Developing Tree Diameter Increment Models at Three Positions Along the Tree Bole for Planted Korean Pine in Northeast China

Korean pine is an important component species of forest vegetation in northeastern China. Research on the diameter increment of Korean pine is of great significance for future forest management. In this study, 48 Korean pine trees were destructively sampled, and the diameter increment was measured at three positions along the tree bole: breast height, crown base, and maximum crown width. Age, tree size, competition, and site factors were used as candidate variables. Thirteen candidate models were fitted, and the successfully converged models were compared using the adjusted determination coefficient (R2adj), root mean square error (RMSE), mean absolute error (MAE), and mean absolute percentage error (MAPE). M11 (logarithmic function) performed best at breast height, whereas M12 (quadratic function) performed best at the crown base and maximum crown width. Based on this, re-parameterization was applied to incorporate additional variables. The final models retained the ratio of HT to DBH (HDR), the height to the crown base (HCB), and crown length (CL) at breast height, and HDR and HCB at both the crown base and maximum crown width. Overall, the final models showed good fitting performance, explaining 61.9%–70.1% of the variation in diameter increment models across the three positions. The models developed in this study can accurately predict growth at different positions along the tree bole and provide support for the precision management of Korean pine plantations.