TIBETAN ALPINE TUNDRA RESPONSES TO SIMULATED CHANGES IN CLIMATE - ABOVEGROUND BIOMASS AND COMMUNITY RESPONSES

High-elevation ecosystems are predicted to be some of the terrestrial habitats most sensitive to changing climates. The ecological consequen ces of changes in alpine tundra environmental conditions are still unc lear especially for habitats in Asia. In this study we report findings from a field experiment where an alpine tundra grassland on the Tibet an plateau (37 degrees N, 101 degrees E) was exposed to experimental w arming, irradiance was lowered, and wind speed reduced to simulate a s uite of potential changes in environmental conditions. Our warming tre atment increased air temperatures by 5 degrees C on average and soil t emperatures were elevated by 3 degrees C at 5 cm depth. Aboveground bi omass of grasses responded rapidly to the warmer conditions whereby bi omass was 25% greater than that of controls after only 5 wk of experim ental warming. This increase was accompanied by a simultaneous decreas e in forb biomass, resulting in almost no net change in community biom ass after 5 wk. Lower irradiance reduced grass biomass during the same period. Under ambient conditions total aboveground community biomass increased seasonally from 161 g m(-2) in July to a maximum of 351 g m( -2) in September, declining to 285 g m(-2) in October. However, under warmed conditions, peak community biomass was extended into October du e in part to continued growth of grasses and the postponement of senes cence. Our findings indicate that while alpine grasses respond favorab ly to altered conditions, others may not. And, while peak community bi omass may actually change very little under warmer summers, the durati on of peak biomass may be extended having feedback effects on net ecos ystem CO2 balances, nutrient cycling, and forage availability.