National growth models for stand basal area, volume, and biomass in Chinese larch plantations: integrating stand structure and species effects

Abstract

Forest growth and yield models are essential tools for forest management decision-making, but most models have neglected the inclusion of stand structure. The relationship between stand structure and stand growth has been widely reported, but the results remain inconsistent. The objective of the study was to develop national growth models for stand basal area, volume, and total biomass of four larch species plantations and quantify the effect of stand structure on basal area, volume, and total biomass. After testing nonlinear mixed effects, we developed stand basal area, volume, and total biomass models for Larix gmelinii, Larix olgensis, Larix gmelinii var. principis-rupprechtii, and Larix kaempferi plantations by incorporating tree size differentiation indices (DBH-based Shannon-Wiener index, D_Sh; DBH-based Simpson index, D_Si). Models were calibrated with 2, 129 permanent sample plots from four periods of China's National Forest Inventory. The national-scale models demonstrated high predictive accuracy, with adjusted R² > 0.95, and 10-fold cross-validation confirmed robust generalization without over/underfitting. Stand basal area, volume, and biomass showed significant positive correlations with D_Sh and D_Si. Our approach successfully captured growth differences among the four larch species. We quantified the effects of tree size diversity on stand growth and enable tree species-specific management. These findings highlight that both tree species selection and structural optimization (e.g., regulating D_Sh/D_Si through density control) are critical for enhancing stand productivity. It is recommended to apply thinning regimes to maintain high tree size diversity for promoting stand growth and quality improvement of larch forests nationwide.