The two lineages monilophytes (ferns and horsetails) and lycophytes (clubmosses, 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 12,000 species. They are reliable bioindicators, closely linked to environmental factors, readily recognizable and monitored, particularly in riparian ecosystems (Paixão et al. 2013; Della and Falkenberg 2019; Bonari et al. 2022).

Madagascar is recognised as one of the world’s biodiversity hotspots (Myers et al. 2000) displaying over 90% endemism for various animal taxa (e.g. 100% for amphibians, 92% for reptiles; Vences et al. 2009) and 82% for vascular plants. Endemism is lower for spore-dispersed plants, e.g. 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 (Aldasoro et al. 2004; Bauret et al. 2017; Rouhan and Gaudeul 2021). Rakotondrainibe (2003) 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 (Humbert 1955) shows that approximately 85% of the pteridophytes found in Madagascar were reported from the Central and Eastern domains (Rakotondrainibe and Raharimalala 1996, 1998; Rakotondrainibe 2000a, 2000b, 2002, 2003; Reeb et al. 2012), which are characterized by humid or subhumid forests and a high altitudinal range (Humbert and Cours Darne 1965; Cornet 1973; Du Puy and Moat 1996). 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 (Humbert and Cours Darne 1965; Cornet and Guillaumet 1976; Du Puy and Moat 1996).

This study was conducted in Isalo National Park, which is located in the southwestern part of Madagascar. Isalo National Park (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 (Sourdat 1970). 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 (Świerkosz 2012; Rakotomalala et al. 2022). A total of 47 species of pteridophytes were recorded by Lebigre (2010) 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) but there has been no dedicated investigation of the pteridological flora of the area. Documenting pteridophyte diversity is crucial for the Isalo National Park since it will enhance its value and potentially open up research avenues such as ethnobotany or studies on ecosystem services.

In order to determine the richness of the pteridophytes of the Isalo massif and how those communities are organised in the canyons, this study aims to achieve the following objectives: (1) produce a checklist of ferns and lycophytes found in Isalo National Park using newly collected samples and herbarium records, (2) investigate the diversity and distribution patterns of pteridological communities of the eastern canyons of Isalo, depending on the ecological gradient.

This study contributes to the understanding of the pteridophyte flora in southeastern 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 (Roubaud et al. 2018). Similarly, the Bemaraha Tsingy, a ruiniform limestone area in the mid-west of Madagascar, was reported to have only 16 species (Madacamp 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 et al. 2018; Tropicos https://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, and 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 that has an influence on the trade winds at both local and regional scales. On a regional level, the absence of significant relief (> 1500 m) 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, D) (Donque 1978; Guillaumet et al. 2008). The favourable moisture conditions found within the canyons of Isalo (Ferry et al. 1998) 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 (Karst et al. 2005; Millerón et al. 2012; Robledo-Arnuncio et al. 2014). Cyclones reaching the Isalo massif, free of any relief obstacles, dense forests, in all directions play a role in spore recruitment and dispersal (Donque 1978; Walker and Sharpe 2010). 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 (Tables 2, 3). 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, Osmolindsaea latisquama); 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) (Humbert 1955; Cornet 1973; Rakotondrainibe 2003; 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 sub-humid climate, similar to that of the Central domain (Fig. 1B). Such an intrusion of a sub-humid climate influence into an arid regional context has also been observed in the Daraina region (Binara forest) in northern Madagascar (Goodman and Wilmé 2006; Rakotondrainibe and Andriambolantsoa 2006) 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 (Walker and Sharpe 2010; Reeb et al. 2012; Rakotondrainibe et al. in press). These riparian species belong to the azonal wetland system (Moat and Smith 2007), which means that specific local ecological conditions (soil, substrate) influence the communities more than regional ecological factors (Macintyre and Mucina 2021).

Ferns are the most abundant tracheophytes found in riverbeds, due to their widely dispersed spores and their high frequency of vegetative reproduction (Walker and Sharpe 2010). Co-inertia analysis confirms the relationships between local ecological conditions (plots) and the presence of fern species (Fig. 7B, C), 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 (Paixão 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 (Walker and Sharpe 2010), and are favoured by activities such as slash-and-burn or deforestation (Autrey and Bosser 2008; Robinson et al. 2010; Reeb et al. 2012; Yang et al. 2021). 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, Yang et al. (2021) 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.

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 structure of pteridophyte communities. The regional conditions (regional climate, position of Isalo in the southwest 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 (Walker and Sharpe 2010), 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 and 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 National Park was investigated for this study and in those of Rakotomalala et al. (2022). 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 (Germinal Rouhan pers. comm.). Our study focused on the valleys of the canyons: the summits, the slope grasslands, and the rocky formations were not investigated by the plot 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 (Ramahandrison et al. 2022), 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.