The control of external and internal canopy structural heterogeneity on diversity and productivity relationship in a subtropical forest

Forest canopy structure is closely related to species diversity, crown packing efficiency, and ecological processes, while influencing ecosystem functions. However, most existing structural diversity indices only consider internal heterogeneity within the canopy but often neglect the heterogeneity of the external canopy. While, the external canopy heterogeneity can increase the exchange surface area between canopy and external atmosphere, capture more light, which is closely linked to tree growth and ecosystem functions. Comprehensively and accurately estimating canopy structural diversity is crucial in understanding the underlying mechanisms of productivity change. Therefore, in this study, we apply a topographic index – canopy surface rugosity (CSR) – to capture canopy external heterogeneity, and use crown complementary index (CCI) and other canopy structural variables to quantify canopy internal heterogeneity. We find that CSR varies significantly among quadrats (20 ​m ​× ​20 ​m) and it decreases with species diversity and wood net primary productivity (NPP_wood). Species diversity increases NPP_wood directly and indirectly through CSR and CCI. Functional diversity increases NPP_wood through CSR and CCI. Community weighted mean of light demand decreases with NPP_wood through CCI. The mediating effect of CSR on the relationship between species (functional) diversity and NPP_wood is greater than the mediating effect of CCI. We find soil fertility positive effects on NPP_wood mainly through species richness and CSR. Conversely, water availability affects NPP_wood by a direct positive effect and indirect negative effects through species richness, CSR and CCI. Overall, our CSR and CCI provide strong support for a space-based niche partitioning mechanism regulating the relationship between species diversity and NPP_wood. Additionally, CSR is sensitive to gap dynamics, suggesting mechanistic relationships between local disturbance, species diversity, and NPP_wood.