Species mixing enhances aboveground biomass via structural heterogeneity in a subtropical forest

Forest productivity often increases with tree species mixing. However, the structural mechanisms through which species mixing reorganizes stand structure across hierarchical levels and thereby regulates forest productivity and carbon storage remain unresolved. Here, we combined high-resolution UAV-LiDAR surveys with Dagum Gini decomposition in a subtropical evergreen broad-leaved forest to partition stand structural heterogeneity into inter- and intra-specific components. We found that species mixing generated contrasting structural responses across hierarchical levels, amplifying size differentiation among species while reducing size variation within species. The resulting increase in inter-specific heterogeneity was the dominant pathway promoting aboveground carbon accumulation, consistent with realized niche complementarity and more efficient space use. By contrast, intra-specific structural convergence exerted a negative effect on carbon storage, likely reflecting growth suppression under intensified neighborhood competition. Overall, species mixing enhanced aboveground biomass because the benefits of species-level structural stratification outweighed the costs of population-level homogenization. Our results highlight hierarchical structural reorganization as a key mechanism linking biodiversity to forest productivity.