Seasonal drought regulates species distributions and assembly of tree communities across a tropical wet forest region

AIM: How seasonal drought influences tree species distributions might shape future vegetation composition with impending changes to rainfall patterns. Theory suggests that across a seasonal drought gradient, dry‐tolerant species might have wider distributions than species sensitive to seasonal drought, which will be restricted to less seasonal conditions. As wet‐associated species drop out with increasing seasonality, the community will contain mainly dry‐tolerant species. TIME PERIOD: Present. LOCATION: Western Ghats Biodiversity Hotspot in peninsular India. MAJOR TAXA STUDIED: Woody angiosperms. METHODS: Across a 1,200 km seasonal drought gradient, we modelled occurrences of 183 species using Gaussian logistic regression in a hierarchical Bayesian framework. We used the first‐order model coefficients to attribute the association of species to seasonality (dry/wet) and second‐order coefficients to assess modal responses. For 92 species, we checked for an interaction between dry/wet association and two traits: wood density and specific leaf area (SLA). Finally, we assessed the consequences of the seasonality associations of species for regional community assembly. RESULTS: Of 183 species, 38% were significantly wet associated, 27% dry associated and 34% indifferent to seasonality. Nearly 80% of species had estimated modes of occurrence within the seasonality range considered here. Species with lower SLA and lower wood density were more likely to occur in more seasonal sites where dry‐associated species formed the majority of co‐occurring species. The proportion of wet‐associated species increased only after intermediate levels of seasonality. The community proportion of extreme wet‐associated species decreased prominently from less to more seasonal sites. MAIN CONCLUSIONS: Dry‐associated species persisted in less seasonal sites, but not vice versa. A shift toward more seasonal rainfall might favour dry‐associated species and shrink the ranges of wet‐associated species. Future work should assess how the intensity and duration of drought and interactions with other drivers of global change regulate the relative performance of species at more and less seasonal sites.