Abstract: Thinning plays a vital role in maintaining plant diversity and community stability. However, the long-term effects of thinning on plant communities remain unclear. Exploring the differences in plant diversity and community stability as well as their temporal variations under different thinning intensities can provide theoretical references for determining appropriate thinning intensities for forests. We established a thinning experiment in a secondary forest of Huoditang in the Qinling Mountains in 2013, with four thinning intensities (CK: 0%, T₁: 15%, T₂: 30%, T₃: 45%, and T₄: 60%). We analyzed the differences in plant diversity and community stability, their relationships, and the influencing factors under different thinning intensities using significance analysis, regression and partial least squares path modeling (PLSPM), based on data from sampling in 2013 (no thinning), 2017 (4 years after thinning), and 2021 (8 years after thinning). The results showed that the diversity indices of plants (shrubs and herbs) under all thinning intensities peaked in 4 years after thinning and then declined, except for the Pielou evenness index. Four years after thinning, the diversity indices of shrub and herb communities followed the order of T₄>T₁>T₂>T₃>CK and T₁>T₄>T₂>T₃>CK, respectively. The increases of Patrick richness, Simpson and Shannon diversity indices in the herb layer of T₁ and T₄ were significantly higher by 88.0%, 54.7%, 73.8%, and 100.0%, 187.0%, 121.2% compared with CK, respectively. In the shrub layer, community stabilities in T₁ (0.14), T₂ (0.28), and T₄ (0.18) were significantly higher than that in CK (0.03) between 4 and 8 years after thinning. Notably, only the T₄ treatment showed a sustained increase in community stability after thinning (shrub layer: 0.26; herb layer: 0.11). Among the four plant diversity indices, only the ΔPielou evenness index was significantly positively correlated with community stability (shrub layer: R²=0.64; herb layer: R²=0.21). Thinning intensity negatively affected community stability only within four years after thinning. Soil organic carbon and total nitrogen contents were the main soil factors affecting community stability after thinning. The T₄ treatment (60% thinning intensity) continuously improved the stability of understory shrub and herb communities, making it the optimal thinning intensity for achieving forest ecosystem stability and sustainable management in the region.

Key words: thinning intensity, plant diversity, community stability, soil nutrient, partial least squares path modeling