Variation Patterns and Affecting Factors of Plant Alpha Diversity, Beta Diversity and Its Components in Restoration Grasslands on Loess Plateau

Understanding spatiotemporal variation in diversity and identifying key external affecting factors are essential for biodiversity conservation. However, community assembly and species diversity in natural grassland (NG) restoration on the Loess Plateau remain unclear. In this study, we examined α diversity (species richness), β diversity (βtotal), and its components (βrepl and βrich) across 89 grassland communities, which were categorized into five restoration stages: recent grassland (RG), early restoration (ER), middle restoration (MR), later restoration (LR), and NG. Linear mixed models (LMM) were used to analyze the effects of topographic, soil, and landscape factors on diversity. Results showed that α diversity followed an increasing-decreasing trend across restoration stages (7–21 species m⁻²), peaking at the MR stage. In contrast, β diversity and its two components declined with duration, with βtotal and βrepl being notably higher in ER than in LR and NG. Differences in βtotal primarily resulted from species replacement (βrepl, 61%–70%), with a smaller contribution from species gain and loss (βrich, 30%–39%). The LMM also revealed that restoration stage was the most important factor affecting plant diversity, explaining 59.3%, 68.4%, 53.5%, and 58.8% of richness, βtotal, βrepl, and βrich, respectively. In comparison, landscape had weaker effects on diversity (17.2%, 24.2%, 46.5%, 39.8%), while topography and soil factors had the least effects. In summary, deterministic processes (restoration stage) dominate natural restoration, but substantial differences persist between restoration grassland and NG. This study provides valuable insights for assessing restoration progress, prioritizing conservation areas, and informing future grassland management in semi-arid and fragmented regions.