Carina Hoorn

Allison Perrigo

Alexandre Antonelli

Mountains, Climate and

Keywords: paleo-phytogeography, pollen, mountain uplift, paleoaltitude, Cenozoic, global cooling, climatic cycles

Mountain ecosystems, and in particular European Alpine ecosystems, have a rich biodiversity, as they represent complex associations controlled by elevation, soils and rocks, and climatic conditions following latitude, longitude, slope orientation and aspect. Vegetation belts are organized with respect to altitude according to a concomitant decrease in temperature and increase in precipitation, and are defined by their dominant plant elements. Plant diversity within the Alps also results from these mountains' location at the transition point between cold-temperate and warm-temperate climates, as well as from their historical biogeography. In order to explain how past environmental changes have shaped the modern plant diversity and the organization of vegetation in altitudinal belts, a temporal dimension may be provided by paleovegetation data. Abundant micro-(pollen) and macro-remains (leaves, fruits, seeds) show that European Cenozoic vegetation exhibits a similar latitudinal and altitudinal organization to the vegetation belts observed today in south-eastern China, where most of the taxa -missing in Europe since the late Neogene -now occur. Late Eocene to Pliocene pollen floras of the south-western and Eastern Alps are characterized by the presence of megatherm plants and an abundance of mega-mesotherm plants. These are typical of moist evergreen low-altitude forests, and are characterized by the presence of Cathaya, a conifer now restricted to subtropical China at mid to high elevations. Pollen data indicate the presence of Abies/Picea forests at high altitudes since at least the Oligocene. Since then, several thermophilous taxa have declined, and some have disappeared from the Alps due to natural or humanrelated processes that are responsible for the modern plant diversity pattern there. These processes include the uplift of the mountain range, which began ca. 35 Ma, late Neogene global cooling, Pleistocene glacials/interglacials and more recent human impact.

Introduction

Mountain vegetation is characterized by the altitudinal stratification of plants related to temperature and precipitation. Vegetation zones change from low to high latitude or from low to high altitude in the same way [START_REF] Ozenda | Le déplacement vertical des étages de végétation en fonction de la latitude: un modèle simple et ses limites[END_REF]: the replacement of taxa occurs as vegetation types adapt to colder conditions.

Mountain plant ecosystems generally have high diversity, as they represent complex assemblages influenced by elevation, climatic conditions -which follow latitude and longitude -and variable amounts of rainfall. Moreover, the physiographic complexity of mountains creates environments that can vary over short distances due to, for instance, exposure to sun or slope angle and aspect.

27

The Alps: A Geological, Climatic and Human Perspective on Vegetation History and Modern Plant Diversity Séverine Fauquette 1 , Jean-Pierre Suc 2 , Frédéric Médail 3 , Serge D. Muller 1 , Gonzalo Jiménez-Moreno 4 , Adele Bertini 5 , Edoardo Martinetto 6 , Speranta-Maria Popescu 7 , Zhuo Zheng 8 and Jacques-Louis de Beaulieu 3

In the Alps, vegetation organization is also complex because of the large size of the mountains and their position at the crossroads of many floristic provinces [START_REF] Ozenda | On the genesis of the plant population in the Alps: new or critical aspects[END_REF]. Moreover, the Alpine arc is oriented north-south in France but west-east for the rest of the chain (Figure 27.1a). The French section of the chain is thus characterized by important differences between the southern and the northern parts, with the Southern European Alps being drier, as they are under Mediterranean influences.

Plant diversity within the Alps mainly results from their location at the transition point between coldtemperate and warm-temperate climates, their landscape heterogeneity, the local climate variability that arises from complex topographies and historical biogeography. A detailed knowledge of the past is thus essential to an understanding of the processes that have controlled changes in plant diversity up to the present.

This chapter, which focuses primarily on the Western Alps, starts with a description of the present flora and vegetation patterns in their physiographic and geological context. It then shows how the intense tectonic activity in this region over the last 40 My has influenced the flora and vegetation. Finally, it shows the role of Pleistocene climatic cycles and human impact on the mountains and their flora.

Present Flora and Vegetation

Patterns in the Physiographic, Climatic and Geological Context of the Alps

Plant Distribution and Endemism Patterns

The European Alps are one of the most important hot spots of plant biodiversity in Europe [START_REF] Ozenda | La végétation de la chaîne alpine dans l'espace montagnard européen[END_REF][START_REF] Ozenda | Perspectives pour une géobiologie des montagnes[END_REF], with a high number of endemic taxa. According to [START_REF] Aeschimann | Analyse de la flore des Alpes. 1: historique et biodiversité[END_REF], the Alps encompass 3983 indigenous taxa of vascular plants, which is more than onethird of the total European flora. The average taxonomic richness of the Alps is estimated at about 2200 taxa per 10 000 km 2 , and despite their relatively restricted area, the Southwestern Alps are home to more than 70% of the entire Alpine flora. In the Alps, 501 endemic taxa are registered, making up 4% of the European flora. This corresponds to an endemism rate of 12.6% within the area's native flora.

The distribution of endemics (Figure 27.1a) is mainly concentrated in the southernmost Western Alps and the southern part of the East Eastern and West Eastern Alps [START_REF] Noble | La flore des Alpes-Maritimes et de la Principauté de Monaco. Originalité et diversité. Turriers[END_REF]. At the junction of the Mediterranean Basin and the Alps, the Maritime and Ligurian Alps constitute a zone of geographic overlap between major biotic assemblages [START_REF] Comes | Spatial and temporal patterns in the evolution of the flora of the European Alpine system[END_REF], an important hot spot of plant biodiversity [START_REF] Médail | Hot-Spots analysis for conservation of plant biodiversity in the Mediterranean Basin[END_REF] and also a major refugium (Figure 27.1b) [START_REF] Médail | Glacial refugia influence plant diversity patterns in the Mediterranean Basin[END_REF]. This region, with 25% of the endemic plants of the whole range, represents one of the most important biogeographical areas in Europe, due to the high concentration of paleoendemic and endemic plants [START_REF] Casazza | Ecological and historical factors affecting distribution pattern and richness of endemic plant species: the case of Maritime and Ligurian Alps hotspot[END_REF][START_REF] Noble | La flore des Alpes-Maritimes et de la Principauté de Monaco. Originalité et diversité. Turriers[END_REF]. Recent molecular investigations of endemic plants from the Maritime and Ligurian Alps show that vicariance events are the most important factor explaining the distribution and genetic patterns of plant populations there [START_REF] Casazza | Phylogeography of Primula allionii (Primulaceae), a narrow endemic of the Maritime Alps[END_REF]. The Maritime Alps do not act as a strong barrier to the migration of plants, and moderate gene flow seems to create an admixture between some plant populations, even when separated by deep valleys [START_REF] Diadema | Geographic structure of molecular variation of Gentiana ligustica (Gentianaceae) in the Maritime and Ligurian regional hotspot, inferred from ITS sequences[END_REF]. The persistence of several endemics throughout various glacial periods appears to be linked to the capacity of mountains to provide a wide diversity of microhabitats within a reduced space.

In the Alps and Carpathian Mountains, species richness and genetic diversity do not co-vary for mountain vascular plants [START_REF] Taberlet | Genetic diversity in widespread species is not congruent with species richness in alpine plant communities[END_REF]; rather, genetic diversity is associated with the glacial and postglacial history. In the Alps, genetic rarity (the presence of rare alleles or restricted haplotypes) is correlated with both species rarity (i.e., rare markers tend to be located in the same areas as rare species) and species endemism (i.e., rare makers in widespread species tend to be situated in the same areas as endemic species). Rare markers are often fixed in long-term isolated refugial populations, and these crucial areas correspond to distinct glacial refuges, in which the highest endemism levels also occur [START_REF] Schönswetter | Molecular evidence for glacial refugia of mountain plants in the European Alps[END_REF].

Vegetation

Vegetation belts (also called altitudinal zones) in the Alps are well characterized by bioclimatic factors and their principal vegetation types, especially forests. Vegetation belts are organized altitudinally according to steep ecological gradients, with a concomitant decrease in temperature and increase in precipitation, extending from the lowlands to the nival zone. In the Central Alps, five vegetation belts are distinguished: the collinean, mountain, sub-alpine, alpine and nival belts (Table 27.1).

In the Southwestern Alps, vegetation is similarly organized in altitudinal belts on both flanks of the mountains, but with more pronounced Mediterranean influences occurring on the southern slopes, from the lowest thermo-Mediterranean to the oro-Mediterranean belt, which corresponds to the alpine belt of the Central Alps (Table 27.1). These southern mountains are heterogeneous from a biogeographical viewpoint, with the southern side clearly influenced by Mediterranean elements and the northern side composed of medio-European vegetation [START_REF] Barbero | Végétation culminale du mont Ventoux; sa signification dans une interprétation phytogéographique des Préalpes méridionales[END_REF]. The altitudinal limits of these vegetation belts vary along a latitudinal gradient, but they are ~700 m wide in vertical elevation [START_REF] Ozenda | Le déplacement vertical des étages de végétation en fonction de la latitude: un modèle simple et ses limites[END_REF]. For collinean to sub-alpine species, especially trees, temperature often represents the limiting factor for survival at the upper limit and moisture at the lower limit.

These vegetation belts are well delimited by modern pollen data collected from moss polsters along two transects across the Southwestern Alps (Figure 27.2). In order to reconstruct Cenozoic vegetation and climate information from pollen data, [START_REF] Fauquette | Quantifying the Eocene to Pleistocene topographic evolution of the southwestern Alps, France and Italy[END_REF] established the relationship between modern pollen data and vegetation to serve as a baseline. To create this reference data set, pollen grains were grouped according to vegetation belts. However, Pinus was separated because its pollen is not identifiable at the species level and may be representative of several vegetation belts. Pollen grains of Larix are scarce; samples containing this genus were collected within its forests or from sites immediately below such forests. Pollen grains of Cupressaceae are grouped together because infra-family identification (genus identification) is impossible, even though their species grow at different altitudes in the Southwestern Alps: Cupressus sempervirens, Juniperus oxycedrus and J. phoenicea in the two lowermost Mediterranean vegetation belts; J. thurifera in the higher supra-Mediterranean belt and in the lower part of mountain-Mediterranean belt; and J. communis and J. sabina in the mountain-Mediterranean belt. Most of the herbaceous pollen grains cannot be distinguished at the genus level, whereas such plants can inhabit different altitudes; their abundance may indicate open and/or undergrowth vegetation. Finally, alpine tundra-like plants (Aconogonum alpinum, Gentiana, Centaurea uniflora) were identified in only a few sites.

These west-east transects show that there is a good correlation between pollen data and observed vegetation distributions along altitudinal and longitudinal gradients, and that the vegetation distribution follows a clear asymmetric topography (Figure 27.2): replacement of pollen groups as elevation increases appears progressive on the western flank, but much more abrupt in the east.

Pollen data may therefore be used to reconstruct Paleogene to Quaternary paleovegetation. Moreover, for these periods, information from pollen may be complemented by macrofossils when they are available in the same region. However, these fossils are produced from plants that lived not more than a few kilometers from the fossilization site, and rarely record those growing in the higher belts. To get around this problem, [START_REF] Bertini | Reconstruction of vegetation transects for the Messinian/Piacenzian of Italy by means of comparative analysis of pollen, leaf and carpological records[END_REF] used the presence of pollen and the concomitant absence of macrofossils in several deposits of the same age to pinpoint those plants that probably grew at higher elevations (e.g., Cedrus, Picea, Tsuga).

Vegetation History of the Alps Since the Late Eocene

The Alps resulted from the convergence of the European and African plates, trapping the Apulian microplate between them, which led to the closure of the Tethys Ocean by the Eocene, followed by a collision between the European and Adriatic plates (e.g., [START_REF] Rosenbaum | The Western Alps from the Jurassic to Oligocene: spatio-temporal constraints and evolutionary reconstructions[END_REF]. This complex collision history led to a nappe stack involving basement and cover rocks of the European margin, of the intervening ocean and micro-continents, and of the Adriatic margin.

The intense tectonic activity in this region over the last 40 My has influenced the vegetation and flora, with the appearance of high mountains and new marine basins, and consequent landscape fragmentation, environmental heterogeneities and new climatic patterns.

Vegetation History Before Human Impact

Abundant pollen records (e.g., [START_REF] Suc | Distribution latitudinale et étagement des associations végétales au Cénozoïque supérieur dans l'aire ouest-méditerranéenne[END_REF][START_REF] Suc | Structure of West Mediterranean and climate since 5.3 Ma[END_REF][START_REF] Jiménez-Moreno | High-resolution palynological analysis in late early-middle Miocene core from the Pannonian Basin, Hungary: climatic changes, astronomical forcing and eustatic fluctuations in the Central Paratethys[END_REF][START_REF] Bertini | Pliocene to Pleistocene palynoflora and vegetation in Italy: state of the art[END_REF][START_REF] Bertini | Pliocene to Pleistocene palynoflora and vegetation in Italy: state of the art[END_REF] show that European Neogene vegetation followed belts that were aligned similarly in latitude and altitude to the belts seen today in south-eastern China [START_REF] Wang | The Forests of China with a Survey of Grassland and Desert Vegetation[END_REF], where most of the taxa that disappeared from Europe by the late Neogene are still extant.

The study by [START_REF] Fauquette | Quantifying the Eocene to Pleistocene topographic evolution of the southwestern Alps, France and Italy[END_REF] shows that in the Southwestern Alps, late Eocene to Pliocene pollen floras (Figures 27.3 and 27.4) are characterized by prevalent mega-mesotherm plants (see the definitions of these plant groups in the legend of Figure 27.4), reflecting subtropical conditions, and sometimes high frequencies of Cathaya, a conifer restricted today to subtropical China at mid to high elevations, below the Abies and/or Picea belts [START_REF] Wang | The Forests of China with a Survey of Grassland and Desert Vegetation[END_REF]. Pollen records suggest that the past representatives of Cathaya in Europe probably occupied a similar ecological niche (e.g., [START_REF] Suc | Distribution latitudinale et étagement des associations végétales au Cénozoïque supérieur dans l'aire ouest-méditerranéenne[END_REF][START_REF] Combourieu-Nebout | What was the late Pliocene Mediterranean climate like: a preliminary quantification from vegetation[END_REF], 2015;[START_REF] Biltekin | Anatolia: a long-time plant refuge area documented by pollen records over the last 23 million years[END_REF]. Megatherm plants supporting an interpretation of tropical conditions were present in all of the Eocene to mid-Miocene samples (Figure 27.4). The maximum frequency of such species occurred during the early Langhian (~15 Ma) [START_REF] Jiménez-Moreno | High-resolution palynological analysis in late early-middle Miocene core from the Pannonian Basin, Hungary: climatic changes, astronomical forcing and eustatic fluctuations in the Central Paratethys[END_REF], corresponding to the mid-Miocene Climatic Optimum (MCO). The pollen record of mega-mesotherm plants decreases from the early Zanclean (ca. 5 Ma) to the Piacenzian (3.6-2.6 Ma), and major disappearance events have occurred since the earliest Pleistocene (2.6 Ma), due to the onset of northern hemisphere glaciations [START_REF] Suc | Origin and evolution of the Mediterranean vegetation and climate in Europe[END_REF][START_REF] Bertini | Pliocene to Pleistocene palynoflora and vegetation in Italy: state of the art[END_REF]. Climate gradients linked to the latitude, altitude and physiography of pollen sites explain the different timings of the extinctions of Taxodium-type, Cathaya and Tsuga species, among others (e.g., [START_REF] Suc | Structure of West Mediterranean and climate since 5.3 Ma[END_REF][START_REF] Bertini | Pliocene to Pleistocene palynoflora and vegetation in Italy: state of the art[END_REF].

On the whole, microtherm trees support the presence of boreal conditions (Abies, Picea) and meso-microtherm trees (particularly Cathaya) occur in significantly higher frequencies on the eastern side of the Southwestern Alps than on the western side (Figure 27.4). This indicates that the present-day asymmetric topography of the Southwestern Alps, with a relatively gentle western flank and steeper eastern flank, was established early, and has been in existence since at least the early Miocene, and possibly since the Oligocene or late Eocene. Therefore, the high topography and asymmetric morphology of this part of the Alps have been maintained throughout the past ~30 My.

In the East Eastern Alps, early to late Miocene pollen data (Figures 27.3 and 27.4) show a very diverse flora [START_REF] Jiménez-Moreno | High-resolution palynological analysis in late early-middle Miocene core from the Pannonian Basin, Hungary: climatic changes, astronomical forcing and eustatic fluctuations in the Central Paratethys[END_REF], 2008). The Burdigalian and Langhian (20.4-13.6 Ma) vegetation was dominated by thermophilous elements such as evergreen trees, typical of a present-day low-altitude evergreen rainforest (e.g., south-eastern China), reflecting the MCO. During the Serravallian and Tortonian (13.6-7.2 Ma), important vegetation changes are observed: Avicennia, which previously populated the coastal areas up to these high latitudes, v anished, and several other megatherm elements (Buxus bahamensis-type, Alchornea, Melastomataceae), typical of broad-leaved evergreen forests, became rarer, before finally disappearing from Central Europe. These changes can be linked to cooling and correlated with global and regional climatic changes. Later, this vegetation was progressively replaced by communities rich in mesotherm plants. Percentages of mid-and high-altitude conifers increased considerably during the Langhian and, later, during the Serravallian and Tortonian, indicating their relocation to a lower altitude, nearer to the sedimentary basin, which is correlated with significant cooling phases. These coniferous belts developed again at higher altitudes during warmer phases, as signaled by their reduced frequency in younger pollen records.

Subsequently, several thermophilous plants declined in abundance, and some of them disappeared from the Alps due to global cooling and progressive amplification of the glacial-interglacial cycles (see further details in Chapter 9). The data from the Stirone site (Figure 27.4) [START_REF] Bertini | Pliocene climatic cycles and altitudinal forest development from 2.7 Ma in the Northern Apennines (Italy): evidence from the pollen record of the Stirone section (~5.1 to ~2.2 Ma)[END_REF]) at the foot of the Apennines provide a good example of this phenomenon. From their onset, glacial-interglacial cycles are marked by changes in both vegetation and reconstructed climate compared to the early Pliocene [START_REF] Fauquette | Quantification of the northern Italy Pliocene climate from pollen data -evidence for a very peculiar climate pattern[END_REF]. During interglacials, the climate was almost as warm as in the early Pliocene, and the vegetation remained a mega-mesotherm to mesotherm plant forest. During glacials, megamesotherm taxa disappeared (e.g., Distylium, Myrica) or declined in abundance and distribution (Taxodium-type, Sequoia-type). Temperatures were markedly lower than during interglacials -nearly equal to modern valuesand not suitable for these thermophilous taxa.

The known macrofloras mostly come from the northern and eastern borders of the Alps for the Eocene to mid-Miocene interval [START_REF] Mai | Tertiäre Vegetationsgeschichte Europas[END_REF], and from the southern Alps for the Messinian and Pliocene [START_REF] Bertini | Reconstruction of vegetation transects for the Messinian/Piacenzian of Italy by means of comparative analysis of pollen, leaf and carpological records[END_REF][START_REF] Martinetto | Pliocene and Early Pleistocene carpological records of terrestrial plants from the southern border of the Po Plain (northern Italy)[END_REF]. In general, these data confirm the picture provided by pollen with respect to the flora and vegetation of the lowland Alpine fringes. The most interesting information added by macrofossils is connected with the marked climatic cooling that has occurred since ~2.6 Ma: several plants now extinct in Europe entered the macrofossil record for the first time during this period [START_REF] Martinetto | Pliocene and Early Pleistocene carpological records of terrestrial plants from the southern border of the Po Plain (northern Italy)[END_REF]. The cones of Picea florschuetzii, which are very similar to a modern spruce that grows in the Himalaya (P. smithiana), or of Tsuga chairugii, whose relatives today live only in America and East Asia, are frequent in a few early Pleistocene sites of northern Italy [START_REF] Ravazzi | Gli antichi bacini lacustri e i fossili di Leffe, Ranica e Piànico-Sèllere (Prealpi Lombarde)[END_REF]. The presence of these taxa indicates either the descent of the Alpine high-vegetation belts or immigration from Northern Europe/Siberia. Reliable records of plant species typical of the modern Alpine high-altitude belt (e.g., Larix cf. decidua and Picea abies) are first found only in the late early Pleistocene [START_REF] Ravazzi | The lacustrine deposits of Fornaci di Ranica (late Early Pleistocene, Italian Pre-Alps): stratigraphy, palaeoenvironment and geological evolution[END_REF]. Records of diagnostic species of the alpine belt are restricted to the late Pleistocene glacials and most commonly occur in the lowlands to the north of the Alps (e.g., Salix herbacea) [START_REF] Alsos | Past and future range shifts and loss of diversity in dwarf willow (Salix herbacea L.) inferred from genetics, fossils and modelling[END_REF]). This appears to be a good example of a taphonomic window, where alpine taxa are only preserved when they descend close to potential depositional environments, which are restricted to lowlands or basin floors.

Very few pollen data are available from the Pleistocene. Pollen data from Pianico-Sellere (Bergamo, Italy; Figure 27.4), attributed to early-middle Pleistocene, represent vegetation dynamics similar to those of the late Pleistocene interglacials, although "Tertiary relicts" (Pterocarya, Tsuga, Rhododendron ponticum) are still present [START_REF] Martinetto | Palaeoenvironmental significance of plant macrofossils from the Pianico Formation, Middle Pleistocene of Lombardy, North Italy[END_REF]. Other pollen successions in Switzerland and Bavaria attributed to the mid-Pleistocene [START_REF] Welten | Stand der palynologischen Quartörforschung am schweizerischen Nordalpenrand[END_REF][START_REF] Grüger | Untersuchungen zur Gliederung und Vegetationsgeschichte des Mittelpleistozäns am Samerberg in Oberbayern[END_REF] The Alps 421

5°E 6 °E 7°E 8 °E 9°E 1 0°E 1 1°E 1 2°E 1 3°E 1 4°E 1 5°E 1 6°E 1 7°E 1 8°E 19°E
Holstein interglacial. Several pollen records from France, Switzerland, Bavaria and Austria (respectively, Les Echets (de Beaulieu & Reille 1984), Gondiswill [START_REF] Wegmüller | Vegetationsgeschichtliche und stratigraphische Untersuchungen am Schieferkohlen des nördlichen Alpenvorlandes[END_REF], Samerberg [START_REF] Grüger | Spätriß, Riß/Würm und Frühwürm am Samerberg in Oberbayern -ein vegetationsgeschichtlicher Beitrag zur Gliederung des Jungpleistozäns[END_REF]) and Mondsee (Drescher-Schneider 2000); Figure 27.4) document the vegetation dynamics of the last interglacial (Eemian), highlighting the significant role played by Carpinus betulus and the quasiabsence of Fagus.

The Onset of Modern Vegetation: An Example from the Western Alps

Numerous pollen records are available in the western Alps for the Holocene, which allow for the depiction of the general trend of vegetation changes during this period.

The Last Glacial Maximum (LGM), between 24 and 17 ka (calibrated ages), constitutes the state of maximal retreat of temperate vegetation types, to the benefit of alpine-boreal plant communities. Temperate taxa were mostly restricted to southern refuges, mainly located in the Iberian, Italian and Balkan peninsulas and around the Black Sea, but some could also have survived within cryptic northern refugia (e.g., Nieto Feliner 2011). Most of the temperate plant recolonizations occurring thereafter originated from southern regions (e.g., [START_REF] Brewer | The spread of deciduous Quercus throughout Europe since the last glacial period Forest[END_REF][START_REF] Muller | Postglacial migration of silver fir (Abies alba Mill.) in the southwestern Alps[END_REF].

The Lateglacial period (17.0-11.3 ka) was marked by an ecological succession arising from an initial climate warming: the cold steppe vegetation dominated by Artemisia and Amaranthaceae was replaced by Juniperus scrub, then Betula woods and finally Pinus forests (de Beaulieu & Reille 1983). The Younger Dryas climatic event, which allowed a widespread expansion of steppe communities throughout Europe, led to a general lowering of the treeline to ~300 m.

The subsequent temperature increase at the beginning of the Holocene induced a second phase of successional dynamics, involving temperate tree communities that replaced Pinus forests. The first trees to establish were Corylus and deciduous Quercus. These communities constituted, from sea level to about 1800 m, diversified forests, including Fraxinus, Ulmus and Tilia. At higher altitudes, these forests were replaced by coniferous ones.

Simultaneously, around 10.1 ka, Abies alba penetrated into the southern French Alps, migrating from its nearest refugium in the northern Adriatic region [START_REF] Muller | Postglacial migration of silver fir (Abies alba Mill.) in the southwestern Alps[END_REF]. Fir forests expanded during the Atlantic period (8.9-5.4 ka), in both the southern (de Beaulieu & Reille 1983) and the northern French Alps [START_REF] David | Vegetation dynamics in the northern French Alps[END_REF]. This range expansion was slower in the dryer internal massifs, and ended about 6.5 ka, when fir penetrated into the extensive pine woods of the most arid region of the Alps, the Briançonnais [START_REF] Muller | Postglacial migration of silver fir (Abies alba Mill.) in the southwestern Alps[END_REF]). Throughout its Alpine range, this highly competitive species found suitable conditions at middle altitudes, progressively forcing pines and Larix decidua to migrate to higher altitudes and deciduous broad-leaved trees to migrate to lower ones. Between 6.5 and 2.8 ka, Pinus cembra formed dense forests between 2000 and more than 2300 m [START_REF] Muller | Paléostructures de végétation à la limite supérieure des forêts, dans les Alpes françaises internes[END_REF]. This occurred at the same time as an increase in the upper treeline, which exceeded 2900 m in the Queyras Valley [START_REF] Talon | Etudes pédoanthracologiques des variations de la limite supérieure des arbres au cours de l'Holocène dans les Alpes françaises[END_REF]. During the Subboreal period (5.4-2.8 ka), fir and deciduous oak declined as Neolithic human populations progressively colonized mountain territories [START_REF] Walsh | A historical ecology of the Ecrins (Southern French Alps): archaeology and palaeoecology of the Mesolithic to the Medieval period[END_REF]). The anthropogenic pressure was intense during the last 2.8 ka, since the beginning of the Subatlantic period, and the forest exploitation of the Bronze and Iron ages focused on some useful species, such as fir, which was intensively exploited to supply coastal shipyards [START_REF] Nakagawa | Pollen-derived history of timber exploitation from the Roman period onwards in the Romanche valley, central French Alps[END_REF]. Such human activity induced major vegetation changes from the shores to the mountains: at low altitudes, evergreen pioneer formations of Quercus ilex and Pinus halepensis replaced the deciduous forests of Q. pubescens, forming the anthropogenic meso-Mediterranean belt. At midaltitudes, above the remnant deciduous oak forests of the supra-Mediterranean belt and within the mountain-Mediterranean belt, Abies alba forests declined to give way to Fagus sylvatica, while in the sub-alpine belt, Pinus cembra declined and Abies disappeared from the internal valleys [START_REF] Muller | Postglacial migration of silver fir (Abies alba Mill.) in the southwestern Alps[END_REF], which allowed larch and mountain pine to disperse to lower elevations.

At the time of the Roman Empire, several cultivated trees were introduced (e.g., Castanea, Juglans), and the industrial forest exploitation [START_REF] Nakagawa | Pollen-derived history of timber exploitation from the Roman period onwards in the Romanche valley, central French Alps[END_REF] allowed for the immigration of Picea abies into the newly opened vegetation zones. This species spread into the sub-alpine belt of the Northern Alps, where it found a suitably wet climate. The period between AD 500 and 1000 was marked by a forest recovery, followed by a new deforestation trend from feudal times onwards, which finally ended during the Industrial Revolution of the 19th century. Thereafter, the abandonment of agriculture in remote valleys resulted in a re-establishment of mountain forests and the significant establishment of firs and oaks in secondary pioneer forests during the 20th century.

Climate and Paleoaltitude Reconstructions of the Alps Since the Late Eocene

Methods Based on Pollen Data

Pollen grains offer a large number of morphological traits (such as aperture, structure and sculpture) that can be analyzed by light microscopy [START_REF] Punt | Glossary of pollen and spore terminology[END_REF]) and scanning electronic microscopy (SEM) for surface features [START_REF] Halbritter | PalDat -Illustrated Handbook on Pollen Terminology[END_REF]). These features can then be used to identify the pollen to species level. The implementation of these techniques in the 1970s led to vastly improved identification rates of the Neogene pollen flora in the Mediterranean region, based on systematic comparative observations of fossil pollen grains with those of living plants [START_REF] Suc | Quelques taxons-guides dans l'étude paléoclimatique du Pliocène et du Pléistocène inférieur du Languedoc (France)[END_REF][START_REF] Suc | A methodology for Neogene palynostratigraphy[END_REF]. This botanical approach afforded the identification of more than 100 new taxa, including 14 robust identifications at the species level in the study area (Avicennia officinalis, A. alba, Hamamelis mollis, Parrotia persica, Parrotiopsis jacquemontiana, Distylium chinense, Microtropis fallax, Juglans cathayensis, Nyssa sinensis, Rhoiptelea chiliantha, Carpinus orientalis, Rhus cotinus, Lygeum spartum and Cedrus atlantica) [START_REF] Suc | L'investigation palynologique du Cénozoïque passe par les herbiers[END_REF]). This achievement significantly enriched the understanding of the regional Neogene flora and contributed to reliable climate reconstructions.

The climate reconstructions assembled from the Neogene data [START_REF] Fauquette | A new approach for paleoaltitude estimates based on pollen records: example of the Mercantour Massif (southeastern France) at the earliest Pliocene[END_REF][START_REF] Jiménez-Moreno | Vegetation, climate and paleoaltitude reconstructions of the Eastern Alps during the Miocene based on pollen records from Austria, Central Europe[END_REF][START_REF] Fauquette | Quantifying the Eocene to Pleistocene topographic evolution of the southwestern Alps, France and Italy[END_REF] are based on the " climatic amplitude method, " which was developed by [START_REF] Fauquette | A method for climatic reconstruction of the Mediterranean Pliocene using pollen data[END_REF] specifically to quantify the climate of periods with no modern analogs for the pollen spectra. In this method, the past climate is estimated by transposing the climatic requirements of the maximum number of modern taxa to the fossil data. The climatic amplitude method takes into account pollen percentages, and allows, despite some biases, for a greater refinement and precision of climatic estimates, because taxa with very low pollen percentages are not always significant. The most probable climate for a set of taxa is estimated as the climatic interval suitable for the highest number of taxa and a "most likely value" corresponding to a weighted mean.

In this method, Pinus pollen grains are excluded from the pollen sum, as they are generally over-represented in marine sediments due to a prolific production and overabundance resulting from long-distance air and water transport [START_REF] Heusser | Pollen distribution in marine sediments on the continental margin of Northern California[END_REF][START_REF] Beaudouin | The significance of pollen signal in present-day marine terrigenous sediments: the example of the Gulf of Lions (western Mediterranean Sea)[END_REF]). Highlatitude/altitude taxa are also excluded from the reconstruction process to avoid a cold bias linked to pollen transport from higher elevations. The excluded taxa are defined on the basis of their occurrence in modern vegetation zones: vegetation types described for the studied time period are found today in south-eastern China. There, the vertical distribution of vegetation is characterized from the base to the top of the massifs by evergreen broad-leaved forest, mixed evergreen and deciduous broad-leaved forest, Picea/Tsuga forest, Abies forest and high mountain meadows [START_REF] Wang | The Forests of China with a Survey of Grassland and Desert Vegetation[END_REF][START_REF] Hou | Vegetation of China with reference to its geographical distribution[END_REF]. This stratification allows us to exclude the modern highaltitude taxa from the climatic reconstruction in order to infer the low-to middle-low-altitude climate [START_REF] Fauquette | A method for climatic reconstruction of the Mediterranean Pliocene using pollen data[END_REF]. This procedure also allows us to discriminate between climatic variations and elevation changes.

Estimates based on this method show that the latitudinal temperature gradient in western Europe was ~0.48 °C/ degree latitude during the mid-Miocene [START_REF] Fauquette | Latitudinal climatic gradients in western European and Mediterranean regions from the Mid-Miocene (~15 Ma) to the Mid-Pliocene (~3.5 Ma) as quantified from pollen data[END_REF]) and ~0.46 °C/degree latitude during the early Oligocene [START_REF] Fauquette | Quantifying the Eocene to Pleistocene topographic evolution of the southwestern Alps, France and Italy[END_REF], and that a modern gradient was established during the Tortonian.

The method used to quantify paleoaltitudes, developed by [START_REF] Fauquette | A new approach for paleoaltitude estimates based on pollen records: example of the Mercantour Massif (southeastern France) at the earliest Pliocene[END_REF], is based on pollen floras, which provide a regional view of the vegetation from the lowest to the highest altitudinal belts feeding into the drainage basin from which the sediments were sourced, and the mean annual temperature (MAT) reconstructed at sea level, which is transposed into a modern latitude. The latitude difference between the Cenozoic site and the modern latitude indicates the shift in altitude of the vegetation belts; in particular, the Abies/Picea belt, which is the best indicator of elevated topography in the fossil pollen floras. This method uses a standard modern relationship established by [START_REF] Ozenda | Le déplacement vertical des étages de végétation en fonction de la latitude: un modèle simple et ses limites[END_REF][START_REF] Ozenda | Perspectives pour une géobiologie des montagnes[END_REF], as the data do not allow the estimation of past terrestrial lapse rates. Ozenda's relationship assumes that, in the Alps, modern vegetation belts shift by 110 m in altitude per degree of latitude. It should be noted that terrestrial lapse rates are highly variable geographically in relation to altitude, slope aspect, prevailing wind directions, continentality and atmospheric composition, among other factors, and they are difficult to apply where the past atmospheric composition differed from the present (see Chapter 7). [START_REF] Meyer | Lapse rates and other variables applied to estimating paleoaltitudes from fossil floras[END_REF] estimated modern terrestrial lapse rates for many areas of the world, and found that they vary between 0.36 and 0.81 °C/100 m for MAT. However, because there is no information concerning paleoterrestrial lapse rates in the Alps, [START_REF] Fauquette | Quantifying the Eocene to Pleistocene topographic evolution of the southwestern Alps, France and Italy[END_REF] hypothesized that the ratio between altitudinal and latitudinal temperature gradients was similar to the present ratio; taking into account the range of modern terrestrial lapse rates, the relationship between the latitudinal and altitudinal gradients would vary, giving different values for the shift of vegetation belts per degree of latitude (Table 27.2). If this range of values were used, the estimates of paleoaltitude would differ as well. However, Table 27.2 highlights that the estimates of the terrestrial lapse rate for each studied period based on the standard modern relationship are included in Meyer's estimated range.

Moreover, paleoelevation estimates correspond to the most probable paleoelevations based on the "most likely values" of MAT and to ranges of paleoelevation based on the minimum and maximum MAT. This partially compensates for the use of the modern altitudinal/ latitudinal ratio.

In the case of sites with unknown elevation [START_REF] Suc | The use of pollen floras as a tool to estimate palaeoaltitude of mountains: the Eastern Pyrenees in the Late Neogene, a case study[END_REF], which were all younger than 10 Ma in [START_REF] Fauquette | Quantifying the Eocene to Pleistocene topographic evolution of the southwestern Alps, France and Italy[END_REF], the latitudinal temperature gradient was similar to the modern one, and the authors chose to keep the terrestrial lapse rate equivalent to the modern rate, taking into account that the position and continentality of the Alps were the same as today.

The record in the fossil pollen flora of microtherm trees such as Abies and Picea indicates elevated topography close to the sampled sites. Nevertheless, pollen data do not permit altitudinal range estimation for this vegetation belt, nor a determination of whether alpine herbaceous and perpetual-snow belts existed above it, due to difficulties in differentiating between the pollen of alpine herbaceous elements and that of plants growing at lower elevations. As a consequence, these pollen-based paleoelevations are minimum estimates, corresponding to the lower limit of the highest forested belt.

We must emphasize the consistency of our previous estimates: the paleoaltitude we inferred for the Mercantour Massif (Southwestern Alps) is consistent with a geomorphological reconstruction [START_REF] Fauquette | A new approach for paleoaltitude estimates based on pollen records: example of the Mercantour Massif (southeastern France) at the earliest Pliocene[END_REF]; the paleoaltitude of the Ganos Mountain in the Dardanelles Strait [START_REF] Melinte-Dobrinescu | The Messinian Salinity Crisis in the Dardanelles region: chronostratigraphic constraints[END_REF]) is in agreement with the tectonic hypothesis of [START_REF] Armijo | Westward propagation of the North Anatolian fault into the northern Aegean: timing and kinematics[END_REF]; and the paleoaltitude of the Canigou Mount in the Eastern Pyrenees [START_REF] Suc | The use of pollen floras as a tool to estimate palaeoaltitude of mountains: the Eastern Pyrenees in the Late Neogene, a case study[END_REF]) is supported by a paleogeographic validation [START_REF] Clauzon | The Roussillon Basin (S. France): a case-study to distinguish local and regional events between 6 and 3 Ma[END_REF].

Resulting Paleoaltitudes

Based on this method, the study by [START_REF] Fauquette | Quantifying the Eocene to Pleistocene topographic evolution of the southwestern Alps, France and Italy[END_REF] shows that the topography of the internal zone of the Southwestern Alps reached elevations of around 1800 m as early as the Oligocene (ca. 30 Ma). In contrast, the external massifs, including the Mercantour Massif, uplifted later, during the late Neogene, while the post-Serravallian (ca. 12-13 Ma) emplacement of the Digne thrust nappe was reinforced. Pollen data show that the present-day topographic asymmetry of the Southwestern Alps, with a relatively gentle western flank and a much steeper eastern one, dates back to at least the early Miocene, and possibly to the Oligocene or late Eocene.

In the East Eastern Alps [START_REF] Jiménez-Moreno | Vegetation, climate and paleoaltitude reconstructions of the Eastern Alps during the Miocene based on pollen records from Austria, Central Europe[END_REF], paleoaltitudes during the Burdigalian were not high enough for Abies and Picea to form forests, but were sufficient to allow Cathaya to survive. Therefore, the relief was less than 1800 m. On the other hand, Serravallian and Tortonian pollen records, rich in conifers typical of high elevations, indicate an intense uplift during the mid-Miocene. The abundance increase in high-elevation trees is due to uplift and not to climatic changes, as the pollen records are still dominated by thermophilous plants. Paleoaltitudes were higher than 1600 m at that time.

How Do Regional Geological Evolution, Global Climatic Changes and Human Pressure Affect Alpine Plant Diversity and Vegetation?

Due to the formation of the Alps since the late Paleogene (40-35 Ma) under warm-temperate to subtropical conditions, boreal-type environments could only develop at the higher elevations. Since the Oligocene, high elevations (~1800 m) have allowed meso-microtherm (Cedrus, Cathaya) and microtherm (Abies, Picea) trees to establish.

The development of plant populations on mountains arises from two major sources [START_REF] Ozenda | La végétation de la chaîne alpine dans l'espace montagnard européen[END_REF][START_REF] Ozenda | On the genesis of the plant population in the Alps: new or critical aspects[END_REF]: an autochthonous (in situ) population, formed of plants already present at the beginning of the uplift, and an allochthonous component, arising from migration from other cold regions.

The autochthonous part formed from a subtropical to tropical flora. Most of these taxa, vulnerable to cold, became extinct later due to glaciations and competition, but a few, such as Juniperus thurifera and Berardia subacaulis, survived in the Western Alps [START_REF] Ozenda | On the genesis of the plant population in the Alps: new or critical aspects[END_REF]. Plants of temperate origin are represented by two groups: Holarctic and Mediterranean plants [START_REF] Ozenda | La végétation de la chaîne alpine dans l'espace montagnard européen[END_REF]. [START_REF] Fauquette | Quantifying the Eocene to Pleistocene topographic evolution of the southwestern Alps, France and Italy[END_REF]. The minimum (0.36 °C/100 m) and maximum (0.8 °C/100 m) lapse rates are those reconstructed by [START_REF] Meyer | Lapse rates and other variables applied to estimating paleoaltitudes from fossil floras[END_REF] The allochthonous part is made up of plants that migrated from the Mediterranean and/or central Asian mountains and Arctic regions [START_REF] Ozenda | L'endémisme au niveau de l'ensemble du Système alpin[END_REF][START_REF] Ozenda | On the genesis of the plant population in the Alps: new or critical aspects[END_REF][START_REF] Kadereit | Asian relationships of the flora of the European Alps[END_REF]. Exchanges with Arctic and Northern European floras during glacials/interglacials is evidenced by the presence of "arctic-alpine" plants. However, the Alpine flora shows less pronounced affinities to the northern (Scandinavia, North America) than to the southern (Mediterranean Basin, Central and Eastern Asia) floras [START_REF] Ozenda | L'endémisme au niveau de l'ensemble du Système alpin[END_REF][START_REF] Ozenda | On the genesis of the plant population in the Alps: new or critical aspects[END_REF][START_REF] Comes | Spatial and temporal patterns in the evolution of the flora of the European Alpine system[END_REF]. The European Alpine system and major Central Asian mountains (Altai, Tibetan Plateau and Himalaya) are connected by a more or less continuous system of mountain ranges. This implies both a northern (boreal, sub-arctic, arctic) and a southern (southern mountains) connection between the European Alpine system and the Central Asian high mountains.

Pleistocene glaciations played a major role in the changing vegetation of the Alps: (i) the pre-existing thermophilous flora almost completely disappeared; (ii) plants of temperate origin only survived in refugia, leading to a disjunct distribution of some plants; and (iii) exchanges between the Alps and the other mountain ranges and/or northern regions were amplified. Pleistocene warming phases led to the expansion of the thermophilous floras, forcing microtherm plants to move towards colder areas to the north or to higher elevations [START_REF] Ozenda | La végétation de la chaîne alpine dans l'espace montagnard européen[END_REF][START_REF] Ozenda | On the genesis of the plant population in the Alps: new or critical aspects[END_REF], which resulted in disjunct distributions of arctic-alpine plants.

The postglacial period of the French Alps is the period in which the different vegetation belts recognized today were established (Table 27.1). Around 10 ka, only two forested belts existed: a lower one, occupied by deciduous oak forest, and an upper one, characterized by coniferous forests (mainly Pinus sylvestris and P. uncinata). The boundary between these two belts was not abrupt, but formed a transition zone with mixed formations of oaks and pines. This intermediate belt only transformed into a belt in its own right with the immigration of fir, from ca. 9 ka. The fir forests, mixed with numerous deciduous trees, forced oak forests downslope and pine forests upslope. The late onset of this belt is clearly due to the migratory dynamics of firs that arrived in the French Alps from their Adriatic and Apennine refuges. Between 7.5 and 5.0 ka, the subalpine belt, which was made up of open woodlands of Pinus uncinata mixed with Larix and Pinus cembra, differed significantly from its modern physiognomy. The modern forest structure of this belt was only achieved around 5 ka, with the development of dense forests of P. cembra up to ~2300 m. The onset of a real forested sub-alpine belt appears to be related to a general aridification, evidenced by low lake levels [START_REF] Digerfeldt | Reconstruction and paleoclimatic interpretation of Holocene lake-level changes in Lac de Saint-Léger, Haute-Provence, southeast France[END_REF].

The last belt to appear in the Southwestern Alps was the lower meso-Mediterranean belt, at around 5-4 ka, characterized by the establishment of typical Mediterranean plants at low altitudes, in particular Quercus ilex and Pinus halepensis. The presence of Neolithic human populations supports the anthropogenic origin of this belt, which corresponds to the replacement of deciduous forests by pioneer disturbance-adapted plant formations. Nevertheless, recent multiproxy attempts to quantify Holocene climate fluctuations have shown that a succession of dry episodes from ca. 5 ka may have also triggered the expansion of sclerophyllous Mediterranean taxa [START_REF] Magny | Assessment of the impact of climate and anthropogenic factors on Holocene Mediterranean vegetation in Europe on the basis of palaeohydrological records[END_REF]. Unlike the other vegetation belts, the meso-Mediterranean belt appears to be a partly humantriggered subdivision of the former (supra-)Mediterranean belt, naturally dominated by deciduous oak forests. At higher altitudes, human activity also had strong effects, such as lowering the treeline by at least 300 m [START_REF] Carnetti | Past uppermost tree limit in the Central European Alps (Switzerland) based on soil and soil charcoal[END_REF]) and creating sub-alpine meadows. Moreover, increased human population sizes in mountains led to the fragmentation of forest ecosystems and thus to erosion events. In the second part of the 20th century, a policy of intensive reforestation started.

Conclusion

The flora of the Alps encompasses more than one-third of the European flora. Endemic taxa, mainly concentrated in the south-westernmost Alps and the southwestern part of the Eastern Alps, represent 4% of the European flora. Maritime and Ligurian Alps constitute an important hot spot of plant species richness and form a major refugium where endemic plants represent 25% of all alpine endemics.

In the Alps, vegetation belts are organized by altitude, governed by decreases in temperature and increases in precipitation at higher elevations. These vegetation belts are well recognized in modern pollen data, established on two transects along the slopes of the Southwestern Alps. The modern pollen data also reflect the asymmetric west-east elevation of the massif: the replacement of pollen groups as elevation increases appears progressive on the western flank but much more abrupt on the eastern one.

The formation of the Alps dates back to ca. 35 Ma and has influenced both the flora and the vegetation of the area, due to the appearance of high mountains and new marine basins, and consequent landscape fragmentation, environmental heterogeneities and new climatic patterns. As early as the Oligocene, high relief allowed boreal-type environments to develop at the higher elevations, and meso-microtherm (Cedrus, Cathaya) and microtherm trees (Abies, Picea) established early in the history of Alpine vegetation. During the Pleistocene, due to glaciations, the earlier, more thermophilous flora became extinct, while arctic-alpine plants developed on the massifs. Holarctic plants only survived in refugia, leading to disjunct distributions. However, warm phases also led to disjunct distributions of the arctic-alpine plants, which moved towards colder areas to the north or to higher elevations. Today, only some "Tertiary relicts, " such as Juniperus thurifera and Berardia subacaulis, still survive in the Alps.

The onset of the modern vegetation patterns started in the early postglacial period, around 10 ka. The different vegetation belts recognized today gradually developed following local climate and soil conditions and the migratory dynamics of the species. The presence of human populations in the Alps since the Neolithic triggered the expansion of some vegetation types, in particular typical Mediterranean plant formations at low altitude, and the lowering of the treeline, which led to the development of the sub-alpine meadows. Finally, the recent abandonment of land cultivation has resulted in the return of mountain forests.

Figure 27 . 1

 271 Figure 27.1 Maps of the five main phytogeographic zones of the Alps. Source: Adapted from Aeschimann et al. (2011). In gray: (a) The main centers of vascular plant endemism, defined by Noble & Diadema (2011), and (b) The main refugia of vascular plants in the Alps, defined by Schönswetter et al. (2005).

Figure 27 . 2

 272 Figure 27.2 Relationship between vegetation belts positioned on a topographic profile, following studies by Noirfalise et al. (1987) and Ozenda (2002), and modern pollen data collected from moss polsters along two transects. (a) Simplified vegetation map of the Southwestern Alps (from Noirfalise et al. 1987), on which the transects are shown (dots correspond to the moss polsters). Pollen localities and altitudes are indicated below. (b) Transect AA′: 1, Nyons (300 m); 2, Luz la Croix haute (1100 m); 3, Le Freney (800 m); 4, Col du Lautaret (1850 m); 5,6, Pinerolese (Lago Villaretto di Roure and Balma di Roure, 980 and 890 m, respectively); 7,8, Cumiana Giaveno (612 and 605 m, respectively); 9,10, Baldissero Torinese (Val Samfrà and Superga road, 383 and 537 m, respectively). (c) Transect BB′: 11, Taulanne (995 m); 12, Sisteron (540 m); 13, Saint-Léger (2000 m); 14, Agnel (2800 m); 15, Pinerolese, Inverso Pinasca (560 m); 16, Moncucco Torinese, Borelli (315 m). Source: Adapted from Fauquette et al. (2015). See also Plate 45 in color plate section.

Figure 27 . 3

 273 Figure 27.3 Synthetic pollen diagrams from late Eocene to Pleistocene localities of the Southwestern and Eastern Alps, and their positions in a chronostratigraphic frame. Note the change of time scale at the Paleogene-Neogene boundary. Taxa have been arranged into 13 different groups (Pinus excluded) based on ecological criteria to visualize the main changes in the paleovegetation cover. Plants without signification indicates families (e.g., Rosaceae, Ranunculaceae, Rutaceae) in which pollen identification is difficult at the genus level and includes cosmopolitan taxa and/or taxa with wide ecological requirements, plus unidentified pollen grains. Some groups are classified according to thermic requirements with respect to the Nix (1982) classification: equatorial and tropical forests are inhabited by megatherm plants (mean annual temperature, MAT >24 °C), subtropical forests by mega-mesotherm plants (MAT 20-24 °C), temperate deciduous forests by mesotherm plants (MAT 14-20 °C), boreal coniferous forests successively by meso-microtherm (MAT 12-14 °C) and microtherm (MAT <12 °C) plants and tundra by microtherm plants. Source: Adapted from Fauquette et al. (2015) and Jiménez-Moreno et al. (2008). See also Plate 46 in color plate section.

  illustrate, as elsewhere in Europe, a moderate expansion of Pterocarya during the * .

Figure 27 . 4

 274 Figure 27.4 Locations of the palynological sites mentioned in the text and in Figure 27.3. The sites of the Torino Hills are: Gassino Torinese, Tetti Civera (samples 1, 2 and 3 in Figure 27.3) and Rio Civera, Pavarolo, Borelli. The numbered sites in Southern France are: 1, Saint Lions, Gambi, Châteauredon, Le Rochassas; 2, La Rosée, Les Mées 1, Les Mées-Périgoite; 3, Villeneuve, Oraison (Flour); 4, La Motte d'Aigues, Vaugines; and 5, Le Gourg, Carros, Saint Martin du Var, Saint-Isidore

Table 27 .1

 27 Structure of the vegetation belts of the Alps in relation to the mean minimum temperatures of the coldest month of the year (m), and their correspondence with two concepts of the Mediterranean vegetation belts.

m (°C) Alpine vegetation belts Mediterranean vegetation belts Ozenda (1985) Main vegetation types Ozenda (2002) Quézel & Médail (2003)

	-10 -9 -7	Nival Alpine	Bryophytes and lichens Grasslands with Poaceae and Cyperaceae; low heathlands with Salix, Juniperus and Vaccinium	Alti-Mediterranean	Oro-Mediterranean
	-6 Sub-alpine -4	Coniferous forests with Picea abies, Larix decidua, Pinus mugo and Pinus cembra	Mountain-Mediterranean	Mountain-Mediterranean
	-3 Mountain -1	Forests with Fagus sylvatica and/or Abies alba; mesophilous forests with Pinus sylvestris	Supra-Mediterranean	Supra-Mediterranean
	0
	1	Collinean	Deciduous forests with Quercus pubescens, Q. robur or Q. petraea; xeric forests with Pinus sylvestris; mesophilous and Prunus Fraxinus, Acer forests with Tilia,	Medio-Mediterranean Per-Mediterranean	Meso-Mediterranean Thermo-Mediterranean
	9 10		Mediterranean vegetation (southern slopes) in the Southwestern Alps
	≥10				Infra-Mediterranean

Table 27 .2

 27 Shifts in altitude of vegetation belts in relation to different terrestrial lapse rates and to latitudinal gradients estimated for different epochs

  for many areas of the world. The lines in bold corresponds to the estimates of terrestrial lapse rate for each studied period based on the standard modern relationship.

		Latitudinal	Terrestrial
		gradient	lapse rate
		(°C/°lat.)	(°C/100 m)	Shift in altitude/°lat.
	Standard modern	0.6	0.55	1 °lat. → 110 m
	Pliocene	0.6	0.36	1 °lat. → 167 m
			0.55	1 °lat. → 110 m
			0.81	1 °lat. → 74 m
	Middle Miocene	0.48	0.36 0.44	1 °lat. → 134 m 1 °lat. → 110 m
			0.55	1 °lat. → 87 m
			0.81	1 °lat. → 60 m
	Early Oligocene	0.46	0.36 0.42	1 °lat. → 128 m 1 °lat. → 110 m
			0.55	1 °lat. → 84 m
			0.81	1 °lat. → 57 m

Acknowledgments

The authors are deeply indebted to the late Professor Pierre Quézel for his teaching, advice and unfailing support for their work on the history of the Mediterranean flora and vegetation. A part of this study has been realized in the framework of the project "Erosion and Relief Development in the Western Alps, " funded by the Agence Nationale de la Recherche. The authors thank the two reviewers, R.A. Spicer and J. Carrión, who helped us to improve the chapter. This is ISEM contribution no. 2016-115.


Gaëtan Drouin

Daniel D Catheline

Charlotte Baudry

Pascale P Le Ruyet

Philippe P Legrand

Impact of n-3 docosapentaenoic acid (DPA) supplementation on n-3 fatty acid composition of tissues in rats

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Sprague Dawley rats (n=8) were fed after weaning with a control diet (Ctrl) or a n-3 DPA-supplemented diet (0.5% n-3 DPA) containing a mix of plant oils during 3 weeks .

Lipids were extracted from 18 differents frozen tissues (Delsal, 1944). FA were then derived into FA methyl esters whose composition was analysed by gas chromatography coupled with mass spectrometry (GCMS).

Results were expressed as mean ± SEM. DPA DHA

Fish oil

DPA was purified by successive purification with a preparative HPLC system to obtain >98% DPA.

n-3 DPA was increased in most tissues when diets were supplemented with this FA, and particulary heart, spleen, lung and bone marrow but not in liver, plasma, brain, retina and muscle.

DHA profile was not impacted by the DPA diet except in spleen, lung and bone marrow where it was increased.

Introduction

Methods

DPA purification

Results

(Ret. Time)

Univariate analysis

Multivariate analysis % of total fatty acid

EPA n-3 DPA DHA

The n-6 PUFA / LA ratio was decreased in some tissues and especially in the liver, suggesting a decrease in the n-6 PUFA conversion. This result could be due to a substract competition between n-3 PUFA and n-6 PUFA for the different enzymes of the conversion pathway, since these enzymes are common for both families.

EPA proportion was increased in some tissues like in the liver. This FA might come from the peroxisomal retroconversion of DPA into EPA.

Conclusion and perspectives

All the rats of each diet are well separated on the score scatter plot. The loadings scatter plot shows the most discriminant FA levels between the 2 diets.

Heart, liver, lung, kidney, spleen then bone marrow and pancreas were the most impacted by the n-3 DPA supplementation.

The changes in FA proportions concerned PUFA metabolism only; suggesting a specific action of n-3 DPA supplementation on these pathways.

Beyond an increase in all n-3 PUFA, the n-3 DPA supplementation induced principally a decrease in n-6 docosapentaenoic acid (22:5 n-6) and arachidonic acid (20:4 n-6).

Model validation

CV

1

  Laboratory of Biochemistry and Human Nutrition -Agrocampus Ouest, Rennes, France 2 Lactalis R&D, Retiers, FranceAfter only 3 weeks of physiological n-3 DPA supplementation, the omega-3 status was improved in most tissues. The impact of n-3 DPA diet was tissue-dependent. It specifically affected heart, lung, spleen, kidney and bone marrow and was specific of PUFA metabolism. These results suggest potential physiological effects specific of DPA metabolism in these organs compared to the EPA and DHA, whose assimilation was less specific from our findings. Furthermore, some studies have shown an increase in n-3 DPA in red blood cells and in some tissues when the diet is partially enriched with dairy lipids compared to diet composed of vegetable oils (Dinel & al., 2016, PLEFA)(Du & al., 2013, PLEFA). It would be interesting to investigate whether the potential physiological effects associated with n-3 DPA supplementation would be found with a diet partially enriched with dairy lipids.

  Bmw : bone marrow ; EpF : epididi.fat ; Hrt : heart ; Kdy : kidney ; Liv : liver ; Lng : lung ; Plm : plasma ; Rbc : red blood cells Skn : skin ; Spn : spleen ; Brn : brain ; Hrt : heart ; Kdy : kidney ; Liv : liver ; Lng : lung ; Skn : skin ; Spn : spleen ; Pan : pancreas ; Mus : muscle ; Ret : retina ; Stc : stomach ; ScF : subcutaneous fat ; Tes : testis.


Francesca Galassi

Olivier Commowick

Christian Barillot

Integration of Probabilistic Atlas and Graph Cuts for Automated Segmentation of Multiple Sclerosis lesions AUTHORS

INTRODUCTION

We propose a framework for automated segmentation of Multiple Sclerosis (MS) lesions from MR brain images. It integrates a priori tissues and MS lesions information into a Graph-Cuts algorithm for improved segmentation results.

METHOD

Pre-processing. The method requires three MR sequences per subject: T1-w, T2-w and FLAIR. MR images are first denoised [START_REF] Coupe | An optimized blockwise nonlocal means denoising filter for 3-D magnetic resonance images[END_REF] , rigidly registered towards the T1-w image [START_REF] Commowick | Block-matching strategies for rigid registration of multimodal medical images[END_REF] , skullstripped [START_REF] Manjón | An Online MRI Brain Volumetry System[END_REF] and bias corrected [START_REF] Tustison | N4ITK: improved N3 bias correction[END_REF] . Then, their intensities are normalized by applying the decile normalization method proposed by Nyul et al. [START_REF] Nyul | New variants of a method of MRI scale standardization[END_REF][START_REF] Shah | Evaluating intensity normalization on MRIs of human brain with multiple sclerosis[END_REF] .

A probabilistic atlas (i.e. CSF, WM, GM and MS lesion probability maps intensity normalized for the four classes, Figure 1) is registered into the subject space. The T1-w template image is registered to the T1-w subject image using a linear registration, based on a block-matching algorithm 2 , followed by a dense non-linear registration [START_REF] Commowick | Automated diffeomorphic registration of anatomical structures with rigid parts: application to dynamic cervical MRI[END_REF] .

Segmentation method. We adopt a Graph-Cuts segmentation approach, where the two terminal nodes, source and sink, respectively represent the MS lesions, or object class, and the Normal Appearing Brain Tissues (NABT), or background class [START_REF] García-Lorenzo | Multiple sclerosis lesion segmentation using an automatic multimodal graph cuts[END_REF] . Given the set of nodes I and the set of connections N between two nodes {i, j}, the GC algorithm minimizes an energy function E(V), where V is the object segmentation:

E(V)=∑ 𝐵 {𝑖,𝑗} {𝑖,𝑗}∈𝑁 +α∑ 𝑅 𝑖 (𝑉 𝑖 ) 𝑖∈𝐼 .
The boundary term 𝐵 {𝑖,𝑗} reflects the similarity of neighbouring voxels {i,j}, and the regional term 𝑅 𝑖 (. ) refers to the probability for i to fit into the object and background models. We compute the boundary weights using the spectral gradient [START_REF] García-Lorenzo | Multiple sclerosis lesion segmentation using an automatic multimodal graph cuts[END_REF] and the object and background weights as described below.

We model the NABT with a 3-class Gaussian Mixture Model. We estimate the NABT GMM using tissue probability maps with a robust EM [START_REF] García-Lorenzo | Trimmed-likelihood estimation for focal lesions and tissue segmentation in multisequence MRI for multiple sclerosis[END_REF] , which optimizes a trimmed likelihood to be robust to the presence of outliers (i.e. lesions). Then, we evaluate the Mahalanobis distance Z 2

between each voxel i and each tissue class m. Assuming that Z 2 follows a χ 2 m distribution, the p-value for a voxel i is obtained as

p im =1-Fχ 2 m (Z 2 )
p im represents the probability that the voxel i does not fit into a class m of the NABT GMM.

Voxels that fit into the NABT GMM must have a high background weight Wi B . We formulate the equation for computing Wi B to include both p im and a priori MS lesion information P iMS as

Wi B =β(1.0-min(p im ))+(1-β)(1.0-P iMS ) ,
where the parameter β controls the amount of information from the Mahalanobis distance and the prior. min(p im ) is the lowest p-value among the m classes for a voxel i. The parameter β was set to 70%.

We formulate the object weights to include information from the MS lesion prior, the Mahalanobis distance and the hyper-intensities on T2-w and FLAIR images:

Woi=min(P iO ,W T2 ,W flair ) ,
where

P iO =βmin(p im )+(1-β)P iMS
W T2 ,W flair are fuzzy weights derived from the T2-w and FLAIR hyper-intensities [START_REF] García-Lorenzo | Multiple sclerosis lesion segmentation using an automatic multimodal graph cuts[END_REF] .

Post-processing. A candidate lesion is discarded if one of the following conditions is verified:

i) it is not sufficiently located within the WM, ii) it touches the brain mask border, iii) its size is lower than 3mm [START_REF] Manjón | An Online MRI Brain Volumetry System[END_REF] .

RESULTS

We evaluated the method on 37 MS subjects. Each subject included T1-w, T2-w and FLAIR images. The ground truth was computed using the LOP STAPLE method [START_REF]A Logarithmic Opinion Pool Based STAPLE Algorithm for the Fusion of Segmentations With Associated Reliability Weights[END_REF] on six independent manual segmentations per patient. Results are illustrated in Figure 2. The average Dice Similarity Coefficient (DSC) was 0.578, the average Positive Predictive Value (PPV) was 0.711. An example of segmentation result (without and with priors) is illustrated in Figure 3.

The computation time to process a subject on a laptop with an Intel Core i7 CPU 2.40GHz (8 cores) was approximately 10 minutes.

DISCUSSION

Results indicate good performance of the proposed method. Three of the MS subjects with low lesion load and low number of lesions obtained a DSC lower than 0.3. This can be partially explained by the low experts' agreement, which becomes more relevant for a small lesion load. When compared to a Graph-Cuts approach that does not include knowledge from MS lesion priors, our approach shows generally improved performance (average DSC=0.480 on the same dataset).

CONCLUSION

We propose a framework that incorporates tissues and MS lesions probability maps into a Graph-Cuts approach for MS lesions segmentation. Results indicate that integrating a priori information with the information derived from the images can improve the segmentation outcome. The performance of the proposed method relies on the accuracy of the probabilistic atlas and parameters that must be accurately tuned on a training dataset. The method can be easily adapted to be used with different MR sequences (e.g. PD images).

Figure 1 .

 1 Figure 1.

Figure 2 .

 2 Figure 2. MS lesions as segmented using the proposed method, without (top) and with (bottom) MS lesion priors in the computations of the term weights. Results are overlaid the FLAIR image. With the use of the MS lesion priors missing lesions (red arrows) can be detected (green arrows) and false positive (orange arrows) can be removed.

Figure 3 .

 3 Figure 3.