Abstract: Plant community structure and influencing factors of species richness were assessed in herbaceous layer grasslands situated beneath forest canopies (understory grasslands) and in abandoned grasslands at successive stages of natural restoration (10, 20, 30, and 40 years) in the Loess Hilly-Gully region. Using plot-based sampling and Levins’ niche breadth index, 60 plant species were classified into four functional groups based on niche breadth. Species with narrow niche breadth (B=1), primarily specialists, exhibited significantly greater dominance in later successional stages, while generalists with broader niches (B>5) dominated early restoration. Species richness reached its maximum at 20 years post-abandonment, significantly exceeding values observed at other timepoints (P<0.05). Stratification by plant height and niche breadth revealed distinct patterns: low-height species (≤16 cm) were underrepresented in early succession, while mid-height species (>16–32 cm) dominated during mid-successional stages. Species richness showed significant unimodal nonlinear relationships with both plant height and niche breadth, with richness declining significantly once plant height exceeded 32 cm or niche breadth surpassed 3 (P<0.05). In contrast, linear associations with grassland type and restoration age explained limited variance, suggesting that species-level traits, particularly plant height and niche breadth, exert a stronger influence on richness patterns. Across all stages, abandoned grasslands consistently harbored higher richness than understory grasslands (P<0.01).