Background and Aims Unregulated grazing is a primary driver of grassland deterioration. Dominant species maintain ecosystem function, yet how their long-term adaptations to grazing scale to influence community structure remains poorly understood. We investigated how grazing intensity and season alter the ontogeny and reproductive strategy of a dominant Stipa grass and how these changes propagate to the plant community.Methods We conducted a 3-year field study in a typical steppe with a 23-year controlled grazing experiment. The dominant perennial grass, Stipa bungeana, was classified into four ontogenetic stages based on basal diameter and vegetative tiller number. Its reproductive traits, biomass allocation and community-level impacts were evaluated under different grazing intensities and seasons.Key Results Long-term grazing accelerated population ontogenetic ageing and shifted reproductive strategy from sexual to asexual. Grazing shortened reproductive tillers, increased their diameter and reduced investment in tiller number. Climatic factors dominated interannual variation in reproductive traits, while grazing intensity strongly affected population stage structure and biomass allocation. Increasing S. bungeana cover enhanced total vegetation and biocrust cover but reduced species richness.Conclusions Our study demonstrates that long-term grazing restructures dominant grass populations via ontogenetic ageing and reproductive trade-offs, with cascading reorganization at the community level. While climate regulates annual trait expression, grazing intensity and season exert lasting control over demographic structure. These findings bridge individual- and community-level responses and support a shift from intensity-based to structure-based grazing management. This approach emphasizes maintaining developmental heterogeneity in dominant populations to balance ecosystem productivity and biodiversity under global change.