Structural dominance and environmental modulation of carbon sequestration potential in subtropical Chinese fir plantations

Cunninghamia lanceolata is an important industrial timber species in the subtropical regions of China, with substantial carbon sequestration value. However, continuous monoculture of Chinese fir plantations results in simplified stand structures and soil nutrient imbalances, reducing their carbon sequestration potential productivity (CSPP). In this study, we estimated the CSPP of 2110 typical Chinese fir plots in subtropical China and analyzed its driving mechanism based on the potential mean annual increment method, machine learning algorithms (random forest), and path models. CSPP demonstrated notable gradient changes across different developmental stages and carbon sequestration grades, ranging from 3.53 to 7.19 Mg ha⁻¹ yr⁻¹. Middle-aged forest stages exhibited the highest carbon sequestration, whereas the peaks for medium and low carbon sequestration occurred at more advanced stages of forest development. Stand structural factors, notably stand age, dominant height, and mean DBH, had a more pronounced effect on CSPP than that of abiotic factors like climate, geo-topographic, and soil, which influenced CSPP indirectly by modifying stand structure. For medium-sized sites and young forests, the combined effect of stand structure on CSPP exceeded 95 %. Therefore, corresponding density management strategies can be applied in this scenario to maximize productivity. For high site grades, with resource saturation, productivity was driven by unquantified genetic sources and intensive management strategies rather than environmental limitations. Overall, the diagnostic framework, focusing on the interaction between stand structure and environment, provides theoretical support for density regulation and adaptive carbon management across developmental stages, thereby enhancing the carbon sink potential and productivity of subtropical plantations. • Young and middle-aged Chinese fir forests are at a high carbon sequestration rate, a critical period for future carbon sink growth. • Stand structural attributes dominated CSPP variation over environmental constraints. • Climatic factors have a significant negative impact on CSPP but a positive impact during the near-mature stages of forest development. • The framework offers density-based management pathways to enhance carbon sequestration potential.