Environmental gradients structure tropical tree assemblages at the regional scale

Questions: Environmental filtering and niche differences influence assembly of tree communities at multiple spatial scales. At regional scales, determining the environmental variables that primarily influence compositional variation in species assemblages reveals ecologically relevant habitat types for conservation. Strength of species' association with these habitats and differences in species occurrence along environmental gradients lend credence to niche-based assembly and help estimate responses of tree communities to natural or human-mediated environmental change. Location: Western Ghats, India. Methods: We analysed an openly available data set of ca. 62 000 trees (>10 cm DBH) in 96 1-ha plots across a 22 000 km(2) landscape, using multivariate regression trees (MRT) to identify compositional groups related to combinations of temperature, rainfall and soil type. Next, we conducted indicator species analysis to assess species associations with habitats. We then compared species richness and diversity among habitats. Finally, to discern niche overlap we assessed pair-wise differences between species in their abundance distributions along environmental gradients. Results: Environmental gradients clearly influenced landscape-scale tree assemblages in this region. Nine ecological habitat types were identified: annual rainfall was the most important variable driving compositional differences, followed by temperature, seasonality and soils. Approximately 17% of 398 species tested were associated with single habitats. Number of species associations and local diversity varied significantly among habitats. Between 29 and 50% of species pairs showed significant differences in their distributions along environmental gradients considered. Conclusions: Relating compositionally defined tree assemblages to combinations of abiotic variables allows ecologically robust recognition of habitat types for conservation planning. Compositional differences structured by annual rainfall and temperature suggests that climate change can impact species persistence and tree community composition in the Western Ghats. Specifically, future research should assess drought response of forests by examining recruitment and survival of species in different combinations of rainfall, temperature and human disturbances.