Objective Shrub encroachment is a key process driving the degradation of alpine meadow ecosystems and the succession of herbaceous communities, yet its impact on the niche characteristics and interspecific relationships within herbaceous communities remains unclear. This study aims to reveal the evolution patterns of herbaceous community composition and niche characteristics during shrub encroachment, and to elucidate the synergistic effects of intershrub soil properties and shrub community structure on herbaceous community assembly.

Method Focusing on the alpine meadows of the Zoige Plateau on the eastern edge of the Tibetan Plateau, study plots were established along a shrub encroachment gradient (potential, light, moderate, severe). Through vegetation surveys and soil analysis, the composition of herbaceous communities, niche breadth and overlap, functional group structure, and their relationships with soil environmental factors were investigated.

Result (1) With intensifying shrub encroachment, the importance values of native dominant herbaceous species such as grasses and sedges significantly decreased, while those of Rosaceae and some forbs significantly increased. Although the overall diversity of the herbaceous community did not decrease significantly, distinct changes were observed in its species composition structure. (2) Species with broader niche breadths (e.g., Potentilla anserina, Polygonum viviparum) maintained high niche overlap, indicating that dominant species tended to utilize resources similarly, leading to intense interspecific competition. Simultaneously, species pairs with moderate niche overlap were the most numerous, accounting for 46.84% of the total pairs, reflecting that most species achieved coexistence across multiple succession stages through niche differentiation. (3) Intershrub-space soil organic carbon was the primary environmental factor influencing herbaceous community structure. Its content initially increased and then decreased with increasing shrub encroachment. In the later stages of encroachment, its depletion coincided with a decline in herbaceous diversity, promoting the expansion of species tolerant to poor conditions with stolons or deep root systems (e.g., Potentilla anserina, Elymus nutans). Increased shrub diversity further intensified resource competition, indirectly strengthening the functional filtering for shade-tolerant and nutrient-stress-tolerant herbs. Ultimately, species with broader niche breadths became dominant in the community.

Conclusion Shrub encroachment drives the transformation of herbaceous communities from native "competitive" functional groups towards "stress-tolerant" functional groups by reducing intershrub soil fertility and intensifying competitive pressure from shrubs. Species composition and structure become simplified, with broad-niche species gradually becoming dominant. These results, from the perspective of synergistic evolution between niches and the soil environment, reveal the intrinsic mechanisms by which shrub encroachment influences herbaceous community assembly in alpine meadows, providing a theoretical basis for the restoration and management of meadow ecosystems.