Background: Tropical old-growth forest ecosystems are essential for global carbon regulation. Even there are mounting evidences for the significance of species and functional composition, stand structure and elevation gradients on aboveground carbon storage, the relative strengths of these drivers and whether elevation effects via biotic factors are not clear. Furthermore, the mechanisms (the mass-ratio hypothesis or niche complementarity hypothesis) are still poorly understood. Methods: We

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... yzed aboveground carbon storage, species diversity, stand structural diversity, community-weighted mean (CWM) of functional traits and functional diversity (FDvar) using date from 56 old-growth forest communities with different elevation gradients in Dawei mountain of southwestern China. Multiple regression models were used to test the relative importance of the predictor variables and structural equation model was used to explore the direct and indirect influences on the aboveground carbon storage. Results: Our optimal multiple regression model show aboveground carbon storage is mostly affected by diameter at breast height (DBH) diversity, followed by FDvar of dry matter concentration in mature leaves and CWM nitrogen concentration in young leaves. The final structural equation model indicates elevation indirectly affected aboveground carbon storage via DBH diversity. The stand structural diversity, but not species diversity or functional diversity, enhanced aboveground carbon storage. Conclusions: Our results indicate mass-ratio and niche complementarity effect promote aboveground carbon storage simultaneously. The complex stand structure and species with leaf nitrogen conservative strategy were the crucial drivers of aboveground carbon storage in tropical old-growth forests.