Grazing Offsets Nitrogen Enrichment Effects on Species Richness by Promoting the Random Colonization of Local Species in an Alpine Grassland

Abstract

Nitrogen (N)-induced declines in species richness can be offset by grazing via the suppression of dominant species to increase ground-level light. However, it is not known whether grazing-mediated locally colonized species correspond to N-induced locally extinct species with regard to taxonomy or traits. Data from 11 years of N addition and winter grazing experiments were analyzed to assess species richness dynamics at community and functional group levels. Regarding N treatments, enclosure and N addition were performed during the first 7 years and were then followed by winter grazing with continuous N addition during the next 4 years. Additionally, all treatments were used to compare N-induced local species extinction and grazing-mediated local species colonization. Then, traits of N-induced locally extinct species and grazing-mediated locally colonized species were further correlated with plant traits. We found that N addition induced a decline in plant species richness as a result of both random and non-random species losses. The lost species were closely associated with traits, including low height, non-clonality, low special leaf area (SLA), and low leaf N concentrations, which are relevant to light capture capacity. In contrast, grazing promoted the random colonization of local rare species, thus offsetting the N-induced decline of species richness. However, non-randomly lost species were not rescued by grazing. Grazing-mediated locally colonized species were generally characterized by high leaf carbon (C) concentrations, SLA, and seed mass. Our study provides experimental evidence indicating although grazing offsets N-induced species reduction, shifts in species composition and traits could cause a divergence in ecosystem function facing anthropogenic alterations to nutrient cycles and grazing.