Objective Thinning is a key silvicultural practice for regulating stand structure and productivity in plantations. This study aimed to quantify the dynamic effects of different thinning regimes on individual tree growth and stand productivity in Larix gmelinii var. principis-rupprechtii plantations, and to determine the optimal stand density threshold for maximizing productivity.

Method Based on long-term permanent sample plots established in 2017, two thinning methods (based on basal area percentage and residual stem density) were implemented to create a gradient of stand densities. Over a 7-year period post-thinning (2017−2024), residual tree growth and stand-level productivity were monitored over two census periods. Regression analysis was primarily used to evaluate the regulatory effects of the post-thinning stand structure (with residual basal area as key indicator) on growth dynamics.

Result (1) Thinning significantly promoted the individual growth of residual trees. The mean annual individual growth in the thinned sample plots ((4.98 ± 3.65) kg/year and (4.08 ± 3.04) kg/year for the 2017−2020 and 2020−2024 periods, respectively) was significantly higher than control ((3.45 ± 3.05) kg/year and (2.49 ± 2.17) kg/year, respectively; p < 0.05). (2) For larger-sized residual trees (DBH > 20 cm), their growth showed no significant difference from the control in 2017−2020 period, but was significantly higher in 2020−2024 period (p < 0.05). (3) The growth of all residual trees and stand productivity exhibited a declining trend over time (p < 0.05). (4) Residual basal area had a significant negative effect on the growth of both all residual trees and larger-sized trees in 2020−2024 period (p < 0.001). (5) At stand level, no significant difference in productivity was found between thinned sample plots and control in either period (p > 0.05). However, during 2017−2020, stand productivity showed a quadratic relationship with residual basal area, peaking at 34.2 m²/ha, whereas no significant relationship was observed in 2020−2024 period.

Conclusion Thinning effectively enhances the individual growth of residual trees in Larix gmelinii var. principis-rupprechtii plantations, particularly for promoting larger-diameter timber. However, its beneficial effect on stand productivity is constrained by an optimal density range. To maximize productivity, we recommend maintaining the residual basal area at 29.8−40.1 m²/ha. The quantitative targets proposed in this study provide a scientific basis for decision-making in the sustainable management of regional forests.