Effects of individual-tree competition and spatial structure on the DBH growth of Cunninghamia lanceolata

To clarify the influence of individual-tree competition and spatial structure on the growth of Cunninghamia lanceolata diameter at breast height (DBH), and to provide theoretical basis and practical recommendations for the fine management and sustainable development of Cunninghamia lanceolata forests.

Taking 22 fixed sample plots of Cunninghamia lanceolata trees in Kaihua County as the research object, based on the 2014-2019 forest resources inventory data, we calculated breast diameter growth indexes (breast diameter growth, breast diameter squared growth and its logarithmic conversion form), 10 competition indexes (7 distance-independent and 3 distance-related) and 4 spatial structure indexes (breast diameter size ratio, breast diameter size differentiation, mixing degree, and angular scale). Variables were screened by optimal subset regression, Pearson correlation analysis was used to investigate the correlation between each factor and the indicators of breast diameter growth, severely collinear independent variables were eliminated, and the relative contribution of each factor to breast diameter growth was quantified by the linear mixed-effects model and variance decomposition method.

The optimal subset regression identified key factors influencing various DBH growth indicators. Spatial structure indices and competition indices (C2, C5, C6, and C17) jointly affected multiple growth indicators, while initial DBH (D₁) and competition index C15 also played important roles. Pearson correlation analysis showed that D₁ was significantly correlated with several DBH growth indicators, while the spatial structure indicator U_D was negatively correlated with all DBH growth indicators. Competition indices reflecting competitive ability were significantly positively correlated with DBH growth, whereas most indices representing competitive pressure showed significant negative correlations. The linear mixed-effects model outperformed the ordinary linear regression model, indicating that differences in site conditions among plots had a significant impact on DBH growth. The relative importance of driving factors varied among different DBH growth indicators, with competition indices generally having the greatest influence, followed by D₁, while the contribution of U_D was relatively small. Interaction effects in the mixed-effects model revealed that competition and spatial structure exhibited synergistic effects. In stands with higher diameter differentiation (higher U_D), the interaction terms C6 × U_D, C2 × U_D and C5 × U_D had significant positive effects on DBH growth, whereas C17 × U_D showed a significant negative interaction effect. These results indicate that increased spatial heterogeneity enhances the positive effects of competitive ability indices on growth while weakening the inhibitory effects of certain competition pressure indices (e.g., Alemdag index).

DBH growth of Cunninghamia lanceolata is jointly influenced by initial DBH, competition indices, and spatial structure, among which competition plays a dominant role. This complex multi-factor interaction mechanism provides a scientific basis for the precision management and sustainable management of Cunninghamia lanceolata stands.