LAUSR

Enhancing forest ecological functioning by optimizing stand structure is essential in high-quality, sustainable forests. We examined this in 38 plots (20 m × 20 m) of Pinus tabuliformis forests in the mountainous area of Beijing. We investigated and analyzed the spatial structure and functional characteristics of these plots. Structural equation modeling and response surface methodology were used to identify spatial structural stand factors affecting soil nutrient levels and understory biodiversity. We examined the pathways and strengths of the coupling relationships between structure and function and the ranges and thresholds of changes in these structural factors. Mingling degree, openness degree, competition index, and forest layer index substantially affected the understory herbaceous species diversity and soil nutrient levels. Mingling degree significantly impacted understory biodiversity and soil nutrient levels (direct path coefficient (DPC), 0.48 and 0.70, respectively). Openness degree significantly affected understory species diversity (DPC, 0.37). The competition index and forest layer index exerted less significant direct impacts on these functions; competition influenced herbaceous diversity primarily indirectly. The optimal features were as follows: mingling degree, 0.8; openness degree, 1.1; competition index, 0.3; and forest layer index, 0.5. Average understory herbaceous species diversity and soil nutrient levels are projected to increase by ca. 47.74% and 52.29%, respectively, post optimization. These findings provide a reference for precise regulated stand structures and establish multifunction management optimization objectives in Beijing’s mountainous Pinus tabuliformis forests.