Aristolochiaceae)[END_REF][START_REF] Givnish | Adaptive radiation, correlated and contingent evolution, and net species diversification in Bromeliaceae[END_REF], the integrative, and yet relevant, framework provided by plant architecture lags behind other approaches (see [START_REF] Bateman | Evolutionary-developmental change in the growth architecture of fossil rhizomorphic lycopsids: Scenarios constructed on cladistic foundations[END_REF][START_REF] Bateman | Architectural radiations cannot be optimally interpreted without morphological and molecular phylogenies[END_REF][START_REF] Meyer-Berthaud | The land plant cover in the Devonian: a reassessment of the evolution of the tree habit[END_REF][START_REF] Isnard | Growth form evolution in Piperales and its relevance for understanding the angiosperm diversification -An integrative approach combining plant architecture, anatomy and biomechanics[END_REF].

Linking functional traits and plant architecture

Growth habit results from the integration of a combination of several individual traits (e.g. branching pattern, body size and shape, position of inflorescences, anatomy…) that have often been studied separately (e.g. [START_REF] Carlquist | Wood and stem anatomy of Lardizabalaceae with comments on the vining habit, ecology and systematics[END_REF][START_REF] Givnish | Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae)[END_REF][START_REF] Isnard | Growth form evolution in Piperales and its relevance for understanding the angiosperm diversification -An integrative approach combining plant architecture, anatomy and biomechanics[END_REF][START_REF] Wagner | Major trends in stem anatomy and growth forms in the perianth-bearing Piperales, with special focus on Aristolochia[END_REF]. As such, growth habit provides some evidence for the correlated evolution (sensu [START_REF] Pagel | Detecting correlated evolution on phylogenies: a general method for the comparative analysis fo discrete characters[END_REF] of two or more traits across lineages. Some of these trait associations, known as global spectra (e.g. [START_REF] Reich | The Evolution of Plant Functional Variation: Traits, Spectra, and Strategies[END_REF][START_REF] Wright | The worldwide leaf economics spectrum[END_REF][START_REF] Chave | Towards a worldwide wood economics spectrum[END_REF][START_REF] Díaz | The global spectrum of plant form and function[END_REF], are considered as primary drivers of plant evolution and functional diversity worldwide [START_REF] Díaz | The plant traits that drive ecosystems: Evidence from three continents[END_REF][START_REF] Poorter | Leaf traits are good predictors of plant performance across 53 rain forest species[END_REF][START_REF] Díaz | The global spectrum of plant form and function[END_REF]. Among the oldest and best documented are Corner's rules [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF](Corner, , 1953(Corner, -1954) ) describing a universal correlation between branching intensity, leaf size, stem size, fruit size, and inflorescence complexity [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]White, 1983b;[START_REF] Bond | Allometry and sexual differences in leaf size[END_REF][START_REF] Lauri | Le mouvement morphogenetique, approche morphometrique et restitution graphique: L'exemple de quelques plantes tropicales[END_REF][START_REF] Brouat | Corner's rules revisited: ontogenetic and interspecific patterns in leaf-stem allometry[END_REF][START_REF] Cornelissen | A triangular relationship between leaf size and seed size among woody species: allometry, ontogeny, ecology and taxonomy[END_REF][START_REF] Brouat | Leaf-stem allometry, hollow stems, and the evolution of caulinary domatia in myrmecophytes[END_REF][START_REF] Westoby | Plant Ecological Strategies: Some Leading Dimensions of Variation Between Species[END_REF][START_REF] Preston | Hydraulic architecture and the evolution of shoot allometry in contrasting climates[END_REF][START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF][START_REF] Pickup | Dry mass costs of deploying leaf area in relation to leaf size[END_REF][START_REF] Sun | The leaf size-twig size spectrum of temperate woody species along an altitudinal gradient: an invariant allometric scaling relationship[END_REF][START_REF] Normand | Hydraulic and mechanical stem properties affect leaf-stem allometry in mango cultivars[END_REF]. As such, selection on a single trait is likely to affect whole plant form and function (Figure 1.2). In this context, evolution of growth habit needs to be FIGURE 1.2 -Schematic illustration of Corner's rules [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]) corresponding to two principles; (i) "Axial conformity. The stouter, or more massive, the axis in a given species, the larger and more complicated its appendages" and (ii) "Diminution on ramification. The greater the ramification, the smaller become the branches and their appendages". Following these principles, a negative correlation is expected between branching intensity and leaf size, fruit size and inflorescence size.

studied through an integrative approach merging functional traits and plant structural construction.

Plant architecture characterizes the spatial arrangement and specialization of structures (morphological origin, branching pattern, axis categorization) and their evolution during ontogeny [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF], see Chapter 2 for more details). It can consequently highlight how plant structure correlates with function and help identify the evolutionary processes behind plant evolution (see [START_REF] Bateman | Evolutionary-developmental change in the growth architecture of fossil rhizomorphic lycopsids: Scenarios constructed on cladistic foundations[END_REF][START_REF] Bateman | Architectural radiations cannot be optimally interpreted without morphological and molecular phylogenies[END_REF][START_REF] Sussex | The evolution of plant architecture[END_REF][START_REF] Meyer-Berthaud | The land plant cover in the Devonian: a reassessment of the evolution of the tree habit[END_REF]. Architectural studies have taught us that plants are modular organisms composed of structural elements that can differ in their organization and function [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF]. For instance in many tree and treelet species, the trunk mainly assumes exploration and support functions while branches are, in comparison, more specialized in assimilation and reproduction. As such, plant architecture provides integrative tools to understand plant spatial and temporal exploitation of resources [START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF][START_REF] Smith | Deviation from symmetrically self-similar branching in trees predicts altered hydraulics, mechanics, light interception and metabolic scaling[END_REF]. Architectural traits have been shown to impact plant fitness either directly [START_REF] Küppers | Ecological significance of above-ground architectural patterns in woody plants: A question of cost-benefit relationships[END_REF][START_REF] Millet | Relationship between architecture and successional status of trees in the temperate deciduous forest[END_REF][START_REF] Charles-Dominique | Architectural strategies of Cornus sericea, a native but invasive shrub of Southern Quebec, Canada, under an open or a closed canopy[END_REF][START_REF] Charles-Dominique | Architectural strategies of Rhamnus cathartica, in relation to canopy openness[END_REF][START_REF] Millan | Analyse de la variabilité des traits architecturaux des formes de croissance dans les communautés végétales[END_REF][START_REF] Charles-Dominique | The architectural design of trees protects them against large herbivores[END_REF] or in interaction with other functional traits [START_REF] Pérez-Harguindeguy | New handbook for standardised measurement of plant functional traits worldwide[END_REF][START_REF] Trueba | Trait coordination, mechanical behaviour and growth form plasticity of Amborella trichopoda under variation in canopy openness[END_REF]. Plant architecture thus has much to offer in comparative studies that aim to decipher the evolution of plant growth habits and their associated traits.

The monocaulous growth habit

From the above architectural background, we know that plant functions are generally partitioned into different axis categories. However, among the diversity of extant and past known architectures (see [START_REF] Galtier | How early ferns became trees[END_REF][START_REF] Hallé | Architectures de Plantes[END_REF][START_REF] Meyer-Berthaud | L'évolution des premiers arbres : les stratégies dévoniennes[END_REF][START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF], some trees that have been referred to as "monocaulous" have such a simple structure that they are constituted by a single stem [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF].

Monocauly classically characterizes woody plants constituted by a single unbranched trunk supporting a distal rosette of large leaves [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]. The term was used in various domains, and formal definitionswhen providedoften differ among studies (see Chapter 3 for a more detailed review). This growth habit, particularly atypical for non-monocots, has fascinated naturalists for a long time (e.g. [START_REF] Humboldt | Ansichten der Natur mit wissenschaftlichen Erläuterungen[END_REF][START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF][START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF][START_REF] Hedberg | Tropical-Alpine Life-Forms of Vascular Plants[END_REF]) and is at the center of highly discussed ecological and evolutionary theories. Monocaulous species were long considered as relicts of the ancestral form for Angiosperms [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]. Recent molecular phylogenies have indicated multiple recent evolution of monocauly in Angiosperms (e.g. [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF] but the evolutionary history of extant monocauls remains unclear. Monocaulous plants were also at the inception of Corner's rules [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF](Corner, , 1953(Corner, -1954) ) (Figure 1.2), whose statements are today among the most widely documented global spectra (leafstem scaling or foliagestem scaling, e.g. [START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF][START_REF] Olson | Universal foliage-stem scaling across environments and species in dicot trees: plasticity, biomechanics and Corner's Rules[END_REF][START_REF] Yang | The Effects of Leaf Size, Leaf Habit, and Leaf Form on Leaf/Stem Relationships in Plant Twigs of Temperate Woody Species[END_REF]. Probably because of their global rarity and restriction to tropical areas, monocaulous species have rarely been included in ecological and evolutionary studies. The most famous case of the evolution of monocauly is represented by "unbranched shrubs with massive leaf rosettes that dominate equatorial alpine zones in many part of the world" [START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. In these rosette-tree species, architecture, anatomy, and leaf features represent adaptation to nightly frost and very high diurnal fluctuations in temperature (Goldstein and [START_REF] Goldstein G | Influence of insulating dead leaves and low temperatures on water balance in an Andean giant rosette plant[END_REF][START_REF] Meinzer | Adaptations for water and thermal balance in Andean giant rosette plants[END_REF][START_REF] Melcher | Determinants of Thermal Balance in the Hawaiian Giant Rosette Plant, Argyroxiphium sandwicense[END_REF][START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. Other monocaulous species are restricted to dry areas, particularly on islands, where their ecology and evolution have been indirectly investigated through secondary woodiness (Mabberley, 1974b, a;[START_REF] Böhle | Island colonization and evolution of the insular woody habit in Echium L. (Boraginaceae)[END_REF][START_REF] Mort | Physiological Evolution and Association between Physiology and Growth Form in Aeonium (Crassulaceae)[END_REF][START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF]. But the majority of monocaulous species seem to occur in tropical rainforests where other abiotic conditions likely favor the evolution of this habit (e.g. [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. This other kind of monocauly has been much less studied, although it has been notedsometime as highly diversifiedin rainforests worldwide such as in New Guinea [START_REF] Hallé | Architecture of Trees in the Rain Forest of Morobe District, New Guinea[END_REF], Gabon [START_REF] Hallé | Présentation de quelques formes ligneuses simples de la forêt de Belinga (Gabon)[END_REF], Panama (D'Arcy, 1973), Madagascar [START_REF] Koechlin | Recently evolved diversity and convergent radiations of rainforest mahoganies (Meliaceae) shed new light on the origins of rainforest hyperdiversity[END_REF], and especially New Caledonia, where monocauly has long attracted the attention of botanists [START_REF] Carlquist | Island Biology[END_REF][START_REF] Veillon | Architecture végétative de quelques arbres de l'archipel Néo-caledonien[END_REF][START_REF] Schmid | Les écosystèmes forestiers mélanésiens (Nouvelle Calédonie, Nouvelles-Hébrides, Fidji et îles Salomon)[END_REF][START_REF] Schmid | Fleurs et plantes de Nouvelle-Calédonie[END_REF][START_REF] Schmid | Conditions d'évolution et caractèristiques du peuplement végétal insulaire en Mélanésie Occidentale: Nouvelle-Calédonie et Vanuatu[END_REF]. [START_REF] Carlquist | Island Biology[END_REF] was probably the first to note that, in New Caledonia, "there is a modal tendency for smaller, sparsely branched shrubs". [START_REF] Schmid | Fleurs et plantes de Nouvelle-Calédonie[END_REF] considered that the preponderance of this growth habit is among the main physiognomical characteristics of New Caledonian rainforest understory. He estimated that at least 20 families include monocaulous or pachycaulous (i.e. sparsely branched) species [START_REF] Schmid | Conditions d'évolution et caractèristiques du peuplement végétal insulaire en Mélanésie Occidentale: Nouvelle-Calédonie et Vanuatu[END_REF]. A more accurate quantification of the diversity of monocaulous plants in New Caledonia was provided by the work of [START_REF] Veillon | Architecture végétative de quelques arbres de l'archipel Néo-caledonien[END_REF], who described the architectural models of 575 vascular plant species. His results indicated that ca. 19% of the studied flora was monocaulous and that this growth habit was found to occur in 23 families. While this study did not include all of the non-monocot flora, the frequency of monocauly in so many different families suggests a potentially remarkable case of convergent evolution.

Problematic and objectives

Convergence in growth habit is a key evolutionary pattern linking plant architecture, functional traits and the abiotic environment. While the phenomenon is a fascinating particularity of island biotas, it has never been deeply described in the megadiverse and distinctive flora of New Caledonia. Monocauly represents a growth habit whose ecology and evolutionary history remain poorly known and the repeated observations of its frequency in several distantly related families could clearly exemplify the first case of evolutionary convergence for the archipelago. The objectives of this thesis are to investigate the diversity, ecology and evolution of monocaulous species in New Caledonia. More precisely, the rationale and design of the study were developed around the following questions:

Ø What is monocauly? How many non-monocot monocaulous species are there in New Caledonia?

Ø How many times did monocauly evolve? What are the evolutionary correlates of monocauly? Are there prerequisites or environmental contingencies that appear to have facilitated its evolution?

Ø Can we use an architectural approach to depict the evolution of monocauly in a species-level phylogeny? What are the functional and ecological implications of shifts in growth habit? Has monocauly been involved in the diversification of lineages?

Ø What are the morpho-anatomical traits associated with the monocaulous habit? Do we find evidence that supports Corner's rule? Ø Which hypotheses could explain convergence toward monocauly in New Caledonia?

Thesis outline

This thesis comprises seven chapters of which four are presented under an article format.

For these four chapters, only the layout was edited from the original manuscripts articles to provide better consistency within the document. Each chapter depicts different and complementary approaches to characterizing monocauly and analysing the diversity, ecology and evolution of monocaulous plants in New Caledonia.

Chapter 2 presents the general methodology used throughout this work. It describes in particular the study sites, the architectural approach, and how it was used to compile a list of New Caledonian monocaulous species.

Chapter 3 presents a bibliographic synthesis of the concept of monocauly and hypotheses about its evolution and associated life history and environmental traits. This chapter does not follow an article format but is planned to be the starting point of a review paper.

In Chapter 4 we describe a new monocaulous species encountered during this thesis. By illustrating architectural diversity in the endemic genus Bocquillonia (Euphorbiaceae), we also aim to show possible variation in growth habit around a single architectural model.

The aim of Chapter 5 is to study the evolutionary history of monocaulous species throughout the phylogeny of New Caledonian woody genera. In particular, we quantified repeated evolution, phylogenetic signal and evolutionary correlates of monocauly in the island. IUCN risk of extinction status and threats to the New Caledonian flora were used to evaluate the major threats to monocaulous species.

Chapter 6 aims to illustrate in detail the evolution from a branched habit toward the monocaulous habit and the functional implication of this transition in the genus Atractocarpus (Rubiaceae). We furthermore investigated how growth habit transition could affect species diversification in the genus.

The aim of Chapter 7 is to analyse morpho-anatomical traits of monocaulous species through a comparative approach that also includes branched close relatives and to provide insights into the functional implications of Corner's rules.

Finally, the results of these chapters are summarized, combined and discussed in Chapter 8. This general discussion deals in particular with the functional attributes and adaptive significance of monocauly, evolutionary pathways leading to monocauly, inferences about the evolution and conservation of the New Caledonian flora, and the general interest of plant architecture to depict plant ecology and evolution. and north of New Zealand (ca. 2000 km). The total land area is about 18,500 km², of which the largest island (Grande Terre) covers 89%. The remaining land area is divided between the Loyalty Islands (Ouvéa, Lifou and Maré), Belep, the Ile des Pins and several smaller islands (Figure 2.1).

Grande Terre (ca. 400 km long and 50 km wide on average) is divided lengthwise (from Northwest to Southeast) by a continuous mountain range that reaches 1628 m in the North (Mont Panié) and 1618 m in the South (Mont Humboldt). This mountain range is closer to the east coast, where it drops steeply into the sea, while the west coast is characterized by relatively broad plains, except on the Northwest, where large isolated mountains occur.

The subtropical climate of New Caledonia is strongly influenced by a marked orography; mean annual precipitation ranges from 800 mm.yr -1 in the lowlands of the leeward west coast to more than 4000 mm.yr -1 on the windward east coast and mountain tops (Météo-France, 2007;[START_REF] Maitrepierre | Les types de temps et les cyclones, les éléments du climat[END_REF]. A dry season (precipitation < 100 mm.month -1 ) occurs from August to November. Mean annual temperatures are comprised between ca. 20 and 25 °C, and frost is supposed to rarely occurs, even on mountain tops [START_REF] Maitrepierre | Les types de temps et les cyclones, les éléments du climat[END_REF]. New Caledonia is frequently subject to cyclones and tropical depressions (3 or 4 per year on average) that can be very intense [START_REF] Maitrepierre | Les types de temps et les cyclones, les éléments du climat[END_REF]Ibanez et al., 2018a). The climate is highly variable from one year to another due to the influence of larger climatic events such as ENSO (El Niño-Southern Oscillation, FIGURE 2.1 -Topographical map of New Caledonia (modified from georep.nc) and location in the South-West Pacific (modified from geographicguide.com). [START_REF] Maitrepierre | Les types de temps et les cyclones, les éléments du climat[END_REF][START_REF] Menkes | Les grandes fluctuations des hydroclimats: le phénomène ENSO[END_REF].

Three main geological substrates are distinguished in New Caledonia [START_REF] Paris | Géologie de la Nouvelle-Calédonie, un essai de synthèse[END_REF][START_REF] Jaffré | The Relationship between Ecological Diversity and Floristic Diversity in New Caledonia[END_REF] (Figure 2.2-A). (i) Ultramafic substrate covers ca. 1/3 of Grande Terre, mainly in the southeast of Grande Terre and on the northwest mountain massifs. Soils derived from ultramafic substrate are generally poor in plant-essential nutrients (P, Ca, K) and rich in elements which are normally toxic for plants (Ni, Co, Mn, Cr) [START_REF] Jaffré | Composition chimique et conditions de l'alimentation minérale des plantes sur roches ultrabasiques (Nouvelle Calédonie)[END_REF][START_REF] Jaffré | Etude écologique du peuplement végétal des sols dérivés de roches ultrabasiques en Nouvelle-Calédonie[END_REF][START_REF] Isnard | How did the ultramafic soils shape the flora of the New Caledonian hotspot?[END_REF]. (ii) Volcanosedimentary substrates cover ca. 2/3 of Grande Terre and are highly diverse in origin and composition [START_REF] Fritsch | Les sols[END_REF]. They comprise mainly micaschists, glaucophtanites and phtanites in the North of Grande Terre, whereas the central metamorphic mountain groups are formed of greywacks [START_REF] Paris | Géologie de la Nouvelle-Calédonie, un essai de synthèse[END_REF][START_REF] Jaffré | The Relationship between Ecological Diversity and Floristic Diversity in New Caledonia[END_REF]. (iii) Calcareous substrate is much less common covering less than 20% of the archipelago [START_REF] Paris | Géologie de la Nouvelle-Calédonie, un essai de synthèse[END_REF][START_REF] Morat | The flora of New Caledonia's calcareous substrates[END_REF] almost exclusively found in the Loyalty Islands and the Ile des Pins (raised coral formations), but also occurs in small areas on Grande Terre (basalt calcareous rocks) [START_REF] Paris | Géologie de la Nouvelle-Calédonie, un essai de synthèse[END_REF][START_REF] Morat | The flora of New Caledonia's calcareous substrates[END_REF].

Geological and Paleoclimatic history

New Caledonia's main island (along with Belep and the Ile des Pins) is a part of the New Caledonian ridge that split and spread from the eastern margin of the Gondwanan supercontinent during the Cretaceous (ca. -120 to -80 Myr) [START_REF] Picard | L'archipel néo-calédonien, 300 millions d'années pour assembler les pièces d'un puzzle géologique[END_REF][START_REF] Cluzel | Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications[END_REF][START_REF] Pelletier | Geology of the New Caledonia region and its implications for the study of the New Caledonian biodiversity[END_REF][START_REF] Cluzel | An outline of the Geology of New Caledonia; from Permian-Mesozoic Southeast Gondwanaland active margin to Cenozoic obduction and supergene evolution[END_REF]. The presence of numerous endemic relictual lineages on the island led some authors to think that this piece of Gondwana remained emerged from the rifting event until today (e.g. [START_REF] Raven | Plate Tectonics and Australasian Paleobiogeography[END_REF]. However geological insights have shown that New Caledonia was submerged from the Paleocene to the Eocene (ca. -62 to -50 Myr) at which time it was obducted under the Pacific plate and covered by oceanic crust [START_REF] Picard | L'archipel néo-calédonien, 300 millions d'années pour assembler les pièces d'un puzzle géologique[END_REF][START_REF] Cluzel | Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications[END_REF][START_REF] Pelletier | Geology of the New Caledonia region and its implications for the study of the New Caledonian biodiversity[END_REF][START_REF] Cluzel | An outline of the Geology of New Caledonia; from Permian-Mesozoic Southeast Gondwanaland active margin to Cenozoic obduction and supergene evolution[END_REF], leading to the formation of metamorphic rocks and to the atypical ultramafic substrate. New Caledonia reemerged during the Eocene (-50 to -35 Myr, and probably ca. -37 Myr) after which the ultramafic layer progressively weathered to the present day resulting in the partial exposure of subjacent volcano-sedimentary substrates. This scenario is coherent with the evolutionary history of several New Caledonian lineages which suggests that local radiations are younger than 37 Myr [START_REF] Murienne | Evolution on a shaky piece of Gondwana: is local endemism recent in New Caledonia?[END_REF][START_REF] Grandcolas | New Caledonia: a very old Darwinian island?[END_REF][START_REF] Pillon | Time and tempo of diversification in the flora of New Caledonia[END_REF]. The Loyalty Islands emerged more recently (Pliocene, ca. -2 Myr) through an uplifting of the Loyalty ridge [START_REF] Picard | L'archipel néo-calédonien, 300 millions d'années pour assembler les pièces d'un puzzle géologique[END_REF][START_REF] Pelletier | Geology of the New Caledonia region and its implications for the study of the New Caledonian biodiversity[END_REF].

Paleoclimatic data indicate that the Southwest Pacific has experienced a general cooling since early , leading to an increase in aridity [START_REF] Gallagher | The Miocene palaeoenvironmental and palaeoceanographic evolution of the Gippsland Basin, Southeast Australia: a record of Southern Ocean change[END_REF][START_REF] Zachos | Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present[END_REF][START_REF] Dodson | Palynological evidence for aridity events and vegetation change during the Middle Pliocene, a warm period in Southwestern Australia[END_REF]. This trend was punctuated by several more or less pronounced oscillations such as the drastic increase in both temperature and precipitation in the Miocene (ca. -15 to -17 Myr, Zachos et al., 2001;[START_REF] Böhme | The Miocene Climatic Optimum: evidence from ectothermic vertebrates of Central Europe[END_REF] coupled with intense cooling [START_REF] Gallagher | The Pliocene climatic and environmental evolution of southeastern Australia: evidence from the marine and terrestrial realm[END_REF][START_REF] Dodson | Palynological evidence for aridity events and vegetation change during the Middle Pliocene, a warm period in Southwestern Australia[END_REF]. This overall climate aridification had important consequences on vegetation, especially a decline of rainforest areas, which probably disappeared in some regions such as Australia [START_REF] Gallagher | The Pliocene climatic and environmental evolution of southeastern Australia: evidence from the marine and terrestrial realm[END_REF][START_REF] Crisp | Radiation of the Australian flora: what can comparisons of molecular phylogenies across multiple taxa tell us about the evolution of diversity in present-day communities?[END_REF][START_REF] Dodson | Palynological evidence for aridity events and vegetation change during the Middle Pliocene, a warm period in Southwestern Australia[END_REF][START_REF] Byrne | Birth of a biome: insights into the assembly and maintenance of the Australian arid zone biota[END_REF][START_REF] Byrne | Decline of a biome: evolution, contraction, fragmentation, extinction and invasion of the Australian mesic zone biota[END_REF]. For New Caledonia, paleoclimatic data are scarce but its small size and isolated position in the Pacific are thought to have buffered the effects of general aridification [START_REF] Barrabé | Systématique et Evolution du genre Psychotria (Rubiaceae) en Nouvelle-Calédonie[END_REF]. Nevertheless, the archipelago is likely to have experienced several glacial episodes during the Neogene (ca. -6.5 Myr) and Quaternary (ca. -2.5 Myr) [START_REF] Chevillotte | Long-term tropical morphogenesis of New Caledonia (Southwest Pacific): Importance of positive epeirogeny and climate change[END_REF][START_REF] Karas | Pliocene climate change of the Southwest Pacific and the impact of ocean gateways[END_REF] and also more recently (-22000 and -12000 yr, Tournebize et al., 2017). Despite these glacial episodes, rainforests seem to have continuously persisted in New Caledonia for quite a long period [START_REF] Hope | Tropical vegetational change in the late Pleistocene of New Caledonia[END_REF][START_REF] Stevenson | A comparison of late Quaternary forest changes in New Caledonia and northeastern Australia[END_REF][START_REF] Tournebize | Two disjunct Pleistocene populations and anisotropic postglacial expansion shaped the current genetic structure of the relict plant Amborella trichopoda[END_REF], contrary to adjacent regions [START_REF] Kemp | Tertiary climatic evolution and vegetation history in the Southeast Indian Ocean region[END_REF][START_REF] Gallagher | The Pliocene climatic and environmental evolution of southeastern Australia: evidence from the marine and terrestrial realm[END_REF]; [START_REF] Jaffré | La végétation[END_REF]. [START_REF] Crisp | Radiation of the Australian flora: what can comparisons of molecular phylogenies across multiple taxa tell us about the evolution of diversity in present-day communities?[END_REF][START_REF] Dodson | Palynological evidence for aridity events and vegetation change during the Middle Pliocene, a warm period in Southwestern Australia[END_REF][START_REF] Byrne | Birth of a biome: insights into the assembly and maintenance of the Australian arid zone biota[END_REF][START_REF] Byrne | Decline of a biome: evolution, contraction, fragmentation, extinction and invasion of the Australian mesic zone biota[END_REF]. In investigating the evolutionary history of rainforest lineages, [START_REF] Pillon | Time and tempo of diversification in the flora of New Caledonia[END_REF] argued that this vegetation type is likely to have persisted in New Caledonia since at least 6.9 Myr.

Flora and vegetation

The New Caledonian flora is known worldwide for its exceptional levels of diversity and endemism. The flora comprises ca. 3400 species of vascular plants, of which ca. 2500 (75%) are endemic [START_REF] Morat | The taxonomic database « FLORICAL » and characteristics of the indigenous flora of New Caledonia[END_REF][START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF]. This high level of endemism is also seen at supra-specific levels as the flora includes between 62 and 91 endemic genera (ca. 13%, [START_REF] Pillon | How many genera of vascular plants are endemic to New Caledonia? A critical review based on phylogenetic evidence[END_REF] and 3 endemic families (Amborellaceae, Oncothecaceae, and Phellinaceae). The taxonomic composition of New Caledonia's flora is highly disharmonic compared to adjacent regions, some lineages being over-represented (e.g. Gymnosperm, Cunoniaceae, Myrtaceae and Basal Angiosperms) while others are under-represented (e.g. Asteraceae, Ericaceae and Lamiaceae) [START_REF] Morat | Richesse et affinités floristiques de la Nouvelle-Calédonie: conséquences directes de son histoire géologique[END_REF][START_REF] Jaffré | Distribution and Ecology of the Conifers of New Caledonia[END_REF][START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF][START_REF] Trueba | Ecology, forms and functions of the basal angiosperms from New Caledonia[END_REF]. This disharmony also involves functional groups, for example a high proportion of metal hyper-accumulators [START_REF] Jaffré | The metal hyperaccumulators from New Caledonia can broaden our understanding of nickel accumulation in plants[END_REF][START_REF] Van Der Ent | The flora of ultramafic soils in the Australia-Pacific Region : state of knowledge and research priorities[END_REF][START_REF] Jaffré | The discovery of nickel hyperaccumulation in the New Caledonian tree Pycnandra acuminata 40 years on: an introduction to a Virtual Issue[END_REF] and dioecious species [START_REF] Carpenter | Reproductive traits of tropical rain-forest trees in New Caledonia[END_REF][START_REF] Schlessman | Incidence, Correlates, and Origins of Dioecy in the Island Flora of New Caledonia[END_REF], as well as a low richness of lianas [START_REF] Bruy | Abundance, richness and composition of lianas in forest communities along an elevation gradient in New Caledonia[END_REF]. Several unusual biological forms are also known, such as the world's only parasitic or mycotrophic gymnosperm [START_REF] Laubenfels | Parasitic Conifer Found in New Caledonia[END_REF][START_REF] Feild | A unique mode of parasitism in the conifer coral tree Parasitaxus ustus (Podocarpaceae)[END_REF][START_REF] Hs | Parasitic Flowering Plants[END_REF], a monocarpic and branched large tree [START_REF] Veillon | Une Apocynacée monocarpique de Nouvelle-Calédonie Cerberiopsis candelabrum Vieill[END_REF][START_REF] Read | Mass flowering and parental death in the regeneration of Cerberiopsis candelabra (Apocynaceae), a long-lived monocarpic tree in New Caledonia1[END_REF], and an aquatic fern that can live completely submerged [START_REF] Veillon | Réhabilitation de l'espèce Blechnum francii Rosenstock, fougère aquatique de la Nouvelle-Calédonie[END_REF]. This astonishing flora is also of conservation importance because it is highly threatened [START_REF] Jaffré | Threatened plants of New Caledonia: Is the system of protected areas adequate?[END_REF][START_REF] Lowry | Diversity, endemism, and extinction in the flora of new Caledonia: a review[END_REF]. The system of protected areas appears to be inadequate, particularly for the highly fragmented rainforest and ultramafic substrate [START_REF] Jaffré | Conservation programmes in New Caledonia, western Pacific: in place for the dry forest, but urgently needed for the ultramafic vegetation[END_REF][START_REF] Jaffré | Threats to the conifer species found on New Caledonia's ultramafic massifs and proposals for urgently needed measures to improve their protection[END_REF][START_REF] Wulff | Conservation priorities in a biodiversity hotspot: analysis of narrow endemic plant species in New Caledonia[END_REF]Ibanez et al., 2017a;Ibanez et al., 2018b). These threats, along with the New Caledonia's outstanding diversity, led scientists to recognize the archipelago as one of the 10 original "Biodiversity Hotspots" [START_REF] Myers | Threatened biotas: "Hot Spots" in tropical forests[END_REF]. Today, the archipelago remains the smallest of the 34 recognized Hotspots [START_REF] Mittermeier | Hotspots revisited[END_REF]. Recently, the risk of extinction of New Caledonian plant species started to be extensively evaluated using the IUCN Red List criteria (IUCN, 2012). Among the 833 species evaluated to date, 43% appeared to be threatened and one species was considered as extinct (RLA-NC Flora, unpublished data). The principal threats are bushfires (ca. 64% of species concerned), mining activities (ca. 40%) and introduced herbivores such as deer (28%).

Several vegetation types have been recognized in New Caledonia, the most general classification used being that of [START_REF] Jaffré | La végétation[END_REF]. Based on climatic, structural and floristic features, this classification includes seven main types: (i) low-and mid-elevation rainforest, (ii) high elevation rainforest and maquis, (iii) sclerophyllous forest, (iv) low-and mid-elevation maquis, (v) savanna (essentially non-native), (vi) halophytic vegetation, and (vii) wetlands (Figure 2.2-B). Among the native vegetation types, maquis, whose occurrence is related to the fire regime [START_REF] Jaffré | Etude écologique du peuplement végétal des sols dérivés de roches ultrabasiques en Nouvelle-Calédonie[END_REF][START_REF] Isnard | How did the ultramafic soils shape the flora of the New Caledonian hotspot?[END_REF], is the most extensive (covering 4600 km²). Rainforests (covering ca. 3,800 km²) contain by far the greatest diversity (more than 2000 native species against ca. 1200 in maquis) [START_REF] Jaffré | Input of the different vegetation units to the richness and endemicity of the New Caledonian flora[END_REF]Birnbaum et al., 2015a;Birnbaum et al., 2015b) and is one of the most threatened formations [START_REF] Jaffré | Conservation programmes in New Caledonia, western Pacific: in place for the dry forest, but urgently needed for the ultramafic vegetation[END_REF][START_REF] Jaffré | Threats to the conifer species found on New Caledonia's ultramafic massifs and proposals for urgently needed measures to improve their protection[END_REF]Ibanez et al., 2017a), along with dry forests [START_REF] Bouchet | Plant extinction in New Caledonia: protection of sclerophyll forests urgently needed[END_REF][START_REF] Lowry | Diversity, endemism, and extinction in the flora of new Caledonia: a review[END_REF]. New Caledonian rainforests are characterized by high beta diversity, very high stem density coupled with small stem diameter (inferior to 10 cm DBH), and low canopy height (about 15-25 m) [START_REF] Jaffré | Etude floristique et structurale de deux forêts denses humides sur roches ultrabasiques en Nouvelle-Calédonie[END_REF][START_REF] Jaffré | Structural and floristic characteristics of a rain forest on schist in New Caledonia : a comparaison with an ultramafic rain forest[END_REF][START_REF] Ibanez | Structural and floristic diversity of mixed tropical rain forest in New Caledonia: newdata from the New-Caledonian Plant Inventory and Permanent PlotNetwork (NC-PIPPN)[END_REF][START_REF] Blanchard | Contrasted allometries between stem diameter, crown area, and tree height in five tropical biogeographic areas[END_REF]Ibanez et al., 2017b).

The floristic and structural distinctiveness of the New Caledonian biota is the result of several factors, of which ultramafic substrate, rainforest refugia and cyclones are considered as the most important. As suggested by New Caledonia's geological history (see above), its flora is the result of post-emergence colonization (mainly from Australia, [START_REF] Morat | Our knowledge of the flora of New Caledonia: endemism and diversity in relation to vegetation types and substrates[END_REF][START_REF] Swenson | Sapotaceae biogeography supports New Caledonia being an old Darwinian island[END_REF][START_REF] Thomas | Molecular dating of Winteraceae reveals a complex biogeographical history involving both ancient Gondwanan vicariance and longdistance dispersal[END_REF] either through long-distance dispersal or shorter-distance dispersal from now submerged intermediate islands [START_REF] Pelletier | Geology of the New Caledonia region and its implications for the study of the New Caledonian biodiversity[END_REF]. Species establishment was largely filtered by the constraining ultramafic substrate, which is considered as the main driver of taxonomic disharmony [START_REF] Jaffré | Changements dans la végétation de la Nouvelle-Calédonie au cours du Tertiaire: la végétation et la flore des roches ultrabasiques[END_REF][START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF][START_REF] Isnard | How did the ultramafic soils shape the flora of the New Caledonian hotspot?[END_REF]. Another important driver is the longterm persistence of rainforests in the archipelago (see above), which are thought to have served as refugia for drought-sensitive lineages such as basal Angiosperms or palms [START_REF] Pintaud | Chorology of New Caledonian palms and possible evidence of Pleistocene rain forest refugia[END_REF][START_REF] Poncet | Phylogeography and niche modelling of the relict plant Amborella trichopoda (Amborellaceae) reveal multiple Pleistocene refugia in New Caledonia[END_REF][START_REF] Pouteau | New Caledonia: a Pleistocene refugium for rain forest lineages of relict angiosperms[END_REF][START_REF] Tournebize | Two disjunct Pleistocene populations and anisotropic postglacial expansion shaped the current genetic structure of the relict plant Amborella trichopoda[END_REF] that disappeared from dryer regions in the western Pacific [START_REF] Bowler | Late Quaternary climates of Australia and New Guinea[END_REF][START_REF] Byrne | Decline of a biome: evolution, contraction, fragmentation, extinction and invasion of the Australian mesic zone biota[END_REF]. Post-glaciation periods provided several ecological opportunities for occurring lineages to diversify and are suspected to have driven the recent diversification of the flora [START_REF] Pillon | Time and tempo of diversification in the flora of New Caledonia[END_REF]. Forest structural characteristics (e.g. high stem density) and composition are believed to have been largely shaped by strong climatic forces (cyclones, see above) that recurrently impact vegetation [START_REF] Read | Population dynamics of canopy trees in New Caledonian rain forests: are monodominant Nothofagus (Nothofagaceae) forests successional to mixed rain forests?[END_REF]Ibanez et al., 2017b;Ibanez et al., 2018a).

Plant architecture

Plant architecture is a domain of plant science that concerns the nature and organization of plant parts and their evolution during ontogeny. It emerged with the fundamental works of Hallé et al. [START_REF] Hallé | Essai sur l'architecture et la dynamique de croissance des arbres tropicaux[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] in which fundamental principles of plant morphology (growth patterns, branching modalities, axis differentiation, and the position of reproductive functions) were combined into a comprehensive and dynamic approach to define 23 architectural models. These models illustrate both the general architecture of a plant and the way it was constructed [START_REF] Hallé | Essai sur l'architecture et la dynamique de croissance des arbres tropicaux[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Barthélémy | Architectural concepts for tropical trees[END_REF][START_REF] Nicolini | Approche morphologique de développement du Hêtre (Fagus sylvatica L.)[END_REF] (Figure 2.3-A). While compelling for the recognition of common overall species forms, architectural models appeared too general to understand fines aspects of complex plant construction [START_REF] Edelin | Image sur l'architecture des conifères[END_REF][START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF].

A deeper characterization of plant architecture came with the development of notions of axis category and architectural units [START_REF] Edelin | Image sur l'architecture des conifères[END_REF][START_REF] Edelin | L'architecture monopodiale: l'exemple de quelques arbres d'Asie tropicale[END_REF][START_REF] Barthélémy | Architectural concepts for tropical trees[END_REF][START_REF] Barthélémy | Levels of organization and repetition phenomena in seed plants[END_REF]. Individuals of each species are made of a limited number of axis categories [START_REF]Inflorescences elongated, typically more than 5 cm long; inflorescences ♀ with spaced flowers; inflorescences ♂ composed of several glomerules, generally distinct[END_REF][START_REF]Inflorescences shorter, up to 4 cm long, sessile (rarely inflorescences ♀ slightly elongated) with flowers solitary or clustered in a single glomerule hiding the axis[END_REF](3)[START_REF]4 Identification key of McPherson & Tirel[END_REF][START_REF]Blades elliptic, ovate or obovate, generally with acute bases (if bases obtuse, longest petioles > 3 cm long)[END_REF](6), each characterized by a non-limitative combination of morphological, anatomical and functional traits (Figure 2.3-B). The number of axis categories, their characteristics and their spatial arrangement determine the so-called architectural unit, i.e. the species-specific expression of an architectural model [START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF]. The higher an axis category (i.e. situated at the periphery of the plant), the more functionally specialized it is [START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF]. For example, in most of tree and treelet species, trunks (axis category 1 = C1) have the general functions of support and storage. The more the axis number increases (C2, C3…), the more specialized the axis is in exploration, photosynthesis and reproductive functions. Extreme examples of axis specialization are provided by some inflorescences for reproduction function [START_REF] Van Steenis | Definition of the concept "inflorescence" with special reference to ligneous plants[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] or by phyllomorphic branches for photosynthesis [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Hallé | Etude biologique et morphologique de la tribu des Gardéniées[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF].

In most cases, a well-defined architectural unit is modified through the morphogenetic process of reiteration. Reiteration is a special branching process by which a plant duplicates its architectural unit [START_REF] Oldeman | A molecular phylogeny of the Solanaceae[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF]. The process of reiteration is complex and not necessarily well understood. Different modalities of reiteration have been recognized according to whether (i) the complete architectural unit is duplicated (total reiteration) or only a part of it (partial reiteration), (ii) the reiterate is produced by the development of a dormant bud (delayed reiteration) or by the transformation of an already growing meristem (immediate reiteration), and (iii) the reiteration process is included in the developmental sequence of the species (sequential reiteration) or is due to either plant damage (traumatic opportunistic reiteration) or to an increase in resource levels (adaptive opportunistic reiteration). All combinations of reiteration modalities are theoretically possible (Figure 2.4) but all of them have never been observed in a single species, each taxon having instead its own reiterative strategy [START_REF] Edelin | Stratégie de réitération et édification de la cime chez les conifères[END_REF][START_REF] Barthélémy | Levels of organization and repetition phenomena in seed plants[END_REF].

Toward a new definition of monocauly: between structure and function

Plant architecture has taught us that plant form is due to the genetically controlled association of several structuro-functionnal entities (phytomers, growth units, axis categories, reiterates…) whose arrangement and differentiation change with age and are shaped by the environment. This integrative approach revealed objective criteria that could be powerful for defining growth habit [START_REF] Millan | Analyse de la variabilité des traits architecturaux des formes de croissance dans les communautés végétales[END_REF]. For instance, among the few attempts to define the monocaulous habit (see Chapter 3), that of [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] is probably the most successful.

These authors made the distinction between the structural definition ("trees with a single axis" = monoaxial) and the physiognomic definition ("trees with a single trunk or visible stem of the plant" = monocaulous). The latter, referring to the general appearance of the plant rather than its structural construction, seems more appropriate for an ecological study since it is directly linked with the space exploration strategy of species. To pursue this functional aspect of growth habit further, monocauls could be defined as "self-supporting woody plants whose cardinal functions rely on a single visible stem". This functional definition, better suited to studying the adaptive aspects of growth habit, is the one we will use here. Beyond giving a clear physiognomic definition of monocauly, the architectural approach of Hallé et al. [START_REF] Hallé | Essai sur l'architecture et la dynamique de croissance des arbres tropicaux[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] was the first to provide clear discriminating morphological criteria. The classification of a species in the monocaulous class relies on the selection of structural types fitting the given definition of monocauly. In the work of [START_REF] Hallé | Essai sur l'architecture et la dynamique de croissance des arbres tropicaux[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], structural types corresponded to architectural models. This classification, while providing a fundamental basis for plant architecture, nevertheless appeared to be too general to understand the precise architecture of plants and more integrative concepts such as architectural units and reiteration were later developed (see section 2.2). The work of [START_REF] Hallé | Essai sur l'architecture et la dynamique de croissance des arbres tropicaux[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] aimed to present the known diversity of developmental plans observed in tropical trees. Our aim is different, since our interest is to segregate plants for which vegetative functions are assumed by one visible stem, from other plants (i.e. branched). For all these reasons, we will not strictly refer to architectural models to define our structural types of monocauly but our classification will, in essence, largely overlap that of [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. Our definition of monocauly, focusing on function rather than structure, includes true woody plants with a single orthotropic entity functioning as a trunk and determined plagiotropic structures functioning as leaves. The only other aboveground structures are those specialized in reproduction (i.e. inflorescences) or are due to exogenous stimuli (i.e. opportunistic reiterates). [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], a single trunk could be constructed in two different ways. (i) In most cases (and quite apart from traumatisms), single trunks are produced by one single meristem functioning throughout the plant's lifespan (Figure 2.5-A). In this case, the trunk is monoaxial and the meristem might stop functioning following flowering (monocarpic plant) or natural senescence (polycarpic plant). (ii) In some cases, a single trunk could be constructed by several meristems, each producing a module, that stack up to form a linear sympodium (Figure 2.5-B). In this case, the trunk is polyaxial and meristems of each module might stop functioning by flowering (hapaxanthic module) but not necessarily (see [START_REF] Cremers | Etude de l'architecture aérienne de quelques plantes tropicales à ramification basitone: vers une révision du modèle de Tomlinson[END_REF].

Trunk entity -As argued by

Photosynthetic entity -

The classical photosynthetic organ is the leaf, an often determinate bilateral organ highly specialized in photosynthesis [START_REF] Harris | Plant identification terminology: an illustrated glossary[END_REF]

(Figure 2.6-A).
However, functional differences between leaves and stems may in some cases be vague and some authors have argued that the transition from branch function toward leaf function is continuous (see [START_REF] Sattler | Homology-A Continuing Challenge[END_REF][START_REF] Sattler | Homeosis in Plants[END_REF][START_REF] Sattler | Multivariate Analysis Confirms the Continuum View of Plant Form[END_REF][START_REF] Sattler | Classical Morphology and Continuum Morphology: Opposition and Continuum[END_REF]. For example, in several species, classical leaf functional characteristics are supported by highly specialized stems (e.g. cladodes, [START_REF] Cooney-Sovetts | Phylloclade development in the Asparagaceae: an example of homoeosis[END_REF][START_REF] Bell | Plant Form -An illustrated guide to flowering plant morphology[END_REF] (Figure 2.6-B). To rigorously consider function and not structure, such stems must be considered as the functional photosynthetic entities of plants. As such, in the present study, photosynthetic entities are defined as "determinate growth structures highly specialized in photosynthesis and with an abscission point" (see [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. This definition is largely fitted by phyllomorphic branches, as introduced by [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF] and defined by [START_REF] Hallé | Etude biologique et morphologique de la tribu des Gardéniées[END_REF], which are plagiotropic stems functionally considered as compound leaves [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] . Similarly, leaves with indeterminate growth, as described for example in Meliaceae [START_REF] Corner | The Life of Plants[END_REF][START_REF] Steingraeber | Indeterminate Growth of Leaves in Guarea (Meliaceae): A Twig Analogue[END_REF], will not be considered as photosynthetic entities but rather as additional exploration structures (but no such leaves are known in New Caledonia).

Reproductive entity -In morphological terms, inflorescences are stems highly specialized in the reproduction function. Accordingly, strictly monoaxial plants do not exist or are very rare [START_REF] Hallé | Essai sur l'architecture et la dynamique de croissance des arbres tropicaux[END_REF]. Once again, the demarcation between inflorescences and vegetative branches could be confusing and they appear to be two extremes of a continuum [START_REF] Van Steenis | Definition of the concept "inflorescence" with special reference to ligneous plants[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. For example, while some inflorescences comprise only a single lateral flower (Figure 2.7-A), others are quite long and bear numerous assimilative leaves (Figure 2.7-B) or comprise a densely branched complex of axes functioning during the entirety of the plant's lifespan ("bud complex" leading to cauliflory, Figure 2.7-C). Our main purpose here is to identify discriminating criteria to classify axes under the inflorescence or branch classes. [START_REF] Van Steenis | Definition of the concept "inflorescence" with special reference to ligneous plants[END_REF] proposed that inflorescences "post-anthesis do not participate in the vegetative extension of the individual". In the present study, we extend this finding by considering inflorescences as "axes or complex of axes highly specialized in reproduction and whose contribution to whole plant vegetative exploration and photosynthesis is negligible". Consequently, species with additional axes that do not fit this definition will be considered as branched.

Reiterates -For a given species, it is common to find individuals exhibiting an architecture that differs from the architectural unit that characterizes the taxon. Similarly, almost all of the species previously described as monocaulous (e.g. [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Hallé | Architectures de Plantes[END_REF] can occasionally be seen in nature with several axes (except for most monocotyledons). In this case, all supernumerary stems are morphologically and functionally identical to each other and to the primary trunk: that is, they are reiterates (see section 2.2). It is consequently crucial to distinguish a reiteration that results from a developmental sequence (i.e. sequential reiteration) from one triggered by exogenous factors (i.e. opportunistic reiteration). In the first case, a species will be considered as branched and in the second as monocaulous. If the theoretical differentiation between opportunistic and sequential reiteration seems obvious [START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF], there are no clear morphological characters to differentiate easily the two processes in the field, particularly for delayed reiteration. In the present study, if reiteration of a species is predictable in space and time, it is considered as sequential and the species is therefore classified as branched (Figure 2.8).

To summarize, in the present study we consider as monocaulous all self-supporting woody species whose cardinal functions rely on a single visible stem, i.e. which are made of (i) a trunk (either monoaxial or pluriaxial), (ii) determinate growth structures highly specialized in photosynthesis and with an abscission point, (iii) axes or complex of axes highly specialized in reproduction and whose contribution to whole plant vegetative exploration and photosynthesis is negligible, and possibly (iv) opportunistic reiterates. This definition encompasses the two monoaxial models of [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]: the Holttum's model for terminal flowering species and the Corner's model for lateral flowering taxa (Figure 2.9-A). Our definition also includes Chamberlain's model except that we consider both hapaxanthic (terminally flowering) and nonhapaxanthic (lateral flowering) modules (Figure 2.9-B), while the initial definition of Hallé et al. [START_REF] Hallé | Essai sur l'architecture et la dynamique de croissance des arbres tropicaux[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] only includes linear sympodia made of terminally flowering modules. The definition of monocauly adopted here also includes the Cook's architectural model in its extreme expression (i.e. with short-living determinate phyllomorphic branches, Figure 2.9-C). [START_REF] Hallé | Architectures de Plantes[END_REF] suggested that the Cook's model should be merged into the Roux's model, their differences being mainly quantitative. In our opinion, differences between classical long shoots (Roux's model) and phyllomorphic branches as considered here (the Cook's model) are more than just quantitative given the fundamental difference in the growth pattern (indeterminate vs. determinate, respectively). Finally, our definition also corresponds to the Tomlinson's model as initially described by [START_REF] Hallé | Essai sur l'architecture et la dynamique de croissance des arbres tropicaux[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] as the basal repetition of two of our above-described monocaulous models (the Holttum's and the Corner's models). [START_REF] Cremers | Etude de l'architecture aérienne de quelques plantes tropicales à ramification basitone: vers une révision du modèle de Tomlinson[END_REF] challenged the validity of this architectural model by showing that basal repetition occurs at least in seven other architectural models, including the monocaulous Chamberlain's model. These authors proposed that such repetitions are reiterations occurring basally [START_REF] Cremers | Etude de l'architecture aérienne de quelques plantes tropicales à ramification basitone: vers une révision du modèle de Tomlinson[END_REF], the process sometimes being predictable in space and time (i.e. sequential reiteration) and sometimes not (i.e. opportunistic reiteration).

Consequently, some plants initially described under the Tomlinson's architectural model will be considered as monocaulous in the present study since they match the definition presented above.

Other theoretical structural types fit our definition of monocauly, such as those constituted by a sympodial trunk and phyllomorphic branches, but that does not conform to any of the described architectural models because they remain unobserved in nature.

List of monocaulous species

To assess the evolutionary history of monocaulous plants in New Caledonia, an exhaustive list of monocaulous species native to New Caledonia has been established. As our definition of monocauly applies to woody plants, the list is limited to the non-monocot flora, which is by far the most diverse group [START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF]. Monocotyledons were not considered because they represent a highly different monocaulous condition since aerial branching is developmentally constrained by shoot growth and the absence of wood [START_REF] Tomlinson | Vascular anatomy of monocotyledons with secondary growth -an introduction[END_REF][START_REF] Tomlinson | Establishment growth in woody monocotyledons native to New Zealand[END_REF][START_REF] Haushahn | Plant ramifications inspire branched lightweight composites[END_REF].

A preliminary list of New Caledonian monocaulous species was compiled based on an extraction of the NOU Herbarium database using appropriate keywords ("monocaul", "peu ramif", "pachycaul", "non ramif", "unbranch"). The list was supplemented through expert opinion and extensive bibliographic analysis, particularly in the series Flore de la Nouvelle-Calédonie (et Dépendances) (Aubréville et al., 1967(Aubréville et al., -1992;;Morat andMacKee, 1992-2004;[START_REF] Hopkins | Cunoniaceae : Flore de la Nouvelle-Calédonie[END_REF] and taxonomic publications. The resulting initial list of taxa was cross-checked with the Florical taxonomic reference [START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF] to ensure that only valid names of species were used. When possible, the architecture of each taxon on this list was studied through extensive field work on Grande Terre (documented by more than 1200 herbarium collections, Figure 2.10) to verify whether each species fit our definition of monocauly. Species were studied using as many individuals and ontogenetic stages as possible. For most widespread species, an attempt was made to investigate several populations. However, these precautions were sometimes not applicable due to time limitation, access or population-size constraints. In such cases, field observations were supplemented with observations of some photographs (UMR AMAP-IAC database and Endemia website). Photographs led to the exclusion of some species from the monocaulous list, but did not permit the validation of whether they had a monocaulous architecture. As such, some species require further investigation to confirm or refute their monocaulous habit. This concerns less than 10 species for which distinction between sequential and opportunistic reiteration was equivocal (belonging to the genera Bocquillonia, Sloanea, Dysoxylum and Pycnandra). Only one species was never observed (Symplocos paniensis) and was therefore coded as NA.

The final list of monocaulous species (Appendix 1) was used in the following chapters to investigate the diversity, ecology and evolution of monocaulous plants in New Caledonia. For this purpose, we used several approaches combining taxonomy, architectural analysis, functional ecology and phylogenies (Figure 2.11). More details about the methods relating to phylogenies and functional traits are provided in the corresponding chapters. Chapter 3 -The monocaulous growth habit: a review

History and definitions

The term monocauly, derived from the Latin monocaulis (mono = one, caulis = stem), has been used since at least the 17 th century (see [START_REF] Mentzelius | Index nominum plantarum universalis[END_REF] to characterize plants or parts of plants (curiously including roots, e.g. [START_REF] Clementi | Sertulum Orientale[END_REF]. As far as we know, this Latin term was anglicized for the first time by E.J.H. Corner (monocauly, monocauls, monocaulous species) to illustrate his Durian theory about the origin of the modern tree [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]. While the Latin term was mainly applied to herbaceous species, the tropical approach of Corner restricted the English term to whole woody plants ("trees"). In this fundamental work [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF], monocauls are considered as an extreme of pachycauls (few branches, thick stems and large leaves) in their total absence of branching. Since then, the term monocauly has been used by a variety of authors to characterize the growth habit of some sparsely-branched plants, the definitions being more or less accurate depending on the study. The vast majority are taxonomic studies in which new species of monocauls are described (e.g. [START_REF] Morat | Contribution à l'étude des Sterculiaceae de la Nouvelle-Calédonie[END_REF][START_REF] Tange | A revision of the genus Mouretia (Etubiaceae)[END_REF][START_REF] Sonké | Synopsis of the genus Mitriostigma (Rubiaceae) with a new monocaulous species from south Cameroon[END_REF][START_REF] Jebb | Five new species of Barringtonia (Lecythidaceae) from Papua New Guinea. Blumea -Biodiversity[END_REF][START_REF] Taylor | A new species of Pentagonia (Rubiaceae: Hippotideae) from southern Peru[END_REF][START_REF] Yu | A taxonomic revision of Trigonostemon (Euphorbiaceae) in Malesia. Blumea -Biodiversity[END_REF]. In this case, monocauls are sometimes considered under a broad physiognomical definition, i.e. sparsely-branched plants with large leaves clustered in distal part of the stem. Such a general definition better matches Corner's pachycauly [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF] than his more restricted monocauly and fits with several other botanical terms used at different times and in different languages: palmenforms or bananenform [START_REF] Humboldt | Ansichten der Natur mit wissenschaftlichen Erläuterungen[END_REF], rosettentrager [START_REF] Reiter | Die Consolidation der Physiognomik[END_REF], tuft-trees [START_REF] Warming | Oecology of plants : an introduction to the study of plant-communities[END_REF]), rosette-trees (Du Rietz, 1931), megaphytes [START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF], cabbage-trees [START_REF] Irvine | Woody plants of Ghana : with special reference to their uses[END_REF], columnar-trees (Zimmermann andBrown, 1971), palmoid (D'Arcy, 1973), caulescent-trees [START_REF] Halloy | A morphological classification of plants, with special reference to the New Zealand alpine flora[END_REF], giant rosette plant [START_REF] Hedberg | Tropical-Alpine Life-Forms of Vascular Plants[END_REF], Schopfbaume [START_REF] Zona | Litter-trapping plants: filter-feeders of the plant kingdom[END_REF], scapose-trees [START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF]. Each of these terms is highly figurative but was often used without an unambiguous definition, making them largely subjective. [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] made the distinction between the structural definition ("trees with a single axis" = monoaxial) and the physiognomic one ("trees with a single trunk or visible stem of the plant" = monocaulous). These two approaches to monocauly were reinforced by the designation of a set of architectural models corresponding to each definition (i.e. the Holttum's and the Corner's in the first case and the Holttum's, the Corner's and the Chamberlain's in the second). In the course of this thesis, monocauly was regarded from a functional perspective, and a preferred definition has been adopted: "self-supporting woody plants whose cardinal functions rely on a single visible stem", i.e. plants (i) made of a trunk (either monoaxial or pluriaxial) and with, (ii) determinate growth structures highly specialized in photosynthesis and with an abscission point, (iii) an axis or complex of axis highly specialized in reproduction and whose contribution to whole plant vegetative exploration and photosynthesis is negligible, and possibly (iv) opportunistic reiterates (see Chapter 2).

Evolution of the monocaulous habit

The oldest known plant macrofossils, dated from ca. 430 Ma (Silurian), were probably isodichotomously branched, producing two daughter branches of similar size (e.g. Cooksonia) with terminal sporangia [START_REF] Meyer-Berthaud | L'évolution des premiers arbres : les stratégies dévoniennes[END_REF]. They presented a mixture of prostrate and erect axes. This simple morphology is suggested to have been dominant up to the Early Devonian (ca. -400 Myr). Devonian is also the period during which pseudomonopodial branching became widespread. In this mode of branching, daughter branches are different in size and orientation, leading to the appearance of a vertical growth and side branches. This mode of branching is suggested to have played an important role in the evolution toward arborescence [START_REF] Meyer-Berthaud | L'évolution des premiers arbres : les stratégies dévoniennes[END_REF][START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF]. These fossil plants were, however, of small size. The tree growth habit evolved in several lineages via convergent evolution [START_REF] Niklas | The evolutionary biology of plants[END_REF], with the earliest known modern tree dating from the Middle Devonian (-390 Myr) [START_REF] Stein | Giant cladoxylopsid trees resolve the enigma of the Earth's earliest forest stumps at Gilboa[END_REF]. In the Earth's "oldest forest", these tree-fern-like plants (Cladoxylopsida) had a trunk bearing large branches that probably abscised (cladoptosis or branch shedding), as a "frond-like module" [START_REF] Stein | Giant cladoxylopsid trees resolve the enigma of the Earth's earliest forest stumps at Gilboa[END_REF]. The architecture of these fossil plants corresponds to Berry's model [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF]. Archaeopteris, another modern tree from the late Devonian, was shown to form excurrent deciduous branches (Type A, in Meyer-Berthaud's model, [START_REF] Meyer-Berthaud | Archaeopteris is the earliest known modern tree[END_REF][START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF]. Thus, large fossil trees seem to be dominated by non-perennial photosynthetic or lateral structures. Among tree-ferns, an advance level of organization of branching has also been described [START_REF] Galtier | How early ferns became trees[END_REF], but most fossil tree-ferns known since the Carboniferous had a monocaulous trunk supporting large compound leaves, comparable to extant tree-ferns of Cyatheaceae and Dicksoniaceae. Plants expressing Corner's architectural model are known from the early Devonian (ca. -400 Myr) and the Holttum's model dates back to at least the Late Devonian (ca. -355 Myr), i.e. before the appearance of most extant and extinct architectural models [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF]. While these first unbranched plants probably do not fit with our definition of true woody monocauly (vascular cambium of extant species appearing in the late Triassic (-220 Myr, Savidge, 2008), they show that unbranched architectures were clearly more represented in the past. It was particularly abundant from the to the early , representing between ca. 20% and 40% of the total fossil record for which information is available [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF]. From the early Cretaceous, the proportion of taxa with an unbranched architecture gradually decreased in favor of architectures expressing axillary branching. This is in agreement with the rise of small-leafed and highly branched angiosperms, which have progressively replaced the often large-leafed monocaulous habit of ferns and progymnosperms [START_REF] Coiffard | Rise to dominance of angiosperm pioneers in European Cretaceous environments[END_REF]. Today, the monocaulous habit is estimated to represent only ca. 2% of the total known architectures [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF].

The abundance and diversity of monocaulous forms in the floras of the Paleozoic and Mesozoic has often led scientists to consider it as primitive for seed plants [START_REF] Galtier | Contrasting diversity of branching patterns in early ferns and early seed plants[END_REF] or Angiosperms [START_REF] Hallier | L'origine et le système phylétique des Angiospermes exposés à l'aide de leur arbre généalogique[END_REF][START_REF] Meeuse | Again: the growth habit of the early Angiosperms[END_REF]. In his Durian theory on the origin of the modern tree, [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF] suggested that "the more remote ancestors [=of Angiosperms] appear to have been monocarpic and monocaulous, with the Cycad-habit". This idea was supported by early classifications in which Cycas was placed as sister to Gymnosperms and Angiosperms. Such a primitive form was thought to have gradually evolved toward branched forms (with smaller and simpler leaves, flowers, seeds and fruits) to colonize drier and colder habitats, leading in the extant Angiosperm architectural diversity. [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF] argued that extant monocaulous species, occurring in several tropical families, are "relicts" of this ancestral form. Other authors have suggested that the occurrence of monocauly in numerous unrelated families is instead evidence of recent convergent evolution [START_REF] Richards | The Tropical Rain Forest[END_REF][START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF], the simplicity of monocaulous forms no being synonymous with "antiquity" (Mabberley, 1974b;[START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. Recently, the rise of molecular phylogenies has largely supported this second view andwhile the ancestral growth habit of Angiosperms is still under debate [START_REF] Doyle | Molecular and Fossil Evidence on the Origin of Angiosperms[END_REF]) -Corner's hypotheses about the evolution of monocauly have been discredited. In extant angiosperms, the monocaulous habit appears to be derived in Senecioneae (Asteraceae, [START_REF] Knox | The origin of Dendrosenecio within the Senecioneae (Asteraceae) based on chloroplastDNA EVIDENCE[END_REF], Lobeliads (Campanulaceae, [START_REF] Givnish | Giant lobelias exemplify convergent evolution[END_REF] and the genera Espeletia (Asteraceae, [START_REF] Rauscher | Molecular phylogenetics of the Espeletia complex (Asteraceae): evidence from nrDNA ITS sequences on the closest relatives of an Andean adaptive radiation[END_REF], Oxera (Lamiaceae, [START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF] and Aeonium (Crassulaceae, [START_REF] Mort | Physiological Evolution and Association between Physiology and Growth Form in Aeonium (Crassulaceae)[END_REF]. Using a phylogenetic tree including more than 20,000 species of vascular plants, [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF] found 118 independent origins of monocauly, strongly suggesting that this habit is not (or not only) relictual in the extant flora.

For several monocaulous species occurring in islands or island-like environments (tropical mountains), evolution of this habit from herbaceous ancestors was suggested early on. [START_REF] Darwin | On the Origin of Species by Means of Natural Selection[END_REF] proposed that herbaceous species, more likely to reach islands than woody taxa, evolved to be taller to outcompete other herbaceous species, ultimately leading to bushes and/or trees (Figure 3.1-A). This hypothesis of so-called secondary woodiness [START_REF] Carlquist | Island Biology[END_REF][START_REF] Nattier | Climate and Soil Type Together Explain the Distribution of Microendemic Species in a Biodiversity Hotspot[END_REF] was first supported by data from comparative anatomy (e.g. [START_REF] Carlquist | A theory of Paedomorphosis in Dicotyledonous woods[END_REF][START_REF] Carlquist | Wood Anatomy of Goodeniaceae and the Problem of Insular Woodiness[END_REF] and later confirmed by molecular phylogenies [START_REF] Knox | The origin of Dendrosenecio within the Senecioneae (Asteraceae) based on chloroplastDNA EVIDENCE[END_REF][START_REF] Rauscher | Molecular phylogenetics of the Espeletia complex (Asteraceae): evidence from nrDNA ITS sequences on the closest relatives of an Andean adaptive radiation[END_REF][START_REF] Givnish | Giant lobelias exemplify convergent evolution[END_REF]. Why these secondary woody plants are often monocaulous or sparsely branched has not yet been investigated, but it probably involves the conservation of an ancestral herbaceous architecture or of tissue too soft to allow branching (i.e. developmental constraints). Such an herbaceous evolutionary pathway involves heterochronic evolution through peramorphosis ("proportionate giantism") since size and lifespan increase in descendants [START_REF] Gould | Ontogeny and Phylogeny[END_REF][START_REF] Alberch | Size and Shape in Ontogeny and Phylogeny[END_REF][START_REF] Smith | Heterochrony revisited: the evolution of developmental sequences[END_REF] (Figure 3.1-A). However, if such a process appears to be nearly certain in some groups, a meta-analysis showed that out of 118 independent evolutions of monocauly, only 34% involved herbaceous ancestors [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF]. Based on the observation that several tropical forest tree species have a long understory monocaulous phase [START_REF] Carlquist | Island Life[END_REF][START_REF] Richards | The Tropical Rain Forest[END_REF][START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF] suggested that monocauls could arise from such trees "which have found it unnecessary to reach the forest canopy". This hypothesis is in accordance with Barthelemy's ideas [START_REF] Barthélémy | Architecture et sexualité chez quelques plantes tropicales: le concept de floraison automatique[END_REF] and suggests heterochronic evolution through paedomorphosis [START_REF] Barthélémy | Architecture et sexualité chez quelques plantes tropicales: le concept de floraison automatique[END_REF] since the descendants (monocauls) evolved to have a smaller size and a simpler form through a truncation of the developmental sequence of their ancestor (which were branched trees). In this case, the paedomorphic processes would be neoteny (decreased growth rate) and possibly progenesis (earlier offset of the vegetative phase) if branching appeared before flowering in the ancestral developmental sequence . This kind of heterochronic evolution was proposed for fossil lycopsids, where Chaloneria (Isoetales), a small unbranched shrub, is thought to have evolved from Sigillaria (Lepidodendrales), a 15 meter branched tree [START_REF] Bateman | Evolutionary-developmental change in the growth architecture of fossil rhizomorphic lycopsids: Scenarios constructed on cladistic foundations[END_REF].

Without phylogenies, however these idea will remain hypothetical. Examples of heterochronic evolution in plants mainly involve fragments of plants (e.g. wood, stem, leaves or flowers) while the importance of such processes in the evolution of growth habit remains largely unknown (reviewed in [START_REF] Li | Heterochrony in plant evolutionary studies through the twentieth century[END_REF].

Life history and environmental traits associated with monocauly

Like all growth habits (see section 1.3), monocauly is not only characterised by an unbranched stem but also by a large set of morpho-physiological traits. The most striking feature, inherent in most terms used to describe monocauls (see section 3.1), is the presence of large leaves. This relationship between branching intensity and leaf size is an element of Corner's rules [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF](Corner, , 1953(Corner, -1954)). These rules concern two fundamental statements: (i) Axial conformity, stipulating that "the stouter, or more massive, the axis in a given species, the larger and more complicated its appendages" and (ii) Diminution on ramification, stipulating that "the greater the ramification, the smaller become the branches and their appendages" [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]. By "appendages", Corner meant leaves, fruits, inflorescences and flowers. Consequently, monocauls are not only expected to have larger leaves but also a thicker stem, larger fruits and more complex inflorescences (Figure 1.2). The relation between leaf area and twig thickness, namely the worldwide leaf sizetwig size spectrum [START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF], and to a lesser extent the relation between leaf area and fruit size, has been extensively investigated (White, 1983b;[START_REF] Bond | Allometry and sexual differences in leaf size[END_REF][START_REF] Brouat | Corner's rules revisited: ontogenetic and interspecific patterns in leaf-stem allometry[END_REF][START_REF] Cornelissen | A triangular relationship between leaf size and seed size among woody species: allometry, ontogeny, ecology and taxonomy[END_REF][START_REF] Brouat | Leaf-stem allometry, hollow stems, and the evolution of caulinary domatia in myrmecophytes[END_REF][START_REF] Westoby | Plant Ecological Strategies: Some Leading Dimensions of Variation Between Species[END_REF][START_REF] Preston | Hydraulic architecture and the evolution of shoot allometry in contrasting climates[END_REF][START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF][START_REF] Pickup | Dry mass costs of deploying leaf area in relation to leaf size[END_REF][START_REF] Sun | The leaf size-twig size spectrum of temperate woody species along an altitudinal gradient: an invariant allometric scaling relationship[END_REF][START_REF] Normand | Hydraulic and mechanical stem properties affect leaf-stem allometry in mango cultivars[END_REF]. On the other hand, the relation with branching intensity (i.e. Diminution on ramification) received much less consideration. Few studies have shown a negative correlation between branching intensity and twig cross-sectional area or leaf size (White, 1983b;[START_REF] Ackerly | Leaf Size, Sapling Allometry, and Corner's Rules: Phylogeny and Correlated Evolution in Maples (Acer)[END_REF][START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF] or inflorescence length [START_REF] Ackerly | Leaf Size, Sapling Allometry, and Corner's Rules: Phylogeny and Correlated Evolution in Maples (Acer)[END_REF] but, as far as we are aware, never with fruit size. Moreover, none of these studies included monocaulous species and the measurement methods used to quantify branching intensity are highly variable in the literature. For example, some authors measured the number of active growing tips on whole plants (White, 1983b;[START_REF] Ackerly | Leaf Size, Sapling Allometry, and Corner's Rules: Phylogeny and Correlated Evolution in Maples (Acer)[END_REF] while others measured the mean length between apices and the first branch [START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF], the proportion of trunk nodes producing branches [START_REF] Ackerly | Canopy Structure and Dynamics: Integration of Growth Processes in Tropical Pioneer Trees[END_REF], or the number of non-branched nodes between two branched nodes [START_REF] Thomasson | Remarques sur la végétation des environs de Tuléar (sud-ouest malgache). II -Superficie foliaire et ramification chez les végétaux ligneux[END_REF]. Such discrepancies call for the need for the definition of a standardized index measuring branching intensity in relation to plants architecture and function.

Consequently, our understanding of the relationships between monocauly and life history trait attributes largely comes from empirical observation and virtually never from attempts to quantify them. Such associations concern cauliflory [START_REF] Hallé | Corner's Architectural Model[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Barthélémy | Architecture et sexualité chez quelques plantes tropicales: le concept de floraison automatique[END_REF][START_REF] Schmid | Conditions d'évolution et caractèristiques du peuplement végétal insulaire en Mélanésie Occidentale: Nouvelle-Calédonie et Vanuatu[END_REF], compound leaves [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Hallé | Etude biologique et morphologique de la tribu des Gardéniées[END_REF][START_REF] Hallé | Corner's Architectural Model[END_REF]White, 1983a), short internodes [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Chuah | Exemples de corrélations quantitatives entre quelques paramètres déterminant la forme végétale[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Sussex | Architectural innovation and developmental controls in some mesozoic gymnosperms, or, why do the leaf crowns in mesozoic forest look tufted[END_REF], dioecy [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], rhythmic growth [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], and high slenderness [START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF]. Their relationship with standard functional traits such as SLA (Specific Leaf Area) or related traits (see [START_REF] Wright | The worldwide leaf economics spectrum[END_REF] are difficult to estimate given that studies have never clearly included monocauls and that the relation with leaf area is unclear [START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF]. In terms of anatomy, studies suggest that pith area along with stem size and leaf area generally increase during ontogeny until the branching point and then progressively decrease [START_REF] Eggert | The ontogeny of Carboniferous arborescent Lycopsida[END_REF][START_REF] Lauri | Le mouvement morphogenetique, approche morphometrique et restitution graphique: L'exemple de quelques plantes tropicales[END_REF]. This in turn suggests a higher pith size in the distal part of the stem for monocaulous species than for branched taxa, as confirmed by several studies [START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF][START_REF] Carlquist | Island Biology[END_REF]Mabberley, 1974a;[START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Meinzer | Adaptations for water and thermal balance in Andean giant rosette plants[END_REF]. Research also suggests that monocauls have a large cortex [START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF]Mabberley, 1974a;[START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Mosbrugger | Tree Habit in Land Plants: A functional comparison of trunk constructions with a brief introduction into the biomechanics of trees[END_REF] and a thin wood layer composed of a high proportion of parenchyma [START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF]Mabberley, 1974a;[START_REF] Aldridge | Anatomy and evolution in the Macaronesian Sonchus subgenus Dendrosonchus (Compositae: Lactuceae)[END_REF]. The relation between monocauly or pachycauly and vessel or fiber size has been investigated indirectly [START_REF] Aldridge | Anatomy and evolution in the Macaronesian Sonchus subgenus Dendrosonchus (Compositae: Lactuceae)[END_REF][START_REF] Aldridge | Anatomy and Evolution in Macaronesian Echium (Boraginaceae)[END_REF] but results are blurred by the variety of sampled environmental conditions. The life history and functional characteristics of monocaulous plants, as suggested by the published literature, are summarized in Figure 3.2.

Each of these morpho-anatomical attributes depends on particular genetic and physiological processes that are undoubtedly linked with the evolution of monocauly. Apical dominance, i.e. the control exerted by the growing apical meristem over the outgrowth of lateral buds [START_REF] Cline | Apical dominance[END_REF], seems of prime importance. While genetics and physiology offer a promising way to understand the evolution of monocauly (see [START_REF] Doebley | The evolution of apical dominance in maize[END_REF], they will not be considered in the present study, which instead focus on macro-ecological processes. The importance of hormonal pathways and gene expression on plant branching have been reviewed elsewhere (e.g. [START_REF] Cline | The role of hormones in apical dominance. New approaches to an old problem in plant development[END_REF][START_REF] Sussex | The evolution of plant architecture[END_REF][START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF].

Monocauls are almost restricted to tropical regions [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], where they are known from areas with diverse environmental conditions, but mainly from rainforests. D' Arcy (1973), for instance, reviewed environmental occurrences of diverse monocaulous plants and concluded that "all are found in wet forests". Such an ecological preference has actually been FIGURE 3.2 -Synthesis of life history trait attributes suggested to be related with monocauly in literature. Numbers point on references in which trait associations were suggested. Bold text represents trait attributes for which association with branching intensity was statistically showed. 1 Corner (1949), 2 Westoby & Wright (2003), 3 White (1983a), 4 Ackerly & Donoghue (1998), 5 Hallé & Mabberley (1976), 6 Hallé et al. (1978), 7 D'Arcy (1973), 8 [START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF], 9 [START_REF] Carlquist | Island Biology[END_REF], 10 Mabberley (1974a), 11 [START_REF] Aldridge | Anatomy and evolution in the Macaronesian Sonchus subgenus Dendrosonchus (Compositae: Lactuceae)[END_REF], 12 Chuah (1977).

suggested for each monocaulous architectural model independently [START_REF] Hallé | Corner's Architectural Model[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] and in more general terms, for monocaulous plants as well [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Hallé | Architecture of Trees in the Rain Forest of Morobe District, New Guinea[END_REF][START_REF] Schmid | Fleurs et plantes de Nouvelle-Calédonie[END_REF]. In rainforests, some insights suggest that pioneer trees, well suited for sunny environments, are less branched than non-pioneer species (White, 1983a;[START_REF] Ackerly | Canopy Structure and Dynamics: Integration of Growth Processes in Tropical Pioneer Trees[END_REF]. This could particularly pertain to compound-leaved trees, for which large dissected leaves are less costly than branches [START_REF] Givnish | Leaf and Canopy Adaptations in Tropical Forests[END_REF][START_REF] Givnish | Plant Stems: Biomechanical Adaptation for Energy Capture and Influence on Species Distributions[END_REF]. Even though monocauly and compound leaves have often been associated in the literature [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Hallé | Etude biologique et morphologique de la tribu des Gardéniées[END_REF][START_REF] Hallé | Corner's Architectural Model[END_REF], no data show whether these pioneer features apply to monocauls. [START_REF] Richards | The Tropical Rain Forest[END_REF] argues that monocauls are "fitted only for a permanently favorable environment", which he considers to include tropical rainforests and some tropical islands. However, while monocauls are obviously present on islands [START_REF] Carlquist | Island Biology[END_REF], recent studies suggest that the evolution of woodiness in these species is associated with greater drought tolerance suited for open habitat [START_REF] Mort | Physiological Evolution and Association between Physiology and Growth Form in Aeonium (Crassulaceae)[END_REF][START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF] and [START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF] even argues that their evolution on islands is linked with high insolation.

Finally, an important part of the literature on monocauly discusses tropical mountain species. This growth habit seems very well adapted to such sunny and cold environments [START_REF] Hedberg | Tropical-Alpine Life-Forms of Vascular Plants[END_REF][START_REF] Meinzer | Adaptations for water and thermal balance in Andean giant rosette plants[END_REF][START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. A detailed morpho-anatomical study of monocaulous plants growing preferentially in rainforest understory conditions might shed light on adaptations and evolutionary constraints related with this more widespread model of monocauly.

Introduction

The genus Bocquillonia Baillon (1862: 225) (Euphorbiaceae) was initially described with two species. Additional species or varieties were subsequently described by Müller Argoviensis (1865), [START_REF] Baillon | Nouvelles observations sur les Euphorbiacées[END_REF], [START_REF] Guillaumin | Species Montrouzieranae seu enumeratio plantarum in Nova Caledonia terrisque adjacentibus a R. P. Montrouzier lectarum[END_REF]publ. 1914) and Airy [START_REF] Shaw | Notes on Malesian and Other Asiatic Euphorbiaceae[END_REF][START_REF] Shaw | Notes on Malesian and Other Asiatic Euphorbiaceae[END_REF]Shaw ( , 1978aShaw ( , 1978b[START_REF] Shaw | Notes on Euphorbiaceae from Indomalesia, Australia and the Pacific[END_REF][START_REF] Shaw | Notes on Asiatic, Malesian and Melanesian Euphorbiaceae[END_REF]. The nomenclature and taxonomy of the genus were finally entirely studied in a global revision [START_REF] Mcpherson | Euphorbiacées I[END_REF], adding one new species and putting two species and three varieties in synonymy. The genus is currently composed of fourteen species all endemic to New Caledonia [START_REF] Mcpherson | Euphorbiacées I[END_REF][START_REF] Govaerts | Euphorbiaceae (with Pandaceae)[END_REF][START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF].

Bocquillonia can easily be distinguished from other New Caledonian Euphorbiaceae by a combination of morphological characters: limb with abaxial laminar glands; plants dioecious or rarely monoecious; glomerulose to narrowly racemiform inflorescences; flowers without corolla; calyx 2-3 lobed in male flowers, calyx shorter than gynoecium in female flowers [START_REF] Mcpherson | Euphorbiacées I[END_REF]. Moreover, recent observations show that Bocquillonia species present internal phloem, i.e. a supernumerary phloem tissue in the stem between pith and wood (Bruy pers. obs.). The only phylogenetic study including Bocquillonia species [START_REF] Wurdack | Molecular phylogenetic analysis of uniovulate Euphorbiaceae (Euphorbiaceae sensu stricto) using plastid RBCL and TRNL-F DNA sequences[END_REF] shows that the genus belongs to the tribe Alchorneeae (Acalyphoideae) and suggests its inclusion in the pantropical genus Alchornea Swartz (1788: 98). However, this study includes too few species (one Bocquillonia and two Alchornea) to invalidate the genus [START_REF] Pillon | How many genera of vascular plants are endemic to New Caledonia? A critical review based on phylogenetic evidence[END_REF].

Bocquillonia exhibits a great diversity of architectures and is therefore included in an ongoing PhD study conducted by David Bruy concerning the diversity, ecology, and evolution of monocaulous New Caledonian species. Some Bocquillonia species such as B. rhomboidea (Schltr.) Airy Shaw (1974: 320) 1913 publ. 1914: 110) are monocaulous (i.e. with a single stem carrying out all essential functions; Corner's model, Figure 4.1-B,C). Other species such as B. grandidens Baillon (1873: 128), can be monocaulous in early maturity but become very ramified when older . In this case, the process of ramification is called 'opportunistic reiteration' (Barthelemy & Caraglio, 2007), which corresponds to the production of a new stem morphologically and functionally identical to the initial stem as a response to stress or increase in resource levels. All of the currently described monocaulous species are small treelets with few reiterations or very reiterated trees. However an unusual population of monocaulous Bocquillonia observed in the extreme northeast of New Caledonia's main island (Grande Terre) differs notably from the other monocaulous species since it is very tall (8 m) and almost not reiterated. Morphological herbarium investigations and field observations confirmed clearly that this population represents a distinct taxon from all other Bocquillonia species. This giant monocaulous taxon is consequently described and illustrated here, and a preliminary IUCN status is proposed. A modification of the Bocquillonia identification key of [START_REF] Mcpherson | Euphorbiacées I[END_REF] is proposed to include this new species.

Material and Methods

Measurements, shapes and colours of the different organs are based on the examination of herbarium material and several field observations. All herbarium specimens of Bocquillonia present at NOU, P and MPU were examined (Herbarium acronyms follow Thiers 2017). All available virtual collections of K were also studied online (http://apps.kew.org/herbcat/). Terminology used for description follows [START_REF] Harris | Plant identification terminology: an illustrated glossary[END_REF]. Small morphological organs were observed using a Leica M165C binocular microscope mounted with a Leica EC3 camera for photography, and measurements were performed using pictures edited in the imageJ software [START_REF] Schneider | NIH Image to ImageJ: 25 years of image analysis[END_REF]. Field pictures were taken with an Olympus Stylus TG-2 camera. The risk of extinction assessment was conducted using the IUCN Red List Criteria (IUCN 2012); Area Of Occupancy (AOO, using a 2 × 2 km grid) and Extent Of Occurrence (EOO) values were calculated using the online "geocat" software [START_REF] Bachman | Supporting Red List threat assessments with GeoCAT: geospatial conservation assessment tool[END_REF]; the number of individuals was established after a half-day of botanical prospection along the single creek where the species is known.

Taxonomy

Bocquillonia corneri Bruy,Barrabé & Munzinger sp. nov. (Figures 4.2,4.3) Type:-NEW CALEDONIA. Province Nord. Commune de Ouégoa: Base du col d'Amoss, 105 m, 20°19 '2.4"S, 164°25'22.8"E, 11 August 2017 (fl.), Bruy, Barrabé & Hattermann 923 [♀] (Holotype P01156371!, Isotypes NOU088193!, MPU310860!, K!, MO!, BRI!).

Diagnosis -Vegetatively, Bocquillonia corneri resembles B. castaneifolia Guillaumin in its monocaulous architecture, its oblanceolate to obovate blade shape and in having very short petioles. Bocquillonia corneri differs most notably from the latter species by the dimensions of the stem (up to 8 cm DBH and 8 m height, vs. 3 cm and 2 m), petiole length (> 9.4 mm, vs. < 5 mm) and blade length (> 25 cm long, vs. < 24 cm) and in having caducous stipules (vs. persistent) and stigmas applied against the ovary in female flowers (vs. erect).

Based on reproductive organs, Bocquillonia corneri resembles B. sessiliflora Baillon (1862: 226), which has also condensed inflorescences and in which the stigmas are oval and applied against the ovary. Bocquillonia corneri differs most notably from the latter species by the strictly monocaulous architecture (vs. well reiterated treelet), the narrowly oblanceolate leaf blade (vs. ovate to oblong), the lack of an upper pulvinus on the petiole (vs. present) and the fushia calyx in vivo (vs. green). Stem glabrous, light grey to white, with brown longitudinal scars; young stems bearing alternatively cataphylls and pseudoverticillate assimilative leaves bundled at the proximal part of the stem.

Stipules caducous, subulate, 3-7 mm long, 0.5-1.2 mm wide, dark brown in sicco, glabrescent. Cataphyll: caducous, narrowly lanceolate, navicular, pectinate, 6-11 (-15) mm long, 0.8-2.1 mm wide, densely strigose with silvery trichomes in sicco. Assimilative leaf: blades narrowly oblanceolate, (24.6-) 36.1-72.5 × 6-16.5 cm, dark purple when young, then pinkish and green when mature, chartaceous to subcoriaceous, adaxial surface glabrous, abaxial surface nearly glabrous; base cuneate to rounded, sometimes asymmetrical; apex acuminate to acute, rarely rounded; margin dentate and minutely revolute; leaf teeth (13-) 21-27 on each side, white on young leaves, black on mature leaves; sunken laminar glands (6-) 13-34 (-49) on each side of the midrib, circular to elliptic, (0.3-) 0.5-1.3 (-1.5) mm in diameter, black in sicco, generally close to the midrib. Venation craspedodromous; midrib not or barely raised adaxially, prominent abaxially, pink to purple when young, green when mature, glabrous adaxially, sparsely and obscurely appressed-puberulent abaxially; secondary veins (16-) 24-30 on each side of the midrib below the acumen, raised adaxially, prominent abaxially, spaced from 8-38 mm, angle with midrib 48-92 °, glabrous; tertiary venation scalariform, not or barely raised on both surfaces, glabrous; quaternary venation obscure adaxially, minutely raised abaxially, glabrous. Petioles without an upper pulvinus, flat adaxially, subtriangular in cross-section, 9.4-34.4 mm long, 3.3-7.1 mm in diameter, pink to purple when young, green when mature, striate in sicco, sparsely and obscurely appressedpuberulent to glabrous. Plants dioecious, sometimes monoecious. Inflorescences reduced to cauliflorous glomerules sometimes axillary to oldest leaves, borne on short shoots; staminate inflorescences (3.5-) 4.4-7.8 mm long, 4.5-14.5 mm in diameter; bracts ovate, with an acuminate to rounded apex, reducing in size from outside to inside of the inflorescence, fushia, sparsely pubescent and ciliate. Staminate flowers glabrous, pedicel ca. 1.5-3.5 mm long, ca. 0.4-1 mm wide, white pinkish; calyx (2-) 3-lobed, fushia, lobes ovate, 0.8-3.1 mm long, 1.5-2.6 (-3) mm wide, with an obtuse to acute apex, with a turbinate tube 2.3-3.8 mm long, 1.8-2.9 mm wide; stamens 2-4, connate at the base, fushia, glabrous, anthers 0.8-1.9 × 1.0-2.0 mm, filaments 1.8-3.6 mm long above the connate section, 0.25-0.60 mm wide, pistillode lacking. Pistillate inflorescences 6.5-13.5 mm long, (7-) 9.9-16.7 (-18.5) mm in diameter, bracts ovate, 1.8-5.6 × 1.7-5.1 mm, with a rounded to acute apex, fushia, ciliate and pubescent, glands present. Pistillate flowers fushia, sessile; sepals 4-5, ovate, 2.2-5.6 mm long, 1.8-4.4 mm wide, with an acuminate apex, ciliate, pubescent to sparsely pubescent; stigmas (2-) 3 (-4), oval, slightly domed, papillate, applied against the ovary; ovary spherical, 2.7-4.6 mm in diameter, densely hirsute. Fruit 3-lobed, ca. 9 mm long, 9-11 mm in diameter, exocarp rugulose, ochre in sicco, sparsely strigose with silvery trichomes; placental column 5.5 mm long; seeds 6-7 × 4.5-5.5 mm, covered with low, blister-like swellings, dark grey in sicco. Etymology -The plant is named in honour of E.J.H. Corner who provided interesting insights about monocaulous plants and whose theories inspired generations of botanists. In addition, the architecture of this plant conforms perfectly to the Corner's architectural model [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF].

Phenology

Species recognition -Bocquillonia corneri is easily recognizable in the field because it is the only species of the genus that is a very few times reiterated monocaulous tree. It is also the only species to combine condensed inflorescences, pistillate flowers with a fuchsia calyx and oval stigmas applied against the ovary.

Conservation status -Bocquillonia corneri is only known from a single population, critically threatened by fire, human activity and invasive species, in particular the deer Rusa timorensis de Blainville (1822: 267). The population corresponds to one unique location sensu IUCN since all individuals could be affected by a single threatening event. Only 56 mature individuals are known, with approximately 25 female individuals. Even though individuals not seen by the authors may be present in the region, it is very probable that the population is smaller than 250 mature individuals. The EOO calculated is 0.092 km² and the AOO is 4 km². We assign consequently a preliminary conservation status of Critically Endangered to B. corneri (CR) B1 ab(iii,v)+2ab(iii,v) using the IUCN Red List criteria (IUCN 2012). Pins.....................................................................B. arborea -. Axils of secondary veins without domatia (trichome tufts); male glomerules spaced; treelet or tree 0.5-8 m tall; species from Grande-Terre.................................................................................................... .............................................................................................................................................................B orders. We found at least 31 independent evolutionary events leading to monocauly. The habit showed a non-random distribution over the supertree that suggests some prerequisites facilitating its evolution. Monocauly evolved preferentially in rainforest and on ultramafic substrate, and triggered the evolution of both cauliflory and marked rhythmic growth. However, no preadaptation was identified in this study although architectural prerequisites appear to be a promising trait for further investigation. Monocaulous species appeared more threatened (IUCN risk of extinction status) than the branched taxa, with a particularly high impact of introduced herbivores.

Additional specimens examined (Paratypes

Conclusions

The evolution of monocauly in New Caledonia is comparable to the most famous cases of convergence on islands. Evolutionary hypotheses explaining this remarkable convergence involve rainforest features and history, ultramafic substrate and long-term absence of browsers.

Keywords: Biodiversity hotspot, Cauliflory, Convergence, Correlated evolution, Disharmony, Growth habit, Islands, IUCN redlist, New Caledonia, Plant architecture, Rainforest, Ultramafic substrate

Introduction

The isolation and environmental heterogeneity of New Caledonia, an archipelago in the Southwest Pacific, and the multiple colonizations from different geographical areas that have taken place since over ~37 Ma have resulted in a flora that harbours many peculiarities. The New Caledonian flora is well known for its exceptional level of richness and endemism [START_REF] Morat | The taxonomic database « FLORICAL » and characteristics of the indigenous flora of New Caledonia[END_REF][START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF], the high incidence of relictual lineages [START_REF] Pillon | Time and tempo of diversification in the flora of New Caledonia[END_REF][START_REF] Pouteau | New Caledonia: a Pleistocene refugium for rain forest lineages of relict angiosperms[END_REF][START_REF] Pillon | How many genera of vascular plants are endemic to New Caledonia? A critical review based on phylogenetic evidence[END_REF], a great diversity of conifers [START_REF] Jaffré | Distribution and Ecology of the Conifers of New Caledonia[END_REF], and some unusual biological forms such as the world's only parasitic Gymnosperm (Parasitaxus usta, [START_REF] Laubenfels | Parasitic Conifer Found in New Caledonia[END_REF][START_REF] Feild | A unique mode of parasitism in the conifer coral tree Parasitaxus ustus (Podocarpaceae)[END_REF] and a monocarpic large tree (Cerberiopsis candelabra, [START_REF] Veillon | Une Apocynacée monocarpique de Nouvelle-Calédonie Cerberiopsis candelabrum Vieill[END_REF]. This astonishing flora is of important conservation concern [START_REF] Jaffré | Threatened plants of New Caledonia: Is the system of protected areas adequate?[END_REF], recognized by [START_REF] Myers | Threatened biotas: "Hot Spots" in tropical forests[END_REF] as one of the 10 original "Biodiversity Hotspots" and the smallest among the 36 currently recognized hotspots [START_REF] Mittermeier | Hotspots revisited[END_REF]).

An important feature of insular biota, considered as major evidence of long-distance dispersal, is their taxonomic disharmony, i.e. the unbalanced representation of plant groups [START_REF] Carlquist | Island Life[END_REF][START_REF] Carlquist | Island Biology[END_REF][START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF]. The presence of novel biotic and abiotic conditions encountered following colonization have, in many instances, triggered functional shifts [START_REF] Patiño | A roadmap for island biology: 50 fundamental questions after 50 years of The Theory of Island Biogeography[END_REF] that can lead to losses in functional diversity [START_REF] Boyer | Extinctions and the loss of ecological function in island bird communities[END_REF] and uneven representation of functional groups, i.e. functional disharmony. In New Caledonia, several examples of functional and taxonomic disharmony have been described recently, including the high incidence of dioecy [START_REF] Carpenter | Reproductive traits of tropical rain-forest trees in New Caledonia[END_REF][START_REF] Schlessman | Incidence, Correlates, and Origins of Dioecy in the Island Flora of New Caledonia[END_REF], the over-representation of relict angiosperms [START_REF] Morley | Why are there so many primitive angiosperms in the rain forests of Asia-Australasia?[END_REF][START_REF] Pouteau | New Caledonia: a Pleistocene refugium for rain forest lineages of relict angiosperms[END_REF][START_REF] Trueba | Ecology, forms and functions of the basal angiosperms from New Caledonia[END_REF], the diversity of metal hyper-accumulator species [START_REF] Jaffré | The metal hyperaccumulators from New Caledonia can broaden our understanding of nickel accumulation in plants[END_REF][START_REF] Van Der Ent | The flora of ultramafic soils in the Australia-Pacific Region : state of knowledge and research priorities[END_REF], and the low richness in liana [START_REF] Bruy | Abundance, richness and composition of lianas in forest communities along an elevation gradient in New Caledonia[END_REF]. The cause of disharmony in the New Caledonian flora is often attributed to the high incidence of ultramafic substrates, covering about a third of the main island, Grande Terre, [START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF][START_REF] Isnard | How did the ultramafic soils shape the flora of the New Caledonian hotspot?[END_REF] and to the persistence of rainforests through periods of paleoclimatic fluctuation [START_REF] Pintaud | Chorology of New Caledonian palms and possible evidence of Pleistocene rain forest refugia[END_REF][START_REF] Poncet | Phylogeography and niche modelling of the relict plant Amborella trichopoda (Amborellaceae) reveal multiple Pleistocene refugia in New Caledonia[END_REF][START_REF] Pouteau | New Caledonia: a Pleistocene refugium for rain forest lineages of relict angiosperms[END_REF][START_REF] Tournebize | Two disjunct Pleistocene populations and anisotropic postglacial expansion shaped the current genetic structure of the relict plant Amborella trichopoda[END_REF], while adjacent regions experienced intense extinction events [START_REF] Kemp | Tertiary climatic evolution and vegetation history in the Southeast Indian Ocean region[END_REF][START_REF] Crisp | Radiation of the Australian flora: what can comparisons of molecular phylogenies across multiple taxa tell us about the evolution of diversity in present-day communities?[END_REF][START_REF] Dodson | Palynological evidence for aridity events and vegetation change during the Middle Pliocene, a warm period in Southwestern Australia[END_REF][START_REF] Byrne | Birth of a biome: insights into the assembly and maintenance of the Australian arid zone biota[END_REF][START_REF] Byrne | Decline of a biome: evolution, contraction, fragmentation, extinction and invasion of the Australian mesic zone biota[END_REF].

Another evolutionary mechanism that has long fascinated biologists in island systems is convergence in plant form and function [START_REF] Elmer | Adaptation in the age of ecological genomics: insights from parallelism and convergence[END_REF][START_REF] Whittaker | Island biogeography: Taking the long view of nature's laboratories[END_REF].

Convergent evolution, i.e. the rise of similar trait attributes among distantly related lineages subjected to similar selection pressures, is a key driver of biological diversity [START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. In many islands, cases of evolutionary convergence have been well documented such as divaricating plants in New Zealand [START_REF] Greenwood | Evolution of divaricating plants in New Zealand in relation to Moa browsing[END_REF][START_REF] Tomlinson | Some qualitative and quantitative aspects of New Zealand divaricating shrubs[END_REF][START_REF] Howell | Moa ghosts exorcised? New Zealand's divaricate shrubs avoid photoinhibition[END_REF] and giant rosette shrubs in Hawaii [START_REF] Carlquist | Island Biology[END_REF][START_REF] Givnish | Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae)[END_REF][START_REF] Givnish | Giant lobelias exemplify convergent evolution[END_REF] and Macaronesia [START_REF] Shmida | Growth Form Diversity on the Canary Islands[END_REF][START_REF] Mes | The evolution of growth-forms in the Macaronesian genus Aeonium (Crassulaceae) inferred from chloroplast DNA RFLPs and morphology[END_REF]. One of the most famous cases of convergence in island floras is the high proportion of woody species that evolved from herbaceous ancestor, a phenomenon referred to as "insular woodiness" [START_REF] Carlquist | Island Biology[END_REF][START_REF] Carlquist | More Woodiness/Less Woodiness: Evolutionary Avenues, Ontogenetic Mechanisms[END_REF][START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF]. A high incidence of insular woodiness, as well as other evolutionary convergence, has, however, not been observed in New Caledonia [START_REF] Carlquist | Island Biology[END_REF][START_REF] Pillon | How many genera of vascular plants are endemic to New Caledonia? A critical review based on phylogenetic evidence[END_REF].

Here we report a previously uninvestigated case of convergence occurring in the New Caledonian flora: the presence of numerous monocaulous (i.e. unbranched) plants in multiple lineages. This phenomenon has been repeatedly described by botanists who have studied the New Caledonian flora and have mentioned its presence in many distinct lineages [START_REF] Veillon | Architecture végétative de quelques arbres de l'archipel Néo-caledonien[END_REF][START_REF] Schmid | Les écosystèmes forestiers mélanésiens (Nouvelle Calédonie, Nouvelles-Hébrides, Fidji et îles Salomon)[END_REF][START_REF] Schmid | Conditions d'évolution et caractèristiques du peuplement végétal insulaire en Mélanésie Occidentale: Nouvelle-Calédonie et Vanuatu[END_REF]. Monocaulous plants, characterized by a thick unbranched trunk and large leaves clustered in the distal part of the stem, have long fascinated naturalists [START_REF] Humboldt | Ideen zu einer Physiognomick der Gewachse[END_REF][START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. Beyond being a botanical curiosity, monocauly has been widely discussed in ecological and evolutionary contexts, and was central to the development of a major biological theory [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]. The monocaulous habit was long considered as primitive in Angiosperms [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF](Corner, , 1953(Corner, -1954) ) until recent phylogenetic work showed a recent origin of this growth habit in many lineages [START_REF] Givnish | Giant lobelias exemplify convergent evolution[END_REF][START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF][START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF]Chapter 6). Several monocaulous species, sometime corresponding to rosette trees, evolved from herbaceous ancestors [START_REF] Carlquist | Island Biology[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF], particularly on islands [START_REF] Carlquist | Wood Anatomy of Goodeniaceae and the Problem of Insular Woodiness[END_REF][START_REF] Böhle | Island colonization and evolution of the insular woody habit in Echium L. (Boraginaceae)[END_REF][START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF]. However, in the New Caledonian flora, the few phylogenetic reconstructions available for groups containing monocaulous species rather suggest that they evolved from woody ancestors [START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF]Chapter 6). The breadth of this pattern remains to be confirmed through the study of other clades. Monocauly has also largely been associated with diverse life history trait attributes such as cauliflory [START_REF] Hallé | Corner's Architectural Model[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Barthélémy | Architecture et sexualité chez quelques plantes tropicales: le concept de floraison automatique[END_REF][START_REF] Schmid | Conditions d'évolution et caractèristiques du peuplement végétal insulaire en Mélanésie Occidentale: Nouvelle-Calédonie et Vanuatu[END_REF], dioecy [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], and the presence of compound leaves [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Hallé | Etude biologique et morphologique de la tribu des Gardéniées[END_REF][START_REF] Hallé | Corner's Architectural Model[END_REF]White, 1983b). From an ecological and evolutionary perspective, some authors have addressed the question of the selective advantage of monocauly [START_REF] Richards | The Tropical Rain Forest[END_REF]. This growth habit appears to be almost exclusively tropical [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. In some giant rosette trees the packing of leaves around terminal buds has been suggested to be adaptation to nightly frosts or short-duration fires [START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. In New Caledonia, as in other tropical regions, monocaulous plants are, however, preferentially found in rainforest [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF][START_REF] Hallé | Architecture of Trees in the Rain Forest of Morobe District, New Guinea[END_REF][START_REF] Hallé | Corner's Architectural Model[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Schmid | Conditions d'évolution et caractèristiques du peuplement végétal insulaire en Mélanésie Occidentale: Nouvelle-Calédonie et Vanuatu[END_REF]Chapter 6), suggesting other selective pressures. Their single growing apical meristem make them particularly sensitive to environmental damage [START_REF] Costes | Plant architecture, its diversity and manipulation in agronomic conditions, in relation with pest and pathogen attacks[END_REF][START_REF] Charles-Dominique | The architectural design of trees protects them against large herbivores[END_REF], implying adaptive growth pattern that would prevent or reduce apical damages [START_REF] Givnish | Thorn-like prickles and heterophylly in Cyanea: adaptations to extinct avian browsers on Hawaii[END_REF].

To examine the evolution of the monocaulous habit in the New Caledonian flora, we compiled a dataset combining architectural, morphological, taxonomic, phylogenetic, and ecological information on monocaulous species. We used this dataset to investigate the evolutionary history of monocaulous plants in New Caledonia. Using a phylogenetic supertree of New Caledonian woody genera, we quantified the phylogenetic signal and estimated the minimum number of independent evolutionary events leading to monocauly in the archipelago. We investigated the relation between the presence of monocaulous species in a genus and species richness in that genus. We then tested for correlated evolution between monocauly and trait attributes related to growth (marked growth rhythms), leaf shape (compound leaves), reproduction (cauliflory, dioecy, plain flower, fleshy fruits) and environment (rainforest, ultramafic substrate).

We also analysed risk of extinction and threats to monocaulous species according to the IUCN Red List criteria. Finally, we explored evolutionary contingencies and selective advantages that could explain the observed convergence of monocauly in the New Caledonian flora.

Materials & methods

Definition of monocauly

The monocaulous habit has long been recognized by numerous authors who have adopted various definitions (e.g. [START_REF] Warming | Oecology of plants : an introduction to the study of plant-communities[END_REF][START_REF] Rietz | Life-forms of terrestrial flowering plants, 1[END_REF][START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF][START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Hedberg | Tropical-Alpine Life-Forms of Vascular Plants[END_REF]. In most of cases, these only concerned woody plant since cambial activity has a considerable incidence on plant morphology and development. As such, monocotyledons represent a highly different monocaulous condition given that aerial branching is developmentally constrained by shoot growth and the absence of wood (Tomlinson and [START_REF] Tomlinson | Vascular anatomy of monocotyledons with secondary growth -an introduction[END_REF][START_REF] Tomlinson | The monocotyledons; their evolution and comparative biology VIII. Branching in monocotyledons[END_REF][START_REF] Haushahn | Plant ramifications inspire branched lightweight composites[END_REF]. In a recent work linking architecture and function, Bruy et al. (Chapter 6) proposed the following definition for monocauly: "selfsupporting woody plants whose cardinal functions rely on one single apparent stem". Following this definition, the structural types corresponding to four architectural models conform to monocaulous plants: Corner's, Holttum's, Chamberlain's and Cook's models [START_REF] Hallé | Essai sur l'architecture et la dynamique de croissance des arbres tropicaux[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. Architectural models describe both the overall architecture of a plant and the developmental growth process generating it [START_REF] Hallé | Essai sur l'architecture et la dynamique de croissance des arbres tropicaux[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF].

While they are too restrictive to understand the precise architecture of complex plants [START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF]Chapter 6), they have proved to be well suited for characterizing simple forms [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]Chapter 6). The Corner's and the Holttum's architectural models correspond to monoaxial systems (i.e. a single axis built up by a single apical meristem) with lateral ( C). Finally, the Cook's model is based on a monoaxial trunk supporting lateral branches that are functionally equivalent to compound leaves ("phyllomorphic branches", which are structure highly specialised in photosynthesis, with determinate growth and an abscission point, [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Hallé | Etude biologique et morphologique de la tribu des Gardéniées[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]

(Figure 5.1-D).
During the life of a plant, a structure that corresponds to one of these well-defined architectural models is often modified by the morphogenetic process of reiteration, corresponding to a repetition of the architectural units [START_REF] Oldeman | A molecular phylogeny of the Solanaceae[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. This process can be sequential, i.e. genetically determined, or opportunistic, i.e. driven by exogenous factors such as an injury or an increase in available resources [START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF]. Consequently, if the process of reiteration is inherent to a plant's development (i.e. sequential), it is not considered as monocaulous; opportunistic reiteration is, however, compatible with the definition of monocauly.

Species list and phylogenetic trees

A comprehensive list of non-monocot species occurring in New Caledonia was extracted from the most recent update taxonomic reference for the island's flora [START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF] supplemented by some unpublished species. Species that are strictly herbaceous or climbing (Bruy et al. unpublished data) were removed from the list. Taxonomy was pruned to species level meaning that infraspecific taxa were not considered. This yielded a final list of 2114 species belonging to 316 genera and 90 families.

Several phylogenetic trees of these woody New Caledonian genera and species were computed using supertree methods [START_REF] Gordon | Consensus Supertrees: The Synthesis of Rooted Trees Containing Overlapping Sets of Labeled Leaves[END_REF][START_REF] Sanderson | Phylogenetic supertrees: assembling the trees of life[END_REF][START_REF] Orp | The evolution of supertrees[END_REF]).

R software (v. 3.4.3) was used through the following packages: ape [START_REF] Paradis | APE: analyses of phylogenetics and evolution in R language[END_REF], adephylo [START_REF] Jombart | adephylo: exploratory analyses for the phylogenetic comparative method[END_REF], ade4 [START_REF] Dray | The ade4 package: implementing the duality diagram for ecologists[END_REF], apTreeshape [START_REF] Bortolussi | apTreeshape: Analyses of Phylogenetic Treeshape. R package version 1[END_REF], phytools [START_REF] Revell | phytools: An R package for phylogenetic comparative biology (and other things)[END_REF], picante [START_REF] Kembel | Picante: R tools for integrating phylogenies and ecology[END_REF], MonoPhy [START_REF] Schwery | MonoPhy: a simple R package to find and visualize monophyly issues[END_REF] and geiger [START_REF] Luke | GEIGER: investigating evolutionary radiations[END_REF]. The phylogenetic tree of angiosperm families produced by [START_REF] Magallon | Absolute diversification rates in Angiosperm clades[END_REF] was initially used, from which all but one of the 90 New Caledonian families were extracted. The missing family, Metteniusaceae, was added in position of Garryaceae and Eucommiaceae, its two closest relatives (APG IV, 2016), which are not represented in New Caledonia. For each family, generic relationships were resolved using published molecular phylogenies (Table 5.1) or taxonomic studies for a few genera that have not been studied using sequence data (Table 5.2). Cupaniopsis and Arytera have been shown to be polyphyletic [START_REF] Buerki | Phylogenetic inference of New Caledonian lineages of Sapindaceae: Molecular evidence requires a reassessment of generic circumscriptions[END_REF] and intrageneric branching times were randomised 100 times to produce a hundred species phylogenetic trees. Given the lack of a robust phylogeny for most New Caledonian genera, these hundred trees were unresolved at the species level, resulting in polytomies.

Character coding

An initial list of New Caledonian monocaulous species was built using an extraction from the NOU Herbarium database. This was then refined through extensive bibliographic analysis, particularly of the series Flore de la Nouvelle-Calédonie (Aubréville et al., 1967(Aubréville et al., -1992;;Morat andMacKee 1992-2004;[START_REF] Hopkins | Cunoniaceae : Flore de la Nouvelle-Calédonie[END_REF] and taxonomic publications. From this expanded list, the architecture of each taxon was studied in the field to determine whether it fit the definition of monocauly (i.e. whether it conforms to the Corner's, the Holttum's, the Chamberlain's and the Cook's architectural models). When field observations were impossible, architecture was checked by examining photographs (UMR AMAP-IAC database and Endemia website, continuously updated). Some monocaulous species were added to the list based on field observation. Of the 2114 species in the final dataset, only one could not be coded for architecture (Symplocos paniensis).

To test evolutionary correlates of monocauly, both life history and environmental traits were scored for each of the 2114 species when possible (Table 5.3). Data on sexual system, fruit type, and flower attraction were extracted from the dataset of [START_REF] Schlessman | Incidence, Correlates, and Origins of Dioecy in the Island Flora of New Caledonia[END_REF]. Data on endemism and vegetation were extracted from the Florical database [START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF] and substrate information was obtained from the dataset of [START_REF] Isnard | How did the ultramafic soils shape the flora of the New Caledonian hotspot?[END_REF]. For environmental traits, a binary approach was used for considering the main drivers of floristic richness and disharmony in New Caledonia: (i) ultramafic (covering 1/3 of the territory) versus non-ultramafic substrate [START_REF] Jaffré | The Relationship between Ecological Diversity and Floristic Diversity in New Caledonia[END_REF][START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF][START_REF] Isnard | How did the ultramafic soils shape the flora of the New Caledonian hotspot?[END_REF] and rainforest versus non-rainforest vegetation [START_REF] Jaffré | The Relationship between Ecological Diversity and Floristic Diversity in New Caledonia[END_REF][START_REF] Poncet | Phylogeography and niche modelling of the relict plant Amborella trichopoda (Amborellaceae) reveal multiple Pleistocene refugia in New Caledonia[END_REF]Birnbaum et al., 2015a;[START_REF] Pouteau | New Caledonia: a Pleistocene refugium for rain forest lineages of relict angiosperms[END_REF]. Each of Missing values for other characters were also obtained when possible using this method.

Data analysis

Richness in monocaulous species -To assess the relative richness of monocauly at the level of genus, we followed the method proposed by [START_REF] Boucher | Evolution and biogeography of the cushion life form in angiosperms[END_REF]. Considering the proportion of monocaulous species in the whole woody flora (9%), null models were built for each genus containing monocaulous species with random binomial distributions. For each model (i.e.

each genus), the number of observation equalled 1000, the number of trials equalled the total number of species in the genus, and the probability of success equalled 0.09. The observed number of monocaulous species in a given genus was then compared to the null distribution to estimate whether it is richer or poorer in monocaulous species than expected by chance, with a confidence limit of 5 %.

Origin, convergence and diversification -

The minimum number of independent origins of monocauly in the New Caledonian flora was estimated using the 100 genus-level supertrees and the phytools package for R [START_REF] Revell | phytools: An R package for phylogenetic comparative biology (and other things)[END_REF]. Each genus was scored as 0 (no monocaulous species)

or 1 (at least one monocaulous species) and ancestor character estimations on this variable were simulated with an Mk model in which transition rates are different ("SYM"). For each 100 genus supertree, 100 simulations were performed and the mean and range of number of the independent origins of monocauly (from state 0 to state 1) were counted. The same method was used independently for each monocaulous architectural model.

The degree of convergence of monocauly in the New Caledonian flora was estimated with the Phylogenetic D Statistic [START_REF] Fritz | Selectivity in mammalian extinction risk and threat types: a new measure of phylogenetic signal strength in binary traits[END_REF], a measure of phylogenetic signal for binary traits. Using the Caper R package [START_REF] Orme | caper: Comparative Analyses of Phylogenetics and Evolution in R. R package version 0[END_REF] we tested the departure of the observed D statistic from the distribution (1000 permutation) of expected D statistic under (i) a phylogenetic randomness model (no phylogenetic signal, D ≈ 1) and (ii) a Brownian threshold model (strong phylogenetic signal, D ≈ 0). Each test was performed on the 100 species supertrees. The phylogenetic signal of each monocaulous architectural models was also calculated using the same method.

To investigate whether the appearance of monocauly could have promoted species diversification, the relation between presence of monocaulous species in a genus and its species richness was tested using phylogenetic regression for binary dependent variables [START_REF] Ives | Phylogenetic Regression for Binary Dependent Variables[END_REF]. The model was fitted using the 100 genus supertrees with the ape R package [START_REF] Paradis | APE: analyses of phylogenetics and evolution in R language[END_REF]. The same method was used to test the relation between monocaulous genera and endemic genera (following [START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF], i.e. whether monocauly is predominant in endemic genera.

Correlated evolution of traits with monocauly -At the specific level, correlated evolution

between monocauly and life history or environmental trait attributes (Table 5.3) was tested using with the method of [START_REF] Pagel | Detecting correlated evolution on phylogenies: a general method for the comparative analysis fo discrete characters[END_REF] as implemented in the phytools R package [START_REF] Revell | phytools: An R package for phylogenetic comparative biology (and other things)[END_REF]. This method fits the Mk model for the dependent and independent evolution of two binary characters and test for significance of correlated evolution, comparing the log-likelihood of the two models.

In the case of significant correlated evolution, transition rates from one character state to another were compared for the two traits in order to investigate which character was likely to appear first (monocauly or the other trait character).

Species endemism, threats and conservation status -

The association between monocauly and endemism, IUCN conservation status and principal threats was tested through permutation tests (Table 5.3). We focused on data deficient and highly threatened species (EX, CR or EN following the IUCN criteria) to test whether monocaulous species were less known or more threatened than expected by chance. Considered threats were bushfires, introduced herbivores (deer, pigs and rats) and mining activities, the three major threats to New Caledonian flora (IUCN, 2017, RLA-NC Flora, unpublished data). A phylogenetic framework was not considered for these analysis because such traits are unlikely to be inherited. For each trait, the attribute (e.g.

"threatened") was permuted 10000 times and for each permutation, the proportion of monocaulous species presenting the attribute was calculated to build the null distribution. The observed proportion of monocaulous species presenting the attribute was then compared to the null distribution to test the departure from the null model (no association between monocauly and the considered attribute).

Results

Taxonomic and phylogenetic distribution of monocaulous plants

Of the 2114 woody self-supporting species present in New Caledonia, a total of 182 (8.6 %) were monocaulous, belonging to 41 genera and 30 families (Appendix 1). Monocaulous plants were widely distributed in the phylogeny and many families (12 of the 30) contained less than 3 monocaulous species. Some phylogenetic clustering was nevertheless observed, as much of the diversity occurred in a few clades (Figure 5.2). The orders Malpighiales and Apiales contained, species, and had significantly more monocauls than expected under the null model (Figure 5.3).

More generally, many genera (19 out of 41) had significantly more monocaulous species than expected under null model, despite many of them (49%) containing only 1 or 2 monocaulous species (Figure 5.3). Only Myrsine and Syzygium had significantly fewer monocaulous species than expected under a null model, even though the latter is the third richest genus in the New Caledonian flora [START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF]. Phyllanthus, the most species-rich genus in New Caledonia, had significantly more species than expected under the null model, while Pycnandra, the fourth richest genus and largest endemic genus, had the same number of monocaulous species as expected under the null model. In general, only a few genera contained a high proportion of monocaulous species.

Those comprising more than ten species, of which at least half were monocaulous, were

Acropogon, Bocquillonia, Meryta and Dysoxylum (Figure 5.3).

The most frequent architectural model observed among monocaulous species was the

Genus diversity and endemism

The presence of monocaulous species in a genus was significantly and positively associated with species richness (phylogenetic regression, pvalue < 0.001) implying that species-rich genera were more likely to have evolved monocauly or that the evolution of monocauly favored genera diversification. The proportion of endemic species in the monocaulous flora (98.9%) was significantly higher than expected by chance (permutation test, pvalue < 0.001). Only two monocaulous species (ca. 1%) were not New Caledonian endemics (Delarbrea paradoxa and Oxera baladica) compared to 9 % for the branched woody flora. By contrast, endemism at the generic level was low (21.9% vs. 22.9% for the branched flora) and unrelated to the occurrence of monocauly (phylogenetic regression, pvalue = 0.75 ± 0.01). Only 9 of the 72 endemic genera in our list (sensu [START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF] contained monocauls (Acropogon, Beauprea, Bocquillonia, Dutaillyea, Mangenotiella, Phelline, Pycnandra, Salaciopsis, and Virotia, Figure 5.3).

Evolution of monocauly and phylogenetic signals

Monocauly appeared independently on average 38.3 times across the genus trees, the number of shifts ranging from 31 to 49 over the 10,000 simulations. Reversion toward a branched habit occurred much less frequently, 7.9 times on average (ranging from 0 to 25 events). The

Contingent and correlated evolution

The evolution of monocauly was significantly correlated with both ultramafic substrate and rainforest (Pagel's model for correlated evolution, pvalue 0.029 and < 0.001 respectively, Table 5.4). Transition rates indicated a higher frequency of shift from branched toward monocauly on ultramafic substrate and in rainforest, meaning that monocauly evolved from branched species preferentially on ultramafics and in rainforest. The environmental preference was strongly marked for rainforest, where 65% of monocaulous species are restricted (Figure 5.5-B) whereas only 11% have never been observed in this habitat (not shown). The proportion of species occurring on ultramafic substrate was, however, similar between branched and monocaulous species (44% vs.

47% respectively).

The evolution of both cauliflory and marked rhythmic growth was strongly correlated with monocauly (Pagel's model for correlated evolution, pvalue < 0.001, Table 5 (Pagel's method) between monocauly and other trait atributes.

P.values in bold indicate trait atributes for which significant correlated evolution is detected. Transition rates in bold indicate the higher transition rates when correlated evolution is significant (A+: Monocaulous, A-: Branched, B+: Presence of the second trait attribute, B-: Absence of the second trait attribute). which were about twice as frequent in the monocaulous flora compared to the branched flora (Figure 5.5-C, D). In both case, transition rates were significantly higher for the evolution of cauliflory and rhythmic growth within monocaulous species. Compared to branched species, monocauls tended to have a slightly higher proportion of species with a dioecious sexual system (29% vs. 26%, Figure 5.5-E), plain flower (62% vs. 55%, Figure 5.5-F), non-fleshy fruits (55% vs.

41%, Figure 5.5-G) and compound leaves (20% vs. 15%, Figure 5.5-H), but none of these trait attributes showed significant correlated evolution with monocauly (Table 5.4).

IUCN risk of extinction status and threats

To date, 728 species (34%) of the woody non-monocot flora of New Caledonia have been assessed according to the IUCN Red List conservation categories and criteria (IUCN, 2017, RLA-NC Flora, unpublished data). This includes 63 monocaulous species (35% of the monocaulous flora), of which 51% were threatened (CR, EN, VU), 33% unthreatened (LC, NT), and 16% insufficiently known to be evaluated ("Data Deficient", Figure 5.6). The proportion of CR and EN species was higher for monocaulous species than branched ones (41% and 32%, respectively), the difference being marginally significant (permutation test, pvalue = 0.08). The number of monocaulous species in the "Data Deficient" class was also higher than expected by chance (permutation test, pvalue = 0.034). Bushfire and mining activities were significant threats for the monocaulous flora, affecting respectively 51% and 43% of species, but they were not more so for monocaulous than for branched species (permutation test, pvalue = 0.19 and 0.82 respectively). By contrast, introduced herbivores, affecting 33% of monocaulous species, were more threatening for monocaulous species than expected by chance (permutation test, pvalue = 0.008).

Discussion

A remarkable evolutionary convergence

Our study identified 182 monocaulous species representing 9% of the self-supporting woody flora and 5.5% of the New Caledonian flora as a whole. This proportion would have been even greater if monocotyledons, not considered in this study, had been included. The monocaulous habit is indeed frequent in monocotyledons (e.g. Arecaceae, Pandanaceae, Asparagaceae, [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], but represents a very different condition as aerial branching is developmentally constrained by shoot growth and the absence of wood in monocotyledons [START_REF] Tomlinson | Vascular anatomy of monocotyledons with secondary growth -an introduction[END_REF][START_REF] Tomlinson | The monocotyledons; their evolution and comparative biology VIII. Branching in monocotyledons[END_REF][START_REF] Haushahn | Plant ramifications inspire branched lightweight composites[END_REF]. Monocaulous species belonged to 41 genera (13% of woody genus) and 30 families (33% of woody families), and was phylogenetically scattered, as illustrated by the low phylogenetic signal and multiple independent origins (at least 31, and as many as 49) of this growth habit. This result, based on a genus-level phylogeny, is conservative as monocauly could have evolved repeatedly within many genera. The only two studies we are aware of, involving two monocaul-rich genera (Atractocarpus and Oxera), indicated multiple independent origins of the monocaulous habit in New Caledonia [START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF]Chapter 6). Additionally, in two genera (Pittosporum and Beauprea), monocauly is expressed by different architectural models (Corner's and Chamberlain's models), also suggesting independent evolution of this habit. These observations are confirmed by molecular phylogenies in which monocaulous architectural models are seen in different clades (He et al., 2016, CEC Gemmill, Waikato University, New Zealand, unpubl. res.). Current knowledge on the phylogenetic relationships in the 41 monocaulous genera in New Caledonia (Table 5.

1) indicates that only

Meryta and Plerandra, two sister genera [START_REF] Plunkett | Phylogeny and Geography of Schefflera: Pervasive Polyphyly in the Largest Genus of Araliaceae[END_REF] with several monocaulous species, could potentially exemplify a common origin of monocauly.

The recurrent convergent evolution of this distinctive growth habit in New Caledonia is remarkable, and similar in its number of independent origins to most famous plant syndromes such as secondary woodiness in the Canary Island (38 origins, [START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF]. In terms of the proportion of the flora in which it is seen, monocauly in New Caledonia is similar to divaricating plants in New Zealand (10% of the woody flora, [START_REF] Greenwood | Evolution of divaricating plants in New Zealand in relation to Moa browsing[END_REF].

Diversity and endemism of monocaulous lineages

We found a significant positive relationship between the presence of monocauly and the total species richness in the genus, implying that monocauly is more likely to evolve in speciesrich genera or that the evolution of monocauly promoted diversification, or both. This positive relationship most probably reflects the fact that six of the ten richest genera in New Caledonia contain monocaulous species. Among these is the archipelago's most speciose genus (Phyllanthus:

116 species, [START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF], which also contains the highest number of monocaulous species. With the exception of some genera containing a large proportion of monocaulous species (e.g. Acropogon, Bocquillonia and Meryta), most (66%) had less than four monocaulous species, which account for a small proportion of their total richness. Globally, the diversity of monocaulous species appeared independent of the number of species within a genus or family (e.g. Syzygium, 71 species and only 2 monocauls), and cannot be considered as a key innovation leading to a large radiation. The evolution of monocauly might instead favour the diversification of ecological roles within lineages, especially related to the occupation of the forest understory. A previous study supported this idea and indicated that the evolution of monocauly on its own did not increase the diversification rates, but might contributed to niche partitioning in the understory of rainforest habitats (Atractocarpus, Chapter 6).

The high rate of endemism in the monocaulous flora (99%), much higher than expected by chance, suggests either an in situ evolution of monocauly in New Caledonia or extinction of monocaulous species in adjacent regions. Interestingly, very few close relatives of New Caledonian monocaulous genera occurring elsewhere (when known, see Table 5.1) contain monocaulous species (the exceptions being Atractocarpus, Casearia, Dysoxylum, Meryta, Phyllanthus, Plerandra, and Tapeinosperma). Conversely, we found no significant relationship between the level of endemism in a genera and presence of monocauls. Only 9 of the 62 to 91 endemic genera (sensu [START_REF] Pillon | How many genera of vascular plants are endemic to New Caledonia? A critical review based on phylogenetic evidence[END_REF] contains monocauls, two of which, Dutaillyea and Mangenotiella should not be recognized [START_REF] Appelhans | A molecular phylogeny of Acronychia, Euodia, Melicope and relatives (Rutaceae) reveals polyphyletic genera and key innovations for species richness[END_REF]Gemmill, unpublished data), and Bocquillonia, Virotia

and Salaciopsis are doubtful (reviewed in [START_REF] Pillon | How many genera of vascular plants are endemic to New Caledonia? A critical review based on phylogenetic evidence[END_REF]. The fact that most monocauls are endemic while they belong to non-endemic genera suggests a rather recent in situ evolution of monocauly in New Caledonia, as found for two genera: Oxera, with two evolution at ca. 2.4 and 1.3-0 Myr [START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF] and Atractocarpus, with at least two apparition at ca. 1.4 and 0.9-0.6 Myr (Chapter 6).

Life history correlates of monocauly

Despite a moderate phylogenetical signal, we found that most monocaulous species belong to few orders (e.g. Apiales, Malphigiales, Sapindales), which are major components of the New Caledonian flora [START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF]. By contrast, monocauly is infrequent in basally branching Angiosperms and Myrtales. This pattern, supported by a D statistic significantly different from random distribution, suggests that there may be underlying morphological or physiological prerequisites [START_REF] Boucher | Evolution and biogeography of the cushion life form in angiosperms[END_REF][START_REF] Saslis-Lagoudakis | Evolution of salt tolerance in angiosperms: A phylogenetic approach[END_REF]. However, none of the morphological characters tested were found to be significantly involved in the evolution of monocauly. Some morphological characters (dioecy, plain flowers, fleshy fruits) were well represented in the monocaulous flora but were not correlated with the evolution of monocauly. As in other island floras, that of New Caledonia has a high incidence of dioecy (21% of the native angiosperm flora, [START_REF] Schlessman | Incidence, Correlates, and Origins of Dioecy in the Island Flora of New Caledonia[END_REF], suggested to have evolved on islands as a response to inbreeding depression in founding colonies [START_REF] Carlquist | The Biota of Long-Distance Dispersal. II. Loss of Dispersibility in Pacific Compositae[END_REF][START_REF] Böhle | Island colonization and evolution of the insular woody habit in Echium L. (Boraginaceae)[END_REF]. In New Caledonia, plain flowers and fleshy fruits are considered as major ecological correlates of dioecy [START_REF] Schlessman | Incidence, Correlates, and Origins of Dioecy in the Island Flora of New Caledonia[END_REF], and these traits are particularly pregnant in rainforest understory [START_REF] Givnish | Outcrossing Versus Ecological Constraints in the Evolution of Dioecy[END_REF] where most of monocaulous species occur. As such, the presence of these characters might rather considered as an insular syndrome that is well represented in the monocaulous flora. Similar results

were found for compound leaves, which were not correlated with the evolution of monocauly but rather with colonization of open/arid habitats [START_REF] Givnish | On the adaptive significance of coumpound leaves, with particular reference to tropical trees[END_REF]. By contrast, the evolution of monocauly appears to have triggered the evolution of cauliflory and marked rhythmic growth.

Cauliflory -Association between monocauly and cauliflory has long been noted [START_REF] Hallé | Corner's Architectural Model[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Barthélémy | Architecture et sexualité chez quelques plantes tropicales: le concept de floraison automatique[END_REF][START_REF] Schmid | Conditions d'évolution et caractèristiques du peuplement végétal insulaire en Mélanésie Occidentale: Nouvelle-Calédonie et Vanuatu[END_REF] but we report for the first time a statistically supported correlated evolution. This result is largely influenced by species conforming to Corner's architectural model, which accounts for 70% of cauliflorous species (vs.

11% and 8% for the Chamberlain's and the Cook's models, respectively). Transitions rates showed that cauliflory appeared more frequently after the evolution of a monocaulous habit. Being singlestemmed, monocaulous plants have fewer leafy nodes than branched plants (White, 1983a;[START_REF] Ackerly | Leaf Size, Sapling Allometry, and Corner's Rules: Phylogeny and Correlated Evolution in Maples (Acer)[END_REF]Chapter 6), constraining sexual reproduction to large axillary inflorescences (e.g. Tapeinosperma, Atractocarpus) or to areas below the leafy parts of the trunk (cauliflory).

Cauliflory is often expressed by short shoots or secondary bud complexes [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], which were observed in 22 of the 28 genera that are both monocaulous and cauliflorous (not shown), implying sites where flowers are borne in multiple years. Moreover, monocauls are known to have larger fruits than branched relatives [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]1953-1954;Chapter 6). Cauliflory, which involves the placement of fruits on woody, perennial stems rather than young, fleshy ones, seems more suited for supporting heavy and long-maturing fruits. These particular flowering and fruiting patterns in monocaulous plants may also be related to pollination and dispersion agents, and are probably also linked to growth in the intermediate strata of the forest understory [START_REF] Yumoto | Pollination systems in a warm temperate evergreen broad-leaved forest on Yaku Island[END_REF][START_REF] Appanah | Plant-pollinator interactions in Malaysian rain forests[END_REF][START_REF] Warren | Reproductive Allocation and Pollinator Distributions in Cauliflorus Trees in Trinidad[END_REF][START_REF] Zjhra | Facilitating sympatric species coexistence via pollinator partitioning in endemic tropical trees of Madagascar[END_REF].

Marked rhythmic growth -Correlated evolution between monocauly and marked rhythmic growth must be interpreted in the context of meristem protection. Transitions rates indicated that this character also appeared more frequently after the evolution of monocaulous habit. Except when reiteration takes place, a process that results in the production of new stems on some older plants [START_REF] Oldeman | A molecular phylogeny of the Solanaceae[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF], vertical exploration of monocaulous species is almost always accomplished by a single meristem. As such, traumatic apex death is particularly critical for unbranched plants. Several modes of meristem protection have been described, including the protection of apices by robust rosette of leaves [START_REF] Potter | Observations on the Protection of Buds in the Tropics[END_REF][START_REF] Groom | On bud-protection in dicotyledons[END_REF]. [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] noted that this mechanism concerned numerous monocaulous species and was often associated with rhythmic growth. However, while this mode of protection is efficient during periods of little or no growth, the meristem and young organs are dangerously exposed during phases of growth. It would therefore be advantageous for this "unprotected" phase to be short as possible, involving the rapid outgrowth of a preformed growth-unit comprising a long section of scale-like leaves and a distal cluster of large assimilative leaves. This marked rhythmic growth, common in monocaulous species, might take place in just a few days while the resting phase could last more than a year (Figure 5.7).

Evolution of plant architecture

Correlated evolution analysis failed to identify preadaptations facilitating the evolution of monocauly. A likely but uninvestigated preadaptation is the inherent architectural background.

Monocauly in New Caledonia is expressed by three different architectural models, each presenting a low phylogenetic signal and numerous independent origins. [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF] showed that some transitions from one model to another are more frequent than others. In the flora of New Caledonia, at least 16 of the 30 genera exhibiting Corner's model also contain species with the Rauh's architectural model, and 7 out of 10 genera expressing Chamberlain's model contain species that conform to the Leeuwenberg's model. A detailed understanding of intra-generic relationships is needed to formulate any rigorous conclusions about the evolution from one model to another but these iterative associations suggest that there are some evolutionary links between the two pairs of models. The relation between the Chamberlain's and the Leeuwenberg's models is obvious given their morphological similarities (unichasial sympodium vs. plurichasial sympodium, respectively). Incidentally, the transition from the Chamberlain's toward the Leeuwenberg's model frequently occurs within an individual during ontogeny [START_REF] Veillon | Architecture végétative de quelques arbres de l'archipel Néo-caledonien[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. The Rauh's model is one of the least differentiated branched models in that branches are morphologically identical to the trunk [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. The Rauh's model could be seen as a sequential repetition of Corner's model or Corner's model as an expression of the Rauh's model in which branches were lost. In the first case, a probable evolutionary mechanism is the integration of an opportunistic reiteration process in the developmental sequence of the plant (see [START_REF] Millan | Analyse de la variabilité des traits architecturaux des formes de croissance dans les communautés végétales[END_REF]. In the second case, a probable mechanism is the structural reduction through heterochronic developmental pathway [START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF][START_REF] Barthélémy | Architecture et sexualité chez quelques plantes tropicales: le concept de floraison automatique[END_REF]Chapter 6). Too few genera expressed the Cook's model and data on the architecture of related branched species are too lacunar to suggest whether one transition is more likely. Nevertheless, we noted that some species of Phyllanthus and Casearia expressed the Roux's model, which is morphologically close to the monocaulous Cook's model also seen in both genera [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Hallé | Architectures de Plantes[END_REF].

Such associations between woody architectural models in monocaulous genera suggest a preferential evolution of this habit from woody ancestors, as shown in Atractocarpus (Chapter 6)

and Oxera [START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF]. However, the evolution of monocauly from herbaceous ancestors is common globally [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF] and particularly on islands [START_REF] Carlquist | Wood Anatomy of Goodeniaceae and the Problem of Insular Woodiness[END_REF][START_REF] Böhle | Island colonization and evolution of the insular woody habit in Echium L. (Boraginaceae)[END_REF][START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF]. New Caledonia was noted to be particularly poor in secondary woody species [START_REF] Carlquist | Island Biology[END_REF][START_REF] Pillon | How many genera of vascular plants are endemic to New Caledonia? A critical review based on phylogenetic evidence[END_REF], known genus only in Scaevola, whose wood anatomy [START_REF] Carlquist | Wood Anatomy of Goodeniaceae and the Problem of Insular Woodiness[END_REF] and phylogeography [START_REF] Howarth | Phylogenetics of the genus Scaevola (Goodeniaceae): implication for dispersal patterns across the Pacific Basin and colonization of the Hawaiian Islands[END_REF] indicate an evolution from herbaceous Australian ancestors toward woody Pacific-islands species. This genus includes one monocaulous species in New Caledonia (S. beckii) that is interestingly closely related with herbaceous to sub-woody Australian species [START_REF] Howarth | Phylogenetics of the genus Scaevola (Goodeniaceae): implication for dispersal patterns across the Pacific Basin and colonization of the Hawaiian Islands[END_REF]. Other potentially secondary woody species that are missing in the Catalogue of Woody Herbs on Islands [START_REF] Carlquist | Island Biology[END_REF] are found in Oxalis, represented by six species in New Caledonia [START_REF] Munzinger | FLORICAL: Checklist of the vascular indigenous flora of New Caledonia[END_REF]) three of which are woody and one monocaulous (O. balansae). Given that the large majority of Oxalis worldwide are herbaceous [START_REF] Cocucci | Oxalidaceae[END_REF], these species could highlight a new case of secondary woodiness, although anatomical and phylogenetic data will be needed to confirm this hypothesis (see [START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF]. As such, evolution of monocauly from herbaceous ancestors has probably occurred in both of these potential secondary woody New Caledonian genera, and the scarcity of this evolutionary pathway there seems further due to the general lack of secondary woodiness in the archipelago. This could also explain the surprising absence of plants exhibiting the Holttum's architectural model, which has principally been described from islands [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. Most of the woody species conforming to this model belong to largely herbaceous families (e.g.

Boraginaceae, Asteraceae, Geraniaceae, Gesneriaceae or Campanulaceae) and are therefore considered as secondarily woody taxa that have retained the ancestral Holttum's architectural model, the latter being frequent in herbaceous plants [START_REF] Jeannoda-Robinson | Contribution à l'étude de l'architecture des herbes[END_REF][START_REF] Chomicki | Deciphering the complex architecture of an herb using micro-computed X-ray tomography, with an illustrated discussion on architectural diversity of herbs[END_REF].

A threatened and poorly known growth habit

More than half (51%) of evaluated monocaulous species were threatened according to the IUCN Red List criteria. This proportion is probably to be moderated given that only 34% of the New Caledonian flora and 35% of monocaulous species have been evaluated to date, and the proportion of highly threatened species (EN and CR following IUCN conservation status) is higher among monocaulous species than in the branched taxa (41% vs. 32% respectively). The higher threat among monocauls is not due to bushfire or mining activities (despite the fact that they concern 55% and 43% of evaluated monocaulous species, respectively), but rather introduced herbivores. Threat from herbivores is indeed higher than expected by chance in the monocaulous flora, probably because the single meristem and few large leaves of these plants make browsing particularly damaging (see [START_REF] Costes | Plant architecture, its diversity and manipulation in agronomic conditions, in relation with pest and pathogen attacks[END_REF][START_REF] Charles-Dominique | The architectural design of trees protects them against large herbivores[END_REF].

Another important factor influencing the risk of extinction of monocaulous species is their preference for the rainforest understory and ultramafic substrate. Habitat conservation is a fundamental element of species protection [START_REF] Rohlf | Six Biological Reasons Why the Endangered Species Act Doesn't Work-And What to Do About It[END_REF][START_REF] Shilling | Do Habitat Conservation Plans Protect Endangered Species?[END_REF] but also for maintaining genetic diversity and evolutionary process [START_REF] Lawler | The theory behind, and the challenges of, conserving nature's stage in a time of rapid change[END_REF]. However, New Caledonian rainforests, and particularly the relictual and fragmented rainforests on ultramafic substrate, are particularly threatened [START_REF] Jaffré | Conservation programmes in New Caledonia, western Pacific: in place for the dry forest, but urgently needed for the ultramafic vegetation[END_REF]. Mining activities on the archipelago increasingly impact the native vegetation on ultramafic substrate [START_REF] Jaffré | Conservation programmes in New Caledonia, western Pacific: in place for the dry forest, but urgently needed for the ultramafic vegetation[END_REF][START_REF] Wulff | Conservation priorities in a biodiversity hotspot: analysis of narrow endemic plant species in New Caledonia[END_REF]Ibanez et al., 2017a).

Rainforests have been drastically reduced [START_REF] Sloan | Remaining natural vegetation in the global biodiversity hotspots[END_REF]Birnbaum et al., 2015b) and are critically fragmented [START_REF] Jaffré | Threatened plants of New Caledonia: Is the system of protected areas adequate?[END_REF]Ibanez et al., 2017a). This habitat loss is of particular concern for rainforest species, of which monocaulous taxa are an important functional group.

Finally, the significantly higher proportion of monocaulous species assessed as data deficient (DD, 16%) illustrates the lack of knowledge of this growth habit. The rarity of monocaulous species worldwide and their peculiar morphology (large leaves, stout stems and a single growing apex) make monocauls particularly relevant for studying ecological and evolutionary processes (see Chapter 6) but constraint their representation in herbarium collections leading to taxonomic shortfalls (see Chapter 4).

Environmental contingency and ecological opportunities in New Caledonia

Whether a high incidence of monocauly is unique to New Caledonia is difficult to assess as comparative data are scarce. In a meta-analysis including more than 20,000 vascular plant species, [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF] found 118 origins of monocauly while we found a mean of 38 (and up to 49 for the New Caledonian flora alone (and this just using a genus-level phylogeny). Current knowledge on plant architecture would suggest that ca. 2% of species are unbranched globally [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF], i.e. less than one third of what occurs in the New Caledonian flora. The question of the evolution of monocauly, however, remains puzzling as it depends on the definition of this habit. For instance, [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF] did not consider the Cook's architectural model in their study but included all monocotyledonous species. While quantitative data for other floras are not available, the strong convergence toward a growth habit that is considered to be rare both globally [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] and regionally [START_REF] Schmid | Les écosystèmes forestiers mélanésiens (Nouvelle Calédonie, Nouvelles-Hébrides, Fidji et îles Salomon)[END_REF][START_REF] Schmid | Conditions d'évolution et caractèristiques du peuplement végétal insulaire en Mélanésie Occidentale: Nouvelle-Calédonie et Vanuatu[END_REF] suggests that monocauly is parts of the New Caledonia's functional disharmony. [START_REF] Schmid | Fleurs et plantes de Nouvelle-Calédonie[END_REF] even considered that the diversity and abundance of this habit are one of the most striking characteristic of New Caledonian rainforests. This is supported by the exceptional rate of endemism in the monocaulous flora (99%) which is by far higher than expected by chance. The repeated evolution of monocauly in New Caledonia among distantly related lineages suggests environmental contingencies peculiar to the archipelago.

Rainforest density -We found a significant pattern of contingent evolution between monocauly and occurrence in rainforest areas, indicating a preferential evolution of monocauly in this habitat. Sixty-five percent of monocaulous species grow exclusively in rainforest and only 11% have never been observed in this vegetation. Consequently, monocaulous species account for a substantial part of rainforest diversity (12.3% of woody species) and much more if we only consider understory species. An ecological preference for the understory of rainforests among taxa whose architecture corresponds to a monocaulous habit has yet been suggested [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF][START_REF] Hallé | Architecture of Trees in the Rain Forest of Morobe District, New Guinea[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Schmid | Fleurs et plantes de Nouvelle-Calédonie[END_REF]. Monocauls support large, mostly simple leaves [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]Chapter 6). This could be an adaptation to low-resource environments where low-cost large leaves could be advantageous compared to more costly branches in buffered conditions [START_REF] Givnish | Sizes and Shapes of Liane Leaves[END_REF][START_REF] Givnish | On the adaptive significance of leaf form[END_REF]. This is particularly true in dense, shady rainforest understory where lateral branches are mechanically constrained by dense neighbouring vegetation. New Caledonian rainforests appear to have higher stem densities than other Southwest Pacific rainforests [START_REF] Jaffré | Etude floristique et structurale de deux forêts denses humides sur roches ultrabasiques en Nouvelle-Calédonie[END_REF][START_REF] Jaffré | Structural and floristic characteristics of a rain forest on schist in New Caledonia : a comparaison with an ultramafic rain forest[END_REF][START_REF] Ibanez | Structural and floristic diversity of mixed tropical rain forest in New Caledonia: newdata from the New-Caledonian Plant Inventory and Permanent PlotNetwork (NC-PIPPN)[END_REF]Ibanez et al., 2017b), probably related to the frequency and intensity of tropical cyclones (Ibanez et al., 2018a). Competition for space and light in this highly congested environment might have favoured the unidirectional space exploration and large leaves of monocaulous species. Moreover, unbranched or sparsely-branched species are less affected by debris fall or the domino effect of falling trees during cyclones. The fact that palms and tree ferns are also particularly abundant in New Caledonian rainforests (Ibanez et al., 2017b) is consistent with these hypotheses, both groups being dominated by unbranched architectures. We believe that these forest structural and dynamic features have been important driver of the convergence toward monocauly in New Caledonia.

Rainforest history -Large simple leaves, inherent to the monocaulous habit, have cheaper construction costs than twigs (see above) but are disadvantageous in arid and cold conditions due to their thicker boundary layer, which slows thermoregulation [START_REF] Givnish | On the adaptive significance of leaf form[END_REF][START_REF] Wright | Global climatic drivers of leaf size[END_REF].

Accordingly, we found that 79% of monocaulous species occurring in dry vegetation had compound leaves or functionally similar phyllomorphic branches (not shown). Large, droughtsensitive leaves have probably constrained the distribution of monocauly to humid and shady environments, which provide buffered thermal and hydric condition [START_REF] Givnish | On the adaptive significance of leaf form[END_REF][START_REF] Givnish | Comparative Studies of Leaf Form: Assessing the Relative Roles of Selective Pressures and Phylogenetic Constraints[END_REF][START_REF] Givnish | Adaptation to Sun and Shade: a Whole-Plant Perspective[END_REF].

New Caledonia has been suggested to have several rainforest refuges for some drought sensitive groups such as Palms [START_REF] Pintaud | Chorology of New Caledonian palms and possible evidence of Pleistocene rain forest refugia[END_REF] or basal Angiosperms [START_REF] Poncet | Phylogeography and niche modelling of the relict plant Amborella trichopoda (Amborellaceae) reveal multiple Pleistocene refugia in New Caledonia[END_REF]Pouteau et al., 2015;[START_REF] Tournebize | Two disjunct Pleistocene populations and anisotropic postglacial expansion shaped the current genetic structure of the relict plant Amborella trichopoda[END_REF] during Pleistocene climatic fluctuations [START_REF] Bowler | Late Quaternary climates of Australia and New Guinea[END_REF][START_REF] Hope | Tropical vegetational change in the late Pleistocene of New Caledonia[END_REF][START_REF] Stevenson | A comparison of late Quaternary forest changes in New Caledonia and northeastern Australia[END_REF]. These paleoclimatic events caused important drought-related species extinctions in other Southwest Pacific rainforests, particularly in Australia [START_REF] Byrne | Decline of a biome: evolution, contraction, fragmentation, extinction and invasion of the Australian mesic zone biota[END_REF], which is considered as the principal source area for New Caledonian flora [START_REF] Morat | Our knowledge of the flora of New Caledonia: endemism and diversity in relation to vegetation types and substrates[END_REF][START_REF] Swenson | Sapotaceae biogeography supports New Caledonia being an old Darwinian island[END_REF][START_REF] Thomas | Molecular dating of Winteraceae reveals a complex biogeographical history involving both ancient Gondwanan vicariance and longdistance dispersal[END_REF]. As such, besides presenting environmental contingencies that promoted the evolution of monocauly, New Caledonian rainforest could have acted as refugia for such drought sensitive plants that disappeared from adjacent regions. After the last glacial episode, ecological opportunities provided by new vacant habitat, are likely to have promoted the diversification of some monocaulous lineages already present and triggered the evolution of monocauly in others through niche partitioning (e.g.

Atractocarpus, Chapter 6).

Ultramafic substrates -Monocauly appears to have evolved preferentially on ultramafic substrate, an unsuspected result given the neglegible difference between the proportion of branched and monocaulous species occurring on this substrate (44% and 47%, respectively). These ultramafic rocks are rare globally but cover ca. one third of New Caledonia [START_REF] Paris | Géologie de la Nouvelle-Calédonie, un essai de synthèse[END_REF]Isnard et al., 2016) and are thought to have initially covered all of the main island [START_REF] Pelletier | Geology of the New Caledonia region and its implications for the study of the New Caledonian biodiversity[END_REF]. Soils derived from ultramafic rocks are diverse but share several characteristics, including their low nutrient content and high concentrations of potentially bioavailable toxic trace elements, implying physiological tolerance among plants that grow on them [START_REF] Jaffré | Composition chimique et conditions de l'alimentation minérale des plantes sur roches ultrabasiques (Nouvelle Calédonie)[END_REF][START_REF] Jaffré | Etude écologique du peuplement végétal des sols dérivés de roches ultrabasiques en Nouvelle-Calédonie[END_REF][START_REF] Isnard | How did the ultramafic soils shape the flora of the New Caledonian hotspot?[END_REF].

As such, ultramafic substrates appear to be largely responsible for the floristic disharmony between the New Caledonian archipelago and adjacent regions [START_REF] Jaffré | The Relationship between Ecological Diversity and Floristic Diversity in New Caledonia[END_REF][START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF]Isnard et al., 2016). [START_REF] Veillon | Architecture végétative de quelques arbres de l'archipel Néo-caledonien[END_REF] suggested that these constraining edaphic conditions could be responsible for the high proportion of orthotropic, slow-growing stems with distally clustered-leaves, as showed by [START_REF] Virot | La Végétation canaque[END_REF]. Deploying large leaves might indeed be no more costly than producing an equivalent photosynthetic area on several twigs [START_REF] Givnish | Plant Stems: Biomechanical Adaptation for Energy Capture and Influence on Species Distributions[END_REF][START_REF] Wright | Cross-species patterns in the coordination between leaf and stem traits, and their implications for plant hydraulics[END_REF]. In this sense, monocauly might be seen as a structural reduction of a more complex branched architecture resulting from selection under constraining edaphic conditions. A phenomena referred to as "architectural pauperization" was proposed for Cecropia obtusa, a branched tree (generally conforming to the Rauh's architectural model) that becomes monocaulous on bare, eroded and humus-free soils [START_REF] Barthélémy | Architecture et sexualité chez quelques plantes tropicales: le concept de floraison automatique[END_REF]. Consequently, the convergence toward monocauly observed in the New Caledonian flora could also have been triggered by preferential evolution of this habit on the locally common but globally rare ultramafic substrate. Interestingly, several clades rich in monocaulous species (e.g. Malpighiales, Apiales, Ericales, Gentianales, Sapindales) have been shown to be over-represented in New Caledonia, probably as a result of exaptation to ultramafic substrate [START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF]. In some of these clades (e.g. Malpighiales, Ericales, Gentianales) nickel hyper-accumulation, a specialization associated with adaptation to ultramafic substrates, is common [START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF][START_REF] Jaffré | The metal hyperaccumulators from New Caledonia can broaden our understanding of nickel accumulation in plants[END_REF].

Lack of browsers -Threats associated with IUCN Red List risk of extinction status suggest that monocaulous species are more sensitive to introduced herbivores than branched taxa. Recent studies actually showed that plant architecture significantly contribute to structural defenses: the more intrinsically branched is a plant, the more effective is the protection [START_REF] Costes | Plant architecture, its diversity and manipulation in agronomic conditions, in relation with pest and pathogen attacks[END_REF][START_REF] Charles-Dominique | An index for assessing effectiveness of plant structural defences against mammal browsing[END_REF][START_REF] Charles-Dominique | The architectural design of trees protects them against large herbivores[END_REF]. As such, the absence of native browsers in New Caledonian rainforests may have provided an ecological opportunity for the evolution of monocauly. This exposed growth habit could then be considered as a syndrome of insular naivety. To our knowledge, horned terrestrial turtles (Meiolaniid) are the only known native browsing vertebrate that were present in New Caledonia and survived until the Holocene [START_REF] Anderson | Faunal Extinction and Human Habitation in New Caledonia: Initial Results and Implications of New Research at the Pindai Caves[END_REF][START_REF] White | Megafaunal meiolaniid horned turtles survived until early human settlement in Vanuatu, Southwest Pacific[END_REF]. As another large terrestrial vertebrate, the bird Sylviornis neocaledonicae [START_REF] Poplin | Sylviornis neocaledoniae (Aves, Galliformes, Megapodiidae), oiseau Géant éteint de l'ile des Pins (Nouvelle-Calédonie)[END_REF], which was probably not a browser [START_REF] Mourer-Chauviré | Description of the skull of the genus Sylviornis Poplin, 1980 (Aves, Galliformes, Sylviornithidae new family), a giant extinct bird from the Holocene of New Caledonia[END_REF][START_REF] Worthy | Osteology Supports a Stem-Galliform Affinity for the Giant Extinct Flightless Bird Sylviornis neocaledoniae (Sylviornithidae, Galloanseres)[END_REF], horned turtles probably lived in the coastal plain and dry forest [START_REF] Anderson | Faunal Extinction and Human Habitation in New Caledonia: Initial Results and Implications of New Research at the Pindai Caves[END_REF], where monocauls are infrequent. Accordingly, monocaulous species of Cyanea (Campanulaceae) in Hawaii where browsing birds were present, have thorn-like prickles to protect the stem [START_REF] Givnish | Thorn-like prickles and heterophylly in Cyanea: adaptations to extinct avian browsers on Hawaii[END_REF]. By contrast, New Caledonian flora account for very few armed species, the vast majority of which are climbers [START_REF] Bruy | Abundance, richness and composition of lianas in forest communities along an elevation gradient in New Caledonia[END_REF].

Conclusions and future directions

Monocauly in New Caledonia appears to represent a remarkable functional convergence, involving different plant architectural models. Environmental contingencies present in the archipelago (e.g. rainforest stem density, ultramafic substrate, glaciation-driven habitat vacancy) might have provide a favorable background for the evolution of monocauly, did not face strong negative selection pressures such as the presence of large browsers or pronounced seasonality.

Changes in growth habit may in turn alter the selective environment of other trait attributes (cauliflory, rhythmic growth) and drive their evolution. The moderate phylogenetic clustering of monocauly indicates some potential prerequisites that have not been highlighted in the present study. An important prerequisite that remains to be tested is plant architecture because the currently available data are insufficient. The New Caledonian flora seems to include many sparsely-branched plants in addition to monocauls, and some architectural models (Corner's, Rauh's, Attims', Leuwenberg's, Chamberlain's, see [START_REF] Veillon | Architecture végétative de quelques arbres de l'archipel Néo-caledonien[END_REF] seem to be particularly well represented. The architectural background of a group could act as a key driver in the diversification of growth habit, and New Caledonia is undoubtedly an appropriate case study to investigate this.

More globally, monocaulous species appear to comprise an important functional group in rainforest understory habitats, as probably occurs in other areas such as Gabon [START_REF] Hallé | Présentation de quelques formes ligneuses simples de la forêt de Belinga (Gabon)[END_REF] and Papua New Guinea [START_REF] Hallé | Architecture of Trees in the Rain Forest of Morobe District, New Guinea[END_REF]. However, the monocaulous habit remains largely overlooked and further comparative studies are needed to gain insight into the ecological and evolutionary history of monocaulous plants throughout the tropics.

Ackowledgements:

We thank Chrissen Gemmill, Thais Vasconcelos and Yohan Pillon for sharing unpublished data on the relationships of some New Caledonian genera. We thank the Endemia association, which coordinates the The Red List Authority for threatened Flora of New Caledonia, for providing data about risk of extinction assessments. We thank all contributors to the UMR AMAP-IAC photograph database and to the Endemia website, for sharing their pictures for the architectural study. We also thank Vanessa Hequet and Daniel Barthelemy for permission to use their high quality photo in the manuscript. The diversification of ecological roles and related adaptations in closely related species within a lineage is one of the most important processes linking plant evolution and ecology. Plant architecture offers a robust framework to study these processes as it can highlight how plant structure influences plant diversification and ecological strategies. We investigated a case of gradual evolution of branching architecture in Atractocarpus spp. (Rubiaceae), forming a monophyletic group in New Caledonia that has diversified rapidly, predominantly in rainforest understory habitats. We used a transdisciplinary approach to depict architectural variations and revealed multiple evolutionary transitions from a branched (Stone's architectural model) to a monocaulous habit (Corner's architectural model), which involved the functional reduction of branches into inflorescences. We propose an integrative functional index that assesses branching incidence on functional traits influencing both assimilation and exploration functions.

We showed that architectural transitions correlate with ecologically important functional traits. Variation in ecologically important traits among closely relatives, as supported by the architectural analysis, is suggestive of intense competition that favored divergence among locally coexisting species. We propose that Pleistocene climatic fluctuations causing expansion and contraction of rainforest could also have offered ecological opportunities for colonizers in addition to the process of divergent evolution.

Keywords: Branching index, Convergence, Corner's rules, Gardenieae, Island, Rainforest, Treelet, Understory

Introduction

Convergent and divergent evolution are widely recognized as important drivers of plant diversification at large scale [START_REF] Givnish | Molecular Evolution and Adaptive Radiation[END_REF][START_REF] Gianoli | Evolution of a climbing habit promotes diversification in flowering plants[END_REF][START_REF] Drummond | Multiple Continental Radiations and Correlates of Diversification in Lupinus (Leguminosae): Testing for Key Innovation with Incomplete Taxon Sampling[END_REF][START_REF] Couvreur | Global diversification of a tropical plant growth form: environmental correlates and historical contingencies in climbing palms[END_REF][START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. Well known examples of large scale convergence in plants (i.e. the appearance of phenotypic similarities among distantly related taxa) include the evolution of a cushion growth habit in alpine environments [START_REF] Boucher | Reconstructing the origins of high-alpine niches and cushion life form in the genus Androsace S.L. (Primulaceae)[END_REF][START_REF] Aubert | 1914-2014: A revised worldwide catalogue of cushion plants 100 years after Hauri and Schröter[END_REF][START_REF] Boucher | Evolution and biogeography of the cushion life form in angiosperms[END_REF], rosette-shrubs in islands [START_REF] Carlquist | Island Biology[END_REF][START_REF] Givnish | Giant lobelias exemplify convergent evolution[END_REF][START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF], climbing mechanisms in lianas [START_REF] Sousa-Baena | Evolution and Development of Tendrils in Bignonieae (Lamiales, Bignoniaceae)[END_REF][START_REF] Sousa-Baena | Convergent Evolution and the Diverse Ontogenetic Origins of Tendrils in Angiosperms[END_REF], and succulence in arid environments [START_REF] Ogburn | The Ecological Water-Use Strategies of Succulent Plants[END_REF][START_REF] Arakaki | Contemporaneous and recent radiations of the world's major succulent plant lineages[END_REF]. These and others examples demonstrate that similar habitats or micro-habitats can produce selective pressures that favor some morphological and physiological trait attributes. Another evolutionary process, sometimes linked with convergence, is the diversification of ecological roles among closely related species, i.e. divergent evolution, which results in the exploitation of different ecological resources (see e.g. [START_REF] Bramwell | Some morphological aspects of the adaptive radiation of Canary Islands Echium species[END_REF][START_REF] Givnish | Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae)[END_REF][START_REF] Givnish | Adaptive radiation, correlated and contingent evolution, and net species diversification in Bromeliaceae[END_REF]. Convergent and divergent evolution have been attributed to morpho-physiological traits, whose gradual evolution or rapid innovation can lead to adaptive radiation, sometimes involving increased diversification rates [START_REF] Givnish | Molecular Evolution and Adaptive Radiation[END_REF][START_REF] Givnish | Adaptive radiation, correlated and contingent evolution, and net species diversification in Bromeliaceae[END_REF][START_REF] Couvreur | Global diversification of a tropical plant growth form: environmental correlates and historical contingencies in climbing palms[END_REF]. A striking feature of convergent and divergent evolution is that it often involves variation in growth habit, which represents the ultimate form of a plant expressed in its physiognomy [START_REF] Warming | Oecology of plants : an introduction to the study of plant-communities[END_REF]. Growth habit results from the integration of a set of traits, e.g. branching pattern and structure, body size and shape, position of inflorescences, and anatomy, among others, which have generally been studied independently (see e.g. [START_REF] Carlquist | Wood and stem anatomy of Lardizabalaceae with comments on the vining habit, ecology and systematics[END_REF][START_REF] Rowe | Plant growth forms: an ecological and evolutionary perspective[END_REF][START_REF] Givnish | Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae)[END_REF][START_REF] Isnard | Growth form evolution in Piperales and its relevance for understanding the angiosperm diversification -An integrative approach combining plant architecture, anatomy and biomechanics[END_REF][START_REF] Wagner | Major trends in stem anatomy and growth forms in the perianth-bearing Piperales, with special focus on Aristolochia[END_REF]. Few work has, however, used the integrative approach provided by plant architecture to study the evolution of plant growth habit.

Plant architecture characterizes the spatial arrangement and specialization of structures (morphological origin, branching pattern, axis categorization) and their evolution during ontogeny [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF]. As such, it can highlight how plant structure impacts plant diversification and can help identify evolutionary processes underlying plant evolution [START_REF] Bateman | Evolutionary-developmental change in the growth architecture of fossil rhizomorphic lycopsids: Scenarios constructed on cladistic foundations[END_REF][START_REF] Bateman | Architectural radiations cannot be optimally interpreted without morphological and molecular phylogenies[END_REF][START_REF] Sussex | The evolution of plant architecture[END_REF][START_REF] Meyer-Berthaud | The land plant cover in the Devonian: a reassessment of the evolution of the tree habit[END_REF].

Architectural studies have shown that plants are modular organisms comprising elements that can differ in their organization and functions [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF]. For instance, in many tree and treelet species, the trunk functions primarily for exploration and to provide support, while branches are involved in assimilation and reproduction. Plant architecture thus influences spatial and temporal exploitation of resources [START_REF] Smith | Deviation from symmetrically self-similar branching in trees predicts altered hydraulics, mechanics, light interception and metabolic scaling[END_REF]. Some architectural traits have been shown to impact plant fitness, either directly [START_REF] Küppers | Ecological significance of above-ground architectural patterns in woody plants: A question of cost-benefit relationships[END_REF][START_REF] Millet | Relationship between architecture and successional status of trees in the temperate deciduous forest[END_REF][START_REF] Charles-Dominique | Architectural strategies of Cornus sericea, a native but invasive shrub of Southern Quebec, Canada, under an open or a closed canopy[END_REF][START_REF] Charles-Dominique | Architectural strategies of Rhamnus cathartica, in relation to canopy openness[END_REF][START_REF] Millan | Analyse de la variabilité des traits architecturaux des formes de croissance dans les communautés végétales[END_REF][START_REF] Charles-Dominique | The architectural design of trees protects them against large herbivores[END_REF] or in interaction with other functional traits [START_REF] Pérez-Harguindeguy | New handbook for standardised measurement of plant functional traits worldwide[END_REF][START_REF] Trueba | Trait coordination, mechanical behaviour and growth form plasticity of Amborella trichopoda under variation in canopy openness[END_REF]. Among the best known examples are Corner's rules, which stipulate that "the greater the ramification, the smaller become the branches and their appendages"

(leaves, flowers, and fruits, [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]. This statement points toward an effect of plant architecture on leaf size and linked architecture and plant ecological strategies (White, 1983a;[START_REF] Ackerly | Canopy Structure and Dynamics: Integration of Growth Processes in Tropical Pioneer Trees[END_REF]. To date, approaches linking plant architecture and function are scarce although some architectural indexes can assess interesting strategies such as defense against herbivores ("Index of caginess", Charles-Dominique et al., 2017) or leaf-to-stem relation that trigger flowering ("Index of axialisation", [START_REF] Lauri | Le mouvement morphogenetique, approche morphometrique et restitution graphique: L'exemple de quelques plantes tropicales[END_REF][START_REF] Lauri | Shoot type demography and dry matter partitioning: a morphometric approach in apple (Malus × domestica)[END_REF].

Despite the functional importance of plant architectural traits, little is known about their evolution [START_REF] Kurmann | The Evolution of Plant Architecture[END_REF][START_REF] Enriquez | The architecture of Phyllanthus acuminatus Vahl: a prelude to understanding the architectural evolution in the Phyllanthaceae[END_REF]. In a recent review, Chomicki et al. ( 2017) investigated the evolution of plant architecture for several groups. Their results showed preferential transitions between models, suggesting pre-requisite morphological ability for the evolution of a given plant architecture. In contrast, some model transitions seem never to occur, suggesting that genetic constraints might prevent them. [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF] did not, however, investigate processes involved in plant architectural evolution. Bateman [START_REF] Bateman | Saltational evolution of form in vascular plants: a neoGoldschmidtian synthesis[END_REF][START_REF] Bateman | Architectural radiations cannot be optimally interpreted without morphological and molecular phylogenies[END_REF] argued that the changes involved in the transition from one architecture to another are too important and deep for such transitions to take place gradually. Rather, such evolution is suggested to occur preferentially by saltation, which involves the direct shift from one competitive architecture to another without passing through intermediate states associated with lower fitness. Other authors have proposed the notion of an "architectural continuum" [START_REF] Oldeman | A molecular phylogeny of the Solanaceae[END_REF][START_REF] Barthélémy | Architectural concepts for tropical trees[END_REF], suggesting a continuous transition from one model to another in some groups [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. In the context of this diversity of possible models to explain evolution in plant architecture, detailed studies within individual clades that combine results from molecular phylogenetic work with detailed architectural analysis may be particularly informative.

Atractocarpus Schltr. & K.Krause (Gardenieae, Rubiaceae), a Pacific genus of about 40 species, of which 32 are endemics to New Caledonia (Mouly et al., submitted), is ideally suited to study the evolution of plant architecture and to test hypotheses regarding the role of architecture in plant ecological diversification. Especially since a recently published phylogeny based on comprehensive sampling is available for the genus (Mouly et al., submitted). Most members of this genus are treelets occurring in the understory of tropical and subtropical rainforests, and they exhibit substantial variation in branching pattern [START_REF] Tirvengadum | La signification taxonomique des modes de ramification de Randia et genres affines[END_REF][START_REF] Robbrecht | A survey of the Gardenieae and related tribes (Rubiaceae)[END_REF], ranging from monocaulous (i.e. unbranched) to well-branched species, including various intermediate states (Figure 6.1). The New Caledonia members of Atractocarpus, which originated from a single colonization event that occurred ca. 2.4 Myr ago (Mouly et al., submitted), has the highest diversification rate of any genus studied to date (1.17 species species -1 Myr -1 ), approaching that of the well-known Hawaiian lobelioid genus Cyanea on the island of Maui (Hawaiian lobelioids; 1.36 species species-1 Myr-1, [START_REF] Givnish | Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae)[END_REF].

The presence of a high diversification rate combined with architectural diversity is thought to provide an indication of ecological diversification, a situation that is often associated with island colonization [START_REF] Silvertown | The ghost of competition past in the phylogeny of island endemic plants[END_REF][START_REF] Silvertown | The monophyly of island radiations: an evaluation of niche pre-emption and some alternative explanations[END_REF]. In groups adapted to growing in the forest understory, light is a variable and limiting resource, and in response, plants have deployed various growth strategies related to leaf photosynthesis and the economization of carbon. To quantify the architectural gradient from monocaulous (unbranched) to well-branched species, we developed a new "functional branching index", which assesses branching incidence on functional traits that influence assimilation and exploration functions. Considering each understory species of Atractocarpus in New Caledonia, we analyzed correlations between architecture and traits associated with key ecologically significant functions (viz., photosynthesis, hydraulic, mechanics, and dispersal). We investigated the evolution of plant architecture (branched vs. unbranched) in forest understory species and its impact on the exceptional diversification rate of the genus. Finally, we explored how ecological opportunities might stimulate the diversification of architecture through spatial differentiation in resource use (niche partitioning) in closely related species following the colonization of New Caledonian rainforests.

Material and methods

Sampling

We sampled all the 27 known rainforest species of Atractocarpus occurring in New Caledonia (list of taxa in Appendix 2). For Ancestral Character Estimation (ACE), three Australian

Atractocarpus species and one species in each of the most closely related genera for which DNA sequences were available [START_REF] Mouly | Phylogenetic structure and clade circumscriptions in the Gardenieae complex (Rubiaceae)[END_REF]Mouly et al., submitted) were included as outgroups.

Individuals were sampled in 20 rainforest sites occurring on Grande Terre, New

Caledonia's main island (Appendix 2).

Branching index and plant architectural traits

Based on the observation that an observed architectural gradient might result from differential allocation in branch length and supported leaf area (Figure 6.1), we used two functional branching indexes based on the differentiation of function from branches to trunk [START_REF] Corner | Transference of function[END_REF].

Given that photosynthetic function can be approximated by leaf area [START_REF] Pérez-Harguindeguy | New handbook for standardised measurement of plant functional traits worldwide[END_REF] and exploration function by stem length [START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF][START_REF] Smith | Deviation from symmetrically self-similar branching in trees predicts altered hydraulics, mechanics, light interception and metabolic scaling[END_REF], branching indexes were calculated as follows:

(i) Photosynthetic branching index:

!"#$%#&"#%'())*&+",%-.%-&#/01"' 2*+#3%3"#$%#&"#%4-&#/01"'5+&(/67
(ii) Exploration branching index:

8&#/01"'%0(9(3#+",%3"/:+1 2*+#3%'+"9%3"/:+1%4-&#/01"'5+&(/67
A value of zero indicates that photosynthesis and exploration are assumed only by the trunk and that branches (lateral exploration) are lacking, which corresponds to the monocaulous habit, physiognomically defined as "trees with a single trunk or visible stem of the plant" [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. The higher the value of the index, the greater the functional role played by branches. We built a single integrative branching index that expresses the architectural gradient by combining these two variables using Principal Component Analysis (correlation between photosynthetic and exploration branching indexes: rho = 0.86) employed in the ade4 package for R [START_REF] Dray | The ade4 package: implementing the duality diagram for ecologists[END_REF].

Species were segregated into three architectural classes (Monocaulous, Intermediate, and Branched) using Wilcoxon signed-rank tests that enable detection of breaks in the distribution of the integrative branching index.

Using morphological criteria reviewed by [START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF], we then described main traits commonly used in architectural analyses (growth process, branching pattern, position of reproductive structures, etc.) for the 27 New Caledonian species of Atractocarpus. The number of individuals studied varied from more than a hundred in some species (e.g. A.

pseudoterminalis, A. ngoyensis and A. bracteatus) to twelve in A. sezitat, a rare species whose habitat is highly disturbed by introduced herbivores. All species were studied at different ontogenetic stages (from very young plants to senescent adults) except A. sezitat and A. sp. nov.

12, for which no young plants could be found.

Plant functional traits

We tested the correlation of 14 traits with branching index. The traits were selected to reflect important features of plant ecological strategies (Table 6.1). They were measured on five individuals per species and, when possible (e.g. for leaf and internode traits), five times per individual. The selection of individuals was standardized for environment (rainforest understory) and for ontogeny following three criteria: (i) recently mature individuals, (ii) non reiterated individuals (see [START_REF] Oldeman | A molecular phylogeny of the Solanaceae[END_REF][START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF], and (iii) non-traumatized trunks.

It was not possible to locate individuals meeting all three conditions for A. sezitat and A. sp. nov. 12 due to small population sizes and habitat degradation, so these taxa were removed from this analysis. We concentrated our sampling in the apical part of the trunk because (i) this enabled standardization of physiological age and (ii) unbranched and branched species can be compared only on the basis of the main stem. Moreover, variation in resource allocation and anatomy between unbranched and branched species might be more prevalent toward the apex, where large leaves are deployed [START_REF] Carlquist | Island Biology[END_REF]. Stem and leaf measurements were standardized as follows. Leaf traits (SLA, leaf area) were measured on the five youngest, fully expended leaves of the main axe (trunk).

Wood was collected below the terminal leaf tuft for the measurement of anatomical traits (proportion of tissues, specific wood density, and specific stem density). Internode length and diameter were measured on the five youngest, well-developed trunk-internodes (before secondary growth). Branch traits (cumulated length, leaf number, leaf area) were measured on two representative branches per individual and the total number of branches was counted.

Most of the individuals studied in the field were infertile, so fruit traits were measured on herbarium specimens (NOU and P, Herbarium acronyms follow Thiers, continuously updated). We selected undamaged and unflattened fruits from which length and diameter were measured. As

Atractocarpus fruits are often ellipsoid to tubular [START_REF] Puttock | Revision of Atractocarpus (Rubiaceae: Gardenieae) in Australia and New Combinations for Some Extra-Australian Taxa[END_REF], these two dimensions are used to approximate fruit volume (Table 6.1). A total of 592 fruits were measured, and only the five largest fruits per taxon (four for A. confertus and A. sessilifolius) were used to ensure that values from only mature fruits were included in our analyses. One of the species, A. sp. nov. 6, is a very rare species whose fruits have never been observed; it was consequently excluded from the fruit volume dataset.

Data analysis

Phylogenetic tree -The molecular phylogeny of Mouly et al. (submitted) was used for analysis. This ultrametric phylogenetic tree was constructed under Bayesian Inference using one nuclear (ITS) and two chloroplastic (trnTL, rpl32) loci. Since the current study focused on rainforest species, we pruned the five dry forest and maquis species using the R software (Ver.

3.4.3).

Phylogenetic signal -We assessed phylogenetic independence of measured traits [START_REF] Felsenstein | Phylogenies and the comparative method[END_REF] using two complementary statistics calculated under a Brownian Motion (BM) model of evolution. Pagel's Lambda [START_REF] Pagel | Inferring the historical patterns of biological evolution[END_REF][START_REF] Freckleton | Phylogenetic Analysis and Comparative Data: A Test and Review of Evidence[END_REF] is widely used for low rates of type I error and robustness, even for poorly or moderately informative phylogenies [START_REF] Freckleton | Phylogenetic Analysis and Comparative Data: A Test and Review of Evidence[END_REF][START_REF] Münkemüller | How to measure and test phylogenetic signal[END_REF]. Because this statistic loses statistical power when used on small phylogenies (< 30 tips, [START_REF] Freckleton | Phylogenetic Analysis and Comparative Data: A Test and Review of Evidence[END_REF], we also used Blomberg's Kappa [START_REF] Blomberg | Testing for phylogenetic signal in comparative data: behavioral traits are more labile[END_REF][START_REF] Kembel | Disentangling niche and neutral influences on community assembly: assessing the performance of community phylogenetic structure tests[END_REF] that is less robust but more appropriate for small taxonomic sampling (until 20 taxa, [START_REF] Kamilar | Phylogenetic signal in primate behaviour, ecology and life history[END_REF]. Both statistics were calculated considering intraspecific variation using the phytools package for R [START_REF] Revell | phytools: An R package for phylogenetic comparative biology (and other things)[END_REF].

Trait correlations and functional characterization -We used phylogenetic regression

based on the method proposed by [START_REF] Ives | Within-species measurement error in phylogenetic comparative methods[END_REF] and implemented in the phytools package for R [START_REF] Revell | phytools: An R package for phylogenetic comparative biology (and other things)[END_REF] to test whether architectural variation was correlated with functional specialization. Using maximum likelihood, this method fits bivariate models taking into account both phylogenetic framework and intraspecific variation. The response variable was the integrative branching index, and the explanatory variables were all other functional traits (Table 6.1).

Significance of relations was tested using a likelihood ratio test between the model and a model constrained with a slope of zero.

To characterize the functional space of each architectural classes, we performed a Principal Component Analysis. We used the species arithmetical mean of each functional traits (branching index excluded), and functional differences between architectural classes (Monocaulous, Intermediate, Branched) were tested with a permanova (Vegan package for R, [START_REF] Oksanen | vegan: Community Ecology Package. R package version 2.4-6[END_REF].

Ancestral Character Estimation -To determine the putative ancestral architectural class of Atractocarpus and infer the evolution of architecture in the genus, Ancestral Character Estimation (ACE) was performed. We assigned an architectural class to each of the 11 outgroup species based on published descriptions [START_REF] Fosberg | The genus Trukia Kanehira (Rubiaceae)[END_REF][START_REF] Smith | Rubiaceae[END_REF][START_REF] Fosberg | Bignoniaceae-Rubiaceae[END_REF][START_REF] Puttock | Revision of Atractocarpus (Rubiaceae: Gardenieae) in Australia and New Combinations for Some Extra-Australian Taxa[END_REF][START_REF] Wong | Bungarimba (Rubiaceae), a new genus distinguished from Porterandia and other allies[END_REF]Zahid andWong, 2004, 2010;[START_REF] Tong | Rubovietnamia sericantha (Rubiaceae: Gardenieae), a new combination and notes on the genus in China[END_REF], herbaria specimens (P, K, BM, E) and available photos. The ACE were performed using a maximum likelihood method under the ape package for R [START_REF] Paradis | APE: analyses of phylogenetics and evolution in R language[END_REF]. Three possible models of evolution fitted the data characteristics: (i) equal transition rates between classes (ER), (ii) different transition rates between classes but with equal rates for reversions (SYM), and (iii) different rates for every transition (ARD). The best model was selected using the corrected Akaike Information Criterion (AICc).

Trait Based Diversification -To test whether one of the architectural classes has contributed more than the others to the diversification of New Caledonian Atractocarpus (by increasing speciation rates and/or decreasing extinction rates), we used the Multiple State Speciation Extinction (MuSSE) framework [START_REF] Fitzjohn | Estimating trait-dependent speciation and extinction rates from incompletely resolved phylogenies[END_REF] as implemented in the diversitree package for R [START_REF] Fitzjohn | Diversitree: comparative phylogenetic analyses of diversification in R[END_REF]. For this analysis, outgroups were dropped from the phylogeny to consider only the 27 rainforest Atractocarpus species. Fifteen models of diversification were used, each differing in whether or not of speciation, extinction and transition rates were equal between classes (Table 6.2). Model selection was done according to AICc. Letters in (C) correspond to the result of the Wilcoxon test; species with shared letters are not significantly different for a given risk of error.

Results

Branching index

Branching indexes confirmed the existence of a morpho-functional gradient from branched to monocaulous species of Atractocarpus, for both exploration and photosynthetic functions (Figure 6.2-A, B). For both indexes, interspecific variation was too gradual to allow the partitioning in distinct classes, but when combined (i.e. using the branching index), three distinct architectural groups could be differentiated (Wilcoxon tests: P < 0.1, Figure 6.2-C), viz. the branched, intermediate and monocaulous architectural classes. On average, branched species had 82% of photosynthesis and 76% of exploration provided by branches, versus 36% and 45%, respectively, for intermediate species, and 3% and 16% for monocaulous species.

Architectural characterization

The architecture of New Caledonian Atractocarpus species involves two axis categories:

(i) a trunk (C1), i.e., an orthotropic monopodium with continuous indeterminate growth, and (ii)

"branches" (C2), i.e., orthotropic sympodia with terminal sexuality conferring rhythmic growth (Table 6.3, Figure 6.3). The sympodial branching of C2 always originates in a hypotonic (i.e. on the lower surface) or amphitonic position, resulting in upward flower exposure (Figure 6.1). The only qualitative architectural variation observed between species concerned the branching position of C1, which was continuous for some monocaulous species while diffuse for others. Two main quantitative architectural traits varied greatly among species, the number of modules per branch and the length of modules, which decreased in monocaulous species (Table 6.3,Figures 6.3,6.4).

For example, A. longistipitatus is a monocaulous species whose C2 comprise many very short modules (apart from the first one). By contrast, A. bracteatus is also monocaulous but its C2 Mean length of module (cm) 5.5 4.8 6.5 8.9 9.2 7.8 7.6 7 5.9 6.9 6.5 7.7 14.8 11.5 6.3 12.9 15 7.1 2.4 1.5 12.1 8.1 4.5 8.5 5.9 5.1 1.7

AC: Axis category, C1: First axis category (trunk), C2: Second axis category ("branches") comprise a single long module (Figure 6.4). The number of internodes per module vary little in the genus but tend to be reduced in monocaulous species, with an extreme case of one node per module in A. confertus (Table 6.3). All species showed the ability to develop delayed reiterate on damaged or aged individuals.

Functional characterization

Phylogenetic signal varied substantially depending on which statistic was used (Lambda or Kappa, Table 6.4). This was probably due to the small size of our phylogeny (25 terminals), since the statistics differ in sensitivity to the number of terminals [START_REF] Freckleton | Phylogenetic Analysis and Comparative Data: A Test and Review of Evidence[END_REF][START_REF] Kamilar | Phylogenetic signal in primate behaviour, ecology and life history[END_REF]). Blomberg's Kappa captures significant phylogenetic signal for stem specific density, proportion of pith, Huber value, and mean leaf area, whereas Pagel's Lambda captures significant signal for internode diameter (Table 6.4). Among all measured functional traits, only the branching index had a highly significant phylogenetic signal calculated with both Pagel's Lambda and Blomberg's Kappa.

Several functional traits were significantly related to the branching index (Table 6.5), including biomechanics (e.g., slenderness ratio, internode diameter) as well as photosynthesis (e.g. leaf area, SLA), hydraulics (e.g. Huber value), and even dispersal (fruit volume). These correlations between architecture and functions were confirmed by PCA and Permanova (P = 0.016), which showed significant differences between the functional space occupied by the three architectural classes (Figure 6.5). Intermediate species were confounded between the two other architectural classes, but monocaulous and branched species appeared to be functionally very different. In comparison to monocaulous species, branched taxa tend to have numerous small leaves, higher SLA, smaller fruits and, smaller internode diameters with a lower proportion of wood, but a higher wood area-leaf area ratio (Huber value). The monocaulous A. confertus (the lowest point on Figure 6.1 for trait abbreviations). Ellipses represent the 95% confidence interval for each architectural classes. Functional differences between architectural classes were tested with Permanova.

6.5) was distinct from the other Atractocarpus species examined in having numerous very large leaves, leading to a disproportionately higher total leaf area (up to 2.4 m² versus a mean of 0.5 m² for the other species).

Ancestral Character Estimation

All considered outgroup species belonged to the branched architectural class, except for the Fijian Sukunia pentagonoides (Seem.) A.C.Sm, which is monocaulous. Among the three possible models of evolution that fitted the data, the best supported one was that of equal transition rates between classes (ER), with an AICc of 84.4 versus 96.1 and 104.2 for SYM and ARD, respectively.

According to this model, the ancestral architecture of Atractocarpus was most likely branched (probability = 0.98, Figure 6.6), and the monocaulous habit evolved two or three times in the two main New Caledonian clades (referred to as "monocaulous clade A" and "monocaulous clade B"

in Figure 6.6), ca. 1.4 and 0.9-0.6 Myr ago respectively. In each clade, species with an intermediate architecture are closely related to monocaulous species, the only exception being A. brandzeanus, whose closest relatives are branched. Conversely, pairs of sister or closely related species never showed branched and monocaulous habit.

Trait based diversification

Following the IACc, models 2 and 10 are the best fitted, in which speciation and transition rates are equal for each state and extinction rates are equal or null (Table 6.2). This means that diversification was not greater among the members of any of the architectural classes during the evolution of Atractocarpus in New Caledonia. The measurement of functionally explicit variables (branch length and leaf area) that link plant architecture and function [START_REF] Lauri | Le mouvement morphogenetique, approche morphometrique et restitution graphique: L'exemple de quelques plantes tropicales[END_REF][START_REF] Lauri | Shoot type demography and dry matter partitioning: a morphometric approach in apple (Malus × domestica)[END_REF][START_REF] Charles-Dominique | The architectural design of trees protects them against large herbivores[END_REF] showed that variation of growth habit in Atractocarpus is correlated with a gradual variation from branched species, for which photosynthesis and exploration are largely assumed by branches, toward monocaulous species, for which these functions are assumed by the trunk. The reduced C2

in monocaulous species supports a limited number of small leaves (on average 4 leaves of 9 cm 2 )

and are dedicated almost exclusively to reproduction. These "axes" are functionally closer to inflorescences than branches [START_REF] Van Steenis | Definition of the concept "inflorescence" with special reference to ligneous plants[END_REF] and are often considered as such by taxonomists (e.g. [START_REF] Fosberg | Bignoniaceae-Rubiaceae[END_REF][START_REF] Puttock | Revision of Atractocarpus (Rubiaceae: Gardenieae) in Australia and New Combinations for Some Extra-Australian Taxa[END_REF]. The similarity of developmental origin and the gradual quantitative architectural variation of lateral axes (C2) across closely related taxa suggest a homologous origin of these axes in the Atractocarpus species studied here (see [START_REF] Nozeran | À propos des phénomènes de transfert des fonctions reproductrices[END_REF][START_REF] Rossignol | Architecture et tendances évolutives dans le genre Phyllanthus (Euphorbiaceae)[END_REF][START_REF] Haicour | Les Phyllanthus: un excellent matériel pour l'étude du polymorphisme[END_REF]. A homologous origin between vegetative and inflorescence axes has been suspected

in several groups such as Ranunculaceae and Papaveraceae [START_REF] Stebbins | Evolutionary Trends in the Inflorescence of Angiosperms[END_REF], as well as several families of Alismatales (Posluszny andCharlton, 1993, 1999) and Pandanales [START_REF] Rudall | Morphological Phylogenetic Analysis of Pandanales: Testing Contrasting Hypotheses of Floral Evolution[END_REF], though none of these studies combined architectural analysis with molecular phylogenetic results.

Only a few studies to date have explored evolutionary links between architectural models (see [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF]. In New Caledonia Atractocarpus, we found recurrent transitions from

Stone's model for branched species (with C2 functioning as branches) to Corner's model for monocaulous species (with C2 functioning as inflorescences), although the limit between them remains fuzzy, as illustrated by species whose architecture is intermediate. The variation observed in branching index across species illustrates an 'architectural continuum' [START_REF] Oldeman | A molecular phylogeny of the Solanaceae[END_REF][START_REF] Barthélémy | Architectural concepts for tropical trees[END_REF] rather than saltational evolution involving rapid and profound change in architecture [START_REF] Bateman | Saltational evolution of form in vascular plants: a neoGoldschmidtian synthesis[END_REF][START_REF] Bateman | Architectural radiations cannot be optimally interpreted without morphological and molecular phylogenies[END_REF]. Nothing suggests that the intermediate architectural class confers lower fitness (i.e., represents a fitness valley) compared to monocaulous or branched architecture, particularly in situations where the environment (rainforest understory) varies gradually. Saltation from one model to another might take place when gradual changes are impossible, for example in the colonization of contrasted habitats [START_REF] Bateman | Saltational evolution of form in vascular plants: a neoGoldschmidtian synthesis[END_REF]. Gradual variation between and within models has already been noted [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], suggesting that preferential transition between models might occur.

The two independent origins of monocauly inferred in Atractocarpus illustrate two different evolutionary pathways involved in the transition from branches into inflorescences. In confertus). This type of morphological reduction observed at the module level in Atractocarpus could affect any elementary level of organization in other plant groups (e.g. annual shoots, growth units, metamers, etc.).

The iterative gradual transition of axillary structure (C2) from branches to inflorescences occurs as a functional reduction of vegetative growth and a change in the timing of flowering. Such evolutionary changes, often result from growth heterochrony [START_REF] Smith | Heterochrony revisited: the evolution of developmental sequences[END_REF]. In the case of New Caledonian Atractocarpus, the axillary structure (C2) of descendants is either a truncated part (in "monocaulous clade A") or a miniaturized copy (in "monocaulous clade B") of that of their ancestors. This suggests heterochronic evolution through "neoteny" in the first case and through "proportional dwarfism" in the second [START_REF] Gould | Ontogeny and Phylogeny[END_REF][START_REF] Alberch | Size and Shape in Ontogeny and Phylogeny[END_REF][START_REF] Smith | Heterochrony revisited: the evolution of developmental sequences[END_REF], but more details of the ontogenetic sequences are required to test these hypotheses in Atractocarpus.

In this context, our original approach showed that a careful study of low organization levels is needed to depict the evolution of plant architecture. This involves a deep characterization of architectural units [START_REF] Edelin | Image sur l'architecture des conifères[END_REF][START_REF] Edelin | L'architecture monopodiale: l'exemple de quelques arbres d'Asie tropicale[END_REF], combining qualitative and quantitative traits.

6.4.2 Branching index, Corner's rules, and ecological strategies

In the original Durian theory, [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]Corner ( , 1953Corner ( -1954) ) proposed two fundamental rules governing plant morphology: (i) Axial conformity, stipulating that "the stouter, or more massive, the axis in a given species, the larger and more complicated its appendages"; and (ii) Diminution on ramification, stipulating that "the greater the ramification, the smaller become the branches and their appendages". Axial conformity is by far the most studied since the leaf size -twig size and fruit size -leaf size spectra are among the best-documented (White, 1983b;[START_REF] Bond | Allometry and sexual differences in leaf size[END_REF][START_REF] Brouat | Corner's rules revisited: ontogenetic and interspecific patterns in leaf-stem allometry[END_REF][START_REF] Cornelissen | A triangular relationship between leaf size and seed size among woody species: allometry, ontogeny, ecology and taxonomy[END_REF][START_REF] Brouat | Leaf-stem allometry, hollow stems, and the evolution of caulinary domatia in myrmecophytes[END_REF][START_REF] Westoby | Plant Ecological Strategies: Some Leading Dimensions of Variation Between Species[END_REF][START_REF] Preston | Hydraulic architecture and the evolution of shoot allometry in contrasting climates[END_REF][START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF][START_REF] Pickup | Dry mass costs of deploying leaf area in relation to leaf size[END_REF][START_REF] Sun | The leaf size-twig size spectrum of temperate woody species along an altitudinal gradient: an invariant allometric scaling relationship[END_REF][START_REF] Normand | Hydraulic and mechanical stem properties affect leaf-stem allometry in mango cultivars[END_REF]. Large leaves and fruit carried by a large single stem, as seen in monocaulous species, illustrate one extremity of this spectrum.. The second rule (diminution of ramification), has received much less consideration (but see White, 1983a;[START_REF] Ackerly | Leaf Size, Sapling Allometry, and Corner's Rules: Phylogeny and Correlated Evolution in Maples (Acer)[END_REF]. Our results agree with this statement, as we have shown that internode diameter, as well as fruit and leaf size, are negatively correlated with branching index, even with phylogenetic corrections.

Beyond Corner's rules, we found a strong correlation between branching index and several functional traits related to various ecological strategies in areas with a similar habitat (i.e. rainforest).

We found that branching index tends to be correlated with ecologically important dimensions [START_REF] Mooney | The carbon balance of plants[END_REF][START_REF] Grime | Vegetation classification by reference to strategies[END_REF][START_REF] Grime | Comparative plant ecology: a functional approach to common British species[END_REF][START_REF] Westoby | Plant Ecological Strategies: Some Leading Dimensions of Variation Between Species[END_REF]. The large leaves of monocaulous species increase light interception in shady understory [START_REF] Poorter | Growth responses of 15 rain-forest tree species to a light gradient, the relative importance of morphological and physiological traits[END_REF][START_REF] Rozendaal | Plasticity in leaf traits of 38 tropical tree species in response to light; relationships with light demand and adult stature[END_REF], their low SLA and Huber value are likely associated with low photosynthetic capacity [START_REF] Field | The photosynthesis-nitrogen relationship in wild plants[END_REF][START_REF] Shipley | Functional linkages between leaf traits and net photosynthetic rate: reconciling empirical and mechanistic models[END_REF][START_REF] Poorter | Leaf traits are good predictors of plant performance across 53 rain forest species[END_REF][START_REF] Rozendaal | Plasticity in leaf traits of 38 tropical tree species in response to light; relationships with light demand and adult stature[END_REF][START_REF] Pérez-Harguindeguy | New handbook for standardised measurement of plant functional traits worldwide[END_REF] and their small internode length can result from a slow growth rate [START_REF] Mooney | The carbon balance of plants[END_REF][START_REF] Chuah | Exemples de corrélations quantitatives entre quelques paramètres déterminant la forme végétale[END_REF][START_REF] Grime | Comparative plant ecology: a functional approach to common British species[END_REF][START_REF] Westoby | Plant Ecological Strategies: Some Leading Dimensions of Variation Between Species[END_REF].

Monocaulous species are likely skewed toward a high resource conservation strategy [START_REF] Grime | Integrated Screening Validates Primary Axes of Specialisation in Plants[END_REF][START_REF] Díaz | The plant traits that drive ecosystems: Evidence from three continents[END_REF][START_REF] Díaz | The global spectrum of plant form and function[END_REF] suited to the shady understory of rainforest.

In sparsely branched to unbranched species, a distal part of the stem with thicker diameter and higher wood proportion was found to be suited to supporting large and numerous energetically costly leaves (higher leaf area and SLA). However, higher wood area does not fully compensate for high total leaf area of the main stem since the Huber value (the ratio of wood area to leaf area)

was positively correlated with branching index. Detailed anatomical studies, particularly on vessel size and density, are needed to understand the hydraulic and mechanical trade-off involved in the pervasive link between leaf area, stem thickness and branching intensity [START_REF] Lehnebach | The pipe model theory half a century on: a review[END_REF].

Divergence and ecological opportunities in New Caledonian rainforests

New Caledonian Atractocarpus appear to be the result of a single colonization event (Mouly et al., submitted) and the archipelago is the center of diversity for the genus. Island colonization is a two steps process involving dispersal and successful establishment [START_REF] Silvertown | The ghost of competition past in the phylogeny of island endemic plants[END_REF]. The large, fleshy fruits of Atractocarpus suggest a rather limited ability for long-distance dispersal. Such niche preemption, claimed to act as a major driver of monophyly and diversification in island floras [START_REF] Silvertown | The ghost of competition past in the phylogeny of island endemic plants[END_REF], could have prevailed in the diversification of the New Caledonian clade of Atractocarpus. Its young age (estimated at 2.4 My) coincides with colonization during late Pleistocene, a period of intense climatic Fluctuation [START_REF] Bowler | Late Quaternary climates of Australia and New Guinea[END_REF][START_REF] Hope | Tropical vegetational change in the late Pleistocene of New Caledonia[END_REF][START_REF] Stevenson | A comparison of late Quaternary forest changes in New Caledonia and northeastern Australia[END_REF]. Increasing evidence supports the persistence of taxa in rainforest refugia during the Pleistocene in New Caledonia [START_REF] Pintaud | Chorology of New Caledonian palms and possible evidence of Pleistocene rain forest refugia[END_REF][START_REF] Pillon | Time and tempo of diversification in the flora of New Caledonia[END_REF][START_REF] Nattier | Climate and Soil Type Together Explain the Distribution of Microendemic Species in a Biodiversity Hotspot[END_REF][START_REF] Poncet | Phylogeography and niche modelling of the relict plant Amborella trichopoda (Amborellaceae) reveal multiple Pleistocene refugia in New Caledonia[END_REF][START_REF] Pouteau | New Caledonia: a Pleistocene refugium for rain forest lineages of relict angiosperms[END_REF][START_REF] Tournebize | Two disjunct Pleistocene populations and anisotropic postglacial expansion shaped the current genetic structure of the relict plant Amborella trichopoda[END_REF]. Climatic fluctuations causing expansion and contraction of rainforests could also have offered ecological opportunities for new colonizers. Other diversified rainforest clades in New Caledonia have been shown to result from recent colonization (e.g. palms, [START_REF] Pintaud | Chorology of New Caledonian palms and possible evidence of Pleistocene rain forest refugia[END_REF][START_REF] Pillon | Time and tempo of diversification in the flora of New Caledonia[END_REF]. Similarities in form and physiology among close relatives, as suggested by our character state reconstruction and architectural analysis, are suggested to involve intense competition that favors divergence among locally coexisting species [START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. In support of this, we found notable variation in module length and number between sisters species, even when they belong to the same architectural class. In Atractocarpus, variation in growth habit is correlated with a gradual switch in assimilation function from branches to trunk, and is associated with a vertical differentiation of major functions.

Leaf arrangement in monocaulous species results in important overlap that impacts photosynthesis, while stem slenderness suggests a rather small structural investment in vertical support. Branched species tend to increase light harvesting and reduce leaf overlap (via increase branch length), a strategy that require more investment in stem tissue [START_REF] Givnish | Plant Stems: Biomechanical Adaptation for Energy Capture and Influence on Species Distributions[END_REF], as allocation to non-leaf structures might increase with ramification. Atractocarpus species thus exhibit a gradient of foraging for light in which leaf size and disposition vary with branching index.

New Caledonian rainforests are characterized by a low canopy (ca. 20 m) and trees with small crowns [START_REF] Blanchard | Contrasted allometries between stem diameter, crown area, and tree height in five tropical biogeographic areas[END_REF], two structural features that increase the number and intensity of sunflecks [START_REF] Chazdon | The Importance of Sunflecks for Forest Understory Plants: Photosynthetic machinery appears adapted to brief, unpredictable periods of radiation[END_REF]. A low canopy and its corollary, reduced forest stratification, could result in stronger competition within a given stratum. These forest features (high sunfleck variability, reduced stratification) favor niche partitioning through a gradient in architecture and related functional traits. Moreover, limited dispersal of the large fruits of Atractocarpus might interact with niche partitioning to promote divergent evolution. Finally, divergent selection may be especially favored in permissive environments where competition prevails over the external environment or in the colonization of islands where resources are underutilized [START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. New Caledonia exhibits both components of divergent selection: (i)

climatically permissive rainforests and (ii) ecological opportunities offered by recent climatic fluctuations. Divergent selection, caused by competition among closely related taxa, leads to adaptive radiation, i.e. the rise of a diversity of ecological roles and related adaptations in different species among a lineage [START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. The theory of niche pre-emption holds that adaptive radiation creates a barrier that inhibits the establishment of closely related taxa, thus reducing the likelihood of repeated colonization.

Conclusions

Despite the fundamental importance of plant architecture, little work has been done to integrate this aspect into key domains of plant science such as evolution and functional ecology.

However, increasing availability of information on ecologically important traits and molecular phylogenies provides a basis for testing and developing new concepts. Our study clearly highlights evolutionary processes behind architectural transitions and their link to plant ecological strategy and perhaps also to diversification. We have shown that gradual transition from one architecture to a very different one is possible through morpho-functional reduction of morphological units.

Quantifying plant architecture through functional indexes appears to offer a promising avenue toward further understand the implications of architectural variation on plant fitness under different environmental conditions. Based on such an index, our study provides a functional definition of monocauly that is ecologically and evolutionary more explicit than one based solely on physiognomy (cf. [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF]: we define monocaulous species as self-supporting woody plants whose cardinal functions (e.g., assimilation and exploration) rely on a single apparent stem.

Although our work focused on a small clade, it opens new perspectives and proposes a general framework for further understanding evolution of plant architecture and its functional implications in other plant groups and other geographical areas, and at larger scales. We believe that Pleistocene climatic fluctuations have played a major role in the evolution of monocauly and more widely in shaping the current diversity of the New Caledonian flora. This hypothesis now needs to be tested at a larger phylogenetic scale in New Caledonia, as the monocaulous habit, which occurs repetitively in many different groups [START_REF] Veillon | Architecture végétative de quelques arbres de l'archipel Néo-caledonien[END_REF][START_REF] Schmid | Les écosystèmes forestiers mélanésiens (Nouvelle Calédonie, Nouvelles-Hébrides, Fidji et îles Salomon)[END_REF][START_REF] Schmid | Conditions d'évolution et caractèristiques du peuplement végétal insulaire en Mélanésie Occidentale: Nouvelle-Calédonie et Vanuatu[END_REF], has much to offer to understand the evolution, biogeography and ecology of this "very old Darwinian island" [START_REF] Grandcolas | New Caledonia: a very old Darwinian island?[END_REF].

Introduction

Monocauly (i.e. unbranched habit) in woody plants is a globally rare phenomenon but represents a case of evolutionary convergence (Chapter 5) that has much to offer to our understanding of metabolic implications and selective advantage of large leaves and limited branching patterns (Chapter 6). Monocaulous plants have a single apparent woody stem with large leaves concentrated toward the apex [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF]. They conform to four architectural models (Corner's, Holttum's, Chamberlain's and Cook's) (Chapter 5). Thus, while the vast majority of woody growth habits (e.g. tree, treelets, lianas) present several more or less specialized axis (axis categories, see [START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF], the cardinal functions (photosynthesis, water transport, mechanics, storage) are carried by a single stem in monocaulous plants (Chapter 6). This particular architectural design might entail a stronger interdependence of traits and contrasting functional behavior of stems and leaves compared to branched species.

Though, morpho-anatomical traits of woody monocaulous plants have virtually not been investigated (but see Chapter 6) and their functional characteristics remain unclear.

Strategic dimensions given by correlation of two or more ecologically important traits [START_REF] Westoby | Plant Ecological Strategies: Some Leading Dimensions of Variation Between Species[END_REF][START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF][START_REF] Wright | The worldwide leaf economics spectrum[END_REF][START_REF] Wright | Relationships Among Ecologically Important Dimensions of Plant Trait Variation in Seven Neotropical Forests[END_REF][START_REF] Osnas | Global Leaf Trait Relationships: Mass, Area, and the Leaf Economics Spectrum[END_REF][START_REF] Olson | Plant height and hydraulic vulnerability to drought and cold[END_REF] are considered as main drivers of plant evolution and functional diversity worldwide [START_REF] Díaz | The plant traits that drive ecosystems: Evidence from three continents[END_REF][START_REF] Poorter | Leaf traits are good predictors of plant performance across 53 rain forest species[END_REF][START_REF] Díaz | The global spectrum of plant form and function[END_REF]. As such, identifying extreme points in these multivariate correlations of functional traits should bring strong insights into the selective advantage of increasing some leading dimension. As an ultimate state of branch reduction, the monocaulous habits stand out at the extremity of the branching orderleaves size spectrum known as an element of Corner's rules [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF](Corner, , 1953(Corner, -1954)). The second statement of Corner's rules, "diminution on ramification", indeed proposed that "the greater the ramification, the smaller become the branches and their appendages" [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]. This statement has however been poorly investigated (but see White, 1983a;[START_REF] Ackerly | Leaf Size, Sapling Allometry, and Corner's Rules: Phylogeny and Correlated Evolution in Maples (Acer)[END_REF]Westoby and Wright, 2003, Chapter 6).

A more widely studied spectrum is the leaf sizetwig size spectrum [START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF], also known as leafstem scaling (e.g. [START_REF] Yang | The Effects of Leaf Size, Leaf Habit, and Leaf Form on Leaf/Stem Relationships in Plant Twigs of Temperate Woody Species[END_REF]. It corresponds to the first statement of Corner's rules, i.e "axial conformity", proposing that "the stouter, or more massive, the axis in a given species, the larger and more complicated its appendages" [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]. It arises from the observation of [START_REF] Sinnott | The Relation between Body Size and Organ Size in Plants[END_REF] and [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF] on the general correlation between individual leaf size and pith (i.e. primary meristem size) or stem diameter respectively. This relationship, confirmed by numerous studies (White, 1983b, a;[START_REF] Bond | Allometry and sexual differences in leaf size[END_REF][START_REF] Brouat | Corner's rules revisited: ontogenetic and interspecific patterns in leaf-stem allometry[END_REF][START_REF] Cornelissen | A triangular relationship between leaf size and seed size among woody species: allometry, ontogeny, ecology and taxonomy[END_REF][START_REF] Brouat | Leaf-stem allometry, hollow stems, and the evolution of caulinary domatia in myrmecophytes[END_REF][START_REF] Preston | Hydraulic architecture and the evolution of shoot allometry in contrasting climates[END_REF][START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF][START_REF] Sun | The leaf size-twig size spectrum of temperate woody species along an altitudinal gradient: an invariant allometric scaling relationship[END_REF]Normand et al., 2008, Chapter 6), has been extended to twig or plant levels as the foliagestem scaling (e.g. [START_REF] Brouat | Corner's rules revisited: ontogenetic and interspecific patterns in leaf-stem allometry[END_REF][START_REF] Sun | The leaf size-twig size spectrum of temperate woody species along an altitudinal gradient: an invariant allometric scaling relationship[END_REF][START_REF] Olson | Universal foliage-stem scaling across environments and species in dicot trees: plasticity, biomechanics and Corner's Rules[END_REF][START_REF] Smith | Convergence in leaf size versus twig leaf area scaling: do plants optimize leaf area partitioning?[END_REF]; see [START_REF] Lehnebach | The pipe model theory half a century on: a review[END_REF]. If the universality of these relationships is obvious, the functional relations linking foliage area and stem area, and individual leaf area and stem area, remain poorly debated. Leaf area and stem diameter are alternatively linked by hydraulic [START_REF] Chuah | Exemples de corrélations quantitatives entre quelques paramètres déterminant la forme végétale[END_REF]White, 1983a;[START_REF] Preston | Hydraulic architecture and the evolution of shoot allometry in contrasting climates[END_REF][START_REF] Sun | The leaf size-twig size spectrum of temperate woody species along an altitudinal gradient: an invariant allometric scaling relationship[END_REF][START_REF] Normand | Hydraulic and mechanical stem properties affect leaf-stem allometry in mango cultivars[END_REF], mechanic (White, 1983a;[START_REF] Niklas | Plant Biomechanics: an engineering approach to plant form and function[END_REF][START_REF] Normand | Hydraulic and mechanical stem properties affect leaf-stem allometry in mango cultivars[END_REF][START_REF] Olson | Universal foliage-stem scaling across environments and species in dicot trees: plasticity, biomechanics and Corner's Rules[END_REF]Levionnois et al., 2018a) and photosynthesis [START_REF] Primack | Relationships Among Flowers, Fruits, and Seeds[END_REF][START_REF] Cornelissen | A triangular relationship between leaf size and seed size among woody species: allometry, ontogeny, ecology and taxonomy[END_REF] functions. Attempts to identify the relative importance of each function remain scarce [START_REF] Normand | Hydraulic and mechanical stem properties affect leaf-stem allometry in mango cultivars[END_REF]Levionnois et al., 2018a). This debate needs further anatomical studies to quantify the structure-function relationships responsible for foliagestem scaling (Lehnebach et al., 2018, Chapter 6).

The selective advantage of monocauly in rainforest has been questioned by several authors (e.g. [START_REF] Richards | The Tropical Rain Forest[END_REF][START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF]. Some insights suggest that sparsely-branched trees with large leaves are fast-growing and preferentially growth in first stage of forest succession (White, 1983a;[START_REF] Ackerly | Canopy Structure and Dynamics: Integration of Growth Processes in Tropical Pioneer Trees[END_REF]. By contrast, in forest understory, monocaulous species might not benefit from growth advantage but would rather exhibit a conservation resource strategy compared to their branched relatives (Chapter 6). Anatomical properties of monocauls has mainly been investigated in rosette-tree and rosette-shrub species through the phenomenon of secondary woodiness. These plants appeared to have large pith and cortex [START_REF] Carlquist | Island Biology[END_REF]Mabberley, 1974a;[START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Meinzer | Adaptations for water and thermal balance in Andean giant rosette plants[END_REF][START_REF] Mosbrugger | Tree Habit in Land Plants: A functional comparison of trunk constructions with a brief introduction into the biomechanics of trees[END_REF] and thin wood characterized by an important proportion of parenchyma [START_REF] Carlquist | A theory of Paedomorphosis in Dicotyledonous woods[END_REF]Mabberley, 1974a;[START_REF] Aldridge | Anatomy and evolution in the Macaronesian Sonchus subgenus Dendrosonchus (Compositae: Lactuceae)[END_REF][START_REF] Mosbrugger | Tree Habit in Land Plants: A functional comparison of trunk constructions with a brief introduction into the biomechanics of trees[END_REF]. But these anatomical features seem different from some rainforest monocaulous species, which showed a thick cylinder of dense wood and have not evolved from herbaceous ancestors (Chapter 6).

In this study, we compared functional traits, from micro-anatomical to whole plant scale, of monocaulous species and their branched relatives growing in the understory of New Caledonian rainforest. This site was selected because of the exceptional diversity in monocaulous plants providing a wide phylogenetic spectrum for the selection of species (Chapter 5). Our aim was to compare the functional strategy of monocaulous plants with closely related species using several dimensional traits associated with mechanical, hydraulic and physiological functions.

Monocaulous species were also compared to the TRY Plant trait Database (Kattge et al., 2011) for important trait coordination. Through multi-level anatomical investigation (from cell to tissue) we also investigated the functional explanation of the universal leafstem and foliagestem scaling. Twenty species in 10 different genera were sampled (one monocaulous and one branched species in each genus). To ensure phylogenetical independence of results, each genus was selected from different families covering seven orders (Table 7.1). Only genera Bocquillonia and Cleidion belong to the same family (Euphorbiaceae) but belong to different tribes [START_REF] Wurdack | Molecular phylogenetic analysis of uniovulate Euphorbiaceae (Euphorbiaceae sensu stricto) using plastid RBCL and TRNL-F DNA sequences[END_REF].

Five individuals per species were sampled between September 2016 and Mars 2017. In order to limit ontogenetic effect and allow for reliable comparison, selection of individuals was standardized as follows: (i) recently mature individuals were selected based on the observation of inflorescences/fruits or scars, (ii) only non-reiterated individuals (see [START_REF] Oldeman | A molecular phylogeny of the Solanaceae[END_REF][START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF] and (iii) non-traumatized trunks were selected. Fully expanded leaves were sampled in distal parts of the plants (Figure 7.2). All samples (Figure 7.2) were placed in hermetic bags, and stored in a cool and wet environment during transportation to the lab. Measurements were carried within max. 24 hours after sampling. Leaves were not separated from stem until measurement.

Traits analyzed

All traits (Table 7.2) were measured from five individual per species (Figure 7.2). Whole plant stem traits (plant height and basal area) were measured in situ with a measuring tape, a caliper and eventually a TRUPULSE 360R Telemeter for taller plants.

For the measurement of leaf traits, fresh mass of lamina were measured with a precision balance. Length of petioles (PL) was measured with a measuring tape. Leaf laminas (petiole removed) were scanned at 300 pp and leaf area (LA) was calculated with the imageJ software through the LeafArea package for R [START_REF] Masatoshi | LeafArea: Rapid Digital Image Analysis of Leaf Area[END_REF]. Laminas were dried at 50 °C for 72 hours and their dry masses were measured to calculate specific leaf area (SLA) and leaf dry matter content (LDMC).

For the measurements of stem traits, sampling was done at Point Of Measurement (POM)

which is located directly under the trunk leaf-tuft (Figure 7.2). This POM was selected because (i) branched and monocaulous species could only be compared for trunks, (ii) it makes a direct link between stem anatomy and distal leaf traits (e.g. Huber Value) and (iii) distal parts of the plant are supposed to be approximately of same age. The portion of stem situated at POM (5-6 cm long) was sampled and separated in (i) one 2-3 cm long portion of stem, (ii) one 2-3 cm long portion of wood (pith and cortex pulled-out), (iii) one cross-section of stem used for anatomical measures (see below) (Figure 7.2). The fresh and dried (at 50°C for 72h) mass, and fresh volume (using the water displacement method) of stem and wood portion were measured to calculate Stem Specific Density (SSD), Stem Dry Matter Content (SDMC) and Wood Specific Density (WSD) (see Table 7.2 for trait values calculations). For leaf anatomical measurements, one leaf per individual was measured (Figure 7.2). Procedure followed the IAWA requirements as close as possible [START_REF] Wheeler | IAWA List of Microcopie Features for Hardwood Identification[END_REF][START_REF] Baas | IAWA List of microscopic features for softwood identification[END_REF]. Anatomical characters were investigated through freehand cross-sections on material fixed in 70 % ethanol. To examine photosynthetic and non-photosynthetic tissues of the limb, unstained transversal cross-sections (Figure 7.2) were mounted in water. Leaf thickness (LT)

and area of palisade and spongy mesophyll were measured on five 1000-μm-wide fields of view.

Stomata density (SD) was measured from leaf impression (nail polish) of a lamina, avoiding veins, on six fields of view of 250000 μm². For all observations we used a Leica DM5000B binocular microscope or Leica M165C stereo microscope depending on anatomical elements. Both were mounted with a Leica EC3 camera for photography. Traits were measured from photographs with the imageJ software [START_REF] Schneider | NIH Image to ImageJ: 25 years of image analysis[END_REF]. The two species of Bocquillonia presented too much ornamented leaf abaxial epiderm to enable leaf impression; in this case, stomata density was measured on photographs performed with scanning electron microscope (FEI Quanta 200 FEG).

Leaf anatomical traits (Figure 7.2, Table 7.2) have not been measured yet on all sampled species. LT, ratio of palisade mesophyll area to spongy mesophyll area (PSR) and SD was only measured on genera Atractocarpus (Rubiaceae), Bocquillonia (Euphorbiaceae), Litsea (Lauraceae), Phelline (Phellinaceae) and Tapeinosperma (Primulaceae). The five other genera will be measured subsequently to complete the dataset.

For stem anatomical traits (Figure 7.2, Table 7.2), different scales were used. Macroanatomy (cross-sectional area, area of pith, wood, phloem and cortex) was measured regarding the total cross-section either with stereo microscope (in this case, section was finely sanded) or binocular microscope depending on stem size. For micro-anatomy, approximately 30-μm-thick transverse sections were made with a vibratome (Microm HM650V) and were prepared according

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/%'-0,.10-#++0#%0!23 /%'-04.'+&0-#++0#%0!23 5#+#"0,(#-'%'. 6 7(+%#"0,(#-'%'. =.'#!94!>(%& 8&9"'!+'?%(9$!#.'# =.'#!94!@99, 8&9"'!+'?%(9$!#.'# =.'#!94!?9.%'A 8&9"'!+'?%(9$!#.'# =.'#!94!>&"9'-8&9"'!+'?%(9$!#.'# 899,!,.1!-#++0#%0!23 899,!4.'+&!;9"<-'0#%0!23 BC-#D9.0;'++'"0,(#-'%'.0 × -($9.0;'++'"0,(#-'%'.E X'#40,.10-#++ X'#404.'+&0-#++ !#"(+#,'0-'+9>&1""0#.'# />9$)10-'+9>&1""0#.'# to the following protocol: (i) 10-20 minutes in 20% bleach to decolorize tissues, (ii) 5 minutes in water to remove bleach, (iii) 2 minutes in acetic acid to facilitate colorant fixation, (iv) 5-10 seconds in Toluidine blue to colorize different tissues and (v) fast rinsing with water. The external portion of wood were photographed to measure wood anatomical traits. Vessel density (VD) was measured in six 60000 μm² fields of view. Fiber density (FD) was measured in six 5000 μm² fields of view.

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Ray frequency (RF) was measured as the number of rays crossing six 200-µm-long tangential segments. Ray thickness (RT) is the maximal thickness of ray for one individual in number of cells as recommended by IAWA [START_REF] Wheeler | IAWA List of Microcopie Features for Hardwood Identification[END_REF][START_REF] Baas | IAWA List of microscopic features for softwood identification[END_REF]. Vessel diameter (Vd) was measured as the geometrical mean of the minimum and maximum vessel diameter (Figure 7.2) for 18 vessels randomly selected in external wood.

We further calculated hydraulically weighted mean vessel diameter (Dm), theoretical conductivity (Kth) and theoretical specific conductivity (Ks-th) (see Table 7.2 for formulas and references) based on micro-anatomical traits.

Data analysis

Comparison between monocaulous and branched species -

The effect of monocaulous architectural models (Corner's: 7 species, Chamberlain's: 2 species, Cook's: 1 species) and leaf composition (simple: 8 species, compound: 1 species, phyllomorphic branch: 1 species) on functional traits were tested beforehand through permanova (Vegan package for R, [START_REF] Oksanen | vegan: Community Ecology Package. R package version 2.4-6[END_REF]. The effect of both factors was non-significant (P > 0.063). Although this absence of difference could be due to the unbalanced representation of architectural models and leaf types, these factors were not statistically controlled in subsequent analyses.

Trait differences between monocaulous and branched species were tested through mixed effects anovas performed with the nlme package for R [START_REF] Pinheiro | nlme: Linear and Nonlinear Mixed Effects Models[END_REF]. Fixed explanatory variable was the growth habit (monocaulous or branched) and response variables were all other functional traits (Table 7.2), and were logarithmically transformed when necessary. Random variables were genera and individual (nested in genera) for traits with several replicates per individual.

Stem sizeleaf size allometries were investigated through linear regression at three different levels; (i) at the level of phytomer by considering distal pith area (primary meristem diameter)mean leaf area relationships [START_REF] Sinnott | The Relation between Body Size and Organ Size in Plants[END_REF], (ii) at individual-leaf level by considering distal stem area (including secondary growth)mean leaf area relationships (leafstem scaling, e.g. [START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF][START_REF] Sun | The leaf size-twig size spectrum of temperate woody species along an altitudinal gradient: an invariant allometric scaling relationship[END_REF] and (iii) at leaf-tuft level by considering distal stem areatotal leaf area relationships (foliagestem scaling, e.g. [START_REF] Brouat | Corner's rules revisited: ontogenetic and interspecific patterns in leaf-stem allometry[END_REF][START_REF] Olson | Universal foliage-stem scaling across environments and species in dicot trees: plasticity, biomechanics and Corner's Rules[END_REF]. For each regression, data was log-transformed and differences in slopes and intercepts between monocaulous and branched species were tested by adding growth habit and their interactions in linear models (Ancovas).

Comparison with the TRY Database -To test if functional differences observed between

New Caledonian monocaulous and branched species were consistent when compared to other species in other geographical areas, functional trait values were extracted from the TRY Plant trait Database [START_REF] Jdiaz | TRY -a global database of plant traits[END_REF]. We focused on two standardized and widely measured plant traits to approximate: (i) resource acquisition strategy in relation to leaf size (SLA -LA relationship),

both traits are at the center of the worldwide leaf economic spectrum [START_REF] Wright | The worldwide leaf economics spectrum[END_REF][START_REF] Osnas | Global Leaf Trait Relationships: Mass, Area, and the Leaf Economics Spectrum[END_REF][START_REF] Edwards | Leaf life span and the leaf economic spectrum in the context of whole plant architecture[END_REF], and (ii) hydraulic investment in relation to deployed leaf area (Huber value -Vd relationship). For each functional dimension, linear regressions were performed with 95% prediction intervals and the functional space occupied by monocaulous plants and their 95% prediction ellipse were added on the global spectrum.

Huber value (HV) represents sapwood area to leaf area ratio and is widely used to investigate hydraulic constraints on wood anatomy [START_REF] Pickup | Dry mass costs of deploying leaf area in relation to leaf size[END_REF][START_REF] Wright | Cross-species patterns in the coordination between leaf and stem traits, and their implications for plant hydraulics[END_REF][START_REF] Pérez-Harguindeguy | New handbook for standardised measurement of plant functional traits worldwide[END_REF]. Vd is widely used to approximate hydraulic efficiency in plants [START_REF] Zanne | Angiosperm wood structure: Global patterns in vessel anatomy and their relation to wood density and potential conductivity[END_REF][START_REF] Beeckman | Wood anatomy and trait-based ecology[END_REF], as hydraulic conductivity is a fourth-power function of the vessel diameter (Hagen-Poiseuille law, [START_REF] Tyree | Xylem structure and the ascent of sap[END_REF]. All freely available data from the TRY database where both traits were measured came from the compilation of [START_REF] Choat | Global convergence in the vulnerability of forests to drought[END_REF] and involve tree or shrub species. After removing some bottle tree species such as Brachychiton australis, for which anatomy is too much atypical, the used dataset included 163 pairs of measures.

Data where LA and SLA were measured such as previously described came from 10 published studies [START_REF] Shipley | Structured Interspecific Determinants of Specific Leaf Area in 34 Species of Herbaceous Angiosperms[END_REF][START_REF] Pyankov | Leaf structure and specific leaf mass: the alpine desert plants of the Eastern Pamirs, Tadjikistan[END_REF][START_REF] Shipley | Dry matter content as a measure of dry matter concentration in plants and their parts[END_REF][START_REF] Cavender-Bares | Phylogenetic structure of Floridian plant communities depends on taxonomic and spatial scale[END_REF][START_REF] Swaine | Ecological and evolutionary drivers of plant community assembly in a Bornean rain forest[END_REF][START_REF] Baraloto | Decoupled leaf and stem economics in rain forest trees[END_REF][START_REF] Laughlin | A multi-trait test of the leaf-height-seed plant strategy scheme with 133 species from a pine forest flora[END_REF][START_REF] Milla | Multi-trait interactions, not phylogeny, fine-tune leaf size reduction with increasing altitude[END_REF][START_REF] Frenette-Dussault | Functional structure of an arid steppe plant community reveals similarities with Grime's C-S-R theory[END_REF][START_REF] Auger | Inter-specific and intra-specific trait variation along short environmental gradients in an old-growth temperate forest[END_REF]. Given that relationships between leaf traits varied little with growth habit [START_REF] Wright | The worldwide leaf economics spectrum[END_REF][START_REF] Milla | The scaling of leaf area and mass: the cost of light interception increases with leaf size[END_REF], herbs and lianas were considered as well as woody autoportant species in analysis. Only arborescent monocotyledons (e.g. palmae), of which a majority is unbranched, were removed from the dataset leading to a total of 18863 observations.

Anatomical implication of foliage -stem scaling -To disentangle functional implication

of the foliage areastem size relationships and particularly relative importance of hydraulic, mechanic and storage constraints, anatomical structures were partitioned following their functions and their relationship with foliage size was independently assessed at different levels.

At the stem level, whether variation in Sarea was more related to total leaf area or to total leaf fresh mass was tested. Univariate and multivariate linear regressions were performed with foliage area and/or foliage mass as explanatory variables and Sarea as the response variable.

Differences in the slope between the two regressions were tested with Pillai's trace test [START_REF] Pillai | Some New Test Criteria in Multivariate Analysis[END_REF] as implemented in the car package for R [START_REF] Fox | An R Companion to Applied Regression[END_REF]. Higher effect of foliage area, with a surrogate of evapotranspiration rate and photosynthetic resource capture, would suggest higher constraints in water conduction and/or metabolic rates on the variation of stem size.

Higher effect of foliage mass would suggest stronger mechanical constraints on the stem, implying resistance adjustment through stem size.

At the tissue level, the relative contribution of the major stem tissues to the increase in foliage area with stem size was analyzed. Pith area, xylem area, phloem area and cortex area were each regressed against total leaf area and slope differences were tested through Pillai's trace tests.

A higher contribution of pith would suggest stronger mechanical constraints (through second moment of inertia, [START_REF] Niklas | Plant Biomechanics: an engineering approach to plant form and function[END_REF] in the foliagestem relationship; a higher contribution of phloem or cortex would suggest a strongest role of photosynthate transport and storage [START_REF] Kozlowski | Carbohydrate sources and sinks in woody plants[END_REF].

Wood is a highly multifunctional tissue [START_REF] Evert | Esau's Plant Anatomy[END_REF][START_REF] Beeckman | Wood anatomy and trait-based ecology[END_REF] and its higher contribution to the foliagestem relationships has to be further explored through cell-types contribution (vessel, fiber and parenchyma).

Therefore, at the wood level the relative contribution of the major cell types to the variation in foliage area was tested. Number of vessels (hydraulic function), fibers (mechanical function)

and rays (storage function) in the total wood cross-section were respectively regressed against total leaf area. Slope differences between these three regressions were tested through Pillai's trace tests.

Results

Functional differences between monocaulous and branched species

Several morpho-anatomical traits are significantly different between monocaulous and branched plants (Table 7.3). Monocaulous plants exhibited slender and less tapered stem, suggesting less mechanical reinforcement. They tended to have thicker apical cross-sections, both at primary growth (Parea) and after the onset of secondary growth (Sarea) (Figure 7.3). This larger Sarea was associated with a higher proportion of xylem, and not pith, while branched species allocated relatively more in cortex tissue. WSD, SSD and SDMC were not significantly different between monocaulous and branched species. Wood of monocaulous species was characterised by larger vessels and a lower FD, VD and RF in comparison to branched species (Figure 7.3). HV was lower in monocaulous plants, implying lower wood area dedicated to the supply of the distal leaf area with water and minerals. This was, at least partially, compensated by higher values of traits related to hydraulic capacity (Vd, Dm, Kth, Ks-th). Leaves were by far larger for monocaulous plants but no significant difference in SLA was found. This could result from higher LT but lower LDMC in monocaulous plants as SLA is an inverse function of LDMC and LT [START_REF] Hodgson | Is leaf dry matter content a better predictor of soil fertility than specific leaf area?[END_REF].

Allocation in the palisade and spongy mesophylls did not differ between growth habits, but monocauls tended to have lower SD. These results need to be confirmed by a complete sampling (currently only half of the species has been measured for these traits).

Leaf area and stem area scaled positively for monocaulous and branched species at stem distal area than mean leaf area.

When data for monocaulous plants was compared to a larger diversity of species, growth habits and environmental conditions from the TRY database, the New Caledonian monocaulous occupied a corner position on the trait coordination spectrum. Globally, HV decreased logarithmically with increasing Vd (logged data, Figure 7.5-A). This trend was mainly due to an increasing variation in HV when Vd increased: since large vessels facilitate hydraulic capacity, they are suited for both high and low ratio of xylem to distal leave area while small vessel elements are more strongly associated with high ratio of wood to distal leaves. Within this global scaling, monocaulous appeared to be extremes in having both relatively large vessels and low leaf area to wood area ratio. LA and SLA scaled positively in a linear way (logged data, Figure 7.5-B): at large scale, the larger the leaf, the lower the structural investment. In this context, New Caledonian monocaulous appeared peculiar in having extreme leaf size but low SLA as shown by ellipse largely exceeding the global 95% prediction interval.

Anatomical implication of foliagestem scaling

Sarea scaled positively with both total leaf area and total leaf mass (Figure 7.6). Both traits were strongly correlated and association with stem size was high in both cases (slopes > 0.97, adjusted R² > 0.74), but slope was significantly higher for foliage mass than for foliage area (Pillai's trace test, P < 0.001) suggesting greater importance of mechanical constraints on the foliagestem size relationship.

The area of each stem macro-anatomical tissues (pith, xylem, phloem and cortex) scaled positively with total leaf area (Figure 7.7). Slopes of regressions were significantly higher for pith and xylem areas than for cortex and phloem areas, even if the adjusted R² of the pith areafoliage area relationship was quite low (adjusted R² = 0.57). Vessel and fiber number showed a significant positive relationship, with similar slope, with total leaf area (logged data, Figure 7.8). The relationship was also significantly positive but much less pronounced for the number of rays.

Discussion

Toward a functional characterization of rainforest monocaulous habit

Whereas major functions of most trees and treelets are partitioned between axis categories (trunks, branches, twigs, see [START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF], monocauls are characterized by a single trunk assuming all essential functions (Chapter 6). This particular architectural design involved functional constraints leading to contrasting functional behavior of stems and leaves between growth habits (branched versus monocaulous).

Biomechanics - [START_REF] Carlquist | Island Biology[END_REF] suggested that distal parts of monocaulous stems might possess a wood cylinder configuration that would maximize strength because of the large and numerous leaves. We found a strong difference in both LA and Sarea between monocaulous and branched species, as well as a significant relation between Sarea and total leaf mass that confirms the mechanical reinforcement of upper stem in response to apical mass for both growth habit.

Thicker apical diameter in monocaulous species however results from higher wood fraction and not pith fraction as previously suggested for other monocaulous models [START_REF] Carlquist | Island Biology[END_REF]Mabberley, 1974a;[START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Meinzer | Adaptations for water and thermal balance in Andean giant rosette plants[END_REF][START_REF] Mosbrugger | Tree Habit in Land Plants: A functional comparison of trunk constructions with a brief introduction into the biomechanics of trees[END_REF]. As shown by the position of monocaulous plants along a two-dimensional spectrum related to hydraulic strategies, they tend to have low wood to distal leaf area ratio (HV). Increasing wood area fraction, compared to their branched counterparts, is probably to be related to the hydraulic and mechanical demand of large and numerous leaves (see below). When considering only monocaulous species, we found only weak correlation between WSD and total leaf area (Spearman correlation test: rho = -0.12, P = 0.41) or wood traits such as FD (rho = 0.10, P = 0.48), Vd (rho = -0.26, P = 0.07) or VD (rho = 0.24, P = 0.09) while these correlations are significant for closely related branched species (not shown) and often supported in literature (e.g. [START_REF] Mencuccini | The ecological significance of long-distance water transport: short-term regulation, long-term acclimation and the hydraulic costs of stature across plant life forms[END_REF][START_REF] Ackerly | Adaptation, Niche Conservatism, and Convergence: Comparative Studies of Leaf Evolution in the California Chaparral[END_REF][START_REF] Cavender-Bares | Multiple trait associations in relation to habitat differentiation among 17 Floridian oak species[END_REF][START_REF] Santiago | Leaf photosynthetic traits scale with hydraulic conductivity and wood density in Panamanian forest canopy trees[END_REF][START_REF] Pickup | Dry mass costs of deploying leaf area in relation to leaf size[END_REF][START_REF] Wright | Relationships Among Ecologically Important Dimensions of Plant Trait Variation in Seven Neotropical Forests[END_REF][START_REF] Zanne | Angiosperm wood structure: Global patterns in vessel anatomy and their relation to wood density and potential conductivity[END_REF]. These results support the view that geometric adjustment of the stem, rather than micro-anatomical variation, is involved in the response to the biomechanical and hydraulic demand of the monocaulous habit.

At the whole plant level, monocaulous species tended to deploy higher slenderness ratio (SR), whose mechanical stability depends on mechanical reinforcement (E, bending modulus) and static-load (P) [START_REF] Niklas | Plant Biomechanics: an engineering approach to plant form and function[END_REF][START_REF] Niklas | A Comparison between the Record Height-to-Stem Diameter Allometries of Pachycaulis and Leptocaulis Species[END_REF]. As for the mechanical stability of a column, SR is proportional to (E/P) 1/2 [START_REF] Niklas | A Comparison between the Record Height-to-Stem Diameter Allometries of Pachycaulis and Leptocaulis Species[END_REF], and reducing P through lack of branching could be sufficient to ensure mechanical stability of slender stems. Higher SR in monocaulous species indeed means that they would be more vulnerable to global elastic buckling induced by selfloading, i.e. if E is low or P is high. But lower tapering of monocaulous stem, resulting for thicker apical portion, suggests lower critical buckling lengths for a given bending modulus [START_REF] Niklas | A Comparison between the Record Height-to-Stem Diameter Allometries of Pachycaulis and Leptocaulis Species[END_REF]. Preliminary results seem to indicate that there is no relationship between slenderness ratio/tapering and bending modulus at the base of monocaulous stems, i.e. no mechanical reinforcement (unpublished data). Monocaulous species are however variables in their biomechanical behaviors, with for instance maximal values of bending modulus ranging from 4000 to 10000 MPa, and different E to diameter relationships (unpublished data). Further biomechanical investigations should bring more information on the diversity of mechanical strategies related to anatomical construction.

Hydraulics -We found that monocauls deployed less xylem per distal leaf area and wider vessels than their branched congeners and species from the TRY database. This condition could involve plant hydraulics, in regard to the deployment of a large distal leaf area, as wider vessels could compensate hydraulically for a proportionally narrower xylem area [START_REF] Ewers | Why vines have narrow stems: histological trends in Bauhinia (Fabaceae)[END_REF][START_REF] Pickup | Dry mass costs of deploying leaf area in relation to leaf size[END_REF][START_REF] Wright | Cross-species patterns in the coordination between leaf and stem traits, and their implications for plant hydraulics[END_REF]. This is because Kth scales with the fourth power of Vd (Hagen-Poiseuille law), thus a slight increase in Vd causes a disproportional increase in conductivity and modifies the wood area needed to conduct water at a given rate [START_REF] Tyree | Xylem structure and the ascent of sap[END_REF]. By contrast, VD is not linked with HV (rho = -0.09). The counterpart of this increased hydraulic efficiency through vessel size rather than density is the potential increase in cavitation risk, as embolism can be (indirectly) related to vessel diameter [START_REF] Tyree | Vulnerability of xylem to cavitation and embolism Annual Review of Plant[END_REF][START_REF] Hacke | Drought experience and cavitation resistance in six shrubs from the Great Basin, Utah[END_REF][START_REF] Poorter | The importance of wood traits and hydraulic conductance for the performance and life history strategies of 42 rainforest tree species[END_REF]. This phenomena is however supposed to be rare in the buffered rainforest understory (see [START_REF] Wright | Cross-species patterns in the coordination between leaf and stem traits, and their implications for plant hydraulics[END_REF] where most monocaulous species are found (Chapter 5).

Leaves economics -As empirically suggested by [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF], monocaulous plants have larger leaves than branched relatives. In a more global spectrum, monocaulous plants from our dataset (6×10 3 -1.5×10 5 mm²) have among the largest leaves recorded worldwide (TRY Database, Figure 7.5). The global data indicate that SLA increases with LA, while the trend is reversed for monocaulous plants (Spearman correlation test: rho = -0.2, P < 0.001, Appendix 3). Several authors found similar relationship when comparing species from similar sites [START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF].

In monocaulous plant, this could be attributed to the greater construction cost of large leaves or long leaf lifespan, both being often interrelated [START_REF] Wright | The worldwide leaf economics spectrum[END_REF][START_REF] Sack | Developmentally based scaling of leaf venation architecture explains global ecological patterns[END_REF]. SLA of monocaulous species did however not significantly differ from congeneric branched species, but was nevertheless in the lower range of the worldwide spectrum for similar leaf size, suggesting low acquisition of nutrient and conservation of resource [START_REF] Wright | The worldwide leaf economics spectrum[END_REF]. As such, SLA does not bring much information on the ecological strategy that differentiates monocaulous and branched habit in similar environment. The components of leaf dry massthickness and density (LDMC as surrogate)adjust more independently from the environment and might provide better insights into the components of functional strategies [START_REF] Niinemets | Research review. Components of leaf dry mass per area -thickness and density -alter leaf photosynthetic capacity in reverse directions in woody plants[END_REF]. We indeed found that leaves of monocaulous species tend to be thicker with a lower LDMC. Low LDMC could mean that the fraction of the leaf volume occupied by mesophyllwhere CO2 fixation takes placeis higher, though photosynthetic activity is more tightly correlated with SLA [START_REF] Nadal | Possible link between photosynthesis and leaf modulus of elasticity among vascular plants: a new player in leaf traits relationships?[END_REF]. But monocaulous species tend to have a lower SD, rather suggesting less efficiency in CO2 capture.

Further works on the economy of light harvesting and biomass partitioning should bring more light on the economy of leaves in monocaulous plants (support investments, leaf chemical and structural characteristics, [START_REF] Niinemets | Research review. Components of leaf dry mass per area -thickness and density -alter leaf photosynthetic capacity in reverse directions in woody plants[END_REF][START_REF] Niinemets | Do we Underestimate the Importance of Leaf Size in Plant Economics? Disproportional Scaling of Support Costs Within the Spectrum of Leaf Physiognomy[END_REF].

For leaves, in a given microclimate and soil watering, stomatal and boundary layer conductance determine the transpiration rate [START_REF] Sack | The 'hydrology' of leaves: co-ordination of structure and function in temperate woody species[END_REF]. Large leaves, with thicker boundary layers, must face slower thermal regulation asall else being equalthey have larger leaf-to-air temperature difference [START_REF] Wright | Global climatic drivers of leaf size[END_REF]. This condition is mostly critical in hot, drier and sunny environment, where large leaves would fail to regulate transpiration rate [START_REF] Givnish | Comparative Studies of Leaf Form: Assessing the Relative Roles of Selective Pressures and Phylogenetic Constraints[END_REF]. In rainforest understory, the physiological constraints of large leaves are largely buffered (perhaps virtually lacking), favoring the deployment of large photosynthetic area, and its corollary reduced branching pattern, to increase light capture at cheaper cost. Using models and different scenarios, [START_REF] Smith | Convergence in leaf size versus twig leaf area scaling: do plants optimize leaf area partitioning?[END_REF] predicted that partitioning a given total leaf area in few large leaves is economically more interesting than in several small leaves (maximization of return on twig-leaf investment). Another selective advantage of the large-leaved monocaulous habit could then be related to growth advantage associated with lower structural cost at plant level (no branching, lower wood fraction and slightly lower stem area per total foliage, though not significant). Such growth advantage is susceptible to be particularly pronounced on the nutrient poor ultramafic substrate [START_REF] Jaffré | The Relationship between Ecological Diversity and Floristic Diversity in New Caledonia[END_REF][START_REF] Huillier | Extractability of nickel and its concentration in cultivated plants in Ni rich ultramafic soils of New Caledonia[END_REF][START_REF] Isnard | How did the ultramafic soils shape the flora of the New Caledonian hotspot?[END_REF] where monocauly preferentially evolved (Chapter 5).

Carbohydrate storage -Carbohydrates are stored in all parts of plants, but the more largely used tissue in ligneous non-liana Angiosperms is ray parenchyma [START_REF] Loescher | Carbohydrate Reserves, Translocation, and Storage in Woody Plant Roots[END_REF][START_REF] Kozlowski | Carbohydrate sources and sinks in woody plants[END_REF][START_REF] Morris | A global analysis of parenchyma tissue fractions in secondary xylem of seed plants[END_REF][START_REF] Carlquist | Living Cells in Wood 3. Overview; Functional Anatomy of the Parenchyma Network[END_REF]. Monocauls presented a lower ray density and ray thickness was not significantly different following growth habit. The relative allocation in ray parenchyma is consequently higher in branched species but monocauls have a higher proportion of wood, leading to a higher absolute number of rays (on average 395 against only 243 for branched species, not shown). Proportion of cortex is higher in branched species possibly involving further implication of this stem tissue in carbohydrate storage. Despite these slight differences in storage traits, storage function remain very difficult to approximate with soft traits [START_REF] Hodgson | Allocating C-S-R Plant Functional Types: A Soft Approach to a Hard Problem[END_REF][START_REF] Weiher | Challenging Theophrastus: A common core list of plant traits for functional ecology[END_REF] and because storage location varies according to species. Actually, carbohydrate storage implies a wide variety of tissues including pith parenchyma, wood ray parenchyma, wood axial parenchyma, cortex parenchyma and bark. And the relative contribution of each tissue is likely to vary following plant systematic position [START_REF] Kozlowski | Carbohydrate sources and sinks in woody plants[END_REF][START_REF] Carlquist | Living Cells in Wood 3. Overview; Functional Anatomy of the Parenchyma Network[END_REF]. Contrary to hydraulics and biomechanics, storage function could be allocated in other organs or axis categories than the trunk (e.g. roots) and if it is the case for some of the studied species, differences of storage allocation between growth habits could be hidden.

Toward a functional explanation of the foliagestem scaling

We found a strong relation between stem (shoot) area and leaf area (individual and total) confirming the universal leafstem and foliagestem scaling. Beyond confirming the consistence of the pattern across organization levels (phytomer level, individual leaf level, twig level), we further showed that the relationship stays mainly unchanged across growth habits (monocauls vs.

branched) in similar environments, for a wide range of leaf areas (from 6×10 2 to 1.5×10 5 mm²).

Only the stem areaindividual leaf area showed differences in slopes and intercepts between monocaulous and branched species: monocauls had higher individual leaf area for a given stem area but leaf area increased more rapidly with increasing stem area in branched species. The linear log-log relation between stem area and leaf area (at all levels) means that leaf area increases disproportionately with stem area [START_REF] Niklas | Plant Allometry: The Scaling of Form and Process[END_REF], as showed in other studies [START_REF] Bond | Allometry and sexual differences in leaf size[END_REF][START_REF] Brouat | Leaf-stem allometry, hollow stems, and the evolution of caulinary domatia in myrmecophytes[END_REF][START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF][START_REF] Normand | Hydraulic and mechanical stem properties affect leaf-stem allometry in mango cultivars[END_REF], but see Brouat et al. 1998). The relationship was stronger at foliage level than at individual-leaf level, confirming that stem-leaves relationship is more strongly governed by total leaf area than individual leaf area [START_REF] Brouat | Corner's rules revisited: ontogenetic and interspecific patterns in leaf-stem allometry[END_REF][START_REF] Westoby | Plant Ecological Strategies: Some Leading Dimensions of Variation Between Species[END_REF].

The foliagestem scaling has been hypothesized to result from a leaf to stem metabolic dependency alternatively involving hydraulics [START_REF] Chuah | Exemples de corrélations quantitatives entre quelques paramètres déterminant la forme végétale[END_REF]White, 1983a;[START_REF] Preston | Hydraulic architecture and the evolution of shoot allometry in contrasting climates[END_REF][START_REF] Sun | The leaf size-twig size spectrum of temperate woody species along an altitudinal gradient: an invariant allometric scaling relationship[END_REF][START_REF] Normand | Hydraulic and mechanical stem properties affect leaf-stem allometry in mango cultivars[END_REF], mechanics (White, 1983a;[START_REF] Niklas | Plant Biomechanics: an engineering approach to plant form and function[END_REF][START_REF] Normand | Hydraulic and mechanical stem properties affect leaf-stem allometry in mango cultivars[END_REF][START_REF] Olson | Universal foliage-stem scaling across environments and species in dicot trees: plasticity, biomechanics and Corner's Rules[END_REF]Levionnois et al., 2018a) and photosynthesis [START_REF] Primack | Relationships Among Flowers, Fruits, and Seeds[END_REF][START_REF] Cornelissen | A triangular relationship between leaf size and seed size among woody species: allometry, ontogeny, ecology and taxonomy[END_REF]. In our study, the metabolic constraint seems to operate mechanically, as we found a stronger response of apical stem area to increasing foliage mass, representing mechanical constraints, than to increasing foliage area, representing hydraulic and photosynthetic constraints.

The strong correlation of foliage area with pith area also supports the mechanical hypothesis.

Actually, the increased contribution of pith put mechanically important tissues (wood, cortical fibers) at the periphery of the stem, where they have a disproportionate mechanical contribution to whole stem stiffness at lower carbon cost [START_REF] Niklas | Plant Biomechanics: an engineering approach to plant form and function[END_REF]. This geometrical strategy was yet suggested as important in the relation between foliage area and stem size (White, 1983a). Wood area is also strongly correlated with the variation in foliage area (more than phloem or cortex). The increase of wood area appears to be an important mechanism for the adjustment of stem size over foliage size because it allows the concomitant increase of several essential functions (hydraulic, mechanic and storage). In our case, this adjustment of wood seems more linked with an increase in the number of vessels and fibers rather than in the number of rays. More generally, the lower relationship between total leaf area and photosynthate-storage related tissues (wood rays, phloem, and cortex) showed that this function only little contributes to the foliagestem scaling.

The stronger importance of the mechanical function in the adjustment of stem size to leaf area is consistent with other studies [START_REF] Normand | Hydraulic and mechanical stem properties affect leaf-stem allometry in mango cultivars[END_REF]Levionnois et al., 2018a) but must depends on the environment. In our study sites, mean annual precipitations were higher than 2200 mm.yr -1 (Météo-France, 2007). Even if a long dry annual period occurs in New Caledonia [START_REF] Maitrepierre | Les types de temps et les cyclones, les éléments du climat[END_REF], this is not critical for rainforest understory where individuals were sampled.

These moist and shaded conditions make hydraulic and photosynthetic constraints on stem size marginal which probably result in prevailing mechanical over hydraulic constraints.

To summarize, our results suggest that the universal foliagestem scaling (in rainforest)

is mainly due to an increase in pith and wood fraction in response to mechanical static load imposed by numerous large leaves. But this does not explain why there is a modal correlation between stem area and individual leaf area [START_REF] Westoby | Plant Ecological Strategies: Some Leading Dimensions of Variation Between Species[END_REF]. The possible reason is that total leaf area (and mass) is more correlated with individual leaf area than with leaf number [START_REF] Falster | Leaf size and angle vary widely across species: what consequences for light interception?[END_REF][START_REF] Westoby | The leaf size-twig size spectrum and its relationship to other important spectra of variation among species[END_REF][START_REF] Smith | Convergence in leaf size versus twig leaf area scaling: do plants optimize leaf area partitioning?[END_REF], as shown by our dataset (linear regressions with total leaf area: Adjusted R² = 0.87 for individual leaf area against 0.28 for leaf number).

Moreover, even if total leaf area is equal, biomechanical constraints are not necessarily similar when leaf area is made up of few large leaves or many small leaves. Whereas mass imposed by small leaves are distributed closely to the supporting axis, mass of large leaves are distributed further away from the axis leading to higher biomechanical constraints on stem (longer level arm).

Such higher biomechanical constraints of leaves appear as a major drivers of foliagestem scaling), and could also explain the universal leaf sizestem size spectrum in rainforest understory.

Another well-developed theory came from the redundant observation of a negative correlation between leaf area and wood density (e.g. [START_REF] Cavender-Bares | Multiple trait associations in relation to habitat differentiation among 17 Floridian oak species[END_REF][START_REF] Santiago | Leaf photosynthetic traits scale with hydraulic conductivity and wood density in Panamanian forest canopy trees[END_REF][START_REF] Pickup | Dry mass costs of deploying leaf area in relation to leaf size[END_REF][START_REF] Wright | Relationships Among Ecologically Important Dimensions of Plant Trait Variation in Seven Neotropical Forests[END_REF] or young modulus [START_REF] Olson | Universal foliage-stem scaling across environments and species in dicot trees: plasticity, biomechanics and Corner's Rules[END_REF][START_REF] Trueba | Trait coordination, mechanical behaviour and growth form plasticity of Amborella trichopoda under variation in canopy openness[END_REF]. [START_REF] Olson | Universal foliage-stem scaling across environments and species in dicot trees: plasticity, biomechanics and Corner's Rules[END_REF] argue that for a constant total leaf area (and therefore a hypothetical similar amount of photosynthates), large-leaved plants have lower stem density because they have longer internodes (i.e. "leaf spacing"), thus needing thicker stems for biomechanical adjustment.

The proposed biomechanical adjustment through stem diameter is coherent with our above hypothesis and our results support the negative relation between leaf size and wood density (Spearman correlation test: rho = -0.31, P = 0.002, Appendix 3). But we do not think that this correlation is due to longer internode in large-leaved species. Virtually all studies that investigated both leaf size and internode length did not find a positive correlation between the two traits [START_REF] Chuah | Exemples de corrélations quantitatives entre quelques paramètres déterminant la forme végétale[END_REF][START_REF] Ashton | Crown characteristics of tropical trees[END_REF][START_REF] De Souza | Évolution de paramètres morphologiques au cours du cycle de croissance de Callisia fragrans (Commelinaceae)[END_REF][START_REF] Lauri | Le mouvement morphogenetique, approche morphometrique et restitution graphique: L'exemple de quelques plantes tropicales[END_REF]Poorter and Rozendaal, 2008, Chapter 6). This hypothesis is also in contradiction with Corner's rules [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF] predicting that small-leaved trees ("leptocaulous") have longer internodes than large-leaved ("pachycaulous") trees. A similar total leaf area does not necessarily imply similar carbon production and carbon allocation strategies could be different between large-leaved and small-leaved species. For example, a significant part of photosynthates is allocated to fruit production (Niklas and Enquist, Caledonia most likely evolved from woody ancestors [START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF]Chapters 5,6) and are suited to moist and shady understory. As such, our study illustrated a virtually unknown ecology of monocaulous plants, occurring in several tropical regions throughout the world (e.g. Gabon, [START_REF] Hallé | Présentation de quelques formes ligneuses simples de la forêt de Belinga (Gabon)[END_REF]Panama, D'Arcy, 1973;or New guinea, Hallé, 1974), where they could represent an important fraction of the understory diversity [START_REF] Hallé | Présentation de quelques formes ligneuses simples de la forêt de Belinga (Gabon)[END_REF]. New Caledonia is well known for the outstanding distinctiveness of its flora, characterized by a marked taxonomic and functional disharmony compared to adjacent regions [START_REF] Morat | Richesse et affinités floristiques de la Nouvelle-Calédonie: conséquences directes de son histoire géologique[END_REF][START_REF] Jaffré | Distribution and Ecology of the Conifers of New Caledonia[END_REF][START_REF] Carpenter | Reproductive traits of tropical rain-forest trees in New Caledonia[END_REF][START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF][START_REF] Van Der Ent | The flora of ultramafic soils in the Australia-Pacific Region : state of knowledge and research priorities[END_REF][START_REF] Trueba | Ecology, forms and functions of the basal angiosperms from New Caledonia[END_REF]. In this study, we found that 182 species are monocaulous, belonging to 41 genera, 30 families and 15 orders. As such, 5.5% of the flora and 9% of the woody flora are monocaulous (Chapter 5). Whether this high diversity of monocaulous species and lineages is part of New Caledonia's functional disharmony is difficult to assess given the lack of data for other floras. We established a reliable list of monocaulous species, but some taxa (about twenty) still have to be thoroughly checked. A similar intensive work (based on a similar definition of monocauly) is needed for other regions such as Australia and New Guinea, recognized as the main sources for New Caledonian flora [START_REF] Morat | Our knowledge of the flora of New Caledonia: endemism and diversity in relation to vegetation types and substrates[END_REF][START_REF] Swenson | Sapotaceae biogeography supports New Caledonia being an old Darwinian island[END_REF]. This could be especially interesting for New Guinea, which was also previously suggested as being particularly rich in monocaulous plants [START_REF] Hallé | Architecture of Trees in the Rain Forest of Morobe District, New Guinea[END_REF].

The occurrence of monocauly in 9% of the woody flora is by far lower than suggested by [START_REF] Veillon | Architecture végétative de quelques arbres de l'archipel Néo-caledonien[END_REF], who found 112 of the 575 species he considered (19.5%) to conform to monocaulous architectural models (Corner's, Holttum's, Chamberlain's and Cook's). This discrepancy might principally result from the inclusion of monocotyledons and ferns in the work of [START_REF] Veillon | Architecture végétative de quelques arbres de l'archipel Néo-caledonien[END_REF]. In addition, his work was based on an opportunistic sub-sampling of the flora, which induced a bias in favor of monocaulous species for which architecture is comparatively easy and quick to describe. However, 5.5% of the entire flora (and 9% of the woody flora) nevertheless represents a significant portion for a growth habit recognized as rare both regionally [START_REF] Schmid | Les écosystèmes forestiers mélanésiens (Nouvelle Calédonie, Nouvelles-Hébrides, Fidji et îles Salomon)[END_REF] and globally [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF]. The contribution of monocauly is even more remarkable when considering only rainforest vegetation, in which it represents respectively 10.7% and 12.4% of rainforest-facultative and rainforest-restricted woody nonmonocot species. For comparison, the monocaulous flora (including monocots and ferns) was suggested to account for only 2% of the world's flora as a whole [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF] and the divaricated flora of New Zealand, recognized as the most outstanding syndrome for this island, accounts for 10% of the woody flora [START_REF] Greenwood | Evolution of divaricating plants in New Zealand in relation to Moa browsing[END_REF].

Whether this outstanding diversity of monocaulous species in New Caledonia translates into a high abundance and density of species with this habit in ecosystems is difficult to assess.

Despite intensive forest inventories carried over the last few years in New Caledonia [START_REF] Ibanez | Structural and floristic diversity of mixed tropical rain forest in New Caledonia: newdata from the New-Caledonian Plant Inventory and Permanent PlotNetwork (NC-PIPPN)[END_REF]Birnbaum et al., 2015a;Birnbaum et al., 2015b;Ibanez et al., 2017b;Ibanez et al., 2017a;Blanchard et al., submitted), abundance and distributional data for monocaulous species are lacking because most ecological censuses considered trees with a DBH > 5 cm or DBH > 10 cm. We investigated the representativeness of monocaulous species in the New Caledonian Plant Inventory and Permanent Plot Network (NC-PIPPN, [START_REF] Ibanez | Structural and floristic diversity of mixed tropical rain forest in New Caledonia: newdata from the New-Caledonian Plant Inventory and Permanent PlotNetwork (NC-PIPPN)[END_REF]. The results indicated that only 24%

of New Caledonian monocaulous species were represented in the database, against 46% for branched tree species [START_REF] Ibanez | Structural and floristic diversity of mixed tropical rain forest in New Caledonia: newdata from the New-Caledonian Plant Inventory and Permanent PlotNetwork (NC-PIPPN)[END_REF]. Most monocaulous species (41%) were represented by less than 3 individuals in the database and 80% of them never exceeded 10 cm in diameter. While this apparent rarity may also be due to restricted distribution and small plot sizes, small DBH indicates that monocaulous plants are mainly represented by old individuals. This confirms that monocaulous species have in general a DBH lower than 5 cm and that standardized forest inventories are not adapted for the study of this growth habit and more generally for the study of understory species. Despite this, monocaulous species were present in 45% of the plots and could represent up to 100% of the understory species in some cases. A less extensive study analyzed structure, diversity and composition of different rainforests measuring all stems > 1.3 m high in nine plots [START_REF] Read | Structural and floristic characteristics of some monodominant and adjacent mixed rainforests in New Caledonia[END_REF]. Plots were located in the south-east of the main island, on ultramafic substrate, from 240 m to 940 m elevation, and ranged from 0.048 to 0.124 ha (total size = 0.576 ha). Extraction of monocaulous species from this dataset confirm that 95.2% of stems are thinner (i)

The establishment of monocaulous species from other regions would imply that the monocaulous habit is an ancestral train in New Caledonian clades. A phylogenetic reconstruction of Atractocarpus did not validate this hypothesis (Chapter 6), nor did the phylogeny of several other lineages such as Oxera [START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF], Pycnandra [START_REF] Swenson | Sapotaceae biogeography supports New Caledonia being an old Darwinian island[END_REF][START_REF] Swenson | Island life -classification, speciation and cryptic species of Pycnandra (Sapotaceae) in New Caledonia[END_REF], Beauprea [START_REF] He | Pre-Gondwanan-breakup origin of Beauprea (Proteaceae) explains its historical presence in New Caledonia and New Zealand[END_REF], Pittosporum (Gemmill, unpublished data), and Tapeinosperma (Gemmill, unpublished data). A careful review of architecture expressed in the presumed sister group of New Caledonian monocaulous genera suggests that among the 41 genera, only Phyllanthus, Casearia, Dysoxylum, Plerandra and Meryta might have been established in New Caledonia via a monocaulous ancestor. Current evidence consequently suggests that this mechanism (primary establishment) did not contributed significantly to extant monocaulous diversity in New Caledonia. This remain to be tested, however, and some monocaulous sister groups could have become extinct.

(ii)

In situ repeated evolution of monocauly in different lineages seems responsible for a large part of the New Caledonian diversity in monocauls, as the habit appears rarely to have been ancestral (see above), and we identified more than 31 independent origins. However, convergent evolution must not be the only mechanism, otherwise there would be as many independent evolutions as monocaulous species.

(iii) The majority of monocaulous genera (64%) contain more than one monocaulous species (Chapter 5) and, while multiple shifts to monocauly are frequent within genera, most monocaulous clades seem to have diversified at least a little. However, there are only a few genera with several monocaulous species and no truly large monocaulous radiation has been identified yet, the largest being seven species derived from a single monocaulous ancestor in Oxera (Lamiaceae, [START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF]. In Atractocarpus, a single monocaulous ancestor diversified into six monocaulous species and two architecturally intermediate species (Chapter 6), and monocauly did not significantly enhance diversification rate in the clade. As such, while in situ diversification has contributed to the current diversity of monocaulous taxa, current evidence although suggests a low rate of species diversification among groups expressing the habit, although this question needs further investigation.

As such, convergent evolution of monocauly in New Caledonia is probably the principal mechanism leading to the remarkable diversity of monocaulous species in the archipelago (i.e.

functional disharmony). Our study revealed that this new case of convergence is particularly exceptional and illustrates this type of phenomenon for the first time in New Caledonia (Chapter 5). At a larger phylogenetic and geographic scale, the monocaulous architecture (sensu [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], see Chapter 3) has been shown to evolve repeatedly, with 118 independent origins, across more than 20,000 vascular species [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF]. Here, we found between 31 and 49 independent origins across the woody non-monocot flora of New Caledonia (2114 species, Chapter 5), using a genus-level phylogeny. This result is no doubt highly conservative since monocauly has often evolved repeatedly within genera [START_REF] Rauscher | Molecular phylogenetics of the Espeletia complex (Asteraceae): evidence from nrDNA ITS sequences on the closest relatives of an Andean adaptive radiation[END_REF][START_REF] Mort | Physiological Evolution and Association between Physiology and Growth Form in Aeonium (Crassulaceae)[END_REF][START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF]Barrabé et al., 2018, Chapter 6). As a comparison, secondary woodiness in the Canary Islandsknown as one of the most striking cases of convergent evolution in islandsevolved 38 times independently (based on species-level phylogenies, [START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF].

This remarkable convergence toward monocauly in New Caledonia suggests that strong environmental contingencies have favored the evolution of this functionally and physiognomically atypical growth habit in the archipelago. To accurately address the question of which environmental factors could have facilitated the evolution of monocauly, i.e. which traits were selected, we first needed to clarify the functional implications of monocauly.

FIGURE 8.2 -Illustration of life-history traits supposed and/or showed to be associated with monocauly. Green ellipses show previously suggested traits for which we confirmed the relationship. Yellow ellipses show previously suggested traits for which we found no relationship. Red ellipses show previously suggested traits for which we found inverse relationship. Blue ellipses show not previously suggested traits for which we found a significant relationship. Letters represent chapters in which the relationship was investigated. mainly driven by mechanical constraints and, to a lesser extent, hydraulic constraints (Chapter 7)

as suggested by previous studies (White, 1983a;[START_REF] Niklas | Plant Biomechanics: an engineering approach to plant form and function[END_REF][START_REF] Normand | Hydraulic and mechanical stem properties affect leaf-stem allometry in mango cultivars[END_REF][START_REF] Olson | Universal foliage-stem scaling across environments and species in dicot trees: plasticity, biomechanics and Corner's Rules[END_REF]Levionnois et al., 2018a). Increasing stem size (and wood fraction) is a response to the static load imposed by numerous apical large leaves, as the mechanical resistance of a structure is proportional to the fourth-power of its radius (Chapter 7). But the higher wood area fraction might not fully compensate hydraulically for the total leaf area of monocauls, as observed by their lower Huber value (sapwood area to leaf area ratio). The increase in wood proportion is consequently accompanied by increase in vessel diameter, leading to higher hydraulic conductivity as hydraulic conductivity is a fourth-power function of the vessel diameter (Chapter 7). (iii) The relation between the above traits (monocauly, leaf size, stem thickness) and fruit size does not appear to be due to a higher production of photosynthates by large leaves contrary to previous suggestions [START_REF] Primack | Relationships Among Flowers, Fruits, and Seeds[END_REF][START_REF] Cornelissen | A triangular relationship between leaf size and seed size among woody species: allometry, ontogeny, ecology and taxonomy[END_REF]. The equal or lower proportions of transport and storage tissues (phloem, wood parenchyma) in monocauls than in branched species suggest similar carbon exchange capacity between leaves and lower organs (Chapter 7). We further think that, as for the observed leaf sizeleaf number trade-off, spatial constraints of monocaulous stem limit the number of fruits, requiring larger fruits to maintain a constant seed number (Chapter 6). Such large fruits further increase the load already imposed by large leaves and probably further influence stem thickening in monocauls [START_REF] Niklas | The allometry of plant reproductive biomass and stem diameter[END_REF] and the higher occurrence of cauliflory (Chapter 5). (iv)

By focusing on vegetative functions rather than reproductive ones, we poorly investigated the relation between monocauly and inflorescence complexity. The homology between branches of branched species and inflorescences of monocaulous species in Atractocarpus involves a change of scale in what is considered the inflorescence. While the simple inflorescence of branched species (a compact dichasium) is terminal on each module, the "inflorescence" of monocaulous species is rather composed of two nested dichasium (the modular axillary complex and the primary dichasium) (Figure 8.4). In this regard, inflorescences of monocauls could be considered as longer and more complex than those of branched species. But this case study appeared as a special case and we did not look further enough into this question to draw general conclusions.

Stem shape and mechanics -Monocauls are often noticed for their slender and little tapered stems [START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF][START_REF] Carlquist | Island Biology[END_REF][START_REF] Lauri | Corner's rules as a framework for plant morphology, architecture and functioning -issues and steps forward[END_REF]. Describing a Hawaiian monocaulous species (Cyanea leptostegia), [START_REF] Carlquist | Island Biology[END_REF] noted that the stem diameter is almost equal between the basis and the apex of the plant. Distal parts were composed by a large pith and a thin cylinder of wood and basal parts by a small pith and a large cylinder of wood [START_REF] Carlquist | Island Biology[END_REF]. This acropetal increase of pith in unbranched plants is also suggested in other studies [START_REF] Eggert | The ontogeny of Carboniferous arborescent Lycopsida[END_REF][START_REF] Lauri | Le mouvement morphogenetique, approche morphometrique et restitution graphique: L'exemple de quelques plantes tropicales[END_REF] and confirmed by unpublished results of this thesis (Figure 8.5). As a result, we found larger pith in distal stems of monocaulous species (absolute value) compared to branched species but this increase was associated with larger wood cylinder, both tissues leading to thicker distal stems in monocauls compared to branched species (see above). But stem slenderness and tapering are also determined by plant basal diameter, which appears to be smaller for a given height in monocauls than in branched species (Figure 8.6). The higher proportion of wood in distal parts with a smaller basal diameter of monocaulous species probably involves a differential cambial activity along the stem. We suppose that this pattern results from the lack of branching which involves a lower static load along the stem of monocauls and slower basipetal diameter increment (cambial activity), leading to higher slenderness andbecause of thick distal stemlower tapering (Chapter 7, Figure 8.3). From mechanical perspectives, slenderness and low tapering can probably occur because of the lower static load inherent to the absence of branches. If true, the geometry of the stem would not require mechanical reinforcement along the stem. Unpublished results of this thesis indicated that young modulus (i.e. bending resistance) measured for basal parts of monocaulous plants (between 4000 and 10000 MPa, Figure 8.7) were in the range of values measured for branches of other trees (between 5000 and 14000 MPa, [START_REF] Niklas | Plant Biomechanics: an engineering approach to plant form and function[END_REF]. Further investigations should integrate variation in bending modulus along the stem, and height to stem allometries [START_REF] Niklas | Evolutionary trends in safety factors against wind-inducted stem failure[END_REF][START_REF] Niklas | A Comparison between the Record Height-to-Stem Diameter Allometries of Pachycaulis and Leptocaulis Species[END_REF]. In this respect, a diversity of mechanical behaviors seem to emerge in monocaulous species (Figure 8.7), whose geometrical and anatomical drivers will be investigated in a future publication.

Photosynthesis and growth rates -Beyond their larger area, investigated leaf traits failed to clearly identify photosynthetic strategy of monocaulous plants (Chapter 7). Monocaulous leaves tend to exhibit lower SLA (Chapter 6), particularly for such large leaves (Chapter 7), higher leaf thickness and lower stomata density (Chapter 7). This combination of trait values suggest lower photosynthetic capacities for monocauls in comparison to their branched relatives, but this has to be confirmed by further measurements. Another publication, focusing on leaf traits diversity in one of the richest monocaulous families (the Araliaceae), is under progress and shows a relation between leaf traits and branching intensity, but divergent allocating patterns according to leaf type (simple, palmate or pinnate) (Gril, Bruy,Heinz & Isnard,in prep.,Appendix 4). Monocauls also tend to have shorter internodes (but only marginally significant in Chapter 6) that, together with higher secondary growth in distal stems, suggests lower primary growth rates. Several monocaulous species of different ages and growing in rainforest understory on ultramafic substrate were marked at the beginning of this thesis. The growth was too slow for the results to be interpreted in the course of the thesis. In 607 days (from 25/07/2016 to 24/03/2018), Pittosporum pronyense individuals produced between 0 and 1 growth unit, Tapeinosperma gracile between 0 and 2 growth units, Atractocarpus bracteatus between one and two phytomers (continuous growth, see Chapter 6) and none of the individuals of Balanops pancheri produced any phytomers. Growth rates seem higher for young plants than for older ones (which is, however, not yet testable). These preliminary results, which need to be further explored (effect of substrate, comparison with cooccurring branched species), together with the putative low photosynthetic capacities, suggest a marked conservative resource strategy for monocaulous species, at least higher than for branched relatives. The comparison of our SLA values with those of the TRY Database [START_REF] Jdiaz | TRY -a global database of plant traits[END_REF] suggests that this strategy is among the most conservative in a global spectrum including other growth habits and regions of occurrence (Chapter 7).

In conclusion, as for other growth habits, monocauly is not only characterized by the lack of lateral branches but by a complex combination of inter-related traits. Spatial constraint on a single stem appeared to be the main origin of all suites of trait attributes, for which Corner's rules are one of the central themes, illustrating functional constraints of plant form evolution (Figure 8.3). Consequently, selection for a given trait attribute in a given environmental is likely to drive the variation of whole plant form and functions.

Environmental constraints on monocauly

Along with unbranched stems, the most remarkable functional characteristics of monocaulous plants are their large, mostly simple leaves (Chapter 5), which are among the largest worldwide (Chapter 7). Large simple leaves are disadvantaged in arid and cold conditions due to their thicker boundary layer that slows thermoregulation [START_REF] Vogel | Leaves in the lowest and highest winds: temperature, force and shape[END_REF][START_REF] Wright | Global climatic drivers of leaf size[END_REF]. This physiological risk constrains the distribution of large simple leaves to humid and shady environments, providing buffered thermal and hydric conditions [START_REF] Givnish | On the adaptive significance of leaf form[END_REF][START_REF] Givnish | Comparative Studies of Leaf Form: Assessing the Relative Roles of Selective Pressures and Phylogenetic Constraints[END_REF][START_REF] Givnish | Adaptation to Sun and Shade: a Whole-Plant Perspective[END_REF].

These physiological vulnerability of large leaves probably explains why monocaulous species are mainly found in rainforest (89%, Chapter 5). Possibly, the ecological limitation of monocaulous plants is reinforced by their larger vessel diameter that could be related to a greater sensitivity to drought-induced embolism. The distribution of vessel size has been shown to be strongly limited by both temperature and precipitation [START_REF] Zanne | Three keys to the radiation of angiosperms into freezing environments[END_REF][START_REF] Pfautsch | Climate determines vascular traits in the ecologically diverse genus Eucalyptus[END_REF][START_REF] Hacke | An ecophysiological and developmental perspective on variation in vessel diameter[END_REF][START_REF] Olson | Plant height and hydraulic vulnerability to drought and cold[END_REF]. Though, drought-induced embolism is also governed by conduit structures (e.g.

pit membrane density and area, [START_REF] Tyree | Vulnerability of xylem to cavitation and embolism Annual Review of Plant[END_REF][START_REF] Hacke | Drought experience and cavitation resistance in six shrubs from the Great Basin, Utah[END_REF][START_REF] Poorter | The importance of wood traits and hydraulic conductance for the performance and life history strategies of 42 rainforest tree species[END_REF] which were not measured here.

Chapter 5 indicates that the branched habit occurs in all types of environments and does not seem to show preference for rainforest contrary to monocaulous species. In Atractocarpus for instance, only branched species "escaped" from rainforest as shown by a study including some results of this thesis (Mouly, Barrabé & Bruy, submitted). Nevertheless, several monocaulous species occur out of rainforest as 35% of them (63 species) have been observed at least once in other vegetation and 11% (21 species) are restricted to maquis or dry forests. The sub-sampling used in Chapter 6 and 7 focused exclusively on rainforest understory as we aimed to detect traits involved in architectural transition in similar habitat. But monocaulous species from dry environments would present interesting case studies to understand the functional limit and trait variations within the monocaulous habit. For instance, we found that 79 % of monocaulous species occurring in dry vegetation had compound leaves or functionally similar phyllomorphic branches (Figure 8.8). Dissected lamina structures are a way to reduce vulnerability inherent to large leaves in open environment by reducing both the boundary layer (accelerating thermoregulation) and drag (limiting wind damages) [START_REF] Givnish | On the adaptive significance of coumpound leaves, with particular reference to tropical trees[END_REF][START_REF] Vogel | Leaves in the lowest and highest winds: temperature, force and shape[END_REF][START_REF] Wright | Global climatic drivers of leaf size[END_REF]. Regarding species with simple leaves (21% of dry vegetation monocauls), an often unconsidered aspect of Corner's rules is that relation between branching intensity and leaf size is effective when comparing similarly sized plants. Consequently, a way to reduce leaf area while remaining unbranched is to reduce whole plant size (Figure 8.8). For example, Argophyllum acinetochromum (Argophyllaceae) and Scaevola beckii (Goodeniaceae), that are among the only monocaulous species with simple leaves from maquis are also among the smallest monocauls of the archipelago (< 1 m high), and their leaves are no larger than leaflets of co-occuring monocaulous species. Beyond reducing leaf size, plant height is also the main driver of vessel diameter worldwide [START_REF] Olson | Plant height and hydraulic vulnerability to drought and cold[END_REF] and small monocaulous plants are then likely to be less sensitive to drought embolism than their taller counterparts.

Modelling studies shown that, for a twig with a given total leaf area, few large leaves are economically more interesting than several small leaves (maximization of return on twig-leaf investment) [START_REF] Smith | Convergence in leaf size versus twig leaf area scaling: do plants optimize leaf area partitioning?[END_REF]. As a result, the monocaulous strategy, being at the extreme end of the leaf sizeleaf number trade off, would be advantaged in resource poor environments (e.g.

shady or with low soil fertility). Other environmental factors known to constrain leaf size are wind and browsing of which tearing probability increases with leaf size [START_REF] Moles | Do small leaves expand faster than large leaves, and do shorter expansion times reduce herbivore damage?[END_REF][START_REF] Vogel | Leaves in the lowest and highest winds: temperature, force and shape[END_REF][START_REF] Charles-Dominique | An index for assessing effectiveness of plant structural defences against mammal browsing[END_REF].

Environmental correlates of monocauly in New Caledonia

If the evolution of monocauly is probably limited by a set of environmental conditions (Figure 8.8), several hypotheses can be proposed in regard to environmental factors that could have facilitated the evolution of monocauly in New Caledonia (Figure 8.9).

Tropical cyclones -New Caledonian rainforests are characterized by an outstanding high density of stems [START_REF] Jaffré | Etude floristique et structurale de deux forêts denses humides sur roches ultrabasiques en Nouvelle-Calédonie[END_REF][START_REF] Jaffré | Structural and floristic characteristics of a rain forest on schist in New Caledonia : a comparaison with an ultramafic rain forest[END_REF][START_REF] Ibanez | Structural and floristic diversity of mixed tropical rain forest in New Caledonia: newdata from the New-Caledonian Plant Inventory and Permanent PlotNetwork (NC-PIPPN)[END_REF]Ibanez et al., 2017b), argued as the main structural characteristic that distinguish New Caledonia from other Southwest Pacific rainforests (Ibanez et al., 2017b). Recent insights suggest that this particularity is due to the high frequency and intensity of tropical cyclones in the region (Ibanez et al., 2018a).

We think that such dense neighboring operate as a space constraint, somehow similar to "crown FIGURE 8.9 -Schematic illustration of main hypotheses explaining the outstanding prevalence of monocauly in New Caledonia.

shyness" [START_REF] Putz | Mechanical Abrasion and Intercrown Spacing[END_REF], in which not abrasion but thigmomorphic response, favor selection toward reducing branching. In these conditions, unidirectional exploration strategy might be advantaged. Moreover, high cyclone frequency involves debris falls and damages through the domino effect, to which monocauls, lacking lateral branches, might be less sensitive (Figure 8.10).

Supporting these hypotheses, palms and tree ferns are also particularly abundant in New

Caledonian rainforests (Ibanez et al., 2017b), both groups being dominated by monocaulous architectures. We believe that the structural and dynamic features of these forests are important environmental contingencies of the convergence toward monocauly in New Caledonia (Chapter 5).

It is the first time that density of New Caledonian rainforest is suggested as a driver of the New Caledonian disharmony. This particularity is actually more likely to affect growth habit distribution which is known to be highly dependent on forest structure (e.g. [START_REF] Schnitzer | Testing ecological theory with lianas[END_REF]. This potential effect on functional disharmony is likely to indirectly affect taxonomical disharmony through the over-representation of typically single-stemmed groups (e.g. Palms: 40 species, Tree ferns: 16 species).

Rainforest history -The long-time persistence of rainforest on the archipelago while they declined from adjacent regions (see Chapter 2) is supposed to be responsible for the high incidence of several drought-sensitive groups such as Palms [START_REF] Pintaud | Chorology of New Caledonian palms and possible evidence of Pleistocene rain forest refugia[END_REF] or basal Angiosperms [START_REF] Poncet | Phylogeography and niche modelling of the relict plant Amborella trichopoda (Amborellaceae) reveal multiple Pleistocene refugia in New Caledonia[END_REF][START_REF] Pouteau | New Caledonia: a Pleistocene refugium for rain forest lineages of relict angiosperms[END_REF][START_REF] Trueba | Ecology, forms and functions of the basal angiosperms from New Caledonia[END_REF][START_REF] Tournebize | Two disjunct Pleistocene populations and anisotropic postglacial expansion shaped the current genetic structure of the relict plant Amborella trichopoda[END_REF]. This mechanism is also likely to drive the current high representation of monocaulous lineages, through different possible scenarios. (i) Monocaulous would have survived in rainforest refugia while their regional relatives disappeared, (ii) the post-Pleistocene expansion of rainforests produced vacant niches for rainforest groups, and favored niche pre-emption in the understory, as suggested for Atractocarpus (Chapter 6). Apart from the genus Atractocarpus, such ecological opportunities are likely to have driven the diversification of several rainforest lineages timely present in New Caledonian rainforest.

Accordingly, three palms clades, strongly associated with rainforest and dominated by unbranched architecture, showed stem ages and crown ages similar to that of Atractocarpus [START_REF] Pillon | Time and tempo of diversification in the flora of New Caledonia[END_REF], and their current distribution was suggested to be related with Pleistocene refugia [START_REF] Pintaud | Chorology of New Caledonian palms and possible evidence of Pleistocene rain forest refugia[END_REF].

As such, alternation in expansion and contraction of rain forests in response to climatic fluctuation during Pleistocene is likely to be an important driver of current New Caledonian biodiversity [START_REF] Jaffré | Structural and floristic characteristics of a rain forest on schist in New Caledonia : a comparaison with an ultramafic rain forest[END_REF][START_REF] Poncet | Phylogeography and niche modelling of the relict plant Amborella trichopoda (Amborellaceae) reveal multiple Pleistocene refugia in New Caledonia[END_REF][START_REF] Pouteau | New Caledonia: a Pleistocene refugium for rain forest lineages of relict angiosperms[END_REF]. More globally, late quaternary climatic change have been suggested to impact current pattern of biodiversity in islands, notably due to variations in islands area [START_REF] Weigelt | Late Quaternary climate change shapes island biodiversity[END_REF].

Edaphic constraints -The presence on one third of the territory of the worldwide rare ultramafic substrate is largely recognized as one of the main drivers of taxonomic disharmony in the New Caledonian flora [START_REF] Jaffré | Changements dans la végétation de la Nouvelle-Calédonie au cours du Tertiaire: la végétation et la flore des roches ultrabasiques[END_REF][START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF][START_REF] Isnard | How did the ultramafic soils shape the flora of the New Caledonian hotspot?[END_REF]. The constraining conditions for plant growth challenge the establishment of most speciesthere are for instance very few invasive species on this substrate [START_REF] Jaffré | Etude écologique du peuplement végétal des sols dérivés de roches ultrabasiques en Nouvelle-Calédonie[END_REF][START_REF] Meyer | Les plantes envahissantes et potentiellement envahissantes dans l'archipel néo-calédonien : première évaluation et recommandations de gestion[END_REF] while favoring the installation of pre-adapted plant lineages [START_REF] Pillon | Ultramafic soils and species sorting in the flora of New Caledonia[END_REF]. Adaptation to ultramafic substrate also drove the functional disharmony with for example an outstanding proportion of metal hyper-accumulator species [START_REF] Jaffré | The metal hyperaccumulators from New Caledonia can broaden our understanding of nickel accumulation in plants[END_REF][START_REF] Van Der Ent | The flora of ultramafic soils in the Australia-Pacific Region : state of knowledge and research priorities[END_REF][START_REF] Jaffré | The discovery of nickel hyperaccumulation in the New Caledonian tree Pycnandra acuminata 40 years on: an introduction to a Virtual Issue[END_REF]. We bring further demonstration of the importance of this substrate on functional disharmony by showing that the monocaulous habit preferentially evolved on ultramafic soils (Chapter 5). Such constraining edaphic conditions are likely to favor the economic "few large leaves, no branches" strategy characterizing monocaulous plants. Effect of soil constraints on plant architecture has little been investigated but recent insights suggest that poor soil fertility reduces the structural development of trees [START_REF] Levionnois | Low functional but large soil-related variations in growth trajectories characterise the widespread Neotropical tree Cecropia obtusa Trecul (Urticaceae). bioRxiv[END_REF]. In extreme cases, low soil fertility was observed to reduce the branched architecture of Cecropia obtusa into monocaulous trees, a phenomena called architectural pauperization [START_REF] Barthélémy | Architecture et sexualité chez quelques plantes tropicales: le concept de floraison automatique[END_REF]. In New Caledonia, some authors also suggested that constraining edaphic conditions linked to ultramafic substrate could be responsible for the frequency of tuft-trees in the island [START_REF] Virot | La Végétation canaque[END_REF][START_REF] Veillon | Architecture végétative de quelques arbres de l'archipel Néo-caledonien[END_REF]. The frequency of ultramafic substrate in New Caledonia could then have contributed to the repeated evolution of monocauly in the archipelago through the reduction of growth and structural expression of branched species, ultimately leading to single stemmed plants. In accordance with this hypothesis, some species growing on ultramafic substrate exhibit few but regular branching (suggesting sequential branching) or a strictly monocaulous habit (e.g. Cunonia macrophylla), possibly depending on soil conditions.

If monocauly evolved principally on ultramafic, the proportion of species occurring on this substrate is not very different than for branched species (47% vs. 44% respectively, Chapter 5),

suggesting further diversification of monocauly on non-ultramafic substrates. This does not seem to pertain to calcareous substrate since only two monocaulous species (Acropogon calcicolus and Plerandra calcicola) are restricted to this substrate and few monocaulous species occur in Loyalty Islands (less than ten and no endemics).

Herbivory -Absence of natural large browsers on islands is well known to allow the evolution of unarmed species, leading to insular naivety [START_REF] Whittaker | Island biogeography: Taking the long view of nature's laboratories[END_REF]. Such process could be particularly striking in New Caledonia as there is no autochthonous large herbivorous species and no extinct browsing giant bird are known from the archipelago (Sylviornis was probably carnivorous or frugivorous, [START_REF] Mourer-Chauviré | Description of the skull of the genus Sylviornis Poplin, 1980 (Aves, Galliformes, Sylviornithidae new family), a giant extinct bird from the Holocene of New Caledonia[END_REF][START_REF] Worthy | Osteology Supports a Stem-Galliform Affinity for the Giant Extinct Flightless Bird Sylviornis neocaledoniae (Sylviornithidae, Galloanseres)[END_REF] contrary to other Pacific islands (e.g. Moanalos in Hawai, [START_REF] Olson | Description of Thirty-two New Species of Birds from the Hawaiian Islands: Part l. Non-Passeriformes[END_REF]; Moas in New Zealand, [START_REF] Worthy | The Lost World of the Moa: Prehistoric Life of New Zealand[END_REF]. Two types of defense against herbivory are recognized: chemical and structural [START_REF] Charles-Dominique | The architectural design of trees protects them against large herbivores[END_REF]. While the former has not been investigated for the New Caledonian flora (but hyper-accumulation of trace elements is suggested to evolve as a response to herbivory, [START_REF] Boyd | The defense hypothesis of elemental hyperaccumulation: status, challenges and new directions[END_REF][START_REF] Pillon | Cryptic adaptive radiation in tropical forest trees in New Caledonia[END_REF], the latter appears to be rare since very few indigenous species are armed with spines or thorns (Carissa ovata, Maclura cochinchinensis, Capparis spp., Mezoneuron spp.) and all are more or less specialized climbers [START_REF] Bruy | Abundance, richness and composition of lianas in forest communities along an elevation gradient in New Caledonia[END_REF]. Recent studies showed that structural defenses can also be achieved through particular architectural design: the more intrinsically branched is a plant, the more effective is the protection [START_REF] Costes | Plant architecture, its diversity and manipulation in agronomic conditions, in relation with pest and pathogen attacks[END_REF][START_REF] Charles-Dominique | An index for assessing effectiveness of plant structural defences against mammal browsing[END_REF][START_REF] Charles-Dominique | The architectural design of trees protects them against large herbivores[END_REF].

In this context, monocauls with their single meristem and large leaves appeared to be disarmed against large herbivores. This insight is confirmed by IUCN redlist data showing that monocaulous species are more threatened by introduced herbivores than branched species (Chapter 5).

Accordingly, monocaulous species of Cyanea (Campanulaceae) in Hawaii where browsing birds were present, have thorn-like prickles to protect the stem [START_REF] Givnish | Thorn-like prickles and heterophylly in Cyanea: adaptations to extinct avian browsers on Hawaii[END_REF].

As such, the absence of large herbivores in New Caledonia, largely present in other areas, released plants from a strong exogenous constraint and from selection against reduction of meristem redundancy, possibly allowing the specific evolution of the monocaulous habit in the archipelago.

Monocauly and species diversification

Evidence showed that evolutionary shift in growth habit often impacts diversification of clades [START_REF] Gianoli | Evolution of a climbing habit promotes diversification in flowering plants[END_REF][START_REF] Roquet | Replicated radiations of the alpine genus Androsace (Primulaceae) driven by range expansion and convergent key innovations[END_REF][START_REF] Couvreur | Global diversification of a tropical plant growth form: environmental correlates and historical contingencies in climbing palms[END_REF][START_REF] Gianoli | Evolutionary implications of the climbing habit in plants[END_REF][START_REF] Frenzke | Evolution of Epiphytism and Fruit Traits Act Unevenly on the Diversification of the Species-Rich Genus Peperomia (Piperaceae)[END_REF]. In New Caledonia, we found a positive relationship between presence of monocauly in a genus and overall diversity of the genus (Chapter 5), but monocauly did not increase diversification rates more than other growth habits in the genus Atractocarpus (Chapter 6). More generally, genera with more than two monocaulous species are infrequent, and genera constituted by a majority of monocaulous species even more (Chapter 5). As such, while large monocaulous genera still have to be investigated (e.g. Phyllanthus, Acropogon, Bocquillonia), dramatic large radiations of monocaulous clades seem uncommon. It remains difficult to assess if this low number of monocaulous species per genera is associated with low speciation rate or with high extinction events, especially in the absence of calibrated phylogeny. Extinction could have been important in New Caledonia, if we considered the high incidence of phylogenetically isolated taxa [START_REF] Pillon | Time and tempo of diversification in the flora of New Caledonia[END_REF][START_REF] Pillon | How many genera of vascular plants are endemic to New Caledonia? A critical review based on phylogenetic evidence[END_REF]. But the recent evolution of the habit in Atractocarpus and Oxera (between 2.4 and 0.6 Myr) rather suggests clades under diversification. Anyway, evidence suggests that monocauly probably did not increase diversification as a key innovation, i.e. by allowing the colonization of large and vacant niches involving rapid diversification of descendants [START_REF] Heard | Key evolutionary innovations and their ecological mechanisms[END_REF]. We nevertheless suggest that, at least in some clades, the evolution of monocauly could have impacted diversification of clades through niche partitioning.

If large scale abiotic constraints drove the convergence toward monocauly in different lineages (see above), small scale biotic interaction between co-occurring species probably drove the divergence among growth habit. In the genus Atractocarpus, we identified a rapid diversification of architecture, and repeated evolution of monocauly, among closely related species that seem concomitant with speciation (Chapter 6). We hypothesized that competition between cooccurring species drove the divergence among growth habit through niche partitioning. For Atractocarpus, field observations further suggest that species with same growth habit are either rarely in sympatry or belong to different clades. As such, our case study could identify an example of adaptive radiation, i.e. "the rise of a diversity of ecological roles and associated adaptations within a lineage" [START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. Three of the four criteria of an adaptive radiation as defined by [START_REF] Schluter | The ecology of adaptive radiation[END_REF] were satisfied: (i) common ancestry with the single colonization of the biota ca.

2.4 Myr ago; (ii) rapid speciation, Atractocarpus being the New Caledonian genera with the highest diversification rate currently known (1.17 species species -1 Myr -1 ); and (iii) trait utility since growth habit was shown to be related to several ecologically important functional traits related to resources acquisition and use. The fourth criteria is the correlation between phenotype and environment and while the correlation between growth habit and ecological strategy suggests different niches, we did not investigate this aspect of adaptive radiation. Species occurring in other environments (dry forests and maquis) and their growth habits may have provided some additional support for the idea that divergent habitats lead to divergence in key functional traits via architectural evolution (Chapter 6). For instance, two species of Atractocarpus occurring respectively in maquis and dry forest seemed to express a very different architecture (3 axis categories) from those of rainforest (2 axis categories, Chapter 6). Unfortunately, environments were too degraded to accurately describe architecture of such species. More fundamentally, we were interested in shifts in growth habit in relation to functional diversification, the consideration of contrasting environments would have blurred the signal. We argue that further investigation linking environmental traits (e.g. vegetation structure, light availability, altitude, soils) and species distribution could accomplish to demonstrate that Atractocarpus represents one of the few cases of adaptive radiation in New Caledonia (see [START_REF] Pillon | How many genera of vascular plants are endemic to New Caledonia? A critical review based on phylogenetic evidence[END_REF]. We suspect that this adaptive radiation in association with growth habit involves reproductive strategy (fruit size and shape, flower shape) that appears to be variable in the genus (Mouly et al., submitted).

The genus Bocquillonia, for which no dated phylogenies are available (Chapter 4), could also exemplify another case of adaptive radiation involving growth habit diversification in relation to forest structure. In this genus, we showed that variation in plant size and branching intensity (here reiteration) led to different growth habits exploiting different strata of the rainforest, dry forest and maquis (Chapter 4, Figure 4.1).

8.3

Plant architecture as a key tool to depict plant evolution 8.3.1 Architectural and functional traits to define growth habits Since Theophraste (372 B.C.), many classifications of plant growth habits have been proposed (reviewed in [START_REF] Rietz | Life-forms of terrestrial flowering plants, 1[END_REF][START_REF] Lacza | A survey of the plant life-form systems and the respective research approaches, I[END_REF][START_REF] Millan | Analyse de la variabilité des traits architecturaux des formes de croissance dans les communautés végétales[END_REF]. Classes and their definitions largely differ among studies, authors disagreeing on the nature and hierarchy of characters to use. The main reason why no consensus has been found so far lies in the difficulty to propose universal classification that would rely on objective and functional criteria [START_REF] Millan | Analyse de la variabilité des traits architecturaux des formes de croissance dans les communautés végétales[END_REF].

One of the most used classifications (see [START_REF] Pérez-Harguindeguy | New handbook for standardised measurement of plant functional traits worldwide[END_REF] is that of Raunkiaer, that considers position of buds in plants [START_REF] Raunkiaer | Biological types with reference to the adaptation of plants to survive the unfavourable season[END_REF]. This definition was functionally meaningful, since based on strategies to survive winter in temperate regions, but cannot be generalized to the tropical regions. Most of the universal classifications proposed to date consider monocaulous plants in a more or less broad definition (e.g. [START_REF] Humboldt | Ansichten der Natur mit wissenschaftlichen Erläuterungen[END_REF][START_REF] Reiter | Die Consolidation der Physiognomik[END_REF][START_REF] Warming | Oecology of plants : an introduction to the study of plant-communities[END_REF][START_REF] Rietz | Life-forms of terrestrial flowering plants, 1[END_REF]. These classifications are however often highly subjective. For instance, "Tuft-trees" in the well-known classification of [START_REF] Warming | Oecology of plants : an introduction to the study of plant-communities[END_REF] was defined as follows:

"Shoots with short internodes ; leaves densely set on the end of the shoot, large, and few ; buds usually naked". Almost all the used terms are subjective ("short", "densely", "large", "few", "usually"), and classification will consequently be largely influenced by observer.

The approach of [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], based on plant architecture, provides more objective criteria and can be generalized to most plants [START_REF] Cremers | Architecture de quelques lianes d'Afrique Tropicale[END_REF][START_REF] Cremers | Architecture de quelques lianes d'Afrique Tropicale[END_REF][START_REF] Jeannoda-Robinson | Contribution à l'étude de l'architecture des herbes[END_REF][START_REF] Cremers | Etude de l'architecture aérienne de quelques plantes tropicales à ramification basitone: vers une révision du modèle de Tomlinson[END_REF][START_REF] Prosperi | Lianas and Hemiepiphytes: Distribution, development, and adaptations[END_REF][START_REF] Hallé | Architectures de Plantes[END_REF][START_REF] Millan | Analyse de la variabilité des traits architecturaux des formes de croissance dans les communautés végétales[END_REF]. [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] considered monocaulous as "trees with a single trunk or visible stem of the plant". This physiognomical definition was reinforced by three distinct structural types, namely the Holttum's, the Corner's, and the Chamberlain's architectural models, each defined by few qualitative morphological characters (see Chapter 2). But this strictly architectural approach also has limitations. (i) [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF] considered morphological origins of structures (e.g. stem or leaf) rather than their functions (e.g. exploration or photosynthesis), making their classification sometimes inappropriate to a given ecological context. For example, architectural analysis of the genus Atractocarpus (Chapter 6) showed that some inflorescences (virtually no photosynthetic contribution, e.g. A. longistipitatus) could have exactly the same structural construction than a large photosynthetic branch (e.g. A. artensis). (ii) Architectural models are known to be stable states among an "architectural continuum" [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Barthélémy | Architectural concepts for tropical trees[END_REF], i.e. an infinity of intermediate forms potentially exist. This phenomena was perfectly illustrated in the genus Atractocarpus where we showed a continuous transition between the Stone's model and the Corner's model, branches gradually becoming inflorescences (Chapter 6).

In this study we combined the architectural and functional approach to propose a definition more adapted to an ecological context; we defined monocauls as "self-supporting woody plants whose cardinal functions rely on a single visible stem". This definition was reinforced by the characterization of structural types that allow the use of objective morphological criteria. We recognize three structural types made of (i) a trunk (either monoaxial or pluriaxial), (ii) determinate growth structure highly specialized in photosynthesis and with an abscission point, (iii) axis or complex of axis highly specialized in reproduction and whose contribution to whole plant vegetative exploration and photosynthesis is negligible, and possibly (iv) opportunistic reiterates.

Finally, to resolve the problem of architectural continuum, we defined a functional branching index directly linking branching intensity and associated ecological functions (Chapter 6). This simple index, assessing the part of photosynthesis and exploration functions assumed by the different axis categories, ranges from 0 to 1 and is very easy to interpret. For example, species of Atractocarpus were considered as monocaulous when trunk assumes more than 80% of exploration functions and 95% of photosynthetic functions, i.e. when these functions are considered as "negligible" on lateral axis based on a a priori statistical analysis (significant grouping of the distribution of function).

Our definition of monocauly, based on a quantitative hierarchization of functions (here the branching index) and reinforced by non-ambiguous morphological criteria, proved to be relevant in our restricted ecological and evolutionary context. While this branching index seems difficult to use on some plant groups (e.g. with polyarchic architecture, [START_REF] Edelin | Nouvelles données sur l'architecture des arbres sympodiaux:le concept de plan d'organisation[END_REF], the general approach proposed here, merging architecture and functions, seems promising for the study of plant growth habit in an ecological and evolutionary context. It further shows that future directions in this field do not only rely on the use of big data, but rather on detailed studies aiming to improve our understanding of mechanisms responsible for plant forms and functions.

Heterochronic evolution

As far as we know, two developmental processes have been proposed for the evolution of monocauly: (i) evolution from herbaceous ancestors (e.g. [START_REF] Carlquist | Island Biology[END_REF], through "proportionate giantism" and (ii) evolution from tree ancestors (D'Arcy, 1973), through "progenesis" and "neoteny" (see Chapter 3, Figure 3.1). Both processes involve heterochronic developmental pathways (i.e. a change in the relative timing of developmental events in one species relative to an ancestral species) on whole plant architecture. In this thesis, an additional evolutionary developmental process involved in the evolution of monocauly has been identified (Chapter 6). This process might also involve heterochrony, not on the whole plant architecture but on axillary structures (C2) only (Figure 8.11). (i) When branching is structurally reduced through the reduction of module number, the resulting axillary structure of the descendant is a truncated part of that of the ancestor, involving neoteny [START_REF] Gould | Ontogeny and Phylogeny[END_REF][START_REF] Alberch | Size and Shape in Ontogeny and Phylogeny[END_REF][START_REF] Smith | Heterochrony revisited: the evolution of developmental sequences[END_REF]. (ii) When branching is structurally reduced through module length, the resulting axillary structure of the descendant is the miniaturized copy of that of the ancestor, involving "proportional dwarfism" [START_REF] Gould | Ontogeny and Phylogeny[END_REF][START_REF] Alberch | Size and Shape in Ontogeny and Phylogeny[END_REF][START_REF] Smith | Heterochrony revisited: the evolution of developmental sequences[END_REF]. Note that some monocaulous species having few and short modules (e.g. Atractocarpus confertus) probably underwent both processes. As such, even if appearing in closely related taxa through a similar process (i.e. reduction of branches), developmental pathways leading to the evolution of growth habits could be much diversified and would need careful observation at lower integration levels.

We think that the evolution of monocauly through the reduction of branches in Atractocarpus is not an isolated case and probably occurred in other plant groups. We particularly recognize the genus Tapeinosperma (7 monocaulous species) where axillary structures range from long orthotropic branches (Scarrone's architectural model) to short determinate inflorescences on unbranched trunk (Corner's architectural model), with several intermediate states (Figure 8.12-A).

Evolution from herbaceous ancestors were hitherto illustrated for other geographic areas (e.g. Macaronesia, [START_REF] Böhle | Island colonization and evolution of the insular woody habit in Echium L. (Boraginaceae)[END_REF][START_REF] Mort | Physiological Evolution and Association between Physiology and Growth Form in Aeonium (Crassulaceae)[END_REF] but probably concerned few clades in New Caledonia (Chapter 5), of which we recognize the genus Oxalis. New Caledonian species actually range from creeping herbaceous species with several short rosettes of leaves, to creeping/semierect woody species with several short rosette of leaves, erect woody species with few large rosette of leaves, and monocaulous species (Figure 8.12-B). This pattern illustrates an interesting architectural gradient where anatomy, biomechanics and architecture seems intrinsically related. Apart from this thesis, the only New Caledonian genus whose growth habit was investigated in a phylogenetic context is Oxera (Lamiaceae) where monocauly evolved from woody liana ancestors [START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF]. We assume that this evolution could also involve heterochronic developmental process since architecture of extant climbing species is constructed by the frequent repetition of a single axis category (Champagnat's architectural model, [START_REF] Veillon | Architecture végétative de quelques arbres de l'archipel Néo-caledonien[END_REF] and species present quite a long juvenile self-supporting phase (Figure 8.12-D).

All these insights remain mainly speculative but suggest that architecture is a robust and perfectly fitted approach to assess heterochrony which has most probably been fundamental in the evolution of growth habit [START_REF] Rowe | Plant growth forms: an ecological and evolutionary perspective[END_REF][START_REF] Wagner | Escaping the lianoid habit: Evolution of shrub-like growth forms in Aristolochia subgenus Isotrema (Aristolochiaceae)[END_REF], at least as important as wood ontogeny [START_REF] Olson | Wood ontogeny as a model for studying heterochrony, with an example of paedomorphosis in Moringa (Moringaceae)[END_REF][START_REF] Carlquist | Xylem heterochrony: an unappreciated key to angiosperm origin and diversifications[END_REF]. The New Caledonian flora, including several young clades with contrasting growth habits and intermediate states, might be well suited for investigating this promising research field.

8.3.3

The reiteration process: a gap that needs to be filled Reiteration is defined as "a morphogenetic process through which the organism duplicates its own elementary architecture, i.e. its architectural unit" [START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF].

Reiteration, of which several modalities have been defined (see Chapter 2), is a key process in plant morphogenesis. For instance, sequential reiteration has been shown as fundamental in the expression of endogenous architecture of several plants, particularly in the development of tree crowns [START_REF] Edelin | L'architecture monopodiale: l'exemple de quelques arbres d'Asie tropicale[END_REF][START_REF] Barthélémy | Canopy architecture[END_REF][START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF]. Opportunistic reiteration is one of the main adaptive processes by which a plant adapts its form after traumatism or increase in resource level [START_REF] Oldeman | A molecular phylogeny of the Solanaceae[END_REF][START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF]. In New Caledonia for example, all studied (non-monocot) monocaulous species showed the ability to reiterate at least after a traumatism. We assume that this process, inherent in most species, is particularly important for the monocaulous habit for which exploration and photosynthesis rely on a single trunk.

While the difference between sequential and opportunistic reiteration seems obvious in theory, the former being genetically determined (endogenous) and the second induced by environment (exogenous), it is not that apparent in the field. This represents the major difficulty we have not been able to solve in our definition of monocauly. We actually found several monocaulous species that could be such reiterated that they lost their monocaulous physiognomy, as illustrated by Bocquillonia grandidens (Chapter 4, Figure 4.1). Due to their sometimes slightly curved stem, the architecture of this species could have also been associated with the Champagnat's architectural model [START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], and therefore not considered as monocaulous. This illustrates a limit of architectural models to finely understand plant architecture and processes involved in their evolution. We actually found that different models such as the Corner's model (e.g. Bocquillonia corneri), the Champagnat's model (e.g. Bocquillonia grandidens) or the Tomlinson's model (e.g. Bocquillonia aff. sessiliflora) could be differentiated only on the basis of number and position of opportunistic reiterates, two features assumed to be rather exogenously than endogenously induced [START_REF] Barthélémy | Plant architecture: A dynamic, multilevel and comprehensive approach to plant form, structure and ontogeny[END_REF]. This is particularly problematic when not only the physiognomy but also the ecological characteristics are lost, as in Dysoxylum aff. roseum (Meliaceae). This species has a long monocaulous phase in the rainforest understory where several flowering and fruiting occur (Figure 8.13). After a longer or shorter period, individuals produce new axes from dormant buds. These axes are morphologically and functionally identical to the primary trunk and their apparition is predictable neither in time nor in space (sometimes nearly not occurring): they are delayed opportunistic reiterates (see Chapter 2). In several cases, these reiterates become more and more numerous until the tree reaches the canopy and has a well-developed crown (Figure 8.13). As such, the inclusion of the opportunistic reiteration process in our definition involves the comparison of small understory monocaulous treelets with tall canopy well-branched trees. But the exclusion of this process is impossible as all of observed monocauls in New Caledonia have the ability to reiterate. A criteria/threshold based on a number of reiteration could have been a solution but unsatisfying from our point of view since the process depends on environmental conditions and is consequently highly variable between individuals for a given species.

Nevertheless, the maximal number of reiterates supported is not equal between taxa, some species or genus having a greater propensity for opportunistic reiteration. This shows that the process is not totally independent from genetic background of plant individual and could be heritable. As such, further investigations are needed on the difference between sequential and opportunistic reiterations that sometimes seem to be two extremes of the same gradient. Besides, fine architectural study in a phylogenetic context (as in Chapter 6) would bring interesting information about the importance of reiteration process in the evolution of growth habit (see Acropogon, Figure 8.12-C). The simple structure of monocaulous plants makes them particularly suited to investigate this question.

Constrained evolution of plant architecture

Convergent evolution is considered as a major evidence of evolutionary constraint, i.e. that Nevertheless, if transition toward monocauly was probably facilitated by some prerequisites and preferentially evolved in some lineages, the evolution of this habit does not seem strongly constrained. It evolved, at least, from treelets (Chapter 6), herbs (e.g. [START_REF] Böhle | Island colonization and evolution of the insular woody habit in Echium L. (Boraginaceae)[END_REF], lianas [START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF] and probably trees [START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF][START_REF] Barthélémy | Architecture et sexualité chez quelques plantes tropicales: le concept de floraison automatique[END_REF]. In our case study, we illustrated a transition from the Stone's model toward the Corner's model through a reduction from branches to inflorescences (Chapter 6). However, such process does not involve special architectural prerequisites and could occur in most of the described architectural models.

As such, while evolutionary constraints seem important in the evolution of growth habit [START_REF] Rowe | Plant growth forms: an ecological and evolutionary perspective[END_REF][START_REF] Monro | The evolvability of growth form in a clonal seaweed[END_REF][START_REF] Wagner | Evolution of shrubs and lianas within Aristolochia subgenus Isotremacombining molecular phylogeny with biomechanical and anatomical approaches[END_REF][START_REF] Wagner | Escaping the lianoid habit: Evolution of shrub-like growth forms in Aristolochia subgenus Isotrema (Aristolochiaceae)[END_REF], this does not seem critically important for the evolution of monocauly, probably because of the simplicity of the form.

Given that heterochronic processes is probably central in the evolution of growth habit (see above)

and that the large majority of woody plants pass through a monocaulous phase during their ontogeny, we argue that transition toward monocauly could occur in many plant groups. Juvenile monocauly can be considered as a developmental enabler (sensu [START_REF] Donoghue | Key innovations, convergence, and success: macroevolutionary lessons from plant phylogeny[END_REF], that probably permitted the evolution of monocauly independently in several lineages. This repeated evolution might have been particularly facilitated in New Caledonia given several environmental and historical contingencies (see section 8.2.3). The diversity of evolutionary scenario that can lead to monocauly (see above) actually suggests a strong environmental forcing in several clades, each dealing with its own architectural background. The evolution of the monocaulous habit is however less advantageous outside of rainforests, emphasizing the role of environmental context in the diversification or iterative evolution of a trait attributes.

In Atractocarpus, evolution of growth habit was gradual and did not evolve by saltation as sometimes proposed for architectural evolution [START_REF] Bateman | Saltational evolution of form in vascular plants: a neoGoldschmidtian synthesis[END_REF][START_REF] Bateman | Architectural radiations cannot be optimally interpreted without morphological and molecular phylogenies[END_REF].

This reflects that profound change in the whole plant form does not necessarily involve deep changes at lower integration levels. Minute but directional quantitative variations from one generation to the next could rapidly (less than 2 Myr in Atractocarpus, Chapter 6) lead to very different growth habits. Extinction of intermediate states could easily mask such a pattern [START_REF] Bateman | Saltational evolution of form in vascular plants: a neoGoldschmidtian synthesis[END_REF]. Whether evolution of plant architecture has to be gradual and if architecture presents adaptive "peaks" in the course of plant evolution is a fascinating question whose exploration will bring more insights into the evolution of plant forms.

Conservation of the flora

New Caledonian flora, as one of the world's hotspots for biodiversity conservation [START_REF] Myers | Threatened biotas: "Hot Spots" in tropical forests[END_REF][START_REF] Mittermeier | Hotspots revisited[END_REF], is known to be highly threatened [START_REF] Jaffré | Threatened plants of New Caledonia: Is the system of protected areas adequate?[END_REF][START_REF] Lowry | Diversity, endemism, and extinction in the flora of new Caledonia: a review[END_REF][START_REF] Jaffré | Conservation programmes in New Caledonia, western Pacific: in place for the dry forest, but urgently needed for the ultramafic vegetation[END_REF][START_REF] Jaffré | Threats to the conifer species found on New Caledonia's ultramafic massifs and proposals for urgently needed measures to improve their protection[END_REF][START_REF] Wulff | Conservation priorities in a biodiversity hotspot: analysis of narrow endemic plant species in New Caledonia[END_REF]Ibanez et al., 2017b;Ibanez et al., 2018b). This threat seems even greater for monocaulous flora according to IUCN redlist criteria (Chapter 5). We actually showed that proportion of EN and CR species was nearly significantly higher than for other woody non-monocot species (41% and 32.5% respectively, Chapter 5). Overall, 17.5%, 23.8% and 9.5% of evaluated species were respectively in the CR, EN and VU classes, leading to more than half of the evaluated monocaulous species being considered as threatened.

Typical threats of the New Caledonian flora such as bushfire (Figure 8.14-A) or mining activity strongly affect monocaulous species (51% and 43% of species concerned, respectively).

Introduced herbivores (deer, pig and rat) were identified as impacting monocauls significantly more than branched species (Chapter 5). We related this vulnerability to the single meristem and large leaves characterizing the monocaulous habit that make a browsing event highly damaging (see [START_REF] Charles-Dominique | The architectural design of trees protects them against large herbivores[END_REF], despite various reiteration capacities. This should be particularly true for deer Rusa timorensis, the most threatening herbivore for New Caledonian flora (de Garine; [START_REF] De Garine-Wichatitsky | The diets of introduced rusa deer (Cervus timorensis russa) in a native sclerophyll forest and a native rainforest of New Caledonia[END_REF]IUCN, 2017), that could rapidly remove a large portion of leaf area and even apical meristems of monocaulous plants . Introduced rats, present in the most isolated forests [START_REF] Rouys | Factors determining the distribution of introduced mammals in nature reserves of the southern province, New Caledonia[END_REF][START_REF] Duron | Caractérisation des interactions entre les rongeurs introduits et la biodiversité des forêts tropicales humides. : vers la mise en place d'indicateurs pour une évaluation rapide des effets du contrôle des populations invasives[END_REF], could also have a dramatic impact on monocauls by eating apical meristems as observed on several individual of Meryta balansae (Figure 8.14-B). Among the three introduced rat species, this most likely concerns the black rat (Rattus rattus) rather than the Polynesian rat (Rattus exulans) or the brown rat (Rattus norvegicus), the second being a poor climber and the third being absent from wild environments [START_REF] Rouys | Factors determining the distribution of introduced mammals in nature reserves of the southern province, New Caledonia[END_REF][START_REF] Munzinger | Novitates neocaledonicae VI : Acropogon mesophilus (Malvaceae, Sterculioideae), a rare and threatened new species from the mesic forest of New Caledonia[END_REF]Vidal, comm. pers.). Impact of feral pig (Sus scrofa) occurs mainly by soil foraging involving predation on seedlings, roots or seeds [START_REF] Caley | Movements, Activity Patterns and Habitat Use of Feral Pigs (Sus scrofa) in a Tropical Habitat[END_REF]. As such, this introduced omnivorous is not expected to impact the monocauls more than the branched species, but the species mainly forages in rainforest understory [START_REF] Rouys | Factors determining the distribution of introduced mammals in nature reserves of the southern province, New Caledonia[END_REF], where monocauls preferentially occur. Monocauls represent an extreme case of insular naivety that seems largely expressed in New Caledonia and more generally in islands.

Despite this, the impact of introduced herbivores on New Caledonian flora remains difficult to quantify (de Garine; [START_REF] De Garine-Wichatitsky | The diets of introduced rusa deer (Cervus timorensis russa) in a native sclerophyll forest and a native rainforest of New Caledonia[END_REF] and IUCN criteria probably underestimate the threats.

This calls for further investigations into the impact of introduced herbivores on native vegetation and additionally highlights the need for concrete solutions to limit their impact.

Another probable threat impacting monocaulous species, although difficult to precisely estimate, is the effect of climate change. On one hand, the predicted increase in tropical cyclone activity [START_REF] Emanuel | Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century[END_REF] could favor the unbranched architecture, potentially less sensitive to falling debris and domino effect. On the other hand, the predicted increase in intensity and duration of drought events [START_REF] Meehl | More Intense, More Frequent, and Longer Lasting Heat Waves in the 21st Century[END_REF][START_REF] Cavarero | Les évolutions passées et futures du climat de la Nouvelle-Calédonie[END_REF]IPCC, 2014;[START_REF] Whan | Trends and variability of temperature extremes in the tropical Western Pacific[END_REF], could impact preferentially drought sensitive species such as monocauls. Drought-induced here Acropogon schumannianus (Malvaceae), the only plant dried in a rainforest community.

mortalities are increasingly observed, even if not documented for New Caledonia in recent reviews [START_REF] Allen | On underestimation of global vulnerability to tree mortality and forest die-off from hotter drought in the Anthropocene[END_REF][START_REF] Hartmann | Research frontiers in drought-induced tree mortality: crossing scales and disciplines[END_REF]. During this study, we noted two drought-induced mortality events for monocaulous species occurring during the drastic dry season of 2017 (August in Diahoué and October at Katalupaik, respectively). These events, involving respectively three individuals of Phelline dumbeensis (Phellinaceae, not shown) and one individual of Acropogon schumannianus (Malvaceae, Figure 8.14-C) from rainforest understory were remarkable since cooccurring species seemed less or not affected. These observations need further quantification but somehow confirm the drought vulnerability of some monocaulous species, which appear as good indicators of drought events as suggested for basal angiosperms in a previous study [START_REF] Trueba | Ecology, forms and functions of the basal angiosperms from New Caledonia[END_REF].

As such monocaulous species are particularly threatened and several monocaulous species could be among the firsts to disappear in response to increasing pressures. These threats are even likely to be underestimated giving the high proportion of monocaulous species in the "Data Deficient" IUCN class (10%, Chapter 5). Their extinction would not be anecdotal given their high contribution to species richness and abundance in rainforest (see section 8.1). This abundance, along with particular functional trait values, suggests that monocaulous plants are an important component of rainforest understories where they probably play a particular role (e.g. litter-trapping, [START_REF] Lachenaud | New and little-known Psychotria (Rubiaceae) from West Africa, and notes on litter-gathering angiosperms[END_REF][START_REF] Zona | Litter-trapping plants: filter-feeders of the plant kingdom[END_REF]. As such, conservation and study of this original growth habit, which is part of the New Caledonian biological heritage, seem of prime importance.

Out of New Caledonia: monocaulies rather than monocauly?

Monocaulous species investigated in this thesis tend to share many functional features (see section 8.2). More generally, their functional and life history traits (in comparison with branched relatives) were largely consistent with hypotheses proposed in literature (Figure 8.2). We however found major differences with other types of monocauly described elsewhere. First, macro-anatomy is different as a large part of the stem section was made of wood with little parenchymatous tissues, and thin cortex and pith. Comparisons are complex due to variation in ontogeny and stem location between studies, but the woody monocauly of New Caledonian species can be confidently differentiated from other anatomical models that mainly concerned secondary woody pachycauls (rosette-trees/shrubs) containing true monocauls (e.g. [START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF][START_REF] Carlquist | Island Biology[END_REF]Mabberley, 1974a;[START_REF] Aldridge | Anatomy and evolution in the Macaronesian Sonchus subgenus Dendrosonchus (Compositae: Lactuceae)[END_REF][START_REF] Aldridge | Anatomy and Evolution in Macaronesian Echium (Boraginaceae)[END_REF][START_REF] Meinzer | Adaptations for water and thermal balance in Andean giant rosette plants[END_REF][START_REF] Mosbrugger | Tree Habit in Land Plants: A functional comparison of trunk constructions with a brief introduction into the biomechanics of trees[END_REF]. Most of these species are not found in rainforest understories but rather in dry (e.g. [START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF]Mabberley, 1974a;[START_REF] Aldridge | Anatomy and evolution in the Macaronesian Sonchus subgenus Dendrosonchus (Compositae: Lactuceae)[END_REF][START_REF] Aldridge | Anatomy and Evolution in Macaronesian Echium (Boraginaceae)[END_REF] or tropical alpine environments (e.g. [START_REF] Hedberg | Tropical-Alpine Life-Forms of Vascular Plants[END_REF][START_REF] Meinzer | Adaptations for water and thermal balance in Andean giant rosette plants[END_REF]Givnish, 2016) (Figure 8.15). (i) For monocaulous species living in dry environment, thick pith and cortex together with parenchymatous wood probably relate to water storage, an adaptation unnecessary in rainforest understory. These species are also smaller than rainforest monocauls and seem characterized by small size vessels [START_REF] Mosbrugger | Tree Habit in Land Plants: A functional comparison of trunk constructions with a brief introduction into the biomechanics of trees[END_REF], probably to limit embolism risks. (ii) For species living in tropical alpine environments with drastic diurnal variations in temperature, the unbranched trunk would be a way to increases the vertical growth and bring the meristem more rapidly away from the ground surface; and the dense rosettes of leaves protect the unique meristem from frost [START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. In these conditions, large pith allows the storage and rapid remobilisation of water for photosynthesis when ground water is frozen and inaccessible [START_REF] Meinzer | Adaptations for water and thermal balance in Andean giant rosette plants[END_REF].

Leaves of these monocaulous species are also long but often narrow to resist high insolation [START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF], pubescent to resist frost, and marcescent to protect the stem after their death [START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF]. In New Caledonia, large leaves are grouped in distal parts of the stem but do not always form dense rosette and no case of marcescence nor extreme pubescence was observed.

As such, apart from the tropical rainforests, where monocauly largely occurs [START_REF] Hallé | Présentation de quelques formes ligneuses simples de la forêt de Belinga (Gabon)[END_REF][START_REF] Richards | The Tropical Rain Forest[END_REF][START_REF] Wg | Correlliana (Myrsinaceae), A New Palmoid Genus of the Tropical Rain Forest[END_REF][START_REF] Hallé | Architecture of Trees in the Rain Forest of Morobe District, New Guinea[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF], species in agreement with our definition of monocauly are also present in other contrasting habitats. They are nevertheless restricted to tropical and subtropical areas since temperate climate with annual seasonality is probably too constraining for their few growing meristem [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF].

Interestingly, there seems to be a correlation between evolutionary history and functional properties of monocaulous plants, reflecting different habitat preferences. Most species from dry and alpine habitat are secondary woody monocauls (e.g. Echium, Aeonium, Dendrosenecio, Lobelia, Argyroxiphium) while species from rainforest could have rather evolved from woody ancestor (e.g.

Atractocarpus, Oxera, Pittosporum, Tapeinosperma), even if phylogenetic insights are lacking for other regions. Interestingly, the two New Caledonian genera that potentially experienced secondary woodiness (Oxalis and Scaevola) contain monocaulous species that are restricted to maquis. This suggests that different selective pressures drove the evolution of monocauly and that their importance for diversification of growth habit largely depends on prerequisite morpho-anatomical features. In this context, it will be interesting to investigate functional characteristics of secondary woody monocauls that secondary gain rainforest (e.g. Hawaian lobeliads, [START_REF] Givnish | Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae)[END_REF].

More generally, other studies are necessary to disentangle effect of growth habit, environment and prerequisites on plant functional diversification. These studies should use a trans-disciplinary approach including architectural characterization to differentiate true monocauly from pachycauly that are often confounded in literature.

Conclusion and perspectives

In this study, we used a transversal approach merging plant architecture, taxonomy, phylogenies and functional ecology to characterize and illustrate the evolutionary history of monocaulous species in New Caledonia. Through an original approach of growth habit we defined monocauls as "self-supporting woody plants whose cardinal functions rely on a single visible stem", i.e. made of (i) a trunk (either monoaxial or pluriaxial), (ii) determinate growth structure highly specialized in photosynthesis and with an abscission point, (iii) axis or complex of axis highly specialized in reproduction and whose contribution to whole plant vegetative exploration and photosynthesis is negligible, and possibly (iv) opportunistic reiterates. This definition proved to be adapted for evolutionary and ecological studies in New Caledonia but remains to be tested in other contexts. Furthermore, the reiteration process showed to be problematic for defining the monocaulous habit, particularly because the differences between opportunistic and sequential reiterations is difficult to assess. In-depth and detailed investigations of these processes in plant groups with a variation in reiterative strategies might provide further insights to refine the definition of monocauly, and perhaps more generally to refine our understanding of reiteration processes in plant diversification. The genus Acropogon would be a suitable model, as growth habit diversity in the genus is mainly achieved through variations in reiteration frequency that range from highly predictable to almost never occurring.

Based on this definition of monocauly, we demonstrated that monocauls represent 5.5% of the New Caledonian vascular flora, and that the monocaulous habit appeared more than 31 times in the archipelago, illustrating one of the most striking cases of convergent evolution on islands.

We identified four possible factors responsible for this strong convergence, namely the frequency and intensity of cyclones, late quaternary climatic changes, nutrient poor ultramafic substrates and the lack of native browsers. Lists of monocaulous species for other areas with contrasting history would provide comparative data to test such hypotheses. Dated phylogenies are needed to confront the apparition of monocauly with recent climatic events. Currently available phylogenies for this purpose are unfortunately poor and concern Atractocarpus (Mouly et al., submitted), Beauprea [START_REF] He | Pre-Gondwanan-breakup origin of Beauprea (Proteaceae) explains its historical presence in New Caledonia and New Zealand[END_REF], Pycnandra [START_REF] Swenson | Island life -classification, speciation and cryptic species of Pycnandra (Sapotaceae) in New Caledonia[END_REF], Melicope/Dutaillyeae [START_REF] Appelhans | A molecular phylogeny of Acronychia, Euodia, Melicope and relatives (Rutaceae) reveals polyphyletic genera and key innovations for species richness[END_REF],

Plerandra [START_REF] Plunkett | Phylogeny and Diversification in the Melanesian Schefflera Clade (Araliaceae) Based on Evidence from Nuclear rDNA Spacers[END_REF] and Oxera [START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF]. Other monocaulous genera are currently under study and phylogenies are expected to be available soon for Pittosporum (Gemmill, unpublished data), Tapeinosperma/Mangenotiella (Gemmill, unpublished data), and

Cupaniopsis (Buerki et al. unpublished data). These nine genera will offer a substantial basis to study in detail the contextual evolution of monocauly.

Monocauly probably mainly evolved from woody ancestors through heterochronic processes and can contribute to the diversification of New Caledonian lineages through niche partitioning. La richesse en espèces monocaules peut être expliquée par 3 scénarios évolutifs nonexclusif (décrits en introduction) : (i) l'établissement en Nouvelle-Calédonie d'espèces monocaules semble être un phénomène rare puisque la monocaulie n'est ancestrale dans aucun des clades calédoniens pour lesquels des phylogénies sont disponibles (appuyé par l'absence de groupes frères monocaules), (ii) l'évolution in situ de la monocaulie semble avoir grandement participé à la diversité totale puisque nous avons identifié au moins 31 apparitions du caractère, (iii) la diversification des clades monocaules a également contribué à la diversité actuelle en espèces monocaules mais aucune grande radiation monocaule n'ait été mise en évidence jusqu'à présent.

Ainsi, l'évolution convergente de la monocaulie en Nouvelle-Calédonie est probablement le mécanisme principal responsable de la diversité en espèces monocaules sur l'archipel. ........................................................................... 12 TABLE 5.1 Résumé: L'évolution convergente des formes de croissance est un phénomène fondamental reliant l'écologie et l'évolution des plantes. Remarquablement illustré dans plusieurs systèmes insulaires, ce phénomène n'a jamais été clairement identifié en Nouvelle-Calédonie, pourtant connue pour la richesse et l'originalité de sa flore. Par une approche combinant architecture des plantes, traits fonctionnels, taxonomie, phylogénie et données environnementales, cette thèse analyse l'histoire évolutive de la monocaulie, une forme de croissance mal connue, en Nouvelle-Calédonie. Les monocaules sont des plantes autoportantes ligneuses dont les fonctions majeures sont assurées par une seule tige apparente. En Nouvelle-Calédonie, elles sont représentées par 182 espèces dicotylédones appartenant à 41 genres et 30 familles et sont souvent menacées d'extinction. L'évolution répétée de la monocaulie en Nouvelle-Calédonie, issue d'au moins 31 événements d'apparition, est l'un des cas les plus remarquables de convergence en milieu insulaire. Dans le genre Atractocarpus, la monocaulie est apparue récemment deux à trois fois via diverses réductions des branches en inflorescences, montrant l'importance des processus hétérochroniques dans l'évolution des formes de croissance. La monocaulie est fortement corrélée à plusieurs traits démontrant des contraintes majeures dans la coordination fonctionnelle. L'évolution de la monocaulie est fortement associée aux forêts denses humides et au substrat ultramafique, et pourrait avoir contribué à la diversification des lignées par des phénomènes de partitionnement de niche. La remarquable convergence de la monocaulie en Nouvelle-Calédonie peut s'expliquer par quatre hypothèses majeures liées (i) à la structure particulière des forêts denses humides (en lien avec les cyclones) favorisant l'exploration unidirectionnelle de l'espace, (ii) aux contraintes édaphiques liées aux substrats ultramafiques induisant une paupérisation architecturale, (iii) à l'absence historique de grands brouteurs, auxquels les monocaules sont particulièrement sensibles, et (iv) à la persistance des forêts denses humides lors des épisodes glaciaires (servant de refuges pour ces espèces sensibles) et leur expansion post-glaciaire (fournissant de nombreuses opportunités écologiques).

List of Figures

List of Tables

Mots-clés :

Architecture des plantes, Convergence évolutive, Forme de croissance, Îles, Phylogénie, Traits fonctionnels Abstract: Convergent evolution in growth habit is a fundamental phenomenon linking plant ecology and evolution. Remarkably illustrated in island biotas, this phenomenon has not clearly been identified in the distinctive and megadiverse New Caledonian biodiversity hotspot. Through an approach combining plant architecture, functional traits, taxonomy, phylogeny and environmental data, this thesis analyses the evolutionary history of the poorly known monocaulous growth habit in New Caledonia. Monocauls are selfsupporting woody plants whose cardinal functions rely on a single visible stem. In New Caledonia, they are represented by 182 dicotyledonous species belonging to 41 genera in 30 families and are often endangered. The repeated evolution of monocauly in New Caledonia, resulting from at least 31 independent events, is one of the most remarkable cases of convergence in insular environments. In the genus Atractocarpus (Rubiaceae), monocauly evolved recently two or three times through branch reductions into inflorescences, emphasizing the importance of heterochronic processes in the evolution of growth habit. Monocauly is strongly correlated with several traits illustrating major constraints in functional coordination. The evolution of monocauly is strongly associated with rainforests and ultramafic substrate, and could have contributed to the diversification of lineages through niche partitioning. The remarkable convergence toward monocauly in New Caledonia can be explained by four major hypotheses: (i) structural features of rainforests (related to cyclone frequency and intensity) favoring unidirectional exploration of space, (ii) the edaphic constraints associated with ultramafic substrates inducing architectural pauperization, (iii) the historical absence of large native browsers to which monocauls are particularly sensitive, and (iv) the persistence of rainforest during glacial episodes -and expansion afterward -that served as refugia and further provided ecological opportunities.

Keywords: Convergent evolution, Functional traits, Growth habit, Islands, Phylogeny, Plant architecture

  UMR AMAP -Centre IRD de Nouméa -B.P. A5 -98848 Nouméa, Nouvelle-Calédonie Contact : david.bruy@ird.fr « C'est l'île aux mille rivières limpides prenant leur source dans des forêts primitives, l'île des falaises et des cimes empanachées de pins colonnaires, l'île des kaoris, des palmiers, des fougères géantes, comme c'est l'île des maquis aux floraisons d'une infinie variété et des niaoulis chatoyant soir et matin aux rayons obliques du soleil. […] mais plus encore, ce sont les détails qui retiennent l'attention, l'architecture étrange d'un arbre, le dessin d'un feuillage, la couleur d'une fleur. On est surpris de la richesse et de la fantaisie de la décoration, car chaque massif a sa flore,

FIGURE 1 . 1 -

 11 FIGURE 1.1 -Illustration of theoretical mechanisms leading to island syndromes. Each form represents a hypothetical attribute and each color represents a hypothetical lineage. High occurrence of an attribute (here ) in the island could be due to (A) establishment of species presenting the trait (dispersion filter and establishment filter), (B) repeated evolution of the trait and/or (C) diversification of lineages presenting the trait.

1

 1 Geography and abiotic environment New Caledonia is a French archipelago located in the Southwest Pacific Ocean slightly north of the Tropic of Capricorn (20-23°S, 164-167°E, Figure 2.1), east of Australia (ca. 1400 km)

FIGURE 2 . 2 -

 22 FIGURE 2.2 -Main environmental characteristics of New Caledonia. (A) Distribution of the three main geological substrate (from DIMENC/SGNC-BRGM and Morat et al. 2001). (B) Distribution of the seven main vegetation types (from Jaffré et al. 2012).

FIGURE 2 . 3 -

 23 FIGURE 2.3 -Illustration of (A) architectural model (from Hallé et al. 1978) and (B) architectural unit for Phyllanthus caudatus var. caudatus (Phyllanthaceae).

FIGURE 2 . 4 -

 24 FIGURE 2.4 -Illustration of the twelve theoretically possible reiteration types for Phyllanthus caudatus var. caudatus combining the three described reiteration modalities. Red entities represent reiterated complexes. Blue cross represent traumatic apices death. Blue arrows represent environmental stimuli (here light). Note that for P. caudatus var. caudatus, only five of the theoretical combination were observed in nature.

FIGURE 2 . 5 -

 25 FIGURE 2.5 -Representation of the two possible modes of trunk construction for monocaulous plants. Pairs of blue segments represent growth rest sites, red crosses represent apex deaths. (A) Bocquillonia species (Euphorbiaceae) exhibit a monoaxial trunk and rhythmic growth, each growth-unit being produced by the same meristem. Growth-rest sites are not sharply bent and only anatomically characterized by slightly narrower and red pith (no pith rupture). (B) Balanops pancheri (Balanopaceae) exhibits a pluriaxial trunk and rhythmic growth, each growth-unit (i.e. module in this case) being produced by a sub-apical axillary meristem of the previous module (sympodial branching). Growth-rest sites are sharply bent and anatomically characterized by pith rupture.

FIGURE 2 . 6 -

 26 FIGURE 2.6 -Example of similar functions carried out by different organs (leaf and twig). (A) Leaf of Agathis lanceolata (Araucariaceae). (B) Cladode (i.e. modified stem) of Semele sp. (Asparagaceae). (C) Compound leaf of Cupaniopsis oedipoda (Sapindaceae). (D) Phyllomorphic branch of Phyllanthus bupleuroides var. ngoyensis (Phyllanthaceae).

FIGURE 2 . 7 -

 27 FIGURE 2.7 -Illustration of the variation in inflorescence complexity. (A) Scaevola coccinea (Goodeniaceae): inflorescence is reduced to a single axillary flower. (B) Tapeinosperma ateouense (Primulaceae): determinate inflorescence supports several small assimilative leaves. (C) Bocquillonia corneri (Euphorbiaceae): annual flowering occurs on contracted axis complexes whose growth is indeterminate.

FIGURE 2 . 8 -

 28 FIGURE 2.8 -Illustration of sequential reiteration (branched species) and opportunistic reiteration (monocaulous species). Pairs of blue traits illustrate the differed character of reiterates, red circles or segments represent inflorescences. (A) Soulamea muelleri (Simaroubaceae): Reiterates appear on all individuals of the species and are regularly clustered on the primary trunk. (B) Bocquillonia sessiliflora (Euphorbiaceae): Reiterates appear very early on some individuals and never on others, their positions on the primary trunk are not predictable.

FIGURE 2 . 9 -

 29 FIGURE 2.9 -Illustration of the three main structural types conforming to the definition of monocauly used in the PhD study. Only vegetative architecture is represented here. (A) Monoaxial architecture exemplified by papaya (Carica papaya, Caricaceae). (B) Sympodial architecture exemplified by several male Cycas (here Cycas seemannii, Cycadaceae). (C) Phyllomorphic-branches architecture exemplified by the "fougère bâtarde" (Phyllanthus mimosoides, Phyllanthaceae).

FIGURE 2 .

 2 FIGURE 2.10 -Satellite map of New Caledonia (from georep.nc) and investigated sites during the thesis (yellow dots).

FIGURE 2 .

 2 FIGURE 2.11 -General method and tools used in each chapter of the PhD study.

FIGURE 3 . 1 -

 31 FIGURE 3.1 -Illustration of the two evolutionary scenarios for monocauly as suggested in literature. (A) Evolution from an herbaceous ancestor through hypermorphosis. (B) Evolution from a branched canopy tree through neoteny (and possibly progenesis).

  are ramified (e.g. close to the Koriba's architectural model, Hallé et al. 1978, Figure 4.1-A) while others such as B. castaneifolia Guillaumin (in Guillaumin & Beauvisage

FIGURE 4 . 1 -

 41 FIGURE 4.1 -Diagram of the different growth habits known for Bocquillonia species at full maturity. (A) Shrub whose architecture is close to the Koriba's architectural model (exemplified by B. rhomboidea). (B) Almost not reiterated shrub conforming to the Corner's architectural model (exemplified by B. castaneifolia). (C) Almost not reiterated tree conforming to the Corner's architectural model (exemplified by B. corneri). (D) Well reiterated tree conforming to the Corner's architectural model (exemplified by B. grandidens). Green cross represents sequential apical death and red points represent inflorescences.

FIGURE 4 . 2 -

 42 FIGURE 4.2 -Pictures of Bocquillonia corneri. (A) Early mature individual in its natural environment. (B) Bark and slash with old staminate inflorescences. (C) Apex and flushing young leaves. (D) Detail of nervation and glands of the abaxial surface. (E) Pistillate inflorescences; (F) Staminate inflorescences.

FIGURE 4 . 3 -

 43 FIGURE 4.3 -Drawing of Bocquillonia corneri. (A) Mature leaf. (B) Detail of venation. (C) Stem apex with stipules. (D) Male flower. (E) Female flower. (F) Fruit. (G) Seed in ventral (left) and dorsal face (right). Illustrator: Ramon L..

FIGURE 4 . 4 -

 44 FIGURE 4.4 -Distribution map of Bocquillonia corneri in the North Province and detail of the distribution of known individuals along Indanou creek. Numbers near the yellow dots represent the number of individuals.

-

  Flowers of Bocquillonia corneri have been observed in March, August and November, and mature fruits have been collected in April and August. Distribution and Ecology -This new species is only known from the Indanou creek, at the base of Col d'Amoss in the northeastern part of Grande Terre, in Province Nord, near Ouégoa village (Figure 4.4), where it occurs in thalweg (gully) forest on volcano-sedimentary substrate, more specifically on micaschists, at around 60-150 meters elevation. Only 56 adult individuals are known but intensive field prospection in the area could potentially locate other individuals.

FIGURE 5 . 1 -

 51 FIGURE 5.1 -Illustration of the fourth monocaulous architectural models. Plain circles represent inflorescences and cross represent apex death. Illustrated taxa (from left to right and top to down): Acropogon aff. austrocaledonicus (Malvaceae); Virotia angustifolia (Proteaceae); Argyroxiphium sandwicense (Asteraceae); Echium pininana (Boraginaceae); Pittosporum paniculatum (Pittosporaceae); Meryta balansae (Araliaceae); Phyllanthus francii (Phyllanthaceae); Casearia silvana (Salicaceae). (A), (C) and (D) from New Caledonia; (B), from Hawaii and California respectively.

Figure 5

 5 terminal (Figure 5.1-B) reproductive structures respectively. Chamberlain's model is characterized by a single trunk build up by several successive determinate modules (sympodial branching, Figure 5.1-

TABLE 5 . 1 -

 51 [START_REF] Potgieter | Phylogenetic Relationships within Apocynaceae s.l. Based on trnL Intron and trnL-F Spacer Sequences and Propagule Characters[END_REF];[START_REF] Meve | Subtribal division of Ceropegieae (Apocynaceae-Asclepiadoideae)[END_REF];[START_REF] Wanntorp | Wax plants disentangled: A phylogeny of Hoya (Marsdenieae, Apocynaceae) inferred from nuclear and chloroplast DNA sequences[END_REF];[START_REF] Endress | Advances in Apocynaceae: the enlightenment, an introduction[END_REF];Livshultz et al. (2007);[START_REF] Simões | Phylogeny and systematics of the Rauvolfioideae (Apocynaceae) based on molecular and morphological evidence[END_REF]; Liede-Schumann et al. (2012); Surveswaran et al. (2014) Araliaceae Plunkett et al. (2005); Nicolas and Plunkett (2009) Bignoniaceae Olmstead et al. (2009) Celastraceae Simmons et al. (2012a); Simmons et al. (2012b) Cunoniaceae Bradford and Barnes (2001); Pillon (2008); Pillon, 'pers. comm.' Elaeocarpaceae Crayn et al. (2006) Ericaceae Kron et al. (2002); Quinn et al. (2005); Wagstaff et al. (2010) Euphorbiaceae Wurdack et al. (2005); Tokuoka (2007); Horn et al. (2012); Costion et al. (2016) Fabaceae Bruneau et al. (2001); Wojciechowski (2003); Wojciechowski et al. (2004); Stefanović et al. (2009); Brown et al. (2012); Manzanilla and Bruneau (2012); Gagnon et al. (2013); Egan et al. (2016); Jabbour et al. List of phylogenetic studies used to build the genus supertree. . (2007); Snow et al. (2011); Thornhill et al. (2015); Vasconcelos, 'pers. comm.' Oleaceae Costion (2011); Guo et al. (2011); Hong-Wa and Besnard (2013) Paracryphiaceae Tank and Donoghue (2010); Soltis et al. (2011); APG IV (2016) Phyllanthaceae Wurdack et al. (2004) Picrodendraceae Tokuoka and Tobe (2006) Primulaceae Anderberg et al. (2002); CEC Gemmill, Waikato University, New Zealand, unpubl. res. Proteaceae Sauquet et al. (2008); Reyes et al. (2015) Rhamnaceae Richardson et al. (2000); Hopkins et al. (2015) Rhizophoraceae Setoguchi et al. (1999) Rubiaceae Achille et al. (2006); Razafimandimbison et al. (2008); Bremer and Eriksson (2009); Rydin et al. (2009); Manns and Bremer (2010); Barrabé et al. (2011); Kainulainen et al. (2013); Wikström et al. (2013); Mouly et al. (2014); Neupane et al. (2015); Wikström et al. (2015) Rutaceae Appelhans et al. (2012); Bayly et al. (2013); Appelhans et al. . (2013); Kim et al. (2015)

  under the older assimilative leaf Not cauliflorous Flowers appear only apically or on the leafy stem portion

FIGURE 5 . 2 -

 52 FIGURE 5.2 -Phylogenetic supertree of New Caledonian woody families. Red branches represent families containing monocaulous species (number of monocaulous species in brackets).

FIGURE 5 . 3 -

 53 FIGURE 5.3 -Number of monocaulous species for the 41 monocaulous genera and per architectural models. Red forms indicate the observed number of monocaulous species in each genera and grey boxplots represent the expected distribution of number of monocaulous species following null model. Asterisks show the significance of over-or underrepresentation of monocaulous species in each genera (*** P < 0.001; ** P < 0.01; * P < 0.05).

  Corner's model (100 species in 30 genus, Figure5.3) followed by the Cook's model (47 species in only 3 genera) and the Chamberlain's model (35 species in 10 genera). The Holttum's model does not appear to be represented in the woody flora of New Caledonia. Only Pittosporum and Beauprea expressed monocauly by two different architectural models (Corner's and Chamberlain's models) (Figure5.3).

FIGURE 5 . 4 -

 54 FIGURE 5.4 -Phylogenetic signals for monocauly and the three architectural models across the phylogeny of New Caledonian woody species. Blue histogram (left) represents the expected distribution of D under a Brownian threshold model (strong phylogenetic signal, 1000 permutation in 100 trees). Green histogram (right) represents the expected distribution of D under a phylogenetic randomness model (low phylogenetic signal, 1000 permutation in 100 trees). Red line represents the observed phylogenetic D statistic.

FIGURE 5 . 5 -

 55 FIGURE 5.5 -Proportion of monocaulous and branched woody species for eight life history and environmental traits.

FIGURE 5 . 6 -

 56 FIGURE 5.6 -IUCN extinction risk of evaluated monocaulous species (n = 63). CR = "Critically endangered"; EN = "Endangered"; VU = "Vulnerable"; NT = "Near Threatened"; LC = "Least Concerned"; DD = "Data Deficient".

FIGURE 5 . 7 -

 57 FIGURE 5.7 -Illustration of marked rhythmic growth in Tapeinosperma gracile (Primulaceae)

FIGURE 6 . 1 -

 61 FIGURE 6.1 -Photographs of different Atractocarpus species in their environment showing variability in growth habit and lateral axis. Monocaulous species: (A) A. confertus, (B) A. bracteatus, (C) A. bracteatus. Intermediate species: (D) A. ngoyensis, (E) A. ngoyensis, (F) A. ngoyensis. Branched species: (G) A. pseudoterminalis, (H) A. sp. nov. 10, (I) A. pseudoterminalis.

FIGURE 6 . 2 -

 62 FIGURE 6.2 -Branching indexes calculated on the base of (A) exploration function, (B) photosynthetic function and (C) the combination of both, for 25 Atractocarpus species.

FIGURE 6 . 3 -

 63 FIGURE 6.3 -Schematic representation of the three main architectural classes found in the rainforest understory species of Atractocarpus: (A) Monocaulous, (B) Intermediate (C) Branched.

FIGURE 6 . 4 -

 64 FIGURE 6.4 -Photographs of branches with their axillary leaves (i.e. from trunk) for height Atractocarpus species with different branching degrees. Arrows indicate apical death, i.e. flowering sites (for A. longistipitatus (E), only half of apical death has been represented). Branched species: (A) A. pseudoterminalis, (B) A. sp. nov. 4. Intermediate species: (C) A. ngoyensis, (D) A. brandzeanus. Monocaulous species: (E) A. longistipitatus, (F) A. pterocarpon, (G) A. bracteatus, (H) A. confertus.

FIGURE 6 . 5 -

 65 FIGURE 6.5 -Projection of (A) species and (B) traits on the two first axis of Principal Component Analysis (seeTable 6.1 for

FIGURE 6 . 6 - 1

 661 FIGURE 6.6 -Estimation of ancestral architectural class in the genus Atractocarpus. Numbers correspond to the Bayesian probability for each reconstituted node. Grey box highlights New Caledonian clade. Letters (A, B) highlight major clades were shifts from branched to monocaulous or intermediate architecture operate.

  The branched architectural class appears to be ancestral in Atractocarpus and our results indicated at least two independent origins of monocauly in New Caledonia. Other examples of derived monocauly have recently been revealed in flowering plants[START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF][START_REF] Barrabé | Changing ecological opportunities facilitated the explosive diversification of New Caledonian Oxera (Lamiaceae)[END_REF]. The intermediate architectural class is evolutionary closer to the monocaulous habit since intermediate and monocaulous species are always sister taxa or very close relatives. There is a high probability that the putative ancestor of "monocaulous clade B", had an intermediate architecture. This insight, as well as the architectural gradient observed among extant species, suggests a gradual evolutionary transition from the branched architecture toward monocauly through an intermediate architectural class. Reversion from a monocaulous architecture toward an intermediate one appears to have been possible (e.g. Atractocarpus sp. nov. 7) but full reversion to a branched architecture was not observed.

"

  monocaulous clade A", reduction in the branching index occurred through a reduction in the number of modules, while in "monocaulous clade B", module length is involved. Architectural analysis actually showed that the two species with intermediate architecture and the two monocaulous species belonging to the clade B are those with the higher number of modules (up to 46 for the intermediate A. sessilifolius and 26 the for monocaulous A. longistipitatus, i.e. as much as certain branched species) and with shorter module length (excluding the monocaulous A.

FIGURE 7 . 1 -

 71 FIGURE 7.1 -Map of New Caledonia and location of study sites in the South of main island. Layer of Ultramafic substrate from DIMENC/SGNC-BRGM (2010). Layer of rainforest from Birnbaum et al. (unpublished data).

FIGURE 7 . 2 -

 72 FIGURE 7.2 -Summary of sampling procedure and trait measurement protocol. DMC: Dry Matter Content, POM: Point of Measurement, SLA: Specific Leaf Area.

FIGURE 7 . 3 -

 73 FIGURE 7.3 -Comparison of micro-anatomy (left) and macro-anatomy (right) between monocaulous and branched species in five of the ten studied genera. (A) Atractocarpus, (B) Bocquillonia, (C) Litsea, (D) Phelline, (E) Tapeinosperma. For macro-anatomical photographs, each bar is 1 mm long.

  phytomer level (Figure 7.4-A), at individual-leaf level (Figure 7.4-B) and at leaf-tuft level (Figure 7.4-C). This observed scaling was not significantly different between growth habit at phytomer (Figure 7.4-A) and leaf-tuft levels (Figure 7.4-C), whether considering slopes (Ancova, P = 0.24 and P = 0.74 respectively) or y-intercepts (Ancova, P = 0.099 and P = 0.94). The scaling was different between growth habit at individual leaf level (Figure 7.4-B), LA increased more rapidlywith Sarea for branched species (Ancova, P = 0.01) that have significantly smaller leaves for a given stem size (lower y-intercept, Ancova, P = 0.007). Total leaf area explained a greater variability of

FIGURE 7 . 4 -

 74 FIGURE 7.4 -Leafstem relationship for monocaulous and branched species considering (A) individual leaf and pith area, (B) individual leaf area and stem area at point of measurement and (C) whole leaf-tuft area and stem area at point of measurement. Green squares correspond to monocaulous species and brown circles to branched species.

FIGURE 7 . 5 -

 75 FIGURE 7.5 -Comparison between monocaulous plants and worldwide plants (TRY database) for (A) hydraulic comportment and (B) photosynthetic comportment. Black lines represent linear regression of branched species with 95% prediction interval in blue. Red points and ellipses represent monocaulous individuals and their 95% prediction ellipses.

FIGURE 7 . 6 -

 76 FIGURE 7.6 -Relationship between stem area and (A) total leaf area or (B) total leaf mass.

FIGURE 7 . 7 -

 77 FIGURE 7.7 -Relationship between total leaf area and (A) pith area, (B) xylem area, (C) cortex area or (D) phloem area. Lowercase letters on graphs indicate the results of Pillai's trace test: regressions with shared letters do not have different slopes following a 0.05 risk of error.

FIGURE 7 . 8 -

 78 FIGURE 7.8 -Relationship between total leaf area and (A) fiber number, (B) vessel number or (C) ray number in wood cross section. Lowercase letters on graphs indicate the results of Pillai's trace test: regressions with shared letters do not have different slopes following a 0.05 risk of error.

FIGURE 8 . 1 -

 81 FIGURE 8.1 -Data of density (A) and frequency (B) for monocaulous plants in nine rainforest plots of 0.048 to 0.124 ha (total size = 0.576 ha). Plots are located in the south-east of the main island on ultramafic substrate from 240 m to 940 m elevation (see Read et al. 2000 for the detailed method).

  than 5 cm DBH and 99.1% thinner than 10 cm DBH. The stem density of monocaulous plants is highly variable (ranging from 417 to 3934 stems per ha) with a mean of 2135 stems per ha (Figure8.1-A). Monocaulous plants account on average for 12.1% (and up to 16.1%) of all stems < 20 cm DBH. The smaller the DBH threshold, the higher the frequency of monocaulous plants with an average of 13.7% (and up to 19.6%) of all stems < 5 cm DBH (Figure8.1-B). Despite the low representability of monocaulous species in the NC-PIPPN database, and the small area sampled by[START_REF] Read | Structural and floristic characteristics of some monodominant and adjacent mixed rainforests in New Caledonia[END_REF] these results suggest that monocaulous plants are an abundant component of rainforest understories.The high incidence of monocaulous species in New Caledonia could be explained by three non-exclusive mechanisms: (i) colonization of many monocaulous taxa, (ii) repeated in situ evolution and (iii) in situ diversification of monocaulous lineages (see Chapter 1 and Figure1.1 for details).

FIGURE 8 . 3 -

 83 FIGURE 8.3 -Functional relationships between trait attributes associated with the monocaulous habit (compared to a branched species of the same size). Bold squares represent fundamental components of Corner's rules. SAM: Shoot Apical Meristem.

FIGURE 8 . 4 -

 84 FIGURE 8.4 -Illustration of change in scale in what is considered the inflorescence for Atractocarpus. (A) In branched species, inflorescences are terminals on each module of the branche. (B) In monocaulous species, inflorescences are axillary on the trunk.

FIGURE 8 . 5 -

 85 FIGURE 8.5 -Relationship between pith area and distance to the basis for five monocaulous species (five individual per species). P represent the p.values for the significance of regressions (*** P < 0.001, ** P < 0.01). Only Myrsine grandifolia (Primulaceae) showed nonsignificant increase of pith area with distance to the base but height data have a more restricted range for this species.

FIGURE 8 . 6 -

 86 FIGURE 8.6 -Relationship between height and basal diameter for monocaulous and branched species (data from chapter 7). Monocaulous species have a significantly lower basal diameter for a given height, leading in higher stem slenderness.

FIGURE 8 . 7 -

 87 FIGURE 8.7 -Relationship between Young modulus and stem diameter for five monocaulous species.

FIGURE 8 . 8 -

 88 FIGURE 8.8 -Illustration of environmental conditions constraining (red arrows) or allowing (blue arrows) the occurrence of large leaves and unbranched stem in rainforest understory and illustration of three evolutionary ways for monocaulous species to colonize open and dry vegetation.

FIGURE 8 . 10 -

 810 FIGURE 8.10 -Illustration of the possible advantage of monocaulous species under forest perturbation. In this high elevation rainforest (Dent de Saint Vincent), only two monocaulous plant and tree ferns escaped the fall of a large tree.

FIGURE 8 . 11 -

 811 FIGURE 8.11 -Illustration of heterochronic processes occurring on axillary structures in Atractocarpus and leading to monocauly.

FIGURE 8 . 12 -

 812 FIGURE 8.12 -Illustration of architectural gradient in (A) Tapeinosperma (Primulaceae): reduction of large branches toward determinate inflorescences (from top left to bottom right), and (B) Oxalis (Oxalidaceae): increasing woodiness and self-supporting while reducing branching (from top left to bottom right).

FIGURE 8 .

 8 FIGURE 8.12 (continuation) -Illustration of architectural gradient in (C) Acropogon (Malvaceae): reducing reiteration frequency and predictability (from top left to bottom right), and (D) Oxera (Lamiaceae): increasing duration of juvenile self-supporting phase and reducing branching (from top left to bottom right).

FIGURE 8 . 13 -

 813 FIGURE 8.13 -Developmental sequence of Dysoxylum aff. roseum, a monocaulous treelet that could become a canopy tree by means of opportunistic reiteration. The fourth stage does not always occurs.

FIGURE 8 . 14 -

 814 FIGURE 8.14 -Illustration of major threats for New Caledonian monocaulous flora. (A) Bushfires. (B) Introduced herbivores: deer that remove large portion of leaves and apical meristems, and rats that eat apical meristem and seeds before maturity. (C) Drought events:

FIGURE 8 . 15 -

 815 FIGURE 8.15 -Examples of monocaulous species from other regions with other evolutionary history and/or habitats. (A) Argyroxiphium sandwicense (Asteraceae), (B) Dendrosenecio keniodendron (Asteraceae), (C) Lobelia deckenii (Campanulaceae), (D) Espeletia killipii (Asteraceae), (E) Lobelia gloria-montis (Campanulaceae), (F) Cyanea shipmanii (Campanulaceae), (G) Plantago princeps (Plantaginaceae), (H) Wilkesia gymnoxiphium (Asteraceae), (I) Biophytum reinwardtii (Oxalidaceae), (J) Brighamia rockii (Campanulaceae), (K) Aeonium urbicum (Crassulaceae), (L) Pachypodium namaquanum (Apocynaceae), (M) Echium pininana (Boraginaceae), (N) Brassica oleracea (Brassicaceae), (O) Melanoselinum decipiens (Apiaceae), (P) Sonchus congestus (Asteraceae).

ØChapitre 4 -Chapitre 5 -

 45 photosynthèse et présentant un point d'abscission, (iii) d'axes ou complexes d'axes hautement

L

  'apparition du caractère plus de 31 fois représente un cas de convergence au moins aussi marquant que celui de la « secondary woodiness » aux îles canaries. D'autant que cette indice sous-évalue probablement le nombre réel d'évolution de la monocaulie, car il se base sur une phylogénie des genres (et non des espèces). Ce phénomène de convergence suggère d'importantes pressions environnementales ayant favorisé l'apparition de la monocaulie en Nouvelle-Calédonie. Les implications écologiques et évolutives de la monocaulie Parmi les 16 caractères supposés être associés à la monocaulie selon la littérature, nous avons confirmé statistiquement la relation pour 8 d'entre eux, trouvé une relation non-significative pour 5 d'entre eux, une relation inverse pour 3 d'entre eux, et mis en évidence une association significative avec 8 nouveaux caractères. Ces caractères sont liés les uns aux autres et à la monocaulie par différentes coordinations fonctionnelles. Les lois de Corner ont été confirmées à plusieurs reprises, sauf en ce qui concerne la relation entre le degré de ramification et la complexité des inflorescences. La relation avec la taille des feuilles implique un compromis entre le nombre de feuille porté (faible chez les monocaules car peu de phytomères) et la taille des feuilles. Ces larges feuilles seraient responsables d'un épaississement distal de la tige, notamment via la proportion de bois, pour répondre aux contraintes mécaniques qui s'exercent sur la partie apicale de la tige. La réponse aux contraintes hydrauliques implique plutôt un élargissement des vaisseaux. La contrainte spatiale liée à la monocaulie (peu de phytomères) serait aussi responsable d'un compromis entre le nombre et la taille des fruits (plus gros chez les monocaules). Les espèces monocaules ont des tiges plus effilées et moins coniques que leurs congénères ramifiés. Ceci serait dû au diamètre distal plus important (augmentation acropète de la taille de la moelle et croissance secondaire plus active) en réponse aux contraintes mécaniques du large toupet de feuille, et au diamètre basal plus faible (croissance secondaire moins active) en réponse à la réduction de la charge statique (et dans une moindre mesure dynamique) résultant de l'absence de branche. Le module de Young (résistance à la flexion) mesuré au cours de ce travail n'est pas très différent de celui renseigné dans la littérature pour les espèces ramifiées. Ces données seront traitées dans une prochaine publication. Outre la taille des feuilles, les valeurs de traits foliaires investigués dans cette étude suggèrent une faible capacité photosynthétique chez les monocaules, bien qu'une étude plus approfondie soit nécessaire. Une publication scientifique en cours de rédaction s'est notamment intéressée à la diversité des traits foliaires dans l'une des familles calédoniennes contenant le plus d'espèces monocaules (les Araliaceae). Les résultats préliminaires montrent un lien entre les traits foliaires et le degré de ramification des individus. D'autres caractères comme des entre-noeuds courts ou la forte croissance secondaire des parties apicales, en combinaison avec les résultats préliminaires d'un suivi phénologique, suggère une croissance très lente des espèces monocaules. L'ensemble de ces caractères suggère une stratégie de conservation de la ressource plus marquée pour les plantes monocaules que pour leurs congénères ramifiés. Ces valeurs de traits particulières des plantes monocaules sont sujettes à de fortes contraintes environnementales. Notamment, leurs très grandes feuilles sont particulièrement sensibles aux températures extrêmes et aux fortes variations environnementales, en raison de leur plus lente thermorégulation et grande surface d'évapotranspiration, ce qui les contraints aux environnements humides, ombragés et non-venteux. Cela est cohérent avec l'occurrence préférentielle des espèces monocaules dans les forêts denses humides en Nouvelle-Calédonie. Néanmoins, certaines espèces monocaules sont présentes en milieux ouverts comme dans le maquis. Celles-ci ont, dans la majorité des cas, des feuilles composées, des rameaux phyllomorphiques ou sont de petite taille, chaque cas représentant un moyen de limiter la surface foliaire individuelle. Finalement, des études montrent que répartir une surface foliaire donnée en peu de grandes feuilles plutôt qu'en beaucoup de petites feuilles est économiquement plus intéressant. La stratégie monocaule est donc susceptible d'être favorisée dans les environnements pauvres en ressources lumineuses et minérales, comme dans le sous-bois et sur substrat ultramafique. Les caractéristiques fonctionnelles et contraintes environnementales associées à la monocaulie nous permettent de poser quatre hypothèses pouvant expliquer la convergence vers cette forme en Nouvelle-Calédonie. (i) Les cyclones sont particulièrement fréquents et intenses dans la région, ce qui est supposé être le facteur principal de la remarquable densité des tiges dans les forêts de Nouvelle-Calédonie. Cette forte densité, ainsi que les débris tombant de la canopée lors des cyclones, est susceptible d'avoir favorisé l'exploration unidirectionnelle de l'espace et donc les espèces monocaules. (ii) La persistance des forêts denses humides sous forme de refuges en Nouvelle-Calédonie lors des épisodes glaciaires aurait permis le maintien des espèces monocaules sur le territoire. Et l'expansion post-glaciaire de ces forêts aurait fourni de nombreuses opportunités écologiques pour ces espèces qui auraient alors subi une diversification importante, comme dans le genre Atractocarpus. (iii) Les contraintes édaphiques du substrat ultramafique auraient favorisé une stratégie plutôt orientée vers l'économie des ressources représentée par les plantes monocaules qui ont une surface foliaire équivalente mais moins coûteuse que les espèces ramifiées. (iv) L'absence de grands herbivores sur le long-terme n'a pas contre-sélectionné les espèces monocaules dont les grandes feuilles et le méristème unique les rendent particulièrement sensible au broutage. La monocaulie est supposé avoir contribué à la diversification des lignées en Nouvelle-Calédonie, non pas à la manière d'une innovation clé (impliquant une importante radiation après apparition du caractère) mais plutôt via le phénomène de partitionnement de niche (exploitation différentielle des ressources entre espèces proches). La monocaulie aurait permis la colonisation de sous-bois particulièrement denses et sombres contribuant à la diversification générale des lignées. Le genre Atractocarpus en est un bon exemple et pourrait représenter un des rares cas de radiation adaptative observés en Nouvelle-Calédonie. L'approche architecturale, un outil clé pour comprendre l'évolution des plantes Notre approche fonctionnelle de la monocaulie, basée sur une hiérarchisation quantifiée des fonctions à travers un indice de ramification, et renforcée par la définition de types structuraux permettant l'utilisation de critères objectifs, s'est révélée être particulièrement appropriée dans le contexte de cette étude. Cette approche mérite d'être testée sur d'autres formes de croissance et dans des études plus globales sur l'évolution des formes de croissance. Dans cette étude, nous avons mis en évidence deux processus non suspectés conduisant à l'évolution de la monocaulie : la réduction des branches en inflorescences par (i) la réduction de la longueur des modules (« nanisme proportionnel ») ou (ii) la réduction du nombre de module (« néoténie »). Nous suggérons que ces phénomènes d'hétérochronie développementale (c'est à dire un changement dans le timing d'un ou plusieurs évènements de la séquence ontogénétique) sont prépondérants dans l'évolution de la monocaulie et plus généralement dans l'évolution des La réitération est définie comme un processus morphogénétique par lequel un organisme duplique son architecture élémentaire, c'est-à-dire son unité architecturale. Plusieurs modalités ont été définies, notamment la réitération séquentielle (génétiquement programmée) qui s'oppose à la réitération opportuniste (dépendante des conditions environnementales). Cependant, notre étude montre que la distinction entre ces deux modalités de réitération n'est pas toujours facile sur le terrain et qu'elles semblent parfois être les deux extrêmes d'un même gradient. Ceci représente la difficulté majeure résidant encore dans notre définition de la monocaulie puisque certaines plantes définies comme monocaules peuvent devenir de grands arbres de canopée. Nous pensons qu'une caractérisation plus fine de la différence entre réitération séquentielle et opportuniste est nécessaire pour mieux comprendre la morphogénèse et phylogénèse des formes de croissance. Conservation de la Flore La Nouvelle-Calédonie est connue pour l'extrême menace qui pèse sur sa flore. Dans cette étude, nous avons montré que la flore monocaule est encore plus menacée d'extinction que la flore ramifiée avec un effet particulièrement dramatique des herbivores introduits. Ceci concerne particulièrement le cerf avec le broutage des feuilles et méristèmes et les rats avec la consommation des fruits et méristèmes. Par ailleurs, les prédictions climatiques suggèrent une augmentation de la fréquence et de la durée des épisodes de sécheresse, qui affecteront en premier lieux les espèces sensibles comme les monocaules. Ainsi, certaines espèces monocaules font probablement partie des espèces les plus menacées par l'augmentation des pressions sur la flore néo-calédonienne. Ceci est particulièrement inquiétant étant donné leur contribution importante aux écosystèmes néocalédoniens en termes de diversité, d'abondance et de fonctions. En dehors de la Nouvelle-Calédonie : des monocaulies plutôt qu'une monocaulie ? Notre étude a montré une certaine cohérence fonctionnelle entre les espèces monocaules étudiées et plus généralement avec les grandes caractéristiques suggérées dans la littérature. Cependant, certaines différences majeures montrent que la monocaulie illustrée dans ce travail de thèse est différente de certaines autres formes de monocaulie, notamment lorsque les espèces ont évolué depuis des ancêtres herbacés et poussent dans des milieux secs ou au sommet des montagnes tropicales. Plus particulièrement au niveau macro-anatomique, ces espèces présentent une moelle et un cortex très large mais un bois en proportion plus fin et parenchymateux. Pour les espèces de milieux secs, ces caractéristiques anatomiques sont des adaptations au stockage de l'eau. Ces espèces sont en général de plus petites tailles et ont des vaisseaux relativement étroits, probablement pour limiter les risques d'embolismes. Pour les espèces des milieux tropicaux alpins, les caractéristiques anatomiques permettent aussi le stockage et la remobilisation rapide de l'eau lorsque celle-ci n'est pas disponible dans le sol en raison du gel. Contrairement aux espèces néocalédoniennes, les feuilles sont le plus souvent étroites, pubescentes et marcescentes, ce qui représenterait des adaptations majeures au froid et à l'insolation. Ainsi notre étude a permis la caractérisation de la monocaulie de sous-bois, qui est probablement la plus commune, mais des espèces fonctionnellement très différentes correspondent aussi à notre définition de la monocaulie. Cela demande de nouvelle étude combinant traits fonctionnels, architecture et phylogénie pour comprendre l'importance relative des différentes pressions environnementales et des prérequis morphologiques conduisant à l'évolution de la monocaulie. Conclusion et perspectives Dans ce travail de thèse, nous avons utilisé une approche transversale combinant architecture végétale, taxonomie, phylogénie et écologie fonctionnelle pour caractériser la monocaulie et illustrer son histoire évolutive en Nouvelle-Calédonie. Notre approche originale de la forme de croissance a permis la mise au point d'une définition de la monocaulie adaptée dans le cadre écologique et évolutif qui était le nôtre. Cette définition doit cependant être appliquée dans d'autres contextes pour pouvoir être affinée. Notamment, le processus de réitération et son implication dans la morphogénèse des plantes a besoin d'être mieux appréhendé. Le genre Acropogon semble être particulièrement adapté pour investiguer cela. À partir de cette définition, nous avons montré que la monocaulie représentait 5,5% de la flore vasculaire néo-calédonienne et qu'elle était apparue plus de 31 fois, illustrant ainsi l'un des cas les plus marquants de convergence évolutive en milieu insulaire. Nous avons identifié quatre hypothèses majeures expliquant l'importance de ce phénomène en Nouvelle-Calédonie : la fréquence et l'intensité des cyclones, la persistance de la monocaulie dans des refuges forestiers au cours des dernières glaciations et sa diversification lors des expansions postglaciaires, l'importance des substrats ultramafiques sur le territoire, et l'absence de grands herbivores autochtones. Des données sur les espèces monocaules dans les flores affines permettraient de tester certaines de ces hypothèses. L'hypothèse concernant les épisodes glaciaires pourrait être testée plus facilement en regardant les dates d'apparition et périodes de diversification des clades monocaules à travers plusieurs phylogénies datées. Notre étude suggère que, en Nouvelle-Calédonie, la monocaulie a principalement évolué à partir d'ancêtres ligneux via des processus hétérochroniques et que son apparition a pu contribuer à la diversification des lignées à travers le partitionnement des niches. Ces hypothèses doivent encore être testées en prenant en compte des données environnementales. Le genre Bocquillonia semble particulièrement prometteur pour cela. La monocaulie en Nouvelle-Calédonie est caractérisée par un ensemble de caractères fonctionnellement reliés les uns aux autres. Ces caractères suggèrent une stratégie de conservation des ressources adaptée mais contrainte aux sous-bois des forêts denses humides où la monocaulie a préférentiellement évoluée, plus particulièrement sur substrat ultramafique. Néanmoins, nous avons encore beaucoup à apprendre sur les caractéristiques fonctionnelles des espèces monocaules et cela demande plus d'études comparatives dans différents groupes phylogénétiques et différents environnements. Nous avons effectivement montré que les espèces monocaules étaient plus que de simples curiosités botaniques et que leur étude pouvait apporter des éléments forts intéressants pour comprendre l'écologie et l'évolution des plantes.

FIGURE 1 . 1 -

 11 FIGURE 1.1 -Illustration of theoretical mechanisms leading to island syndromes ......................

FIGURE 1 . 2 -

 12 FIGURE 1.2 -Schematic illustration of Corner's rules .................................................................

FIGURE 2 . 1 -

 21 FIGURE 2.1 -Topographical map of New Caledonia .................................................................

FIGURE 2 . 2 -

 22 FIGURE 2.2 -Main environmental characteristics of New Caledonia ........................................

FIGURE 2 . 3 -

 23 FIGURE 2.3 -Illustration of the concepts of architectural model and architectural unit ............

FIGURE 2 . 4 -

 24 FIGURE 2.4 -Illustration of the concept of reiteration ...............................................................

FIGURE 2 . 5 -

 25 FIGURE 2.5 -Representation of the two possible modes of trunk construction for monocaulous

FIGURE 2 . 6 -

 26 FIGURE 2.6 -Example of similar functions carried out by different organs (leaf and twig) ......

FIGURE 2 . 7 -

 27 FIGURE 2.7 -Illustration of the variation in inflorescence complexity ......................................

FIGURE 2 . 8 -

 28 FIGURE 2.8 -Illustration of sequential and opportunistic reiteration .........................................

FIGURE 2 . 9 -

 29 FIGURE 2.9 -Illustration of the three main structural types conforming to the definition of

FIGURE 2 . 10 -

 210 FIGURE 2.10 -Investigated sites .................................................................................................

FIGURE 2 . 11 -

 211 FIGURE 2.11 -General method and tools ...................................................................................

FIGURE 3 . 1 -

 31 FIGURE 3.1 -Illustration of the two evolutionary pathways for monocauly as suggested in

FIGURE 3 . 2 - 4 FIGURE 4 . 1 -

 32441 FIGURE 3.2 -Synthesis of life history trait characters suggested to be related with monocauly in

FIGURE 4 . 2 -

 42 FIGURE 4.2 -Pictures of Bocquillonia corneri........................................................................... 5

FIGURE 4 . 4 -FIGURE 5 . 1 -FIGURE 5 . 2 -FIGURE 5 . 3 -FIGURE 5 . 4 -FIGURE 5 . 5 -

 445152535455 FIGURE 4.4 -Distribution map of Bocquillonia corneri ............................................................ 58

FIGURE 5 . 6 -FIGURE 5 . 7 -FIGURE 6 . 1 -FIGURE 6 . 2 -FIGURE 6 . 3 -

 5657616263 FIGURE 5.6 -IUCN extinction risk of evaluated monocaulous species ..................................... 9

FIGURE 6 . 4 -FIGURE 6 . 5 -FIGURE 6 . 6 -FIGURE 7 . 1 -FIGURE 7 . 2 -FIGURE 7 . 3 -FIGURE 7 . 5 -FIGURE 7 . 6 -FIGURE 7 . 7 -FIGURE 7 . 8 - 1 FIGURE 8 . 1 - 1 FIGURE 8 . 2 -FIGURE 8 . 3 -FIGURE 8 . 4 - 1 FIGURE 8 . 5 - 1 FIGURE 8 . 6 -FIGURE 8 . 7 -FIGURE 8 . 8 -

 6465667172737576777818118283841851868788 FIGURE 6.4 -Photographs of branches for eight Atractocarpus species .................................. 12

FIGURE 8 .

 8 FIGURE 8.10 -Illustration of the possible advantage of monocaulous species under forest

FIGURE 8 . 11 - 2 FIGURE 8

 81128 FIGURE 8.11 -Illustration of heterochronic processes occurring in the evolution of architecture

-TABLE 5 . 2 -TABLE 5 . 4 - 1 TABLE 6 . 5 -TABLE 7 . 2 -

 525416572 List of phylogenetic studies used to build the genus supertree .............................. 72 List of taxonomic studies used to build genus supertree ........................................ TABLE 5.3 -List of characters scored on the 2114 New Caledonian woody species and associated analyses .......................................................................................................................................... Results of correlated evolution between monocauly and other traits ..................... 90 TABLE 6.1 -Functional traits measured for 25 Atractocarpus species ..................................... TABLE 6.2 -Models fitted for trait based diversification analysis ............................................ TABLE 6.3 -Architectural traits of 27 Atractocarpus species .................................................. TABLE 6.4 -Phylogenetic signals of measured traits across the phylogeny of 25 Atractocarpus species .......................................................................................................................................... Results of phylogenetical generalised least square ............................................... TABLE 7.1 -List of sampled taxa, study sites and architectural models ................................... List of measured traits .......................................................................................... TABLE 7.3 -Results of mixed effect anovas ............................................................................. 162

  ..............................................................................................................................................................B. codonostylis -. Stigmas ca. 1 mm long; blades generally chartaceous, with ± marked teeth.......................B. brevipes

	6. Axils of secondary veins often with domatia (tufts of trichomes); male glomerules often tightly
	clustered; tree 9-15 m tall; species from Ile des
	) -NEW CALEDONIA. Ouégoa. Route
	Koumac-Ouégoa, peu avant le Col d'Amoss, 28 November 2008 (fl.), Barrabé & Létocart 788
	[♂] (NOU033871); Ouégoa: Base du col d'Amoss, 100 m, 20°19'40.6"S, 164°25'34"E, 11 August
	2017 (fl.), Bruy, Barrabé & Hattermann 921 [♂] (BRI, K, MO, MPU310861, NOU088196,
	P01156372); Païta: Chez D. et I. Létocart, 1 June 2017 (fl.) [Cultivated plant], Bruy, Barrabé &

  7 7. Apical pulvinus of petiole smaller than the basal one......................................................B. goniorrachis -. Apical pulvinus as large as the basal one..........................................................................................................8 8. Stigmas > 6 mm long, erect; mature male flower-buds > 1 mm in diameter, ovoid or cylindrical; blades often glossy on both faces........................................................................................................B. lucidula -. Stigma ± spreading, ca. 1 mm long; mature male flower-buds 0.5-1 mm in diameter, spherical; abaxial blade faces dull.............................................................................................................................................9 East on schists...........................................................................................................................................B. nervosa West........................................................................B. longipes 11. Petioles absent or without pulvini.................................................................................................................12 -. Leaves clearly petiolate and with pulvini......................................................................................................1312. Petioles < 7 mm long and < 3 mm wide; blades < 24 cm long; monocaulous shrub up to 2 m height; stipules persistent; stigmas erect; species from peridotitic massif of extreme North-West....

	9. Blades 4 times longer than wide, abaxial faces generally glaucous; petiole slightly flattened
	above; species from peridotitic mountains of South, West and North-West.............................B. spicata
	-. Blades 2-3 times longer than wide, abaxial faces green; petioles generally cylindrical..................10
	10. Petioles < 11 cm long; blades 3 times longer than wide, up to 13 cm wide; species from North-
	-. Petioles (10-) 14-28 cm long; blades 2 times longer than wide, the wider ones > 13 cm wide;
	species from peridotitic mountains of North-

corneri, sp. nov. 13

  East.................................................................................................................B. . Longer petioles < 18 mm; shrub up to 3 m height, sequentially well ramified; stipules filiform ...............................................................................................................................................................B. brachypoda -. Longer petioles > 18 mm; shrubs or trees up to 7 m height, well ramified by reiteration; stipules with enlarged bases.................................................................................................................................................14 ..............................................................................................................................................B. sessiliflora

	. castaneifolia
	-. Petioles > 7 mm long and > 3 mm wide; blades > 24 cm long; monocaulous shrubs or trees up to
	8 m height; stipules caducous; stigmas applied against the ovary; species from micaschists of
	extreme North-14. Blades 1.7-2.5 times longer than wide, 10-23 cm wide; petioles 4-16 cm long; stipules shortly
	triangular; inflorescence generally elongated up to 3.5 cm; stigmas laciniate.................B. grandidens
	-. Blades (2.7-) 3-5 times longer than wide, 2-11 cm wide; petioles 1.8-10 cm; stipules subulate;
	inflorescence generally spherical, sometimes slightly elongated (up to 2 cm); stigmas not
	laciniate..

TABLE 5 . 2 -

 52 List of taxonomic studies used to build genus supertree for genera that have never been sequenced.

TABLE 5 . 3 -

 53 List of characters scored on the 2114 New Caledonian woody species and associated analyses. the considered genus are endemic to New Caledonia these datasets was thoroughly reviewed based on our own observations and herbarium material (NOU). IUCN conservation status and threats for each evaluated species (728 in total) were extracted from the IUCN Red List website (IUCN, 2017) and from the local Red List Authority dataset (RLA-NC Flora, unpublished data, on 24 April 2018). Finally, leaf type, position of reproductive structures and growth rhythm were compiled from field observations, herbarium material (NOU), and the literature.

			Marked	Growth rhythm marked with long portion of cataphylls-boring stem
		Growth		alternating with portion of assimilative leaves-boring stem
		rhythms	Not marked	Growth rhythm not marked by an important portion of cataphylls-boring
				stem
		Vegetation	Rainforest	Species only found in rainforest
			Not rainforest	Species not only found in rainforest
		Substrate	Ultramafic	Species only found on ultramafic substrate
			Not ultramafic	Species not only found on ultramafic substrate
		Species	Endemic	Species only found in New Caledonia
		endemism	Autochthonous Species native of New Caledonia but occurring in other countries
			Very threatened	Species evaluated as Extinct, Critically Endangered or Endangered
	Permutation			following IUCN red list criteria
	test	IUCN status	Not very	Species evaluated as Least Concerned, Near Threatened or Vulnerable
			threatened	following IUCN redlist criteria
			Data deficient	Species for which data has not been sufficient to permit an evaluation
				following IUCN red list criteria
			Bushfire	Species for which fire has been considered as threat by IUCN

  , about 34% and 16% of all monocaulous species, followed by Malvales, Sapindales and Ericales, each accounting for ca. 9% of monocaulous species. By contrast, the basally branched lineages of the phylogeny (e.g. Magnoliids) contained very few monocaulous species (ca. 1 %). At shallower phylogenetic level, monocauly was clustered in several families and genera, and was achieved through various architectural models(Figure 5.2, 5.3). The richest family was by far Phyllanthaceae, with 43 species of Phyllanthus expressing the Cook's architectural model,

e , endemic genera. respectivelyfollowed by Araliaceae, with 3 genera and 20 species (Chamberlain's model) and Malvaceae, with 16 species restricted to the endemic genus Acropogon (Corner's model). Most families contained only one genus with monocaulous species, with the exception of Euphorbiaceae, Rubiaceae, Sapindaceae, Proteaceae, Salicaceae, which contained two each, and Araliaceae, Primulaceae and Rutaceae, which contained three each. The six richest genera, Phyllanthus, Acropogon, Bocquilllonia, Meryta, Atractocarpus and Oxera, together contained 52% of all monocaulous

TABLE 5 . 4 -

 54 Results of correlated evolution for binary traits

			Growth on UM	Growth in rainforest	Cauliflory	Marked rhythmic growth	Dioecy	Flower plain	Fruit fleshy	Compound leaf
	p.values		0.029 ± 0.002	<< 0.001	<< 0.001 << 0.001	0.15 ± 0.01	0.916 ± 0.023	0.26 ±0.02	0.131 ± 0.298
	Transition	A+:B-→ A+:B+	0.11 ± 0.002	0.10 ± 0.001	0.23 ± 0.001	0.36 ± 0.006	0.15 ± 0.001	0.11 ± 0.001	0.14 ± 0.002	0.30 ± 0.030
	rates	A-:B+ → A+:B+	1.62 ± 0.013	1.95 ± 0.006	0.01 ± 0.001	0.04 ± 0.044	0.00 ± 0.00	0.06 ± 0.003	0.01 ± 0.02	0.10 ± 0.058

.4)

. This result was corroborated by the proportions of cauliflorous species and those exhibiting rhythmic growth,

Chapter 6 -Evolution of plant architecture, functional diversification and divergent evolution in the genus Atractocarpus (Rubiaceae) for New Caledonia Published article Frontiers in Plant Science 9, 1-17. doi: 10.3389/fpls.2018.01775 D. Bruy 1,2 , T. Hattermann 1,2 , L. Barrabé 3 , A. Mouly 4,5 , D. Barthelemy 1,6 , S. Isnard 1,2 (2018)

1 AMAP, IRD, CIRAD, CNRS, INRA, Université de Montpellier, Montpellier, France 2 AMAP, IRD, Herbier de Nouméa, Nouméa, New Caledonia 3 Endemia, Plant Red List Authority, Nouméa, New Caledonia 4 Laboratoire Chrono-Environnement UMR 6249 CNRS, Université Bourgogne Franche-Comté, Besançon, France 5 Jardin Botanique de la Ville de Besançon et de l'Université de Franche-Comté, Besançon, France 6 CIRAD, UMR AMAP, Montpellier, France Abstract:

TABLE 6 . 1 -

 61 Functional traits measured for 25 Atractocarpus species. References are given to justify the interest of each trait for corresponding functions.

	Trait	Abbr eviati on	Unit	Formula	Related function	References
	Trunk traits
						Alméras et al.
	Slenderness ratio	sr	Ø	!"#$%!&'()&% !"#$%!*#+#"!,(#-'%'.	Geometric feature of buckling	(2004); Niklas et al. (2006);
					resistance	Homeier et
						al. (2010)
					Growth rate,	Pérez-
					drought	Harguindegu
	Specific wood density	swd	g.cm -3	/00,!,.1!-#++ /00,!2.'+&!30"4-'	resistance and mechanical	y et al. (2013); Wright et al.
					reinforcemen	(2007); Olson
					t	et al. (2009)
					Growth rate,	Pérez-
	Specific stem density	ssd	g.cm -3	5%'-!,.1!-#++ 5%'-!2.'+&!30"4-'	capacitance, mechanical reinforcemen	Harguindegu y et al. (2013); Díaz
					t	et al. (2016)
						Vertessy et
					Growth rate,	al. (1995);
	Internode diameter	intern ode_d	cm	67($!89!,(#-'%'. × 7#:!89!,(#-'%'.	mechanical reinforcemen	Schuerger et al. (1997);
					t	Olson et al.
						(2009)
						Weijschedé
	Internode length	intern ode_l	cm	Ø!	Growth rate, leaf spacing	et al. (2007); Dong et al.
						(2010)

IN: internode, POM: point of measurement, LF: leaf, BR: branch, TR: trunk

TABLE 6 . 2 -

 62 Models fitted for trait based diversification analysis. Bold lines represent models with lowest AICc.

TABLE 6 . 3 -

 63 Architectural traits of 27 Atractocarpus species.

TABLE 6 . 4 -

 64 Phylogenetic signals of measured traits across the phylogeny of 25Atractocarpus species (24 for fruit volume). Signal were tested with Pagel's Lambda (left) and

	Blomberg's Kappa (right). Bold lines represent significant phylogenetic signal following a
	0.05 risk of error.
	Trait	Lambda	p-value	Kappa	p-value
	Branching index	1.013	< 0.001	1.132	0.002
	Slenderness ratio	0.748	1	0.251	0.539
	Specific wood density	0.447	1	0.429	0.242
	Specific stem density	0.813	0.265	0.607	0.029
	Internode diameter	0.442	0.047	0.533	0.076
	Internode length	0	1	0.127	0.946
	Proportion of pith	0.277	0.427	0.603	0.021
	Proportion of wood	0.595	1	0.399	0.39
	Proportion of cortex	0.488	1	0.418	0.27
	Huber Value	0.589	0.998	< 0.001	0.035
	Total leaf area	0.765	1	0.006	0.165
	Leaf number	1.105	1	0.001	0.157
	Mean leaf area	1.105	0.064	0.031	0.003
	Specific Leaf Area	0	1	0.042	0.405
	Fruit volume	0	1	0.002	0.356

TABLE 6 . 5 -

 65 Results of phylogenetical generalised least square. Bold lines represent significant relation between corresponding trait and branching index following a 0.05 risk of error.

TABLE 7 . 1 -

 71 List of sampled taxa, study sites and architectural models. Asterisks indicate monocaulous architectural models.

	Taxon	Family	Voucher	Study site	Architectural model
	Atractocarpus bracteatus Schltr. & K.Krause Atractocarpus ngoyensis (Schltr.) Mouly	Rubiaceae	Bruy 133 Bruy 139	Plaine des lacs Plaine des lacs	Corner* Stone
	Melicope lasioneura (Baill.) Baill. ex Guillaumin Melicope glaberrima Guillaumin	Rutaceae	Bruy 137 Bruy 597	Plaine des lacs Koghis	Corner* Rauh
	Litsea ripidion Guillaumin Litsea triflora Guillaumin	Lauraceae	Bruy 610 McPherson 4599 Haute Kuébini Haute Kuébini	Corner* Rauh
	Bocquillonia spicata Baill. Bocquillonia rhomboidea (Schltr.) Airy Shaw	Euphorbiaceae	Bruy 131 Bruy 229	Plaine des lacs Plaine des lacs	Corner* Aff. Koriba
	Cleidion lasiophyllum Pax & K.Hoffm. Cleidion vieillardii var. vieillardii Baill.	Euphorbiaceae	Bruy 169 Bruy 140	Démazures Plaine des lacs	Corner* Rauh
	Phelline comosa Labill. Phelline lucida Vieill. ex Baill.	Phellinaceae	Bruy 604 Bruy 414	Plaine des lacs Koghis	Corner* Rauh
	Tapeinosperma gracile Mez Tapeinosperma robustum Mez	Primulaceae	Bruy 172 Bruy 122	Démazures Koghis	Corner* Rauh
	Balanops pancheri Baill. Balanops vieillardii Baill.	Balanopaceae	Bruy 132 Pillon 610	Plaine des lacs Koghis	Chamberlain* Aff. Koriba
	Pittosporum pronyense Guillaumin Pittosporum deplanchei Brongn.& Gris	Pittosporaceae	Bruy 164 Bruy 607	Plaine des lacs Haute Kuébini	Chamberlain* Leeuwenberg
	Phyllanthus francii Guillaumin Phyllanthus koghiense Guillaumin	Phyllanthaceae	Bruy 128 Bruy 603	Plaine des lacs Koghis	Cook* Roux

were on ferralitic soils (i.e. Ferralsols) overlying peridotite. These soils are characterized by low concentration of P, K, and Ca, imbalanced Mg/Ca ratio, high concentration of potentially bioavailable toxic trace elements (Ni, Mn, and low ion exchange capacity;

[START_REF] Jaffré | The Relationship between Ecological Diversity and Floristic Diversity in New Caledonia[END_REF][START_REF] Huillier | Extractability of nickel and its concentration in cultivated plants in Ni rich ultramafic soils of New Caledonia[END_REF][START_REF] Isnard | How did the ultramafic soils shape the flora of the New Caledonian hotspot?[END_REF]

. These locations were chosen because all taxa are abundant, allowing more reliable comparisons. Elevation ranged from 200 m (Haute Kuébini) to 550 m (Koghis) and mean annual pluviometry from 2200 mm.yr -1 (Koghis) to 3000 mm.yr -1 (Plaine des Lacs)

(Météo-France, 2007)

.

TABLE 7 . 2 -

 72 List of measured traits. References are given to justify the interest of each trait for corresponding functions. n=replicate(s) per individuals

	Trait	Abbrev iation	n	Units	Formula	Related function	References
					Whole plant traits
	Slenderness ratio	SR	1	Ø		Geometric feature of buckling resistance	Alméras et al. (2004); Niklas et al. (2006); Homeier et al. (2010)
	Tapering	T	1 cm.m -1		Geometric feature of buckling resistance	Poorter and Werger (1999); Niklas (2000)
							Penfound
	Huber value	HV	1	Ø		Allocation of wood to leaf area deployed	(1931); Gleason et al. (2012); Pérez-Harguindeguy
							et al. (2013)
					Apical (POM) stem traits
							Vertessy et al.
	Distal stem area	Sarea	1	μm²	Ø	Mechanical reinforcement (geometrical)	(1995); Westoby and Wright (2003); Olson et al.
							(2009)
	Pith area	Parea	1	μm²	Ø	Primary stem diameter	Sinnott (1921); Eggert (1961); Chapter 6
	Stem dry matter content	SDMC 1	Ø		Growth rate, mechanical reinforcement	Shipley and Vu (2002); Pérez-Harguindeguy et al. (2013)
	Specific stem density	SSD 1 g.cm -3		Growth rate, capacitance, mechanical reinforcement	Pérez-Harguindeguy et al. (2013);

  These hypotheses have yet to be tested in other plant groups considering fine environmental data (e.g. forest structure) which was lacking in this study. New Caledonian flora holds an interesting set of young lineages with potentially ongoing diversification, and in which evolutionary process are easy to capture, as shown in Atractocarpus. Bocquillonia, one of the Correlation table (method of Spearman) for 32 morpho-anatomical traits measured on twenty species (Chapter 7).

	richest monocaulous genera that offers remarkable diversity in growth habit and environments, arises as particularly promising to study architectural transitions and their relation to species diversification. New Caledonian monocauls are characterized by a complex set of interrelated traits that point toward a resource conservation strategy suited and constrained for dense, humid and shady understory. Most species actually occurred in rainforest and on ultramafic substrate where they preferentially evolved. Despite this apparent homogeneity in form and function, our study brought only incomplete insights on only one kind of monocauly and therefore calls for further research on different plant groups in various geographical areas and environments. We effectively demonstrated that monocaulous plants are more than anecdotic botanical curiosities and that their Appendices Family Species Architectural model Anacardiaceae Euroschinus aoupiniensis M.Hoff Chamberlain Anacardiaceae Euroschinus rubromarginatus Baker f. Chamberlain Anacardiaceae Euroschinus verrucosus Engl. Chamberlain Araliaceae Meryta balansae Baill. Chamberlain Araliaceae Meryta coriacea Pancher ex Baill. Chamberlain Araliaceae Meryta heleneae Lowry, ined. Chamberlain Araliaceae Meryta koniamboensis Lowry & F.Tronchet, ined. Chamberlain Araliaceae Meryta lecardii (R.Vig.) Lowry & F.Tronchet, ined. Chamberlain Araliaceae Meryta oxylaena Baill. Chamberlain Araliaceae Meryta pachycarpa Baill. Chamberlain Araliaceae Meryta schizolaena Baill. Chamberlain Araliaceae Meryta sonchifolia (Linden) Linden & André Chamberlain Araliaceae Plerandra calcicola Lowry & G.M.Plunkett, ined. Chamberlain Araliaceae Plerandra leptophylla (Veitch ex T.Moore) Lowry, G.M.Plunkett & Frodin Chamberlain Araliaceae Plerandra letocartiorum Lowry & G.M.Plunkett, ined. Chamberlain Araliaceae Plerandra osyana (Veitch ex Regel) Lowry, G.M.Plunkett & Frodin Chamberlain Araliaceae Plerandra pouemboutensis Lowry & G.M.Plunkett, ined. Chamberlain Araliaceae Polyscias balansae (Baill.) Harms Chamberlain Araliaceae G.M.Plunkett, ined. Araliaceae Polyscias mackeei Lowry & G.M.Plunkett Araliaceae Polyscias munzingeri Lowry & G.M.Plunkett, ined. Araliaceae Polyscias otopyrena (Baill.) Lowry & G.M.Plunkett Araliaceae Polyscias pancheri (Baill.) Harms Argophyllaceae Argophyllum acinetochromum Guillaumin Argophyllaceae Argophyllum ellipticum Labill. Balanopaceae Balanops pancheri Baill. Celastraceae Salaciopsis megaphylla (J.Poiss. ex Guillaumin) Loes. Cunoniaceae Cunonia macrophylla Brongn. & Gris Cunoniaceae Cunonia schinziana Däniker Elaeocarpaceae Sloanea billardierei (Vieill.) A.C.Sm. Elaeocarpaceae Sloanea magnifolia Tirel Corner Malvaceae Acropogon pilosus Morat & Chalopin Corner Phyllanthaceae Phyllanthus margaretae M.Schmid Cook Corner Malvaceae Acropogon paagoumenensis Morat & Chalopin Corner Phyllanthaceae Phyllanthus mangenotii M.Schmid Cook Proteaceae Virotia rousselii (Vieill.) P.H.Weston & A.R.Mast Corner Corner Malvaceae Acropogon moratianus Callm., Munzinger & Lowry Corner Phyllanthaceae Phyllanthus macrochorion Baill. Cook Proteaceae Virotia angustifolia (Virot) P.H.Weston & A.R.Mast Corner Corner Malvaceae Acropogon merytifolius Morat & Chalopin Corner Phyllanthaceae Phyllanthus loranthoides Baill. Cook Proteaceae Beauprea penariensis Guillaumin Corner Corner Malvaceae Morat Corner Phyllanthaceae Phyllanthus ligustrifolius S. Moore Cook Proteaceae Beauprea filipes Schltr. Chamberlain Chamberlain Acropogon megaphyllus (Bureau & J.Poiss. ex Guillaumin) Phyllanthaceae Phyllanthus jaffrei M.Schmid Cook Proteaceae Beauprea balansae Brongn. & Gris Chamberlain Corner Malvaceae Acropogon jaffrei Morat & Chalopin Corner Phyllanthaceae Phyllanthus guillauminii Däniker Cook Primulaceae Tapeinosperma sessilifolium Mez Corner Corner Malvaceae Acropogon grandiflorus Morat & Chalopin Corner Phyllanthaceae Phyllanthus golonensis M.Schmid Cook Primulaceae Tapeinosperma grandiflorum Guillaumin Corner Chamberlain Malvaceae Acropogon francii (Guillaumin) Morat Corner Phyllanthaceae Phyllanthus francii Guillaumin Cook Primulaceae Tapeinosperma gracile Mez Corner Chamberlain Malvaceae Acropogon fatsioides Schltr. Corner Phyllanthaceae Phyllanthus favieri M.Schmid Cook Primulaceae Tapeinosperma ellipticum Mez Corner Chamberlain Malvaceae Acropogon domatifer Morat Corner Phyllanthaceae Phyllanthus dracunculoides Baill. Cook Primulaceae Tapeinosperma canalense Guillaumin Corner 280 Chamberlain Malvaceae Acropogon chalopiniae Morat Corner Phyllanthaceae Phyllanthus dorotheae M.Schmid Cook Primulaceae Tapeinosperma ateouense M.Schmid Corner Chamberlain Malvaceae Acropogon calcicolus Morat & Chalopin Corner Phyllanthaceae Phyllanthus conjugatus M.Schmid Cook Primulaceae Tapeinosperma amplexicaule Mez Corner Polyscias calophylla Guillaumin ex Lowry & Appendix 1 -List of monocaulous species for New Caledonia. Elaeocarpaceae Sloanea montana (Labill.) A.C.Sm. Corner Elaeocarpaceae Sloanea raynaliana Tirel Corner Euphorbiaceae Bocquillonia castaneifolia Guillaumin Corner Euphorbiaceae Bocquillonia corneri Bruy, Barrabé & Munzinger, ined. Corner Euphorbiaceae Bocquillonia goniorrhachis AiryShaw Corner Euphorbiaceae Bocquillonia grandidens Baill. Corner Euphorbiaceae Bocquillonia longipes McPherson Corner Euphorbiaceae Bocquillonia lucidula AiryShaw Corner Euphorbiaceae Bocquillonia nervosa AiryShaw Corner Euphorbiaceae Bocquillonia phenacostigma AiryShaw Corner Euphorbiaceae Bocquillonia sessiliflora Baill. Corner Euphorbiaceae Bocquillonia spicata Baill. Corner Euphorbiaceae Cleidion lasiophyllum Pax & K.Hoffm. Corner Euphorbiaceae Cleidion macrophyllum Baill. Corner Fabaceae Sophora jabandao Montrouz. Chamberlain Goodeniaceae Scaevola beckii Zahlbr. Corner Lamiaceae Oxera baladica Vieill. Corner Lamiaceae Oxera comptonii S.Moore Corner Lamiaceae Oxera doubetiae Gateblé, ined. Corner Lamiaceae Oxera garoense Gateblé, ined. Corner Lamiaceae Oxera ounemoa Gateblé, ined. Corner Lamiaceae Oxera papineaui Gateblé, ined. Corner Lamiaceae Oxera rugosa Guillaumin Corner Lamiaceae Oxera sessilifolia Dubard Corner Lauraceae Litsea ripidion Guillaumin Corner Linaceae Hugonia racemosa Schltr. Corner Malvaceae Acropogon austrocaledonicus (Hook.f.) Morat Corner Malvaceae Acropogon bosseri Morat & Chalopin Corner Malvaceae Acropogon sageniifolius Schltr. Corner Malvaceae Acropogon schumannianus Schltr. Corner Meliaceae Dysoxylum kouiriense Virot Corner Meliaceae Dysoxylum macranthum C.DC. Corner Meliaceae Dysoxylum macrostachyum C.DC. Corner Meliaceae Dysoxylum roseum C.DC. Corner Meliaceae Dysoxylum rufescens Vieill. ex Pancher & Sebert Corner Moraceae Ficus asperula Bureau Corner Moraceae Ficus auriculigera Bureau Corner Moraceae Ficus otophoroides Corner Corner Myodocarpaceae Delarbrea longicarpa R.Vig. Chamberlain Myodocarpaceae Delarbrea montana R. Vig. Chamberlain Myodocarpaceae Delarbrea paradoxa Vieill. Chamberlain Myrtaceae Syzygium acre (Pancher ex Guillaumin) J.W.Dawson Corner Myrtaceae Syzygium toninense (Baker f.) J.W.Dawson Corner Oxalidaceae Oxalis balansae Guillaumin Corner Phellinaceae Phelline comosa Labill. Corner Phellinaceae Phelline dumbeensis Guillaumin Corner Phyllanthaceae Phyllanthus aoupinieensis M.Schmid Cook Phyllanthaceae Phyllanthus artensis M.Schmid Cook Phyllanthaceae Phyllanthus baladensis Baill. Cook Phyllanthaceae Phyllanthus baraouaensis M.Schmid Cook Phyllanthaceae Phyllanthus boguenensis M.Schmid Cook Phyllanthaceae Phyllanthus bupleuroides Baill. Cook Phyllanthaceae Phyllanthus carlottae M.Schmid Cook Phyllanthaceae Phyllanthus casearoides S.Moore Cook Phyllanthaceae Phyllanthus chamaecerasus Baill. Cook Phyllanthaceae Phyllanthus comptonii S.Moore Cook Phyllanthaceae Phyllanthus moorei M. Schmid Cook Phyllanthaceae Phyllanthus moratii M.Schmid Cook Phyllanthaceae Phyllanthus pancherianus Baill. Cook Phyllanthaceae Phyllanthus parangoyensis M.Schmid Cook Phyllanthaceae Phyllanthus pronyensis Guillaumin Cook Phyllanthaceae Phyllanthus pseudotrichopodus M.Schmid Cook Phyllanthaceae Phyllanthus pterocladus S.Moore Cook Phyllanthaceae Phyllanthus quintuplinervis M.Schmid Cook Phyllanthaceae Phyllanthus salicifolius Baill. Cook Phyllanthaceae Phyllanthus sarasinii Guillaumin Cook Phyllanthaceae Phyllanthus serpentinus S.Moore Cook Phyllanthaceae Phyllanthus sylvincola S.Moore Cook Phyllanthaceae Phyllanthus tireliae M.Schmid Cook Phyllanthaceae Phyllanthus tixieri M.Schmid Cook Phyllanthaceae Phyllanthus torrentium Müll.Arg. Cook Phyllanthaceae Phyllanthus trichopodus Guillaumin Cook Phyllanthaceae Phyllanthus tritepalus M.Schmid Cook Phyllanthaceae Phyllanthus valeriae M.Schmid Cook Phyllanthaceae Phyllanthus veillonii M.Schmid Cook Phyllanthaceae Phyllanthus vespertilio Baill. Cook Pittosporaceae Pittosporum artense Guillaumin Corner Pittosporaceae Pittosporum leratii Guillaumin Corner Pittosporaceae Pittosporum morierei Vieill. ex Guillaumin Chamberlain Pittosporaceae Pittosporum muricatum Tirel & Veillon Chamberlain Pittosporaceae Pittosporum paniculatum Brongn. & Gris Chamberlain Pittosporaceae Pittosporum pronyense Guillaumin Chamberlain Primulaceae Mangenotiella stellata M.Schmid Corner Primulaceae Myrsine grandifolia (S.Moore) Ricketson & Pipoly Corner Taxon Vouche r Site GPS point (DD) Photos ynthet ic BI Expl orati on BI Fruit volum e (cm 3 ) SR Total LA (cm 2 ) Mea n LA (cm 2 ) Leaf num ber IN lengt h (cm) diame IN ter (cm) Atractocarpus sp. nov. 1 Mouly, ined Bruy 964 Dia hou é 164.6 894 -20.48 51 0,92 ± 0,02 0,81 ± 0,08 53,5 ± 31,3 10 1 ± 17 1881 7 ± 4392 154,8 ± 22,3 243, 6 ± 125, 2 5,1 ± 1,13 0,73 ± 0,03 Atractocarpus aragoensis Guillaumin Bruy 615 Pic Vin cent 165.7 737 -21.60 31 0,29 ± 0,19 0,49 ± 0,15 13,6 ± 6,6 12 1 ± 32 1477 ± 1175 84,3 ± 29,1 74 ± 39,2 2,37 ± 0,74 0,36 ± 0,04 Atractocarpus artensis (Montrouz.) Mouly Bruy 922 Cre ek bam bou 164.3 410 -20.46 07 0,82 ± 0,04 0,73 ± 0,14 109,6 ± 13,2 11 0 ± 29 1039 6 ± 6387 164,1 ± 19,9 219, 4 ± 211, 7 6,72 ± 2,99 0,68 ± 0,12 Atractocarpus sp. nov. 2 Mouly, ined Bruy 889 Dog ny 165.8 778 -21.62 17 0,84 ± 0,07 0,67 ± 0,13 11,1 ± 1,6 13 3 ± 32 3770 ± 2558 53,6 ± 12,9 154 ± 89,5 4,03 ± 1,64 0,45 ± 0,05 Atractocarpus baladicus (Montrouz. ex Guillaumin & Beauvis.) Mouly Bruy 650 Tch amb a 165.2 315 -21.01 78 0,01 ± 0,01 0,13 ± 0,07 120,7 ± 61,3 11 7 ± 17 4619 ± 537 305,6 ± 30,9 21,8 ± 4,9 3,91 ± 1,04 0,81 ± 0,11 Atractocarpus bracteatus Schltr. & K.Krause Bruy 133 Plai ne des lacs 166.9 035 -22.27 51 0,02 ± 0,01 0,11 ± 0,03 218,6 ± 53,2 16 4 ± 25 4188 ± 1755 144,5 ± 35,5 42,2 ± 8,2 2,46 ± 0,59 0,62 ± 0,04 Atractocarpus brandzeanus (Baill.) Mouly Bruy 920 Paa gou men e 164.1 931 -20.49 08 0,46 ± 0,19 0,4 ± 0,2 29 ± 9,4 10 6 ± 25 2299 ± 1116 103 ± 30,5 49 ± 33,5 3,32 ± 1,16 0,47 ± 0,06 Atractocarpus sp. nov. 6 Mouly, ined. Bruy 1012 Mo nt Mo u 166.3 349 -22.07 51 0,6 ± 0,11 0,82 ± 0,06 NA 88 ± 23 5252 ± 2014 190,1 ± 30,3 115, 6 ± 39,7 2,64 ± 1,05 0,71 ± 0,05 Atractocarpus sessilifolius Guillaumin Bruy 645 Tch amb a 315 -21.01 78 0,15 ± 0,14 0,53 ± 0,13 6,5 ± 5,6 98 ± 16 1872 ± 738 113,7 ± 36 80,4 ± 25,7 1,99 ± 1,3 0,45 ± 0,08 165.2 Atractocarpus sp. nov. 13 Mouly, ined. Barrabé 1513 Bar autê 165.2 303 -20.97 98 0,89 ± 0,03 0,85 ± 0,04 7,4 ± 0,8 11 4 ± 15 3743 ± 1773 49,9 ± 4,2 181 ± 114, 2 4,57 ± 3 0,47 ± 0,04 Atractocarpus longistipitatus Baill. ex Guillaumin Bruy 612 Mo nts Kog his 165.5 086 -22.17 8507 0,0 ± 0,0 0,3 ± 0,12 15,4 ± 3,2 15 1 ± 39 6417 ± 2650 361,3 ± 110,1 52,2 ± 19,8 1,88 ± 0,7 0,74 ± 0,07 Atractocarpus mollis (Schltr.) Mouly Bruy 683 Pon and ou 165.2 173 -20.81 71 0,89 ± 0,05 0,74 ± 0,08 8,6 ± 1,1 10 2 ± 32 8543 ± 5432 86,3 ± 17,3 191, 4 ± 48 1,89 ± 0,88 0,4 ± 0,04 Atractocarpus pseudoterminalis (Guillaumin) Mouly Bruy 162 Plai ne des lacs 166.9 035 -22.27 51 0,78 ± 0,12 0,76 ± 0,14 13,5 ± 4,1 16 0 ± 13 2933 ± 1461 44,4 ± 7,6 186, 2 ± 138, 1 3,46 ± 1,28 0,32 ± 0,01 Atractocarpus pterocarpon (Guillaumin) Puttock McPher son 3003 Plai ne des lacs 166.9 035 -22.27 Atractocarpus sp. nov. 7 Mouly, ined. Hatterm ann 01 Mo nt Do 166.0 008 -21.75 36 0,48 ± 0,08 0,4 ± 0,11 33,8 ± 17,2 17 7 ± 31 1610 ± 581 63,5 ± 10 46 ± 18,5 3,37 ± 1,77 0,58 ± 0,12 *: P<0.05, **: P<0.01, ***: P<0.001 SD -0.35* 0.36* -0.35* 0.23 0.19 0.20 -0.43** PSR 0.16 -0.05 0.03 -0.17 -0.18 -0.18 0.26 LT 0.43** -0.10 0.04 -0.12 -0.14 -0.06 0.38** 51 0,01 ± 0,01 0,26 ± 0,11 30,4 ± 11,6 12 7 ± 28 2321 ± 701 189 ± 22,5 38,8 ± 21,6 2,57 ± 1,02 0,6 ± 0,05 PL 0.15 0.1 -0.14 -0.47*** -0.46*** -0.24* -0.19 LDMC -0.29** 0.04 -0.01 0.47*** 0.45*** 0.34*** 0.09 Kth 0.63*** -0.26** 0.20* -0.13 -0.16 -0.32** 0.41*** Ks-th 0.85*** -0.42*** 0.34*** -0.13 -0.16 -0.37*** 0.58*** FD -0.55*** 0.27** -0.21* 0.29** 0.28** 0.30** -0.39*** RF -0.16 0.09 -0.09 -0.02 -0.01 0.16 -0.06 VD -0.54*** 0.18 -0.12 0.09 0.10 0.28** -0.32** SLA -0.30** 0.33*** -0.33*** -0.45*** -0.43*** -0.18 -0.40*** *: P<0.05, **: P<0.01, ***: P<0.001 SD -0.26 -0.31* -0.09 0.11 0.50*** -0.24 -0.02 -0.15 SLA (cm² .g -1 ) 75,9 ± 4,1 107, 8 ± 6,7 98 ± 10,9 91,1 ± 6,6 69,7 ± 6,8 60,2 ± 2,8 83,8 ± 14,4 70,6 ± 9,5 67,4 ± 10,8 99,6 ± 15,4 94,3 ± 15,4 159, 3 ± 17,3 88,9 ± 5,5 63,2 ± 12,2 -0.09 -0.01 0.23 76,3 ± 4,3 0.24 -0.20 0.64*** -0.20*** m -3 ) SW D (g.c 0,71 ± 0,11 0,65 ± 0,09 0,7 ± 0,09 0,71 ± 0,05 0,82 ± 0,04 0,77 ± 0,06 0,81 ± 0,04 0,72 ± 0,13 0,8 ± 0,03 0,75 ± 0,06 0,76 ± 0,04 0,82 ± 0,04 0,81 ± 0,03 0,72 ± 0,08 0,75 ± 0,06 -0.54** -0.19 SSD (g.c m -3 ) 0,49 ± 0,04 0,5 ± 0,07 0,49 ± 0,09 0,5 ± 0,05 0,5 ± 0,05 0,56 ± 0,05 0,52 ± 0,08 0,53 ± 0,07 ± 0,03 0,59 0,5 ± 0,06 0,57 ± 0,03 0,45 ± 0,03 0,58 ± 0,04 0,48 ± 0,05 0,5 ± 0,02 0.03 PSR 0.15 0.11 -0.11 0.23 -0.27 -0.06 0.19 -0.21 0.26 -0.26*** LT 0.38** 0.43** 0.18 -0.16 -0.56*** 0.32* -0.14 0.43** 0.40** 0.81*** PL 0.13 0.17 0.16 -0.08 -0.22* -0.01 -0.16 0.01 0.20 Atractocarpus sp. nov. 12 Mouly, ined. Bruy 1042 Nod ela 165.3 531 -21.43 92 NA NA NA N A NA NA NA NA NA NA NA NA Atractocarpus colnettianus (Guillaumin) Mouly Bruy 154 Roc he Oua ièm e 164.8 630 -20.64 01 0,02 ± 0,01 0,15 ± 0,09 156,1 ± 109,9 11 2 ± 39 3791 ± 770 196,2 ± 64,2 32,6 ± 8,4 1,87 ± 0,46 1,09 ± 0,17 41,8 ± 2,7 0,7 ± 0,08 0,43 ± 0,02 Atractocarpus confertus (Guillaumin) Mouly Veillon 5733 De maz ure 166.6 251 -22.19 62 0,01 ± 0,01 0,02 ± 0,01 44,8 ± 8 10 6 ± 18 1639 3 ± 5504 922,5 ± 151,5 24,4 ± 6,1 2,46 ± 0,48 1,3 ± 0,12 64,2 ± 5,6 0,7 ± 0,02 0,48 ± 0,03 Atractocarpus sp. nov. 8 Mouly, ined. Bruy 953 Tch amb a 165.2 315 -21.01 78 0,68 ± 0,06 0,66 ± 0,07 9,4 ± 0,7 11 7 ± 17 5245 ± 3215 120,4 ± 24,3 99 ± 33,3 3,96 ± 0,62 0,45 ± 0,08 139, 9 ± 18 0,77 ± 0,04 0,52 ± 0,05 Atractocarpus heterophyllus (Montrouz.) Guillaumin & Beauvis. Bruy 632 Paa gou men e 164.1 931 -20.49 08 0,12 ± 0,06 0,22 ± 0,09 147,2 ± 36,3 13 1 ± 30 2349 ± 1368 140,3 ± 47,5 30,2 ± 13,8 2,5 ± 0,63 0,59 ± 0,07 54,1 ± 7,4 0,67 ± 0,07 0,55 ± 0,01 Atractocarpus ngoyensis (Schltr.) Mouly Bruy 139 Plai ne des lacs 166.9 035 -22.27 51 0,4 ± 0,13 0,49 ± 0,16 169,4 ± 39,1 14 0 ± 13 6580 ± 2245 228,9 ± 36,9 85 ± 47 3,64 ± 1,63 0,77 ± 0,1 70,7 ± 7,8 0,81 ± 0,04 0,58 ± 0,16 Atractocarpus nigricans (Schltr.) Mouly Bruy 662 Roc he Oua ièm e 164.8 630 -20.64 01 0,86 ± 0,07 0,66 ± 0,16 17,3 ± 4,3 11 5 ± 13 6714 ± 3161 135,3 ± 31,8 79,4 ± 42,6 4,79 ± 3,41 0,63 ± 0,05 92,4 ± 5 0,73 ± 0,11 0,43 ± 0,04 Atractocarpus sp. nov. 4 Mouly, ined. Bruy 929 Man djeli a 164.5 421 -20.40 38 0,69 ± 0,11 0,68 ± 0,06 3,9 ± 0,6 14 4 ± 37 1294 ± 200 36,9 ± 7,5 85,2 ± 33 2,39 ± 1,56 0,35 ± 0,01 82,6 ± 3,7 0,79 ± 0,05 0,51 ± 0,04 Atractocarpus sezitat (Guillaumin) Mouly Bruy 900 Roc he d'A 165.2 422 -21.24 NA NA NA Appendix 3 -LAtotal Parea Pithp Xylemp Phloemp Cortexp HV RT Vd Kth Ks-th FD RF VD SLA LDMC PL LT PSR N A NA NA NA NA NA NA NA NA Sarea SR T SDMC SSD WSD leaf nb LA Parea 0.69*** Ks-th 0.86*** dio 05 Atractocarpus vaginatus Guillaumin Bruy 621 Sarr ame a 165.8 072 -21.66 58 0,09 ± 0,1 0,07 ± 0,03 360,7 ± 110,9 12 8 ± 16 3754 ± 1003 229,7 ± 26,8 24 ± 7,3 6,58 ± 1,13 0,82 ± 0,08 40,7 ± 5,3 0,82 ± 0,15 0,59 ± 0,23 BI: branching index, SR: slenderness ratio, LA: leaf area, IN: internode, SLA: specific leaf area, SWD: specific wood density, SSD: HV % Pith % Wo od Cor % tex ± 0,2 4 ± 0,0 8 0,3 5 ± 0,1 0,4 1 ± 0,0 7 ± 0,2 4 ± 0,0 5 0,3 ± 0,0 5 0,4 6 ± 0,0 1 ± 0,3 ± 0,0 7 0,3 1 ± 0,1 5 0,3 9 ± 0,1 ± 0,2 8 ± 0,0 6 0,3 ± 0,0 7 0,4 0,0 4 ± 0,2 2 ± 0,1 0,4 1 ± 0,1 4 0,3 7 ± 0,0 5 ± 0,1 7 ± 0,0 3 0,3 7 ± 0,0 9 0,4 6 ± 0,0 7 ± 0,2 7 ± 0,0 3 0,2 6 ± 0,0 9 0,4 0,0 8 ± 0,2 6 ± 0,0 6 0,2 5 ± 0,1 2 0,4 9 ± 0,0 8 6 4 6 0,0 0,0 0,0 ± ± 7 ± 3 ± 0,1 0,3 0,5 7 ± ± 74 0,3 4 ± 0,0 7 0,2 6 ± 0,0 9 0,4 1 ± 0,0 9 ± 0,1 5 ± 0,0 5 0,3 8 ± 0,0 8 0,4 7 ± 0,0 7 ± 0,2 5 ± 0,0 8 0,2 4 ± 0,0 5 0,5 1 ± 0,0 5 ± 49 0,2 1 ± 0,0 7 0,3 7 ± 0,1 1 0,4 2 ± 0,0 6 5 9 7 ± 0,2 2 ± 0,0 0,3 3 ± 0,0 0,0 5 ± 0,4 3 5 2 ± 0,1 9 ± 0,0 0,2 5 ± 0,0 0,5 0,0 5 ± -0.16 0.25 0.30* 0.21* 2 ± NA NA NA NA ± 0,1 8 ± 0,0 2 0,3 ± 0,0 1 0,5 2 ± 0,0 3 ± 0,3 3 ± 0,0 3 0,2 8 ± 0,0 3 0,3 9 ± 0,0 3 ± 0,3 ± 0,0 7 0,3 2 ± 0,0 6 0,3 8 ± 0,0 3 ± 0,2 1 ± 0,0 3 0,2 9 ± 0,0 8 0,5 ± 0,0 6 ± 0,2 ± 0,0 6 0,3 8 ± 0,0 6 0,4 2 ± 0,0 4 ± 0,2 9 ± 0,0 6 0,2 ± 0,0 7 0,5 1 ± 0,0 4 ± 0,2 5 ± 0,0 7 0,2 6 ± 0,0 6 Kth NA NA NA NA 335 6 ± 718 0,1 9 ± 0,0 4 0,3 5 ± 0,0 4 0,4 6 ± 0,0 3 SR -0.63*** T 0.54*** Pithp 0.16 0.66*** FD -0.26** -0.41*** -0.99*** SDMC -0.26** 0.11 Xylemp 0.21* -0.34*** -0.64*** RF -0.13 -0.11 0.40*** -0.06 SSD -0.27** 0.11 -0.06 Phloemp -0.24* -0.62*** -0.57*** 0.45*** VD -0.71*** -0.59*** 0.19 0.11 0.97*** WSD -0.48*** 0.27** -0.21* 0.73*** Cortexp -0.25* -0.06 0.00 -0.68*** -0.29** SLA -0.20 -0.30** 0.05 0.10 0.08 0.75*** Leaf nb 0.51*** -0.20* 0.16 0.1 0.07 -0.04 HV -0.29** -0.09 -0.40*** 0.24* 0.08 -0.03 LDMC -0.21* -0.23* 0.34*** 0.35*** 0.18 -0.58*** 0,5 ± 0,0 LA 0.74*** -0.43*** 0.35*** -0.1 -0.12 -0.31** 0.32** RT -0.09 0.16 0.11 -0.35*** -0.36*** 0.40*** 0.13 PL 0.20* 0.17 -0.06 0.06 -0.14 0.35*** -0.42*** specific stem density, HV: Huber value LAtotal 0.82*** -0.39*** 0.31** -0.07 -0.09 -0.26** 0.71*** 0.86*** Vd 0.69*** 0.56*** 0.10 0.22* -0.28** -0.28** -0.04 -0.03 LT 0.36** 0.39** -0.62*** -0.53*** -0.24 -0.30* -0.62*** 0.12 3 Atractocarpus sp. nov. 10 Mouly, ined. Bruy 944 Pwa ala Parea 0.92*** -0.67*** 0.60*** -0.39*** -0.40*** -0.54*** 0.35*** 0.67*** Dm 0.66*** 0.57*** 0.16 0.18 -0.26** -0.31** -0.06 -0.04 0.96*** PSR 0.26 0.26 -0.11 0.03 -0.27 0.26 -0.21 0.41** 0.17 164.5 087 -20.37 50 0,92 ± 0,04 0,85 ± 0,02 11,2 ± 6,6 10 1 ± 16 1092 5 ± 3546 95,5 ± 34 148 ± 60,4 1,96 ± 0,61 0,45 ± 0,05 157, 4 ± 7,6 0,79 ± 0,02 0,53 ± 0,03 ± 0,2 6 ± 0,0 3 0,3 ± 0,0 3 0,4 4 ± 0,0 4 Atractocarpus sp. nov. 3 Mouly ined. Barrabé 699 Aou pini é 165.2 769 -21.17 80 0,87 ± 0,07 0,84 ± 0,06 10,5 ± 3,3 12 2 ± 22 9511 ± 3455 142,2 ± 44,1 140 ± 62,8 5,25 ± 2,39 0,7 ± 0,1 172, 1 ± 36,6 0,68 ± 0,09 0,42 ± 8 2 6 Dm 0.62*** -0.26* 0.19 -0.17 -0.20* -0.34*** 0.38*** 0.64*** LDMC -0.17 -0.29** -0.18 0.12 0.47*** -0.11 0.02 -0.51*** -0.23* -0.19 0,05 0,0 0,1 0,0 Vd 0.63*** -0.24* 0.18 -0.12 -0.15 -0.31** 0.44*** 0.64*** SLA -0.30** -0.24* 0.01 -0.03 -0.08 -0.00 -0.08 0.28** -0.20* -0.18 ± 0,3 4 ± 0,2 7 ± 9 ± RT 0.15 -0.08 0.06 -0.39*** -0.36*** -0.34*** -0.09 -0.06 VD -0.59*** -0.48*** -0.09 -0.30** 0.19 0.39*** -0.09 0.14 -0.70*** -0.71*** 0,3 Pithp 0.35*** -0.42*** 0.40*** -0.48*** -0.47*** -0.32** -0.03 0.24* Xylemp -0.12 0.30** -0.31** 0.46*** 0.44*** 0.29** 0.25* Kth 0.68*** 0.57*** 0.13 0.21* -0.27** -0.30** -0.05 -0.05 0.99*** 0.98*** SD -0.18 -0.27 0.34* 0.09 0.06 -0.27 0.29* -0.26 -0.11 -0.60*** 0.14 Phloemp -0.52*** 0.48*** -0.45*** 0.42*** 0.40*** 0.36*** -0.13 Cortexp -0.05 -0.05 0.06 -0.19 -0.16 -0.07 -0.13 HV 0.02 -0.06 0.06 0.16 0.16 -0.03 -0.25* -0.23* RF 0.02 -0.07 0.19 0.03 0.10 -0.22* -0.36*** -0.47*** -0.19 -0.08 -0.27** FD -0.41*** -0.45*** -0.11 0.30** 0.40*** -0.41*** 0.02 -0.42*** -0.30** -0.23* -0.24* Ks-th 0.88*** 0.72*** 0.14 0.21* -0.32** -0.27** -0.04 -0.01 0.87*** 0.85*** *: P<0.05, **: P<0.01, ***: P<0.001

Remerciements

Acknowledgements:

We thank Laurence Ramon for the illustrations and the curators of NOU, P and MPU, particularly Jacqueline Fambart, for permissions and help with the collections. Thank you to Daniel & Irène Létocart and Jean-Marie Veillon for allowing us to make good observations, photos and collections in their gardens. Thank you to Fabian Carriconde for the use of his binocular microscope and to Gildas Gâteblé for the training in the use of Leica software. We also thank Gordon McPherson for help in translating and useful comments on the manuscript.

Acknowledgments:

We are grateful to all those who helped with field data collection, particularly D. Fleurot and the staff of the Parc Naturel des Grandes Fougères. We also thank S. Lavergne for advice about data analysis and Y. Caraglio for sharing thoughts about our work. Thanks to P. Lowry for the thorough revision of the manuscript. Thanks also to H. Caze, C. Létocart, G. Gateblé and H.

Vandrot for allowing us to use their excellent photographs, and to the NOU and P herbaria for permitting fruit measurements on specimens.

In preparation article

To be submitted to American journal of Botany D. Bruy 1,2 , A. Estarague 1,3 , C. Heinz 1 , J. Bahormal 3,4 , W. Nigote 2,3 , J. Munzinger 3 and S.

Isnard 4 Université de la Nouvelle-Calédonie, Nouméa, Nouvelle Calédonie 2003) which seems higher in large-leaved species [START_REF] Primack | Relationships Among Flowers, Fruits, and Seeds[END_REF]Cornelissen, 1999, Chapter 6).

7.4.3 New Caledonian monocauls, a special case of monocauly?

Despite being achieved through various structural ways (Corner's, Holttum's, Chamberlain's and Cook's architectural models) (Hallé et al., 1978, Chapter 5), the monocaulous habit remains consistent in both physiognomical and functional aspects. We considered a reasonable phylogenetical diversity but only a single habitat (rainforest), where most of the New Caledonian monocaulous taxa occur (89%, Chapter 5 The few published data about monocauls comes from studies about rosette-trees, of which the majority is monocaulous. However, beyond their large leaves and thick stems, their stem anatomy seems very different from our monocaulous species. Rosette-trees present a disproportionate proportion of pith [START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF][START_REF] Carlquist | Island Biology[END_REF]Mabberley, 1974a;[START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Meinzer | Adaptations for water and thermal balance in Andean giant rosette plants[END_REF][START_REF] Mosbrugger | Tree Habit in Land Plants: A functional comparison of trunk constructions with a brief introduction into the biomechanics of trees[END_REF], a thin and highly parenchymatous wood [START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF]Mabberley, 1974a;[START_REF] Aldridge | Anatomy and evolution in the Macaronesian Sonchus subgenus Dendrosonchus (Compositae: Lactuceae)[END_REF] and a large cortex [START_REF] Cotton | Presidential address. The megaphytic habit in the tree Senecios and other genera[END_REF]Mabberley, 1974a;[START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF][START_REF] Mosbrugger | Tree Habit in Land Plants: A functional comparison of trunk constructions with a brief introduction into the biomechanics of trees[END_REF]. These contrasting strategies obviously result from different evolutionary backgrounds and environmental factors. Most previously studied monocaulous species evolve from herbaceous ancestors (e.g. Espeletia for Meinzer & Goldstein, Cyanea for Carlquist, Dendrosenecio for Cotton, Echium for Aldridge) and are adapted to dry [START_REF] Böhle | Island colonization and evolution of the insular woody habit in Echium L. (Boraginaceae)[END_REF][START_REF] Lens | Insular Woodiness on the Canary Islands: A Remarkable Case of Convergent Evolution[END_REF] or frosty [START_REF] Hedberg | Tropical-Alpine Life-Forms of Vascular Plants[END_REF][START_REF] Meinzer | Adaptations for water and thermal balance in Andean giant rosette plants[END_REF][START_REF] Givnish | Convergent Evolution, Adaptive Radiation, and Species Diversification in Plants[END_REF] We tested all of these associations either through taxonomical work in a genera (Chapter 4), correlated evolution in the New Caledonian woody flora (Chapter 5), phylogenetic generalized least square in a genera (Chapter 6) or mixed effect anovas between pairs of co-generic species (Chapter 7). Among the 16 trait characters potentially linked to monocauly, we confirmed association for 8 of them, found no significant relationship for 5 of them and inverse relationship for 3 of them (Figure 8.2). We additionally found eight other features that appeared significantly linked to monocauly while not previously mentioned to our knowledge (Figure 8.2). The suite of trait attributes associated with monocauly appeared to be coordinated by different, sometimes conflicting, functions (Figure 8.3).

Corner's rules -The assertions of [START_REF] Corner | The Durian Theory or the Origin of the Modern Tree[END_REF]Corner ( , 1953Corner ( -1954) ) on the negative relation between branching intensity and (i) leaf size, (ii) stem thickness, (iii) fruit size and (iv) inflorescence complexity were confirmed by our study. (i) The relationship between branching intensity and leaf size involves a coordination between individual leaf area and leaf number since the total leaf area stays constant (Chapter 6); monocauls, being single-stemmed, have less spatial opportunity to insert leaves (less phytomers) than branched plants of the same height and, consequently, deploy larger leaves. (ii) We further showed that leaf sizestem size relationship is evolutionary pathways are constrained by a limited set of functional and developmental possibilities [START_REF] Mcghee | Convergent Evolution: Limited Forms Most Beautiful[END_REF]. We showed that the monocaulous habit is coherent in terms of functional properties but is achieved in different, though limited, structural ways. We recognize three different structural types in New Caledonia in agreement with our functional definition of monocauly (Chapter 2). Some theoretical structural types fitting this definition were however lacking, such as the combination of sympodial trunk and phyllomorphic branches. This restricted diversity of structural types is more obvious for architectural models, of which 23 have been found

in nature among a high number of theoretical forms [START_REF] Hallé | Essai sur l'architecture et la dynamique de croissance des arbres tropicaux[END_REF][START_REF] Hallé | Tropical trees and forests an architectural analysis[END_REF].

Such patterns suggest strong genetic and/or environmental constraints in the evolution of plant architecture [START_REF] Monro | The evolvability of growth form in a clonal seaweed[END_REF]. In the same vein, correlated evolution of some architectural trait attributes seems impossible, reflecting genetic constraints or implying that resulting architecture are maladapted [START_REF] Chomicki | Evolution and ecology of plant architecture: integrating insights from the fossil record, extant morphology, developmental genetics and phylogenies[END_REF]. As such, we found some association between some monocaulous and branched architectural models, even if it needs to be tested with the completeness of architectural data for branched species. Most of genera achieving the Corner's, the Chamberlain and the Cook's models also achieve respectively the Rauh's, the Leeuwenberg's and the Roux's model (Chapter 5). These theoretical preferential evolutionary pathways are coherent with the non-random distribution of monocauly through the phylogeny (Chapter 5), often interpreted as evidence for prerequisites [START_REF] Boucher | Evolution and biogeography of the cushion life form in angiosperms[END_REF]. Atractocarpus for instance, as parts of the Gardenieae, is susceptible to present some evolutionary facilities in the branch reduction.

Actually, the phenomena occurs at least two times in the genus (Chapter 6), but also in related genera such as Sukunia (Chapter 6), Pentagonia, Schumanniophyton, Duroia or Rothmannia [START_REF] Hallé | Etude biologique et morphologique de la tribu des Gardéniées[END_REF]. In Randia and affine genera, which some New Caledonian Atractocarpus was for long considered a part of (Mouly et al., submitted), this variation in branch length was also noted and considered of taxonomical importance [START_REF] Tirvengadum | La signification taxonomique des modes de ramification de Randia et genres affines[END_REF].

thorough examination could bring up innovative insights about plant ecology and evolution.