Objective This study aimed to analyze the regulatory effects of thinning on the stand structure of Larix principis-rupprechtii plantations, identify the optimal thinning intensity for middle-aged stands, and provide a scientific basis for the structural optimization and quality improvement of regional plantations.

Method Taking twelve standard sample plots of Larix principis-rupprechtii middle-aged stands (28 a) in Saihanba Mechanical Forest Farm of Hebei Province as the research objects, four treatments were set up: control (CK, 0% thinning), light thinning (LT, 21% thinning), moderate thinning (MT, 40% thinning), and heavy thinning (HT, 56% thinning). The driving effects of thinning on 11 structural factors in both non-spatial and spatial stand structures were systematically quantified. Based on the entropy weight-multiplication and division coupling model, the Forest Stand Structure Index (FSSI) was established to evaluate the comprehensive effects of different thinning treatments.

Result (1) Compared with the control, thinning treatments significantly increased the diameter at breast height, tree height, crown width, and productivity of the stand, and the magnitudes of increase varied among these indicators. (2) MT and HT significantly reduced the competition index and stand layer index of the stand, while all treatments had no significant effect on the uniform angle index, crowding degree, neighborhood comparison, and leaf area index. (3) Among the 11 structural indicators, the stand layer index had the highest weight (18.39%), while the neighborhood comparison had the lowest (3.96%). The FSSI first increased and then decreased with increasing thinning intensity, reaching the maximum value (0.46) under the moderate thinning, indicating that this treatment yielded the optimal comprehensive stand structure.

Conclusion Moderate thinning at 40% intensity achieved the best balance between promoting the growth of residual trees and controlling stand volume loss, and thus represents the optimal threshold for structural regulation in Larix principis-rupprechtii plantations. These results can provide a quantitative basis for sustainable forest management in this region.