Objective This study aimed to clarify the combined effects of thinning intensity and neighborhood competition on the multi-dimensional growth of residual trees in Larix gmelinii var. principis-rupprechtii (Mayr) Pilg. plantations, and to reveal how thinning modulates competition effects, so as to provide a scientific basis for precise density management of planted forests.

Methods Twenty-four permanent monitoring plots (20 m × 20 m) were established in Larix gmelinii var. principis-rupprechtii plantations in the Guancen Mountain Forest Region, Shanxi Province, China. Thinning treatments were implemented in late 2017 under two schemes: (1) Based on basal area ratio: control, 15% thinning intensity, 30% thinning intensity, and 45% thinning intensity; (2) Based on residual tree density: control, 1800 trees·hm⁻², 1350 trees·hm⁻², and 900 trees·hm⁻². Using full-inventory measurements from 2017 and 2024, we quantified the annual mean increments of individual tree diameter at breast height (DBH), tree height, crown width, and aboveground biomass, as well as the changes in height-to-DBH ratio and crown-to-DBH ratio. Linear models and linear mixed effects models were constructed to analyze the effects of thinning intensity, initial post-thinning competition, and their interaction on each response variable, with initial tree characteristics included as covariates.

Results (1) Thinning intensity significantly promoted DBH growth and biomass accumulation of residual trees, and this effect depended strongly on initial tree size. (2) Neighborhood competition highly significantly suppressed height and crown width growth, and significantly increased both the height-to-DBH ratio and crown-to-DBH ratio, indicating reduced morphological stability and slender stem form. (3) The inhibitory effect of neighborhood competition on height and crown width increments became highly significantly stronger as thinning intensity increased.

Conclusion Growth responses of residual Larix gmelinii var. principis-rupprechtii trees reflect the combined outcomes of direct resource release due to thinning and indirect changes in competitive regimes. Although heavy thinning significantly promotes radial growth, it may intensify competition among residual trees, thereby limiting height growth and influencing tree form. Therefore, practical management should balance density control and spatial configuration, while adopting relatively low residual densities, managers should optimize the spatial distribution of retained trees to alleviate local competition, thereby supporting both large-diameter timber production and stand stability.