Dominant role of stand structure in biomass carbon storage of subtropical natural Quercus forests in South China

• Stand structure has a dominant effect on carbon storage of natural Quercus forests. • Interactions Interaction between stand structure factors, environmental factors, and the two factors themselves affects the carbon storage of natural Quercus forests. • Stand structure factors affecting the carbon storage capacity of natural Quercus forests have specific ecological thresholds. Forest carbon sequestration plays a crucial role in mitigating climate change. Understanding the mechanisms driving biomass carbon storage of subtropical natural Quercus forests is vital for improving their carbon sequestration capacity. This study examined 170 natural Quercus forest plots in Hunan Province, a subtropical region of China, to elucidate the regulatory influence of stand structure and environmental factors on forest carbon storage, including aboveground, belowground, and total forest carbon storage. We systematically gathered plot data and employed Spearman’s correlation analysis, machine learning method, SHAP analysis, and threshold analysis to quantify the contributions and interaction effects of stand structure and environmental factors, identifying the threshold ranges of key determinants. The Spearman’s correlation results revealed that stand structure shows an overall positive relationship with forest carbon storage. However, the machine learning (XGBoost) and SHAP analysis further demonstrated that this impact is nonlinear and stage-dependent, accompanied by distinct threshold effects. The findings demonstrate that stand structural attributes explain over 90 % of the variation in forest carbon storage. Key drivers include mean diameter at breast height (13.54–13.86 cm), stand density (1082–2407 trees hm −2 ), age, tree height, stand layering, and Quercus basal area ratio, exhibiting significant threshold effects. Additionally, second-order interactions among stand structure factors, environmental factors, and between the two categories exert important influences on forest carbon storage dynamics. The research concludes that stand structure is pivotal in regulating forest carbon storage in subtropical natural Quercus forests. Identifying its threshold offers a precise foundation for forest management, enhances the understanding of forest carbon sink mechanisms, and provides scientific support for achieving carbon neutrality in forest management.