Abstract: Understanding how multidimensional biodiversity influences forest biomass carbon storage is crucial for sustainable forest management. However, empirical evidence on the mechanisms and relative contributions of three dimensions of tree diversity (species, functional, and structural) to forest biomass stocks across varying environmental gradients remains limited. This study used forest inventory data from 94 subtropical natural forest plots with an area of 0.6 hectares, and analyzed the impacts of multidimensional diversity, climate, soil, and topography on above-ground biomass across wider geographical gradients. We found that species, functional, and structural diversity significantly promoted aboveground biomass. Structural diversity showed a better association with above-ground biomass relative to other single variables and was linked to the species richness-aboveground biomass relationship. These findings support the niche complementarity effect and selection effect simultaneously. High water availability and soil fertility promoted species richness, functional diversity, and above-ground biomass. Three dimensions of tree diversity and biomass displayed either no trend, a linear increase, or a hump-shaped pattern with rising elevation, failing to support the species-energy hypothesis. Our findings highlight that structural diversity is a critical driver promoting forest biomass stocks across broad spatial ecogeographical gradients. Moreover, adequate water availability and favorable soil resource conditions are essential for maintaining biodiversity and ecosystem functioning.

Key words: biodiversity-ecosystem function, biomass, elevation, function traits, structural diversity, subtropical forest, soil phosphorus