Abstract: Knots are common wood defects that severely reduce the mechanical performance and market value of timber. To identify the key drivers of knot diameter in Larix olgensis and determine its nonlinear response mechanisms, this study, within the framework of tree resource allocation trade-offs and stand competition, integrates a generalized additive model (GAM), SHapley Additive exPlanations (SHAP)–based machine learning models, and structural equation modelling (SEM) to build a multi-dimensional analytical framework spanning nonlinear effect identification, threshold detection, and causal pathway analysis. The results show that diameter at breast height (DBH), tree height (H), sound knot length (SKL), knot height (KH), and knot tilt angle (TA) are the main positive drivers of KD, whereas increasing stand density (SD) exerts a significant suppression on knot expansion through intensified resource competition. By organically combining predictive and mechanistic models, this study reveals several biologically meaningful thresholds and further clarifies how stand structure influences knot development. These findings provide a quantitative basis for optimizing stand density and crown structure to control knot size, thereby improving wood quality and promoting the sustainable management of L. olgensis plantations, and also offer a transferable methodological framework for quantitative studies of knot characteristics in other plantation tree species.

Key words: Larix olgensis, Knot diameter, GAM, Machine learning, SEM