RESPONSES OF ARCTIC TUNDRA TO EXPERIMENTAL AND OBSERVED CHANGES IN CLIMATE

We manipulated light, temperature, and nutrients in moist tussock tund ra near Toolik Lake, Alaska to determine how global changes in these p arameters might affect community and ecosystem processes. Some of thes e manipulations altered nutrient availability, growth-form composition , net primary production, and species richness in less than a decade, indicating that arctic vegetation at this site is sensitive to climati c change. In general, short-term (3-yr) responses were poor predictors of longer term (9-yr) changes in community composition. The longer te rm responses showed closer correspondence to patterns of vegetation di stribution along environmental gradients. Nitrogen and phosphorus avai lability tended to increase in response to elevated temperature, refle cting increased mineralization, and in response to light attenuation, reflecting reduced nutrient uptake by vegetation. Nutrient addition in creased biomass and production of deciduous shrubs but reduced growth of evergreen shrubs and nonvascular plants. Light attenuation reduced biomass of all growth forms. Elevated temperature enhanced shrub produ ction but reduced production of nonvascular plants. These contrasting responses to temperature increase and to nutrient addition by differen t growth forms ''canceled out'' at the ecosystem level, buffering chan ges in ecosystem characteristics such as biomass, production, and nutr ient uptake. The major effect of elevated temperature was to speed pla nt response to changes in soil resources and, in the long term (9 yr), to increase nutrient availability through changes in N mineralization . Species within a growth form were similar to one another in their re sponses to changes in resources (light or nutrients) but showed no con sistent response to elevated temperature. Species richness was reduced 30-50% by temperature and nutrient treatments, due to loss of less ab undant species. Declines in diversity occurred disproportionately in f orbs, which are important for animal nutrition, and in mosses, which m aintain soil thermal regime. There was no increased abundance of initi ally rare species in response to any treatment. During our 9-yr study (the warmest decade on record in the region), biomass of one dominant tundra species unexpectedly changed in control plots in the direction predicted by our experiments and by Holocene pollen records. This sugg ests that regional climatic warming may already be altering the specie s composition of Alaskan arctic tundra.