Ecosystem and plant community consequences of climate warming in a high-altitude meadow

Global climate warming has the potential for significantly altering species composition and community structure through its effect on ecosystem function. Compositional changes, in turn, can further affect ecosystem function. Ecosystems such as the high-altitude meadow used in this study are expected to be more vulnerable to temperature changes than are other ecosystems because of their dependence on the timing and duration of snowmelt, the short growing season, and the steep climatic gradients. I have investigated how simulated warming affects rates of nitrogen cycling, nitrogen availability, plant physiology, and plant community composition in two distinct soil microclimates, one mesic and one xeric. Infrared heating lamps were used to simulate the predicted increase in heat flux in a {dollar}rm 2{lcub}times{rcub}lbrack COsb2rbrack{dollar} atmosphere.; I examined the effect of warming on the aboveground biomass of three growth forms: shrubs, grasses and forbs. Total aboveground biomass did not change, but there was a compositional shift that favored shrubs over forbs. The shrub aboveground biomass doubled in the xeric habitat and increased by 50% in the mesic habitat with warming. Forb biomass declined with warming and there was no significant change in grass aboveground biomass. In the xeric zone, the shrub Artemisia tridentata accumulated more biomass in the heated plots by increasing photosynthetic rates early in the growing season. There were no changes in photosynthetic rates for the mesic zone shrub, Pentaphylloides floribunda. In both zones, the increase in biomass in the heated plots may be partially attributable to the earlier snowmelt in those plots.; Warming altered nitrogen cycling directly, and indirectly through the changes in litter quality. Gross mineralization and NH{dollar}sb4sp+{dollar} consumption increased with warming in the first two years of the study, though there were no measurable heating effects in these rates by the fourth year. Warming had no measurable impact on nitrogen cycling in the mesic zone. There were no direct effects of heating on litter decomposition rates, though there was an indirect effect due to a shift in litter quality and production attributable to the changes in plant community composition. These results will aid in the understanding of complex ecosystem responses to climate warming and in the predictions regarding climate-ecosystem feedbacks.