Abstract: Understanding the mechanisms governing plant community assembly is crucial for developing effective vegetation restoration and management strategies. However, studies examining plant community assembly during the restoration of severely degraded alpine meadows remain limited. In this study, we investigated the processes shaping plant community structure and the associated environmental drivers during the early stages of restoration in severely degraded alpine meadows. Our results showed that differences in plant community composition between mixed sowing and control treatments increased over time. Additionally, both species richness and the dominance of forb species originating from the soil seed bank progressively declined. Across all mixed sowing treatments, plant community assembly showed stochastic assembly patterns, with homogeneous dispersal increasing and ecological drift decreasing annually. By the third year of restoration, heterogeneous selection was more pronounced in the grass + legume mixture than in the grass-only and grass + legume + sedge mixture treatments. Further analyses identified under-canopy photosynthetically active radiation intensity and soil available phosphorus as the key environmental drivers of plant community composition. Therefore, we infer that the stochastic assembly observed after mixed sowing arises from the counterbalancing effects of environmental filtering and niche differentiation. This study highlights that reducing soil nutrient availability to limit the dominance of upper-layer plant communities is crucial for maintaining plant species diversity during the early stages of alpine meadow restoration. Overall, our findings provide valuable insights for improving vegetation restoration strategies in degraded alpine grasslands.

Key words: community assembly, alpine meadow, grassland management, vegetation restoration, plant diversity, Qinghai-Tibetan Plateau