NUTRIENT AVAILABILITY, PLANT ABUNDANCE, AND SPECIES-DIVERSITY IN 2 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 sed ge-dominated alpine plant communities that differed in soil resource a vailability 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 c ommunity than in the wet-meadow community. In the dry meadow, mycorrhi zal forbs increased with N+P fertilization, nonmycorrhizal forbs incre ased with P and N+P, and N-2-fixing forbs increased with P alone. Gras ses increased with N and N+P, whereas sedges, the dominant functional group, were unaffected by fertilization. In the wet meadow, the domina nt sedges exhibited abundance increases in response to N, whereas gras ses increased with P and N+P. Wet-meadow forb abundance was not signif icantly influenced by fertilization. Shifts in relative abundances of species resulted in an increase in species diversity following N+P fer tilization in the dry meadow, and a decrease in species diversity foll owing N+P fertilization in the wet meadow. This study, therefore, allo wed a direct comparison of diversity responses between communities tha t differed primarily in soil resource availability, substantiating the theory that plant species diversity is greatest under intermediate le vels of fertility.