Stand Structure and Type Drive Productivity in Chinese Fir Forests: Comparison of Pure and Mixed Stands.

Understanding the main factors that influence productivity across stand types is essential for effective management strategies. Chinese fir (Cunninghamia lanceolata) is a key afforestation species in subtropical China. However, comparative studies on productivity differences and the underlying mechanisms among different Chinese fir stand types remain scarce. In the present study, using second-class forest resource survey data from Ganzhou, southern China, we classified plots dominated by Chinese fir into three stand types: pure Chinese fir (541 plots), Chinese fir-broadleaf mixed (351 plots), and Chinese fir-conifer mixed stands (232 plots). We then assessed productivity differences among stand types and applied the Boruta feature selection and Random Forest modeling to identify the main influencing factors, followed by structural equation modeling (SEM) to evaluate their direct and indirect effects. The results indicated that stand productivity differed significantly among stand types, with stand density, dominant diameter, stand age, Gini coefficient, and mean tree height being the most influential factors. The productivity of pure stands was more sensitive to climatic and site conditions, whereas mixed stands were more strongly influenced by structural heterogeneity. These findings highlight the critical role of stand structure in regulating productivity and suggest that enhancing density regulation in all stands and promoting structural heterogeneity in mixed stands can improve productivity and stability, with mixed-stand management offering greater potential for long-term productivity gains and resilience to climate change.