LB 10-120 - CANCELLED - Local and landscape-scale drivers of terrestrial herbaceous plant diversity along a tropical rainfall gradient in Western Ghats, India

Background/Question/Methods:

Much of our understanding of the patterns and drivers on tropical plant diversity comes from trees, despite the fact that trees form only a quarter of all tropical-forest plant species. While studies have found water availability to be the main driver of tropical tree diversity, it is unclear whether this can be extrapolated to other lifeforms, especially understory herbaceous angiosperms (herbs), which in spite of forming up to 40% of plant biodiversity in some forests, have received relatively little attention. We hypothesized that herb diversity may be equally or more limited by factors besides water, namely light availability. To test the relative importance of water and light in shaping tropical-forest herb diversity, we surveyed understory herb communities in 13 one-hectare plots along a rainfall gradient in a seasonally dry tropical forest in India. We used simple linear regression to examine how landscape-level variation in understory herb diversity and plant cover corresponded with rainfall, and a linear mixed model to examine whether their micro-site level variation was driven by soil moisture and light availability.

Results/Conclusions:

We found that understory herb diversity and plant cover decreased linearly with increasing rainfall at landscape scale (R² = 0.81 and 0.54, respectively), indicating that water was not limiting. Instead, herb diversity and plant cover were likely driven by understory light, which negatively correlated with rainfall (r = -0.97). This is supported by our microsite-scale results, where herb diversity increased with increasing light and decreased with increasing soil moisture (marginal R² = 0.16), and plant cover increased with increasing light (marginal R² = 0.24). Our results contrast with those of previous studies of trees across the same gradient, as well as in other tropical forests, which found mostly increasing tree diversity with increasing water availability. Light limitation appears to be the main stress for tropical-forest understory herbs, limiting their diversity at both local and landscape scales. This study shows that (1) tree-diversity patterns cannot be assumed to hold for other plant lifeforms, and (2) plant conservation efforts may need to employ different strategies for different lifeforms.