Chronic disturbance of moist tropical forests favours deciduous over evergreen tree communities across a climate gradient in the Western Ghats

Abstract

It is well established that climatic factors such as water stress and chronic anthropogenic disturbances such as biomass extraction influence tropical forest tree community structure, richness and composition. However, while the standalone effects of these two drivers on plant communities are well-studied, their interactive effects are not. Moist tropical forests in Indias Western Ghats face a dual threat from increasingly erratic precipitation (and consequent water stress), and an intensifying anthropogenic footprint. Here, we sampled 120 tree plots (0.05 ha each) across forests with varying histories of biomass extraction and a gradient in climate water deficit (CWD, a proxy for water stress) within a 15,000 km2 landscape in the northern Western Ghats and examined whether and how disturbance history modulates relationships of tree community structure and composition with climate. As expected, tree species richness increased with decreasing water stress in less- and historically-disturbed forests but remained low in repeatedly-disturbed forests. The increase in evergreen species richness with decreasing water stress was far slower in repeatedly-disturbed forests than other categories, and the relative abundance of evergreens in the repeatedly-disturbed forests (25%) was half that in less-disturbed forests (50%) of comparable water stress in the driest parts. Overall, we show that disturbance can amplify threats from climate change to wet forest-associated evergreen tree species, many of which are threatened, while benefiting more widely distributed dry forest-associated deciduous species. In the northern Western Ghats, where much of the remaining forest cover is disturbed and dominated by deciduous tree species, the persistence of evergreen tree flora hinges on protecting existing evergreen forest patches from future disturbances and restoring locally appropriate evergreen species in secondary forests.