Tree structural diversity mediates vegetation carbon storage in dry-hot valley savannas along an elevational gradient

Understanding the drivers of vegetation carbon pool (VCP) distribution is critical for climate change mitigation. While abiotic factors are well-studied, the biotic mechanisms, central to the biodiversity-ecosystem functioning (BEF) relationship, and their interaction with the environment remain less explored, especially in savanna ecosystems. This study investigates these BEF relationships and mechanisms along an elevational gradient (400-1700 m) in the dry-hot valleys of Southwest China, a savanna region with high carbon storage potential. Based on data from eighty 10 m×10 m plots, we assessed how environmental factors (climate and soil), plant diversity (species richness and Shannon index), and stand structure diversity (variation in tree diameter and height) shape VCP distribution. We found no significant elevational trend in total VCP, while its components exhibited divergent distributional patterns: tree VCP decreased while shrub VCP increased with elevation. Crucially, biotic factors outweighed abiotic factors in driving VCP patterns. We found significantly positive BEF relationships: both species diversity and structural diversity were positively correlated with VCPs of woody plants and trees. Notably, structural diversity was a stronger predictor than species diversity and mediated the latter's relationship with carbon storage, highlighting a key mechanism—optimized spatial resource use—through which biodiversity enhances carbon storage. In contrast, these positive diversity effects were not observed for shrub VCP. The total VCP was predominantly driven by tree structural diversity rather than shrub factors, affirming the dominant functional role of trees in this ecosystem. Furthermore, elevation indirectly shaped VCP patterns by regulating both soil properties and the key biotic drivers, demonstrating the environmental dependency of these BEF relationships. These findings provide strong evidence for BEF relationships in a savanna ecosystem, establishing plant diversity, particularly structural diversity, as a critical regulator of carbon sequestration. This underscores the importance of integrating biodiversity conservation, with a focus on maintaining complex stand structure, into carbon-oriented management strategies for dryland ecosystems.