Structural Diversity Amplifies the Effects of Functional Diversity on Forest Carbon Storage Across Stand Development

Aim Forest structural diversity is recognized as a key driver of ecosystem functioning, yet its role in translating functional strategies into ecosystem processes remains poorly understood. This study examines whether structural diversity amplifies the effects of functional diversity (FD) and functional identity (FI) on aboveground carbon storage (AGC) and evaluates the relative contributions of niche complementarity and selection effects across forest types and stand‐development stages. Location Southeastern US, Japan, and Hunan, China. Time Period 1997–2023. Major Taxa Studied Forest trees. Methods We used national forest inventory data (24,023 natural forest plots; 7530 conifer plantation plots) to quantify relationships among AGC, FD, FI, and structural diversity. Analyses were conducted using linear mixed‐effects models, piecewise structural equation modelling, and hierarchical Bayesian methods. Results FD effects on AGC increased with age in natural conifer forests across regions, but diverged in broadleaf forests (declining in the southeastern US and Hunan, increasing in Japan). In natural forests, FD was generally a stronger predictor of AGC than FI. Structural diversity mediated a substantial proportion of FD effects on AGC (33%–100% in broadleaf forests), with mediation peaking earlier in broadleaf than in conifer and mixed forests. Across climatic gradients, indirect effects of FD via structural diversity prevailed over direct effects, whereas FI showed negative indirect effects on AGC associated with conservative strategies. In conifer plantations, FD effects were primarily mediated by structural diversity, whereas FI exerted stronger direct effects on AGC. Main Conclusions The biodiversity–carbon relationship is age‐dependent and forest‐type specific. Structural diversity is a key mediator linking FD to AGC, supporting niche complementarity. Management practices that enhance structural diversity may amplify the carbon benefits of FD. Promoting forests that are both functionally and structurally diverse may therefore enhance long‐term carbon sequestration.