The effect of climatic gradients, topographic variation and species traits on the beta diversity of rain forest trees

Aim We assessed the rates of turnover of tree species with distance (beta diversity) in wet forests of the Western Ghats (WG) complex of India to see whether climate, topographic variation or species traits influence beta diversity. Location The Western Ghats is a chain of mountains about 1600 km in length, running parallel to the western coast of the Indian Peninsula from above 8 degrees N to almost 21 degrees N latitude. Methods We used data from 60 small plot inventories concentrated in three regions: the southernmost part of the Western Ghats (SWG) (8 degrees 24' to 9 degrees 37' N), the Nilgiri Hills (11 degrees 12' to 11 degrees 14' N), and the central Western Ghats (CWG) (12 degrees 32' to 14 degrees 51' N). We used Sorensen's index (SI) to estimate the similarity in species composition between two plots and regressed SI against the logarithm of the distance between plots to assess beta diversity. A bootstrapping procedure provided confidence intervals for regression coefficients. To test for the effects of climate, we regressed seasonality differences between plots against SI for low-elevation (< 800 m) plots along the north-south axis, and all plots in the SWG. We assessed the impact of the rainfall gradient in the Kogar region. Results Among all three regions, beta diversity was highest along the latitudinal axis, and along the rainfall gradient in the Kogar region. Differences in seasonality between sites were strongly related to beta diversity along the north-south seasonality gradient and within the SWG. Within the three regions, beta diversity was highest in the region with the strongest rainfall gradient and lowest for the topographically heterogeneous SWG. Beta diversity did not differ between forest strata and dispersal modes. Main conclusions We conclude that climate, particularly seasonality, is probably the primary driver of beta diversity among rain forest trees of the Western Ghats complex.