Assembly mechanisms of epiphyte communities in a lowland tropical forest (Yunnan, SW China): distribution patterns, microclimatic drivers, and conservation

Forest canopy, known as the "last frontier of biology", harbours approximately 40% of the
world's terrestrial biodiversity, making it one of the most species-rich yet least-known habitats in the biosphere. Forest canopies function as typical examples of habitat islands, characterized by a range of micro-niches that segregate along sharp vertical gradients, the isolation of host trees from each other, and the variation of host trees over time during their ontogenetical development. Epiphytes therefore offer an ideal framework to test the contributions of environmental filters, biotic interactions and dispersal limitations to community assembly and species coexistence at fine spatial scales. While the vertical structures of epiphyte communities and, to a lesser extent, associated variations in microclimatic conditions, have long been documented, no analysis has examined the relationship between microclimatic variation and species composition in a spatially explicit framework due to the unavailability of high-resolution microclimatic data. The development of a network of canopy cranes, coupled with recent advances in microclimatic modelling, opens an avenue of research for an enhanced understanding of the ecological functioning of forest canopies.
In order to examine how environmental variation in time and space impacts the distribution and diversity of epiphytes at different nested spatial scales, I took advantage of a tropical canopy crane to document the distribution of bryophyte and vascular epiphytes, record and model spatiotemporal variation of microclimatic, light and photosynthetically active radiation in a 1.44 ha tropical rainforest, Yunnan, SW China. More specifically, the aims are to: (1) Determine whether variations in light, microclimatic conditions and host tree size affect the variation in species composition and phylogenetic structure of epiphytic communities, and hence, assess the contribution of environmental filtering, phylogenetic constraints and competition to community assembly; (2) Disentangle the contribution of ecological factors, dispersal limitations and biotic interactions to epiphytic species distributions; (3) Integrate these results to determine whether we can evaluate the factors that predict epiphyte species richness and abundance based on site-specific and tree-specific features, in order to provide a scientific basis to identify key drivers for the conservation and management of epiphytes in forest ecosystems.
Generalized Dissimilarity Models implemented to analyse the relationship between taxonomic and phylogenetic turnover among epiphytic communities, host-tree characteristics and microclimatic variation, highlighted the crucial role of microclimates in determining the composition and phylogenetic structure of epiphytic communities. The analyses evidenced the role of phylogenetic niche conservatism in community assembly, raising concerns about the evolutionary capacity of epiphyte communities to adapt to climate change.
Community modelling analyses revealed that niche preference explains more than 78% of the non-random species pairs formed by epiphytic bryophytes. Biotic interactions contributed less than 20% of the observed co-occurrence patterns, with a strong prevalence of facilitation over competition. The results suggest that epiphytic bryophyte community composition is primarily driven by niche-based processes, whose importance increases with niche complexity and diversity. Biotic interactions play a secondary role, with a very marginal contribution of competitive exclusion. This challenges the idea that competition would be important enough to have a negative impact on the accumulation of species richness on trees following an optimum on middle age trees. The results further suggest that biotic interactions vary depending on lineages and life forms, facilitation prevailing among species from the same lineage and life form and competition among species from different lineages and life forms.
Finally, I integrated these results to evaluate the factors that predict epiphyte species richness and abundance based on site-specific and tree-specific features. The results confirm the utmost importance of large trees with emergent canopies for the conservation of the epiphytic flora, but also indicate that epiphytic diversity assessments in tropical forests must also include small understorey trees, which should be further considered for conservation. The importance of the micro-climatic conditions that prevail at the level of each individual host tree further points to the necessity of maintaining a buffer zone around large host trees targeted for conservation.