Climate and soils determine aboveground biomass indirectly via species diversity and stand structural complexity in tropical forests

Theoretical and empirical studies have suggested that climate and soils are the main drivers of biodiversity, stand structure and aboveground biomass in natural forests. Yet, the direct effects of climate and soils on aboveground biomass versus the indirect effects mediated by species diversity and stand structural complexity remain unclear in forest ecosystems across large-scale ecological gradients. Here, we hypothesized that (1) climate and soils would influence aboveground biomass through strong indirect effects; (2) stand structural complexity rather than species diversity would strongly mediate the response of aboveground biomass to climate and soils; and (3) species diversity and stand structural complexity would promote each other under the niche differentiation and facilitation effects, and that stand structural complexity would have positive effects on aboveground biomass across large-scale ecological gradients. To test these hypotheses, we quantified climatic water availability, soil total exchangeable bases, species diversity, stand structural complexity including tree DBH (diameter at breast height) diversity and height diversity, and aboveground biomass across 907 plots in tropical forests of Hainan Island, Southern China. We tested 126 structural equation models to examine the direct and indirect effects of climate and soils on aboveground biomass via species diversity and stand structural complexity. Climatic water availability and soil fertility did not affect aboveground biomass directly but did affect indirectly via increasing stand structural complexity rather than species diversity. Species diversity and stand structural complexity promoted each other, and both increased with increasing climatic water availability. Stand structural complexity increased aboveground biomass directly, whereas species diversity increased it indirectly via increasing stand structural complexity. The total effects of climatic water availability, soil fertility, stand structural complexity and species diversity on aboveground biomass were significantly positive. This study shows that climatic water availability exerts a strong direct effect on stand structural complexity, indicating that any decrease in climatic water availability (i.e. increasing atmospheric drought) may directly diminish stand structural complexity and hence indirectly reduce aboveground biomass and carbon storage. This study suggests that maintaining high stand structural complexity can enhance aboveground biomass under favourable climate and soils while maintaining the benefits of species diversity on stand structural complexity for better ecosystem services such as carbon storage.