Objective Long-term vegetation restoration in the loess area of western Shanxi Province of northern China has significantly improved the ecological environment. However, the factors influencing understory vegetation diversity following the restoration of different tree species remained unclear. This study aimed to explore the differences in species diversity of understory vegetation between typical plantations and Quercus liaotungensis secondary forests, to reveal the regulatory effects of stand growth parameters and soil properties on herb layer diversity, and to provide a scientific basis for vegetation restoration and ecosystem management in the Loess Plateau.

Method Random plots of typical plantations (Pinus tabuliformis forest, Platycladus orientalis forest, and Robinia pseudoacacia forest) and natural forests (Quercus liaotungensis secondary forest) in the loess area of western Shanxi Province were selected as study sites. Stand growth parameters (biomass, plant height, crown width, cluster diameter, etc.) and soil properties (bulk density, porosity, total nitrogen, total phosphorus, total potassium, available nutrients, etc.) were measured. The α-diversity and β-diversity of understory vegetation in 2017, 2020, and 2023 were calculated. Principal component analysis, correlation analysis, and redundancy analysis (RDA) were employed to identify the driving factors of diversity.

Result (1) The α diversity index of herb layer in Pinus tabuliformis forest and Platycladus orientalis forest was relatively stable, and their Simpson index were significantly higher than that in Quercus liaotungensis secondary forest (P < 0.05). (2) The species diversity of herb layer in Quercus liaotungensis secondary forest was significantly lower than that in plantations, but the average biomass and crown width of understory vegetation were the highest, indicating that competition from dominant species suppressed diversity. (3) The soil quality of four stands improved steadily under long-term vegetation restoration. Soil total potassium was significantly negatively correlated with PIE index (r = − 0.334, P < 0.01) and Margalef index (r = − 0.394, P < 0.01). (4) Herb canopy (contribution rate of 24.6%) and belowground biomass (contribution rate of 20.9%) were identified as the primary factors driving the decline in diversity.

Conclusion Plantations can optimize understory vegetation diversity by regulating soil nutrient balance, while Q. liaotungensis secondary forests require moderate disturbance to alleviate competition from dominant species. These findings provide theoretical support and practical guidance for vegetation restoration and biodiversity maintenance in the Loess Plateau.