Objective To comprehensively understand the driving mechanisms of thinning disturbance on the biomass of understory woody plants in forests.

Method This study focused on coniferous and broad-leaved mixed forests in Northeastern China, employing mixed-effects models and structural equation modeling (SEM) to analyze the factors influencing the biomass of understory woody plants.

Result (1) Thinning released growing space for understory woody plants, leading to a significant increase in understory biomass with higher thinning intensity (p < 0.05). (2) Understory species diversity and structural diversity were the primary factors influencing understory biomass, accounting for 51.00% of the variance explained. Thinning disturbance ranked second (29.48%), while the combined effects of overstory species diversity and stand structure explained 16.92%. The direct impact of light environment on understory biomass variation was minimal, explaining only 2.53%. (3) Thinning disturbance significantly affected (p < 0.05) changes in tree-like shrub biomass of woody plants in the understory, whereas it did not significantly affect (p > 0.05) the biomass of treelets and typical shrubs. (4) SEM further revealed that thinning disturbance not only directly promoted understory biomass accumulation but also indirectly influenced it by regulating overstory stand structure and species diversity.

Conclusion This study elucidates the direct and indirect mechanisms of thinning disturbance on understory biomass, highlighting the central roles of understory diversity and logging intensity in enhancing biomass accumulation. These findings provide a theoretical foundation for scientifically evaluating the long-term ecological effects of thinning disturbance and optimizing forest management strategies, and provides practical guidance for achieving sustainable management goals.