Local- and landscape-scale drivers of terrestrial herbaceous plant diversity along a tropical rainfall gradient in Western Ghats, India

In tropical forests, understory herbaceous angiosperms (herbs), which can comprise up to 40% of plant species richness, have received relatively little attention compared with trees, and their diversity patterns and drivers remain poorly understood. While tropical tree diversity has been shown to be driven primarily by water availability, we hypothesized that herb diversity may be equally or more limited by light availability. To test the importance of water and light in shaping herb diversity, we surveyed herb communities in 13 one-ha plots along a rainfall gradient in a seasonally dry tropical forest landscape in India. In each plot, herbs were censused thrice during the year in 47-50 one-m(2) subplots. We examined drivers of herb diversity and ground cover at the landscape scale (across plots) and local scale (among subplots) using simple linear regression and linear mixed models, respectively. At the landscape scale, herb diversity and ground cover were negatively related to rainfall and soil moisture, in contrast with previous studies of tropical forest trees. This indicates that water was not limiting for herbs. Instead, light availability, which was negatively correlated with rainfall, appears to drive patterns of herb diversity and ground cover across the gradient. Herb diversity and ground cover were uniformly low across the gradient during the dry season, indicating that water limitation does have a seasonal impact. When water was abundant (rainy season), herbs took advantage of higher light availability at more open (lower-rainfall) plots. Consistent with landscape-scale patterns, herb richness at the subplot level was positively related to light availability, with its effect stronger at higher-rainfall (more closed-canopy) sites. Herb species richness declined with increasing subplot soil moisture in all sites and seasons, suggesting negative impacts of water logging and high moisture. Synthesis. Diversity of understory herbs is limited by light availability at both local and landscape scales in this region, with the impact of water limitation restricted to the dry season. Our study shows that tree diversity patterns cannot be assumed to hold for other plant lifeforms, and conservation and restoration efforts must consider unique strategies for different lifeforms.