Phylogenetic diversity in the Western Ghats biodiversity hotspot reflects environmental filtering and past niche diversification of trees

Aim We examined how contrasted climatic conditions influenced the ecological and phylogenetic diversity of tropical trees at the regional scale. Beyond the basic expectation of greater environmental filtering in currently stressful contexts, we addressed how biogeographic history and past climates contributed to shaping the distribution and diversity of extant taxa. Location Evergreen forests of Western Ghats, India. Methods We evaluated the relative importance of niche-based, historical, and spatial processes on community phylogenetic structure and turnover in 297 plots including 459 species. If niche conservatism prevails, we expected greater phylogenetic clustering under harsher or historically less stable climates, and overdispersion in opposite conditions. To test the role of environmental filtering in shaping local communities, we assessed species' climatic niches and congruence between niche and phylogenetic diversity. To assess the imprint of ancient versus recent evolutionary constraints, we calculated mean pair-wise (MPD) and nearest-taxon (MNTD) phylogenetic distances, respectively. Results We found non-random phylogenetic structure depending on both current and past climatic variation. Basal community phylogenetic structure (MPD) was related to niche diversity and varied according to hydric stress: (a) northern environments filtered more closely related species, while (b) phylogenetic overdispersion suggested greater niche differentiation in stable, least-seasonal southern habitats and at high elevations. Terminal phylogenetic structure (MNTD) did not show overdispersion. Phylogenetic turnover was driven by current abiotic factors, not space. Main conclusions The patterns of ecological and phylogenetic diversity likely reflect the Miocene climate shift to increasing seasonality northwards. Phylogenetic clustering under hydric stress or historical instability suggests the influence of functional conservatism, whereas overdispersion in southern forests supports the persistence of old lineages in an ancient rain forest refugium as an evolutionary museum. Overdispersion in fragmented montane forests reflects a mixture of biogeographically distinct species pools, both tropical and temperate, that characterise environmentally contrasted cores and fringes. Low phylogenetic turnover along the seasonality gradient suggests recent ecological diversification across forests with contrasted rainfall seasonality.