Objective This study aimed to investigate the growth characteristics of Pinus tabuliformis plantations at different densities and their impacts on understory vegetation diversity, providing a scientific basis for sustainable management of Pinus tabuliformis forests in the loess region of western Shanxi Province.

Methods Eleven plantation densities of Pinus tabuliformis (the forest age is 30 years.) were selected as study objects. Field surveys and laboratory experiments were conducted to measure growth traits, biomass, and vegetation diversity. Univariate analysis of variance (ANOVA) and structural equation modeling (SEM) were employed to analyze the effects of stand density on growth traits of Pinus tabuliformis and understory vegetation diversity.

Results The findings revealed: (1) The average tree height of Pinus tabuliformis fluctuates with increasing stand density, while the average diameter at breast height (DBH) exhibit a decreasing trend. (2) High-density stands promoted biomass allocation to stems, cones, whereas low-density stands favored branches and leaves. As stand density increases, the individual biomass of Pinus tabuliformis shows a decreasing trend, whereas the total stand biomass reaches its maximum value of 148.81 t/hm² at a density of 1 750 trees/ha. (3)The Shannon-Wiener diversity index of the understory shrub layer reaches its maximum at a density of 1 750 trees/ha, while the Margalef richness index peaks at 1 250 trees/ha. For the herbaceous layer, both the Shannon-Wiener diversity index and the Margalef richness index attain their maximum values at a density of 1 250 trees/ha. (4) SEM analysis indicated that density indirectly enhanced herbaceous diversity by positively influencing Pinus tabuliformis growth traits and individual tree biomass but inhibited shrub diversity.

Conclusion By thinning the Pinus tabulaeformis forests in the loess region of western Shanxi to achieve a density within the range of 1 250–1 750 trees/ha, it is possible to achieve a synergistic enhancement of both stand productivity and understory biodiversity.