Grazing exclusion through fencing is widely used for vegetation restoration in degraded alpine meadows. However, the dynamic responses of plant communities to grazing exclusion remain poorly understood, especially from an integrated perspective of species diversity, niches, and interspecific associations. In this study, we investigated four proximate alpine meadows on the Tibetan Plateau with different fencing durations (0, 2, 6, and 12 years). We assessed the responses of plant diversity, niche characteristics, and interspecific associations to fencing duration, along with relationships among these dimensions. The results showed a unimodal response of plant diversity to fencing duration, with the Patrick richness index varying in coordination with niche and interspecific association metrics. After 2 years of fencing, the community niche breadth expanded, accompanied by increased niche overlap and Ochiai association index among major species. By 6 years of fencing, the niche breadth shifted toward lower values, and niche overlap of major species decreased significantly, with the proportion of species pairs with high overlap and high association reduced by 21.95% and 25.93%, respectively. After 12 years of fencing, niche overlap rebounded significantly, and the proportion of species pairs with high niche overlap and high association increased by 18.79% and 16.84%, respectively. Our findings support identifying 6 years of fencing as a critical intervention point. At this stage, the community achieves a dynamic balance between competition and coexistence through niche differentiation, maintaining high species diversity. We suggest moderate disturbance should be implemented in alpine meadows thereafter to prevent retrogressive succession.