Nutrient availability, plant abundance, and species diversity in two alpine tundra communities

The long-term (5-yr) responses of plant absolute abundance and species diversity to N, P, and N + P fertilization were investigated in two sedge-dominated alpine plant communities that differed in soil resource availability but not in macroclimate: a resource-poor dry meadow and a more resource-rich wet meadow. Prior to analysis, species were grouped into functional groups based on growth form, potential developmental constraints, and presence or absence of mutualisms. Absolute abundance changes of functional groups were more pronounced in the dry-meadow community than in the wet-meadow community. In the dry meadow, mycorrhizal forbs increased with N + P fertilization, nonmycorrhizal forbs increased with P and N + P, and N2-fixing forbs increased with P alone. Grasses increased with N and N + P, whereas sedges, the dominant functional group, were unaffected by fertilization. In the wet meadow, the dominant sedges exhibited abundance increases in response to N, whereas grasses increased with P and N + P. Wet-meadow forb abundance was not significantly influenced by fertilization. Shifts in relative abundances of species resulted in an increase in species diversity following N + P fertilization in the dry meadow. This study, therefore, allowed a direct comparison of diversity responses between communities that differed primarily in soil resource availability, substantiating the theory that plant species diversity is greatest under intermediate levels of fertility.