CONSTRAINTS OF NUTRIENT AVAILABILITY ON PRIMARY PRODUCTION IN 2 ALPINE TUNDRA COMMUNITIES

A nutrient amendment experiment was conducted for two growing seasons in two alpine tundra communities to test the hypotheses that: (1) prim ary production is limited by nutrient availability, and (2) physiologi cal and developmental constraints act to limit the responses of plants from a nutrient-poor community more than plants from a more nutrient- rich community to increases in nutrient availability. Experimental tre atments consisted of N, P, and N+P amendments applied to plots in two physiognomically similar communities, dry and wet meadows. Extractable N and P from soils in nonfertilized control plots indicated that the wet meadow had higher N and P availability. Photosynthetic, nutrient u ptake, and growth responses of the dominants in the two communities sh owed little difference in the relative capacity of these plants to res pond to the nutrient additions. Aboveground production responses of th e communities to the treatments indicated N availability was limiting to production in the dry meadow community while N and P availability c olimited production in the wet meadow community. There was a greater p roduction response to the N and N+P amendments in the dry meadow relat ive to the wet meadow, despite equivalent functional responses of the dominant species of both communities. The greater production response in the dry meadow was in part related to changes in community structur e, with an increase in the proportion of graminoid and forb biomass, a nd a decrease in the proportion of community biomass made up by the do minant sedge Kobresia myosuroides. Species richness increased signific antly in response to the N+P treatment in the dry meadow. Graminoid bi omass increased significantly in the wet meadow N and N+P plots, while forb biomass decreased significantly, suggesting a competitive intera ction for light. Thus, the difference in community response to nutrien t amendments was not the result of functional changes at the leaf leve l of the dominant species, but rather was related to changes in commun ity structure in the dry meadow, and to a shift from a nutrient to a l ight limitation of production in the wet meadow.