β-diversity of herbaceous versus woody plant communities across a tropical rainfall gradient

Aim: Most of our current knowledge of tropical forest plant communities is based on trees, despite the substantial contribution of other life forms to plant diversity in these systems. In particular, studies of understory herbaceous plants are limited. With their lower dispersal abilities, higher rates of evolution, and lower drought tolerance than trees, herbs are expected to exhibit different patterns of species composition across space. The aim of the study is to compare the patterns and drivers of variation in species composition (i.e., β-diversity) of trees and understory herbaceous plants.
Location: Mudumalai, Western Ghats, India.
Time period: 2018 - 2019.
Major taxa studied: Angiosperms.
Methods: We surveyed tree and understory herb communities in 13 one-ha plots along a strong rainfall gradient in a seasonally dry forest landscape in the Western Ghats, India. In both groups, we estimated among-plot β-diversity, which we decomposed into two components: turnover and nestedness. Then we partitioned the relative influences of spatial and environmental predictors, including rainfall, temperature, soil, and fire frequency, on β-diversity.
Results: Contrary to our expectations, β-diversity was remarkably similar for herbs and trees, and both groups exhibited high turnover along the gradient. Rainfall and temperature explained the most variation in composition within both groups, while fire and soil explained less variation, and their effects differed between groups.
Main Conclusions: While trees and herbs show contrasting patterns of α-diversity across this rainfall gradient, our study suggests that both life forms are impacted strongly by environmental filtering, predominantly rainfall and temperature, resulting in similar patterns of β-diversity. The high turnover observed in tree and herb communities, and the influence of rainfall and temperature in structuring these communities, should be considered when designing conservation and restoration strategies in the face of ongoing global changes and other anthropogenic pressures on tropical forests.