The synergistic effect of species diversity and functional diversity in subtropical naturally regenerating forests promotes environmental adaptability of biomass accumulation

• Environmental heterogeneity drives niche differentiation and biodiversity patterns in subtropical forests. • Planted forests exhibit 5.4% higher biomass (93.41 ± 3.87 Mg·ha⁻¹) but greater vulnerability to environmental stress. • Naturally regenerating forests leverage phylogenetic diversity and functional trait complementarity to enhance ecological stability • Synergistic effects of climate, soil, biodiversity, and functional diversity promote biomass accumulation. • Forest management prioritizes protecting northwest naturally regenerating forest hot-spots and optimizing vulnerable southern plantations. Ecological ground-based monitoring is essential for understanding the interactions between biodiversity and carbon sequestration, providing insights to address biodiversity loss and improve forest quality. This study aimed to explore the synergies and driving mechanisms of carbon sequestration and biodiversity in subtropical naturally regenerating and planted forests of different ages or successional stages. Based on 326 forest ecosystem plots, we quantified the spatial-temporal heterogeneity of tree species biodiversity and biomass, as well as the driving effects of environmental factors and stand structure, using Mantel test, multiple linear regression model, partial least squares path modeling (PLS-PM), and Hot-spot analysis. Our findings revealed that species richness in subtropical forests of Hunan ranged from 5.04 to 5.71. Mantel test indicated that stand structure and environmental factors influenced biodiversity by enhancing environmental heterogeneity and niche differentiation. The planted forests (93.41 ± 3.87 Mg·ha -1 ) exhibited higher biomass than naturally regenerating forests (88.21 ± 4.60 Mg·ha -1 ), but were more directly influenced by environmental factors due to reduced environmental adaptation. PLS-PM results revealed that in naturally regenerating forests, species diversity (standardized path coefficient: 0.19, p < 0.05) and functional diversity (0.24, p < 0.05) combined with soil (0.43, p < 0.001) promoted biomass accumulation. This study explored how niche theory and complementarity effects drove biomass accumulation in naturally regenerating forests by enhancing environmental adaptation. Hot-spot analysis advocated the need for forest management to prioritize naturally regenerating forests, ensuring the sustainability of biodiversity and ecosystem functions. Our study emphasized the adoption of a dual strategy of optimizing planted forests structure and protecting naturally regenerating forests to enhance subtropical forest ecosystems’ role in addressing global change and regional sustainability.