Initial Species Evenness Modulates Divergent Stability Responses to Nitrogen Enrichment in Tibetan Alpine Grasslands

Anthropogenic nitrogen enrichment significantly alters plant community structure and productivity, often undermining long-term stability. However, the role of initial community structure, particularly species evenness, in mediating these stability responses remains inadequately understood. In this study, we conducted an 8-year nitrogen-addition experiment across four alpine grasslands on the Tibetan Plateau, which represent a natural gradient in initial evenness. Our results indicate that initial species evenness is crucial in mediating community stability under nitrogen enrichment. In low-evenness communities, stability exhibited a linear decline with increasing nitrogen, primarily driven by the instability of dominant species, specifically Carex parvula O. Yano in alpine meadows and Stipa purpurea Griseb. in alpine desert steppes. At lower nitrogen levels, dominant species biomass increased; however, as nitrogen levels rose, biomass variability increased, resulting in diminished overall stability. Conversely, high-evenness communities displayed nonlinear stability responses, buffered by species asynchrony. In the alpine meadow steppe, stability initially increased at low nitrogen levels but subsequently declined at higher nitrogen due to reductions in species evenness and functional redundancy. In the alpine steppe, stability first decreased at low nitrogen due to community imbalance, but compensatory dynamics among nitrogen-tolerant species, such as Stipa purpurea Griseb., restored stability at elevated nitrogen levels. These findings emphasize the diverse responses of alpine grasslands to nitrogen enrichment and underscore the significance of initial species evenness for ecosystem resilience. This study offers critical insights for forecasting ecosystem responses and formulating targeted management strategies in the context of global change.