Abstract: Exploring the changes in stand structure and stability of forest plantations under different thinning intensities can provide theoretical basis for the sustainable management. In this study, four thinning intensities in Larix gmelinii var. principis-rupprechtii plantations in Saihanba, North China were established: light thinning (LT, 21%), moderate thinning (MT, 40%), heavy thinning (HT, 56%), and a control without thinning (CK). Stand structure and vitality were characterized using eight variables, including number of forest layers, angular scale, diameter structure, stand congestion, Simpson index, stand vigor, proportion of healthy trees, and seedling abundance, together with adjacent environmental types. An improved unit circle method was used to analyze the differences in stand conditions under different thinning intensities. A combined entropy weight-internal/external evaluation approach was used to quantify the response of community stability to thinning. The results showed that pure stands generally exhibited simple diameter structure and canopy stratification, with poor regeneration ability and a uniform spatial distribution of trees. Heavy thinning showed the best stand congestion, growth vigor, and health condition, while moderate thinning yielded the highest species diversity. Both additivity and equilibrium were improved after thinning, and the comprehensive evaluation of stand condition ranked as HT (0.485) > MT (0.414) > LT (0.400) > CK (0.275), indicating that all stands were at a relatively poor condition. According to the entropy weight analysis, stand condition and neighborhood environment contributed 82.3% and 17.7% of the total weight, respectively, suggesting that stand condition played a dominant role in stability assessment. The overall stability index ranked as HT (0.547) > MT (0.503) > LT (0.477) > CK (0.374). Both stand condition and stability improved with increasing thinning intensity, whereas neighborhood environmental differences were minor. Our results indicated that thinning enhanced stand stability primarily by improving stand condition. Although heavy thinning showed the best effect, thinning intensity should be reasonably adjusted according to site conditions and management objectives to balance ecological and economic benefits, thereby promoting sustainable forest development.

Key words: thinning intensity, improved unit circle method, stand quality assessment, stand stability