Rivoharifara Randrianarimanana

France Rakotondrainibe

Elodie Boucheron-Dubuisson

Lovanomenjanahary Marline

Mijoro Rakotoarinivo

Catherine Reeb
email: catherine.reeb@mnhn.fr

Ferns and clubmosses in the eastern canyons of Isalo National Park Diversity and distribution of ferns (Filicophytes) and clubmosses (Lycophytes) in the eastern canyons of Isalo National Park, Madagascar

Keywords: ecological factors, habitat diversity, Madagascar, pteridophytes, species diversity

Background and aims -In contrast to the flowering plants, the pteridophyte flora of Madagascar is still understudied. While several studies have been published on the eastern and central parts of the island, there are currently few works or scientific publications dedicated to the pteridophytes of southwestern Madagascar. The aim of the present work is to increase the knowledge of the pteridophyte flora of the Isalo massif in southwestern Madagascar. It presents 1) a checklist of Isalo's pteridophytes and 2) an analysis of the diversity and distribution patterns of pteridophyte communities across ecological gradients in the eastern canyons of Isalo. Material and methods -Eighty plots were placed in six canyons. In each plot, pteridophyte species abundance was inventoried, as well as several ecological and geomorphological variables. A census in the field and observations on the specimens in the herbarium were carried out in order to make a check-list of all the pteridophyte species known for Isalo. Statistical analysis was carried out to understand the pteridophyte diversity and distribution patterns in the Isalo's canyons: (1) exploratory analysis (FAMD and HCPC) highlights the general patterns of ecological gradients, (2) a rarefaction curve was used to compare species diversity, and (3) co-inertia analysis investigated the relationship between ecological gradient and pteridophyte communities. Key results -In total, sixty species of ferns and lycophytes have been recorded in the massif, including ten endemic species to Madagascar and eleven species reported for the first time in the Isalo massif. Species diversity is especially high in the northern canyons (Anjofo, Andramanero, Antsifotra) in contrast to the middle (Maki and Rats) and southern (Namaza) canyons. Fern distribution patterns were correlated to a combination of environmental factors, highlighting species-specific ecological preferences.

INTRODUCTION

The two lineages Monilophytes (ferns and horsetails) and Lycophytes (club mosses, spike mosses and quillworts) were the first vascular plants to colonise land (Ranker and Haufler 2008). They are traditionally referred to as "pteridophytes", or ferns and allies, a group that cannot be recognised as a taxon due to its paraphyly but is used for convenience (PPG I 2016). Pteridophytes are the second most diverse group of vascular plants, comprising around 12000 species. They are reliable bioindicators, closely linked to environmental factors, readily recognizable and monitored, particularly in riparian ecosystems [START_REF] Paixão | More than light: distance-dependent variation on riparian fern community in Southern Amazonia[END_REF][START_REF] Della | Pteridophytes as ecological indicators: an overview[END_REF][START_REF] Bonari | Fine-scale fern ecological responses inform on riparian forest habitat conservation status[END_REF].

Madagascar is recognised as one of the world's biodiversity hotspots [START_REF] Myers | Biodiversity hotspots for conservation priorities[END_REF] displaying over 90 % endemism in various animal taxa (e.g. 100 % for amphibians, 92 % for reptiles; [START_REF] Vences | Madagascar as a model region of species diversification Trends in[END_REF]) and 82 % in vascular plants. Endemism is lower for sporedispersed plants (50 % for ferns) (Rakotondrainibe et al. in press). Madagascar hosts the most diversified pteridophyte flora in the African region, with a species richness equivalent to that of the entire continental Africa, which is approximately 50 times larger in area [START_REF] Aldasoro | Diversity and distribution of ferns in sub-Saharan Africa, Madagascar and some islands of the South Atlantic: Diversity and distribution of African ferns[END_REF][START_REF] Bauret | Madagascar sheds new light on the molecular systematics and biogeography of grammitid ferns: New unexpected lineages and numerous long-distance dispersal events[END_REF][START_REF] Rouhan | Exploration and Origins of Biodiversity in Madagascar: The Message of Ferns[END_REF]. [START_REF] Rakotondrainibe | Checklist of the pteridophytes of Madagascar[END_REF] reported 586 taxa of ferns and lycophytes, including 38 infraspecific taxa, in Madagascar with 40 % endemism. A new version of the "Flora of the Pteridophytes of Madagascar'', led by F. Rakotondrainibe, reports 739 taxa with more than 50 % endemism (Rakotondrainibe et al., in press). The distribution of the Malagasy pteridophyte species across the five defined bioclimatic domains in Madagascar [START_REF] Humbert | Les Territoires Phytogéographiques de Madagascar. Leur Cartographie. Colloque sur les Régions Ecologiques du Globe[END_REF] shows that approximately eighty-five (85 %) of the pteridophytes found in Madagascar were reported from the Central and Eastern domains (Rakotondrainibe andRaharimalala 1996, 1998;Rakotondrainibe 2000aRakotondrainibe , 2000b[START_REF] Rakotondrainibe | Liste commentée des ptéridophytes de la Réserve Spéciale de Manongarivo, Madagascar. Inventaire floristique et faunistique de la Réserve spéciale de Manongarivo (NW Madagascar)[END_REF][START_REF] Rakotondrainibe | Checklist of the pteridophytes of Madagascar[END_REF][START_REF] Reeb | Ecological survey of the Lycophytes and Ferns of the Vohimana Reserve, Madagascar[END_REF], which are characterized by humid or subhumid forests and a high altitudinal range (Humbert and Cours-Danes 1965;Cornet 1972;[START_REF] Puy | A refined classification of primary vegetation of Madagascar based on the underlying geology: using GIS to map its distribution and assess its status[END_REF]. However, little is known about the diversity and endemism of pteridophytes in the Western and Southern domains of Madagascar, subject to arid or subarid conditions [START_REF] Humbert | Carte Internationale du tapis végétal et des conditions écologiques à 1/1 000 000. Madagascar. Pondichéry[END_REF][START_REF] Cornet | Divisions floristiques et étages de végétation à Madagascar[END_REF][START_REF] Puy | A refined classification of primary vegetation of Madagascar based on the underlying geology: using GIS to map its distribution and assess its status[END_REF]. This study was conducted in Isalo National Park, which is located in the southwestern part of Madagascar. Isalo National Park (NP) (Fig. 1A) experiences a subhumid climate similar to the Central domain (Fig. 1B) which is an exception in the southwest region of Madagascar, exposed to semi-arid and hot climate. The Isalo massif is a combination of ruiniform sandstone formations, sand domes and tabular structures [START_REF] Sourdat | Sur l'evolution du massif de l'Isalo et du bassin de l'Onilahy (Sud-Ouest de Madagascar)[END_REF]. The summits are mainly covered by sclerophyllous woodland and shrubs, pseudo-steppes and rocky vegetation, and in the damper and shadier canyons of the ruined reliefs a narrow evergreen humid forest may grow and develop [START_REF] Świerkosz | Vegetation of southern part of the Isalo Sandstone Massif (Central Madagascar, Africa) -differentiation and threats[END_REF][START_REF] Rakotomalala | Grasses of the Isalo National Park, Madagascar -checklist, origins, and significance[END_REF]. A total of forty-seven (47) species of pteridophytes were recorded by [START_REF] Lebigre | Répertoire des plantes du Sud-Ouest de Madagascar[END_REF] for the entire of Southwestern Madagascar (Tulear province to north of Tsingy de Bemaraha and east to Isalo). Several herbarium specimens of pteridophytes from Isalo massif are held in various herbaria (TAN, P, MO) (see locations below) but there has been no dedicated investigation of the pteridological flora of the area. Documenting pteridophyte diversity is crucial for the Isalo massif National Park since it will enhance its value and potentially open up research avenues such as ethnobotany or studies on ecosystemic services. In order to determine richness of the pteridophyte of the Isalo massif and how those communities are organised in the canyons, this study aims to achieve the following objectives: (1) produce a check-list of ferns and lycophytes found in Isalo National Park using our new collected samples and herbarium records, (2) investigate the diversity and distribution patterns of pteridological communities of the eastern canyons of Isalo, depending on canyons and the plots characteristics.

MATERIAL AND METHODS

Study area

Isalo National Park is located in southwestern Madagascar, lying between 22°13 -22°45 South and 45°13 -45°29 East and belongs to the Ihorombe region (Ihosy district) and the Atsimo-Andrefana region (Ankazoabo district) (Fig. 1A). Covering an area of 815,4 km 2 , the Isalo National Park is the fourth largest protected area in Madagascar, and is managed by Madagascar National Parks since 2017 (MNP 2017). It was one of the first protected areas in Madagascar, since 1962, and was declared a national park in 1991 [START_REF] Rakotomalala | Grasses of the Isalo National Park, Madagascar -checklist, origins, and significance[END_REF]. The Isalo massif belongs to the youngest series (Triassic-early Jurassic) of the Malagasy Karoo formation [START_REF] Sourdat | Sur l'evolution du massif de l'Isalo et du bassin de l'Onilahy (Sud-Ouest de Madagascar)[END_REF][START_REF] Wescott | Depositional history and stratigraphical evolution of the Sakamena group (Middle Karoo Supergroup) in the southern Morondava Basin, Madagascar[END_REF], and consists of crossbedded sandstones and conglomerates up to 6000 m thick which are considered a fluviatile origin [START_REF] Geiger | Reappraisal of the timing of the breakup of Gondwana based on sedimentological and seismic evidence from the Morondava Basin, Madagascar[END_REF]). The deposits have been sculpted by both water and wind, creating a majestic ruiniform relief, where the plateau grassland is dissected by a network of deep canyons, up to 200 m deep (Fig. 2A,B). The elevation of the Isalo National Park ranges from 510 to 1268 m (Świerkosz 2012) with two alternating contrasted seasons, a wet season (October-March) and a dry season (April-September), hence its subhumid climate [START_REF] Koechlin | Flore et végétation de Madagascar[END_REF]) (Fig. 1B,C). On the coldest days, fog can form in the canyons, buffering the effects of the dry season (Fig. 2C). The vegetation of the Isalo massif belongs to the western slopes of the Central Domain [START_REF] Humbert | Les Territoires Phytogéographiques de Madagascar. Leur Cartographie. Colloque sur les Régions Ecologiques du Globe[END_REF], in a regional subarid climate [START_REF] Cornet | Divisions floristiques et étages de végétation à Madagascar[END_REF]. Świerkosz (2012) recognised six vegetation types, based on a general transect of the different geomorphological micro-reliefs of the Isalo: secondary bush on the lowest slopes accessible to anthropogenic activities, rock communities (Pachypodium, Aloe) on the top of the highest ruiniform relief, sclerophyllous woodland (Uapaca) and pseudosteppe (Aristidia group) on the plateau, gallery forest (Pandanus) or evergreen humid forest in the canyons or river valley [START_REF] Rakotomalala | Grasses of the Isalo National Park, Madagascar -checklist, origins, and significance[END_REF]) (Fig. 2A,D). This study explored six canyons that open on the southern part of the eastern face of the Isalo Massif, which is a very steep cliff abruptly cut by a few East-West canyons. From south to north these canyons are named as Namaza canyon, Maki canyon, Rats canyon, Anjofo canyon, Andramanero canyon, Antsifotra canyon (Supplementary material 1A).

Taxonomic sampling and analysis

The checklist of the pteridophytes from Isalo was compiled from all herbarium specimens held at the Malagasy National Nerbarium Antananarivo, Madagascar (TAN) and the MNHN herbarium, Paris, France(P), the Missouri Botanical Garden, St Louis, USA (MO), new collections from this study and online databases (Tropicos, Sonnerat, Jstor). Family nomenclature follows PPG I (2016). Species nomenclature follows Rakotondrainibe et al. (in press) for the most recent update and [START_REF] Rakotondrainibe | Checklist of the pteridophytes of Madagascar[END_REF] for distributional accuracy.

Ecological survey

Sampling design

Ecological surveys were adapted from [START_REF] Rakotondrainibe | Contribution à la connaissance de la flore ptéridologique de Madagascar: étude floristique, biologique, phytogéographique et écologique des fougères de la forêt d'Ambohitantely (forêt tropicale d'altitude[END_REF], [START_REF] Rakotondrainibe | The pteridophytes of the eastern slope of the Reserve Naturelle Intégrale d'Andringitra, Madagascar. A floral and faunal inventory of the eastern slope of the Reserve Naturelle Integrate d'Andringitra, Madagascar, with reference to elevational gradient[END_REF]1998[START_REF] Reeb | Ecological survey of the Lycophytes and Ferns of the Vohimana Reserve, Madagascar[END_REF]. A plot is an area established in the riverbed of the canyon. Its central axis was determined by a 20 m string placed in the centre of the riverbed (Fig. 2D). The lateral limits of the plot are defined by the highest flood marks. Abiotic characteristics and fern abundance were recorded (see below). Four plots of 20 m long were successively sampled in a homogeneous part of the canyon, both in terms of vegetation and topology. Finally, an area of at least 800 m 2 in similar conditions is reached, which is considered the minimal area for fern studies, even along a river [START_REF] Rakotondrainibe | Contribution à la connaissance de la flore ptéridologique de Madagascar: étude floristique, biologique, phytogéographique et écologique des fougères de la forêt d'Ambohitantely (forêt tropicale d'altitude[END_REF][START_REF] Reeb | Ecological survey of the Lycophytes and Ferns of the Vohimana Reserve, Madagascar[END_REF].

Plot type definition

The openness of a canyon can influence the variation in shading of pteridophyte community patterns due to differences in preferences between heliophilous and sciaphylous species [START_REF] Dai | Taxonomic, Phylogenetic, and Functional Diversity of Ferns at Three Differently Disturbed Sites in Longnan County, China[END_REF]. Five types of plots were defined geomorphologically, from open plots (Type 1) to semi-open plots (Types 2, 3) and closed plots (Types 4, 5) (Supplementary material 2).

Fern abundance

The number of individuals of each fern species was recorded for each plot. In the case of ferns with long creeping rhizomes, each frond was considered as an individual, as the "true" individual is born from the development of a spore, data which is not accessible in the field. To limit observer bias, individuals of each species in each plot were counted 3 to 4 four times by 3 to 4 pairs of observers. The mean of the observers' counts was then calculated to estimate the number of individuals for each species in each plot.

Ecological factors

To understand the factors that shape the diversity and distribution of pteridophyte communities, we considered ecological factors related to the vegetation, morphology and riverbed characteristics of a canyon (Table 1). The riverbed is the area permanently or temporarily occupied by water. Two main parts can be defined in a riverbed [START_REF] Piegay | Learning to live with rivers, a matter of geomorphology, Encyclopedia of the Environment[END_REF]: the minor riverbed which is permanently occupied by flowing water in the lowest parts of the canyon, and the major riverbed which is only filled with water during floods. These higher limits are generally easy to find in the canyons because the debris load of the highest floods is still embedded in the terrestrial vegetation and because of the change in terrestrial vegetation between flood affected/non-affected zones.

To assess the effect of light penetration in the canyon on pteridophyte diversity, we developed an index L (L-index) which is related to the combination of canopy and rock/cliff cover that act as barriers to light penetration in the canyon. To measure this L index, we developed a simple and repeatable method based on the analysis of canopy images using Fiji-Image J (Supplementary material 3) [START_REF] Schindelin | Fiji: an open-source platform for biological-image analysis[END_REF].

Descriptions of the ecological factors are given in Table 1. The data analysed in this paper are presented in two different data tables (Supplementary materials 4 and 5). The plots are the individuals (rows); in Supplementary material 4, the ecological factors are the variables, quantitative and qualitative; in Supplementary material 5, the abundance of fern species is the variable.

Statistical analysis

The analysis of the pteridophyte community and its relationships with ecological factors is mainly based on ordination methods. These methods allow the transformation of multivariate data into an interpretable form that revealing patterns and relationships within the data (Thioulouse et al. 2018a). As fern abundances ranged from 0 to 6406 (due to a few very common tufted species or to small Hymenophyllaceae), the range of the variation in species abundance was reduced by applying a square root transformation to the abundance table [START_REF] David | Data treatment in environmental sciences: multivaried approach[END_REF].

Analysis of similarity between plots according to ecological factors

An exploratory analysis of the ecological data was first carried out using a Hierarchical Clustering on Principal Components (HCPC). The HCPC is a clustering method that we use to generate a dendrogram that groups plots into a class according to the similarity of their ecological gradients [START_REF] Garderner | Community ecology. Analytical Method using R and Excel[END_REF]. We first performed a Factor Analysis for Mixed Data (FMAD), which is an ordination method belonging to the family of principal components used to identify patterns in data containing both numerical and categorical variables [START_REF] Kassambara | Practical Guide to Principal Component Methods in R: PCA, M (CA), FAMD, MFA, HCPC, Factoextra[END_REF]. This FAMD reduces the dimension in the data to produce a matrix output that can be used by the HCPC to generate clusters. The FMAD and the HCPC were performed using the factoextra and FactomineR R package [START_REF] Lê | FactoMineR : An R Package for Multivariate Analysis[END_REF]) with a maximum category max = 6, as there were six geographical catchments for the canyons. The geographical localisation of each plot (the canyon watershed) was plotted against the tree obtained.

Pteridophyte community analysis

First, we defined the general pattern of species composition in the canyon catchment and then explored the abundance of each species in each plot. To do this, we performed a Correspondence Analysis (CA) from the ade4 package (Thioulouse et al. 2018b) on the fern abundance dataset (Supplementary material 5). CA is an ordination method that maximises the correspondence between species score and plot scores [START_REF] Garderner | Community ecology. Analytical Method using R and Excel[END_REF]) revealing patterns of plot-species affinities. The abundance dataset was first reordered according to the coordinates of the first CA axes to produce a figure showing the distribution of each species in each plot. Secondly, we measure the similarity of species composition at two levels: at the 6 canyons catchment level and at the plot types level (Supplementary material 1 & 2) by using Non-Metric Multidimensional Scaling (NMDS). This produces ordination graphs that highlight dissimilarities between classes (i.e. dissimilarities between the 6 canyon catchments on one hand and between plot types on the other hand in terms of species composition). NMDS was performed on a matrix of Bray-Curtis distance metrics. Species abundances was used to generate a Bray-Curtis distance metrics matrix which is then used for NMDS. Bray-Curtis distance is a dissimilarity metric suitable for species abundance data collected at different sampling locations [START_REF] Garderner | Community ecology. Analytical Method using R and Excel[END_REF]). The NMDS process was performed using the metaMDS function from the R-vegan package [START_REF] Oksanen | vegan: community ecology package[END_REF], with 999 numbers of random starts and three dimensions were conserved to obtain a meaningful stress value. A non-parametric test, one-way analysis of similarity (ANOSIM, with 9999 permutations) was performed to test the significance of NMDS ordination result.

Canyons' flora diversity comparison

To understand the distribution of species in the pteridophyte community, we use diversity indices to compare species diversity in the six canyons (Maki, Rats, Andramanero, Namaza, Anjofo, Antsifotra). The sample size differs from canyon to canyon, due to the difference in sampling effort. A rarefaction method was used because it provides a standardisation of the sample size to overcome the influence of sample size differences when comparing species diversity [START_REF] Gotelli | Quantifying biodiversity: procedures and pitfalls in the measurement and comparison of species richness[END_REF][START_REF] Colwell | Models and estimators linking individual-based and sample-based rarefaction, extrapolation and comparison of assemblages[END_REF]). An individual-based abundance rarefaction curve for each canyon was performed, using iNExtPD package in R [START_REF] Hsieh | iNEXT: an R package for rarefaction and extrapolation of species diversity ( H ill numbers)[END_REF]. Three Hill Numbers q (effective number of species) were calculated: species richness (q = 0), Shannon diversity (q = 1, the exponential of Shannon entropy) and Simpson diversity (q = 2, the inverse of Simpson concentration) [START_REF] Chao | Rarefaction and extrapolation with Hill numbers: a framework for sampling and estimation in species diversity studies[END_REF]). These three Hill numbers allow us to compare community diversity by considering species richness, species evenness, and species dominance in the community.

Investigation of the links between the pteridophyte communities and the ecological factors of the canyons

Co-inertia analysis was used to explore the relationship between fern community patterns and the factors that may control them. Co-inertia analysis takes two multivariate data-sets as an input and searches for the pair of new axes that maximises the concordance between the two data-sets. The optimisation criterion in co-inertia analysis is that the resulting sample scores (ecological scores and floristic scores) are the most covariant [START_REF] Dolédec | Co-inertia analysis: an alternative method for studying speciesenvironment relationships[END_REF].

The ecological dataset contains both quantitative and qualitative data, so a Hillsmith analysis was used [START_REF] Dray | The ade4 Package: Implementing the Duality Diagram for Ecologists[END_REF]. A PCA was performed on the fern abundance dataset. We maximise covariance between the two data tables and calculate the correlation coefficient R (between 0 and 1; the closer R is to 1, the stronger the correlation between the tables). The test of the significance of co-inertia was performed using a 100 co-inertia analysis after random permutation of the rows in both tables. For each run, the R 2 value was calculated and the frequency distribution of these values for the two first co-inertia axes was projected. If the observed value was higher than 95% of the values obtained for the permuted data-sets, then the co-inertia was considered significant [START_REF] Dolédec | Co-inertia analysis: an alternative method for studying speciesenvironment relationships[END_REF]. Data analyses were performed using the ade4 package in the R software.

RESULTS

Species composition

Checklist

Sixty (60) species of ferns (Table 2A) and lycophytes (

Comparison of ecological gradient at the level of canyon and at plot (or station) scale

Five clusters are identified by hierarchical clustering of plots according to ecological factors (Fig. 3A), which are well separated when individuals are projected (Fig. 3B). Most of the series of successive plots are in the same clade (e.g. [7-10], [24-28], [62-65]), confirming the choice of similar conditions of successive plots, in order to reach the 800 m 2 minimum area. Most of the plots are grouped by canyon: the majority of the plots within the Maki and Rats canyons are grouped together (Fig. 3A cluster 4) as well as the Namaza plots (Fig. 3A cluster 5). Series [24 to 28] from Rats canyon and series [68 to 71] from Antsifotra canyon are open plots (type 1) outside of the cliff area (Fig. 3B). However, a few plots are not grouped with their neighbours (e.g. plot 2 regarding [START_REF] Rakotondrainibe | Contribution à la connaissance de la flore ptéridologique de Madagascar: étude floristique, biologique, phytogéographique et écologique des fougères de la forêt d'Ambohitantely (forêt tropicale d'altitude[END_REF]3,4]). The similarities between the composition of the fern communities at the scale of the six canyons catchments is analysed with NMDS (Fig. 5A). The stress value s = 0,11 (< 0,2) for k = 3, indicates that the ordination is not random. The ANOSIM value for the similarity analysis with the grouping by canyon is R = 0.2486, p = 0, 0001; the p-value (significant value) is lower than 0,05, and R is higher than p, which means that there are significant statistical differences between the fern communities of the six canyons. Some superposition is observed, but the all the centrtes of gravity of each catchment are distinct.

Comparison of fern communities at the level of canyon and at plot scale

In a second analysis, when the composition of the fern communities is projected with NMDS according to plot type (Fig. 5B), the open plots (type 1, Supplementary material 2) clearly host different communities from plots in the internal parts of canyons. The ANOSIM value for analysis of similarities with grouping by type of plots is R = 0,2533, p = 0,0001; here again, there is significant statistical differences between the fern communities according the plot type. The communities of type 2 and 3 communities overlap, and the communities of type 4 and 5 appear to be very similar (Fig. 5B).

Figure 5. A. NMDS analysis according to the six canyons. B. NMDS analysis according the type of plots. The horizontal axis x represents the first dimension extracted by the NMDS analysis. This dimension is constructed so that the distance or dissimilarity between the points on this axis reflects as closely as possible the original dissimilarity between plots. In A colored according to the canyons, in B colored according to the type of plot. The vertical axis is the second dimension extracted from the NMDS analysis, also designed to reflect the dissimilarity between the plots, but it is orthogonal to the X axis, which means that it captures another source of dissimilarity relative to the X axis. It is important to remember that the axes in an NMDS plot are used to visualise the relative similarity or dissimilarity of the objects analysed (here plots) in a reduced dimensional space, and have no direct meaning in terms of specific variables.

Species diversity comparison

Rarefied species diversity values (Table 3) give an overview of the diversity pattern between the six canyons. The species richness accumulation curve (Fig. 6, q = 0) shows that Anjofo's canyon has the highest number of pteridophyte species. Then, Andramanero, Maki and Rats have almost the same species richness, and finally, Namaza and Antsifotra canyon have the lowest number of species. However, the accumulation curve of the Shannon diversity index (Fig. 6, q = 1) shows that despite its low species richness, the pteridophyte community of Antsifotra canyon has a higher species evenness and is therefore more diversified than the communities of Andramanero, Maki, Rats and Namaza. Of all of the canyons, Anjofo's pteridophyte community is the most diversified because of it has the highest species evenness and species richness. Furthermore, in contrast to the central (Maki + Rats) and the southern (Namaza) canyons, the pteridophyte flora in the northern canyons (Anjofo, Andramanero, Antsifotra) is more diverse.. The same pattern is seen in the accumulation curve of the Simpson diversity index (Fig. 6, q = 2), which indicates that the pteridophyte communities in the central and southern canyons tend to be dominated by a few species.

Exploring the links between the pteridophyte communities and the ecological factors

The co-inertia analysis (Fig. 7) measures the concordance between the two datasets and shows a covariance between the two tables (Fig. 7D); the first axis accounts for 42 % of coinertia and the second for 22 % (64 % for cumulative projected inertia for the first two axes). The Monte-Carlo test shows a consistent correlation between the ecological factors of the plots and the fern communities (RV = 0.29, p = 0.0001). The correlation is significant, fern distribution is correlated with a combination of environmental factors and not specifically with just one, although the influence of a few major factors can be highlighted (Fig. 7A,B).

On the first axis the influence of the canopy cover is inversely correlated with the orientation and the presence of rocks in the minor or major riverbed (Fig. 7A,B). On the second axis, the difference D between the width of the major and minor riverbeds is inversely correlated with the distance to the mouth of the canyon. The presence of vegetation (trees, pandanus, grasses) is positively correlated on axis 1, with lower values.

In open plots, species such as Equisetum ramosissimum and Thelypteris interrupta are clearly associated with a high L-index, sandy substratee and the presence of vegetation. Marsilea minuta occurs in pools in open areas, and is negatively correlated with the distance from the mouth of the canyon. It means that Marsilea minuta becomes rarer as we move further towards the bottom of the canyon.

On the other hand, on the negative part of axes 1 and 2, species such as Ptisana fraxinea, Cyathea sp. are strongly correlated with the very narrow canyons, surrounded by high vertical cliffs that shade the canopy. Also assiociated with the prensence of rocky outcrops that provide anchorage are Ptisana fraxinea and Didymoglossum erosum. Osmunda regalis and Stenochlaena tenuifolia are associated with large major riverbeds. Odontosoria chinensis, one of the most common ferns, is probably more linked to rock presence. Blotiella isaloensis and Lygodium lanceolatum, two lianeceous species seem to be correlated with the presence of rocks in the riverbed, both minor and major. It confirms the sensitivity of fern species to environmental factors, such as rocks, sand or trees, the presence of grasses, and orientation which can influence the light intensity during the day.

DISCUSSION

This study contributes to the understanding of the pteridophyte flora in south-eastern Madagascar. The Isalo massif has a significant richness of 60 species compared to other geomorphologically similar areas. For example, the Makay massif, a larger sandstone massif to the north of Isalo, was reported to have only 38 species [START_REF] Roubaud | Mise en place d'un dispositif de connaissance, suivi et évaluation socio-économique et environnemental de la Nouvelle Aire Protégée (NAP) du Makay, Madagascar[END_REF]. Similarly, the Bemaraha Tsingy, a ruiniform limestone area in the mid-west of Madagascar, was reported to have only 16 species (Tsingy de Bemaraha National Park https://www.madacamp.com/Tsingy_de_Bemaraha_National_Park [accessed 20.12.2022]). The low number of species recorded in these areas may be due to the lack of comprehensive surveys, with the exception of exploratory studies conducted in the Makay Massif in 2011 and 2017 (Roubaud el al. 2018, Tropicos http://legacy.tropicos.org/Person/100099987). The high diversity of pteridophytes in Isalo can be attributed to the patchwork of ecosystems of the massif ranging from shady, cool and humid environments in narrow canyons to disturbed riverbanks, dry and hot open savannas, and rocky slopes (Fig. 2). The lower community diversity in the southern canyon (Namaza, Maki, Rats) might be explained by the higher human activity in these frequently visited areas.

The low rate of endemism in the Isalo massif (16 % for Isalo, ~ 50 % for the Madagascar fern flora) may be attributed to the geomorphology of the area which has an influence on the tradewinds at both local and regional scales. On a regional level, the absence of significant relief (> 1500m) between the Isalo massif and the eastern coast of Madagascar allows the trade winds blowing from the east to reach the massif, thus facilitating the wind-borne transport of pteridophyte spores even from other countries or continents (Fig 1C ,1D) [START_REF] Donque | Contribution géographique à l'étude du climat de Madagascar[END_REF][START_REF] Guillaumet | Renaud Paulian et le programme du CNRS sur les hautes montagnes à Madagascar: étage vs domaine[END_REF]. The favorable moisture conditions found within the canyons of Isalo [START_REF] Ferry | Water Resources Variability in Africa during the XXeme Century[END_REF] create a suitable environment for the germination of these spores. On a local scale, the low vegetation (compared to a dense forest where trees and leaves act as significant barriers) and the absence of physical barriers on the slopes, domes, and plateaus or in the through valleys of the Isalo canyons may promote dispersal of spores by wind [START_REF] Karst | Fern community assembly: the roles of chance and the environement at local and intermediate scales[END_REF][START_REF] Millerón | Effect of canopy closure on pollen dispersal in a wind-pollinated species (Fagus sylvatica L.)[END_REF][START_REF] Robledo-Arnuncio | Space, time and complexity in plant dispersal ecology[END_REF].

Cyclones reaching the Isalo massif, free of any relief obstacles, dense forests, in all directions play a role in spore recruitment and dispersal [START_REF] Donque | Contribution géographique à l'étude du climat de Madagascar[END_REF][START_REF] Walker | Ferns, disturbance and succession[END_REF]).

Together, these factors reduce genetic isolation, which is a major driver of speciation and endemism.

In particular, the fern communities of Isalo show a variety of affinities with the different phytogeographical domains (Table 2). About 92 % of the fern communities in Isalo are found in the Central Domain of the high plateaus, 42 % in the eastern dense rainforests and the Sambirano region (e.g. Didymoglossum erosum, Asplenium erectum, Gymnosphaera boivinii, Osmonlinsaea latisquamata); 38 % in the Western domain with dense dry forests (e.g. Selaginella helicoclada, Selaginella digitata); 10 % in the Southern Domain with bushes and shrubs (e.g. Cheilanthes perrieri) [START_REF] Humbert | Les Territoires Phytogéographiques de Madagascar. Leur Cartographie. Colloque sur les Régions Ecologiques du Globe[END_REF]Cornet 1972;[START_REF] Rakotondrainibe | Checklist of the pteridophytes of Madagascar[END_REF]Rakotondrainibe et al., in press). The strong affinity (92%) with the central and eastern domains is again due to the geographical position and geomorphology of the Isalo massif, which allows the humid easterly trade winds to reach the massif and pour into the canyon valleys of east-facing canyons that we have studied, which are then subject to a fairly subhumid climate, similar to that of the Central Domain (Fig. 1B). Such an intrusion of a subhumid climate influence into an arid regional context has also been observed in the Daraina region (Binara forest) in northern Madagascar [START_REF] Goodman | Inventaires de la faune et de la flore du nord de Madagascar dans la région Loky-Manambato, Analamerana et Andavakoera[END_REF][START_REF] Rakotondrainibe | Premières observations sur la flore ptéridologique de la forêt de Binara (région de Daraina, nord-est de Madagascar): modèle de distribution altitudinale des espèces dans une formation de transition entre les régions orientale et occidentale in S Goodman & L Wilmé, Inventaire de la faune et de la flore du nord de Madagascar dans la région Loky-Manambato, Analamera et Andavakoera[END_REF] or in the Sambirano region.

Apart from these strong affinities, some species such as Odontosoria chinensis, Stenochlaena tenuifolia are present from north to south in many riparian ecosystems, at the interface between the flowing water and the beginning of the major riverbed as well as in other tropical countries [START_REF] Walker | Ferns, disturbance and succession[END_REF][START_REF] Reeb | Ecological survey of the Lycophytes and Ferns of the Vohimana Reserve, Madagascar[END_REF]Rakotondrainibe et al. in press). These riparian species belong to the azonal wetland system [START_REF] Moat | Atlas of the vegetation of Madagascar Kew[END_REF] which means that specific local ecological conditions (soil, substrate) influence the communities more than regional ecological factors [START_REF] Macintyre | The biomes of Western Australia: a vegetation-based approach using the zonality/ azonality conceptual framework[END_REF].

Ferns are the most abundant Tracheophytes found in riverbeds, due to their widely dispersed spores and their high frequency of vegetative reproduction [START_REF] Walker | Ferns, disturbance and succession[END_REF]. Coinertia analysis confirms the relationships between local ecological conditions (plots) and the presence of fern species (Figs. 7B, 9C), with canopy cover and riverbed width exerting a stronger influence. The L-index (canopy and rocks/cliff cover) is a factor that influences the presence of pteridophytes in a plot, which contrasts with the conclusions of other studies of riparian pteridophyte communities that focused on a single river portion each (Paixo et al. 2013). This difference may be attributed to the environmental heterogeneity of plots within each canyon (Fig. 3) and their different geomorphological characteristics, where shading is provided by high cliffs surrounding narrow plots (narrow riverbeds) or by tree shade in open areas or by a combination of tree shade and cliffs.

Finally, ferns also reflect human disturbance, even in a privileged, protected environment such as the Isalo massif. The creeping fern Pteris aquilinum, the scrambling ferns Dicranopteris linearis and Sticherus flagellaris are characteristic of anthropogenic disturbance [START_REF] Walker | Ferns, disturbance and succession[END_REF], and are favoured by activities such as slash-and-burn or deforestation [START_REF] Autrey | 1 Flore des Mascareignes: La Réunion[END_REF][START_REF] Robinson | Problem ferns: their impact and management[END_REF][START_REF] Reeb | Ecological survey of the Lycophytes and Ferns of the Vohimana Reserve, Madagascar[END_REF][START_REF] Yang | Rethinking the Ecosystem Functions of Dicranopteris, a Widespread Genus of Ferns[END_REF]. Although these ferns are regularly present in the Isalo canyons, they occur in small populations. Monitoring the presence of such ferns can provide insights into ecosystem evolution, with an increase indicating an increase in anthropogenic disturbance. However, [START_REF] Yang | Rethinking the Ecosystem Functions of Dicranopteris, a Widespread Genus of Ferns[END_REF] argues that the invasive colonization of barren areas by Dicranopteris could contribute to ecosystem recovery, providing an interesting nuance to the general perception of the negative effects of Dicranopteris invasion.

CONCLUSION AND PERSPECTIVES

The Isalo massif hosts a mosaic of original fern communities, including both common and rare species that exploit specific local conditions. The study confirms the complex interactions between a multiplicity of factors at different scales to explain the structutr of pteridophyte communities. The regional conditions (regional climate, position of Isalo in the south-west of Madagascar), the local scale level caused by the different canyon conditions (geomorphology, orientation etc.), the fine-scale influences at the plot level, the biology and niche requirements of each fern species, their evolutionary history (e.g. dispersal) ultimately influence the presence or absence of a fern species in a particular area. Thanks to the protected status of Isalo National Park, the canyons of Isalo are mostly subject to natural disturbances, such as extreme flooding. The biology of ferns gives them advantages to survive in such unstable environments [START_REF] Walker | Ferns, disturbance and succession[END_REF], but indirect threats can affect the canyon environment, such as siltation due to erosion of domes, plateau or slopes, after deforestation, or repeated fires. We have witnessed several fires in the Andramanero area during the fieldwork. Sand favours the establishment of species such as Equisetum ramosissimum, Pteridium aquilinum which can be good indicators of siltation. Other species such as Didymoglossum erosum, a filmy fern, could be good candidates for studying the evolution of the canyon's climate and the impact of climate change on species biodiversity; this species has a low tolerance to dry atmosphere, and its potential loss could alert to such changes. Tree ferns could also be monitored, especially to control potential and illegal exploitation (no data were found on this subject).

The accessible eastern part of the Isalo Park was investigated for this study and in those of [START_REF] Rakotomalala | Grasses of the Isalo National Park, Madagascar -checklist, origins, and significance[END_REF]. Considering the geomorphology of the studied area (exposure to trade-winds, foehn effect on the eastern facade), it seems necessary to explore the western façade, which is not exposed to the same conditions. The fern flora there may reveal surprises and very different patterns of diversity, as shown by a first exploratory study in 2023 (Rouhan, pers. com.). Our study focused on the valleys of the canyons: the summits, the slope grasslands and the rocky formations were not investigated by the plots protocol, but only by exploratory collections. In the dry season, ferns in these areas are quite difficult to see and to collection and exploration during or after the rainy season will probably reveal new information about this amazing sandstone massif. Finally, it would be very interesting to compare the fern diversity of the Isalo massif with the larger sandstone massif of the Makay north of the Mangoky river, in a very similar geomorphological and geological context. It has recently shown that the aquatic Adephaga (aquatic Coleoptera) diversity was lower in the Makay than in Isalo [START_REF] Ramahandrison | The aquatic Adephaga of the Makay, central-western Madagascar, with description of two new diving beetle species (Coleoptera, Gyrinidae, Haliplidae, Noteridae, Dytiscidae)[END_REF], and climatic or site considerations (Makay is much drier than Isalo, the substrate of the canyons is mostly sandy, enrichment of the water with iron bacteria…) have been proposed to explain this difference. The study of the fern flora of Makay would allow interesting comparisons to be made. At the same time, the exploration of the northern and western parts of the Isalo National Park could reveal more diversity including endemic species.

Figure 1 .

 1 Figure 1. A. Location of Isalo National Parc in Southwestern of Madagascar. B. Distribution of the phytogeographic domain in the Southern Madagascar showing the incursion of the Central Domain (subhumid) in Isalo Massif and Analavelona area; the elevation gap < 1500 m between northern and southern mountains let the trade winds blow from East to West. C. Simplified isohyets (mm/year) for the Southern Madagascar, from the eastern escarpment to the Western coast, showing the highest humidity in Isalo National Parc (and Anavelona area) (modified from Guillaumet et al 2008). D. Profile view of a West-East transect passing through Isalo massif to the Indian Ocean (Google earth profile view).

Figure 2 .

 2 Figure 2. Landscapes of Isalo National Parc A. Andramanero canyon seen from the summit. B. Entrances of the Makis and Rats canyons on the eastern face of Isalo. C. Fog in early morning along the eastern face of the massif. D. View of a plot in Andramanera canyon.

  the proportion of the sky not obscured by cliffs and canopies River regime in the dry season Ordinal: (1) water completely evaporated, (2) still stagnant river, (3) running water, without stream ripples, (4) running water with stream ripple or small rapids. herbaceous layer, (2) scattered to discontinuous herbaceous layer, (3) continuous herbaceous layer Seepage presence, used as a supplementary variable Binary: (1) presence (2) absence Orientation Discrete, not ordinal: (1) N-S, (2) NW-SE, (3) W-E, (4) SW-NE, (5) S-N Slope Numeric Distance to opening Numeric (meters); the position of the origin is just the opening of the cliff; negative values are considered in the plain, positive values are considered in the canyon

Figure 3 .

 3 Figure 3. A. Hierarchical clustering tree of the plots according to ecological factors, 5 clades are numbered; the plots labels are highlighted according to their canyon (legend below the plots); the plot type is reported in the bottom row. B. Projection of the plots grouped by clades of the HCPC on the first two dimensions of the FMAD.Ordination of pteridophyte species abundanceDifferent patterns of fern abundance emerge with respect to plot ordination (Fig.4. The pteridophyte flora of the eastern canyon's of Isalo is mainly dominated by Odontosoria chinensis and Stenochlaena tenuifolia. However, some canyons differ from this pattern: this is the case of Antsifotra's canyon which is dominated by Thelypteris interrupta and Nephrolepis bisserata and in which Stenochlaena tenuifolia and Odontosoria chinensis have very low abundances (see further discussion). Some species are restricted to a single canyon: Osmunlindaea squamata and Thelypteris arbuscula only in the southern Namaza catchment; Didymoglossum erosum and Asplenium buettneri only in the Maki canyon; Doryopteris kitchingii in Anjofo.

Figure 4 .

 4 Figure 4. Ordination of the fern abundance table on the first axis of the CA analysis, the x-axis shows the species identified, the y-axis shows the number of plots, colored according to their location (= the canyons they were sampled). The size of the black squares are proportional to species abundance.

Figure 6 .

 6 Figure 6. Rarefaction curves for the six canyons showing the diversity detected compared with the predicted total diversity. The x axis represents the number of individuals recorded while the y axis represents a measure of the species richness detected, estimated with the Chao1 index.

Figure 7 .

 7 Figure 7. Co inertia analysis. A. Scatter plot representing the coefficients of the combinations of the variables for ecological variables. The colors are only informative for easier reading: "green" = vegetation presence; "brown"= riverbed dimensions; "violet" = type of plot. B. Scatter plot representing the coefficients of the combinations of the variables for fern species abundance. C. Result of the permutation test; the RV coefficient appears as different that RV expected by chance. See Supplementary material 8 for plot position in the coinertia.

Table 1 .

 1 List of ecological factors considered and their description.

  Table2B) are recorded for the Isalo massif including ten fern species endemic to Madagascar (16 % of endemism for the Isalo massif). Thirty-eight (39) species were inventoried during the fieldwork of which 11 species are reported for the first time in the Isalo massif: Asplenium erectum, Asplenium aff. aethiopicum, Doryopteris kitchingii, Equisetum ramosissimum, Lygodium kerstenii, Macrothelypteris torresiana, Nephrolepis biserrata, Sticherus flagellaris, Thelypteris arbuscula, Thelypteris dentata, Thelypteris unita. Pteridaceae, Thelypteridaceae, Denstaediaceae families are the most represented in terms of richness (Supplementary material 7A). At the genus level, Pellea then Thelypteris are the two most important by their number of species (Supplementary material 7B).The pteridophyte flora of the Isalo Park shows a high affinity with that of the Central Domain. (Table2A, 2B): 92 % of Isalo's fern communities are found in the Central Domain, 42 % in the eastern dense rainforests and Sambirano; 38 % in the Western Domain with dense dry forests; 10 % in the Southern Domain with scrub.

Table 2A .

 2A Check-list of ferns from the Isalo massif. The nomenclature of species and family is based onRakotondrainibe et al. (in press) using PPG1 (PPG1 2016). When possible, the corresponding nomenclature from[START_REF] Rakotondrainibe | Checklist of the pteridophytes of Madagascar[END_REF] is given, together with the distribution in Madagascar, outside the Isalo massif(data 2003). Family names are fromRakotondrainibe et al. (in press). A selection of specimens is given in Supplementary material 6

			Gleichenia polypodioides (L.) Sm.	Gleicheniaceae
	Gymnosphaera rouhaniana (Rakotondr. & Janssen) S.Y.Dong	END	not recorded	Cyatheaceae
	Histiopteris incisa (Thunb.) Sm.		Histiopteris incisa (Thunb.) Sm.	Dennstaediaceae
	Macrothelypteris torresiana (Gaudich.) Ching		Macrothelypteris torresiana (Gaudich.) Ching	Thelypteridaceae
	Marsilea minuta L.		Marsilea minuta L.	Marsileaceae
	Microlepia speluncae (L.) T.Moore		Microlepia speluncae (L.) T.Moore	Dennstaediaceae
	Species (Rakotondrainibe et al. in press) Nephrolepis biserrata (Sw.) Schott Odontosoria chinensis (L.) J.Sm.	Endemic (END = endemic in Madagascar)	Species (Rakotondrainibe 2003) Nephrolepis biserrata (Sw.) Schott Sphenomeris chinensis (L.) Maxon	Family Nephrolepidacea e (Rakotondrainib e et al, in press) Lindsaeaceae
	Actiniopteris dimorpha Pic.Serm. Oleandra distenta Kunze		Actiniopteris dimorpha Pic.Serm. Oleandra distenta Kunze	Pteridaceae Oleandraceae
	Adiantum capillus-veneris L. Alsophila dregei (Kunze) R.M.Tryon Osmolindsaea latisquama Lehtonen & Rouhan Alsophila hyacinthei R.M.Tryon var. Osmunda regalis L. var. obtusifolia (Kaulf.) Milde hyacinthei Pellaea angulosa (Bory ex Willd.) Baker Anemia madagascariensis C.Chr. Pellaea boivinii Hook.		Adiantum capillus-veneris L. Cyathea dregei Kunze Lindsae odorata auct. Cyathea isaloensis C.Chr. Osmunda regalis L. Pellaea angulosa (Bory ex Willd.) Baker Pellaea boivinii Hook.	Pteridaceae Lindsaeaceae Cyatheaceae Cyatheaceae Osmundaceae Pteridaceae Pteridaceae
	Pellaea calomelanos (Sw.) Link		Pellaea calomelanos (Sw.) Link	Pteridaceae
	Pellaea dura (Willd.) Hook.		Pellaea dura (Willd.) Hook.	Pteridaceae
	Pellaea pectiniformis Baker		Pellaea pectiniformis Baker	Pteridaceae
	Pityrogramma argentea (Willd.) Domin		Pityrogramma argentea (Willd.) Domin	Pteridaceae
	Pityrogramma calomelanos (L.) Link Asplenium formosum Willd. Psilotum nudum (L.) P.Beauv. Asplenium pseudostuhlmanii Viane Pteridium aquilinum (L.) Kuhn subsp. capense (Thunb) C.Chr.		Pityrogramma calomelanos (L.) Link Asplenium formosum Willd. Psilotum nudum (L.) P.Beauv. Asplenium stuhlmannii Hieron. Pteridium aquilinum (L.) Kuhn subsp capense (Thunb) C.Chr.	Pteridaceae Aspleniaceae Psilotaceae Aspleniaceae Aspleniaceae Dennstaediaceae
	Blechnum attenuatum (Sw.) Mett. Pteris pseudolonchitis Bory ex Willd.		Blechnum attenuatum (Sw.) Mett. Pteris pseudolonchitis Bory ex Willd.	Blechnaceae Pteridaceae
	Blechnum tabulare (Thunb.) Kuhn Ptisana fraxinea (Sm.) Murdock		Blechnum tabulare (Thunb.) Kuhn Marattia fraxinea Sm.	Blechnaceae Marattiaceae
	Blotiella isaloensis (Tardieu) P.Roux Stenochlaena tenuifolia (Desv.) Moore	END	Blotiella isaloensis (Tardieu) P.Roux Stenochlaena tenuifolia (Desv.) Moore	Dennstaediaceae Blechnaceae
	Cheilanthes bonapartei P.Roux Cheilanthes hirta Sw. Sticherus flagellaris (Bory ex Willd.) Ching Cheilanthes perrieri P.Roux Thelypteris arbuscula (Willd.) K.Iwats.	END END	Adiantopsis linearis Bonap. Cheilanthes hirta Sw. Sticherus flagellaris (Bory ex Willd.) Ching not recorded Sphaerostephanos arbuscula (Willd.) Holltum	Pteridaceae Gleicheniaceae Pteridaceae Pteridaceae Thelypteridaceae
	Dicranopteris linearis (Burm.f.) Underw. Didymoglossum erosum (Willd.) P.Roux Thelypteris confluens (Thunb.) C.V.Morton Thelypteris dentata (Forssk.) E.P.St. John		Dicranopteris linearis (Burm.f.) Underw. Trichomanes erosum Willd. Thelypteris confluens (Thunb.) C.V.Morton Christella dentata (Forssk.) Holttum	Gleicheniaceae Thelypteridaceae Hymenophyllace ae Thelypteridaceae
	Doryopteris kitchingii (Baker) Bonap. ex C.Chr. Thelypteris interrupta (Willd.) K.Iwats. Elaphoglossum poolii (Baker) Christ. Thelypteris unita (L.) C.V.Morton	END	Doryopteris kitchingii (Baker) Bonap. ex Cyclosorus interruptus (Willd.) H.Itô C.Chr. Sphaerosytephanos unitus (L.) Holltum	Pteridaceae Thelypteridaceae Thelypteridaceae

END Anemia madagascariensis C.Chr. Anemiaceae Arthropteris orientalis (G.F.Gmel.) Posth. var. orientalis Arthropteris orientalis (G.F.Gmel.) Posth. var. orientalis Tectariaceae Asplenium aff. aethiopicum (Burm.f.) Bech. Asplenium aethiopicum (Burm.f.) Bech. Aspleniaceae Asplenium blastophorum Hieron Asplenium blastophorum Hieron Aspleniaceae Asplenium buettneri Hieron. ex Brause Asplenium buettneri Hieron. ex Brause Aspleniaceae Asplenium erectum Bory ex Willd. Asplenium erectum Bory ex Willd. END Elaphoglossum poolii (Baker) Christ. Dryopteridaceae Equisetum ramosissimum Desf. Equisetum ramosissimum Desf. Equisetaceae Gleichenia polypodioides (L.) Sm.

Table 2B .

 2B Check-list of Lycophytes from the Isalo massif. The nomenclature of species and family is based onRakotondrainibe et al. (in press) using PPG1 (PPG1 2016). When possible, the corresponding nomenclature from[START_REF] Rakotondrainibe | Checklist of the pteridophytes of Madagascar[END_REF] is given, together with the distribution in Madagascar(data 2003), outside the Isalo massif(data 2003). Family names are fromRakotondrainibe et al. (in press). A selection of specimens is given in Supplementary material 6

	Species (Rakotondrainibe et al. in press)	Endemic (END = endemic in Madagascar)	Species (Rakotondrainibe 2003)	Family (Rakotondrai nibe et al, in press)	Western Domain
	Lygodium kerstenii Kuhn		Lygodium kerstenii Kuhn	Lygodiaceae	X
	Lygodium lanceolatum Desv.		Lygodium lanceolatum Desv.	Lygodiaceae
	Palhinhaea cernuea (L.) Franco & Vasc.		Lycopodiella cernua (L.) Pic.Serm	Lycopodiacea e
	Pseudolycopodiella caroliniana (L.) Holub		Lycopodiella caroliniana (L.) Pic.Serm.	Lycopodiacea e	X
	Selaginella digitata Spring.	END	Selaginella digitata Spring.	Selaginellacea e	X
	Selaginella echinata Baker	END	Selaginella echinata Baker	Selaginellacea e	X
	Selaginella helicoclada Alston ex Alston	END	Selaginella helicoclada Alston ex Alston	Selaginellacea e	X
				11

Table 3 .

 3 Rarefied species diversity values of the 6 canyons. In brackets, asymptotic diversity estimate values of the 6 canyons: first number = LCL, 95% associated Lower Confidence interval Limit; second number = UCL: 95% Upper Confidence interval Limit.

	Canyon	Rarefied species richness q = 0	Rarefied Shannon diversity q = 1	Rarefied Simpson diversity q = 2

ACKNOWLEDGEMENTS

This study was carried out thanks to the Direction General des forêts de Madagascar which provided the research permit N°1146/19/MEDD/SG/DGEF/DGRNE. The authors thank the Madagascar National Park (MNP) and the Isalo National Park for their welcome and the help in organising this study. We would also like to mention Christophe de Comarmond and Parson from the Satrana Lodge, Ranohira, who facilitated our stay in Ranohira, and our transport. Thanks to the department DEBV (Department of Biology and Plant Ecology), University of Antananarivo, Madagascar. A special mention to Benjamin L. Rice for improving the English writing of the manuscript and to the reviewers for their constructive remarks, that helped us to improve the manuscript. This study was carried out with the support of the French naturalist association TIMARCHA, including all the students and colleagues who participated in the sampling on the field: Elianne Andriamiaja Raharisoa, Juliette Bruneau, Delphine Chang, Noe Diamante, Heloise Duprat, Antoine Lafont, Erwan Lejeune, Pauline Michel, Aurelia Pourriau, Alix Thiebault. We also thank our the MNP guides and our cooks.