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

Understanding the factors that drive distributional variation in terrestrial vegetation carbon pools (VCPs) is crucial for assessing the potential of carbon sequestration and mitigating climate change. Although previous studies have extensively examined elevational patterns of VCPs and their abiotic drivers, the biotic mechanisms that drive VCPs remain poorly understood, especially in savanna ecosystems. Characterized by a unique "tree-shrub-grass" plant composition, the savannas in the dry-hot valleys have high productivity and carbon storage capacity. In this study, we investigated VCP distribution and the factors that drive these distributions along an elevational gradient (400-1700 m) in the dry-hot valleys of Southwest China. We found no significant elevational pattern in the total VCP, whereas tree and shrub VCPs exhibited divergent distributional patterns. We also found that VCP distribution was more strongly affected by structural diversity than by species diversity, indicating that structural diversity mediates the relationship between species diversity and VCP distribution. These findings suggest that optimized spatial resource use enhances carbon storage. Notably, the total VCP was more influenced by tree structural diversity than by shrub structural diversity, highlighting the dominant role of trees in savanna carbon sequestration. Furthermore, elevation shaped the VCP patterns by regulating both soil and biotic factors. These findings provide valuable insights for carbon-oriented conservation and restoration practices in dryland ecosystems.