RESPONSES OF SUBALPINE MEADOW VEGETATION TO FOUR YEARS OF EXPERIMENTAL WARMING

Abstract

Ecosystems at high elevations may be especially sensitive to global warming, because productivity is limited to a snow-free growing season, and warming is expected to cause earlier snowmelt. Here we report on vegetation responses to experimental warming in a subalpine meadow in the Colorado Rocky Mountains. We found no evidence that the plant community changed during four years of warming. Species composition in warmed plots did not change more through time than in control plots, nor did warmed plots diverge from adjacent control plots through time. Contrary to an earlier report, we found no evidence that warming facilitated adults or seedlings of sagebrush, a shrub characteristic of lower elevation ecosystems; nor did it facilitate short-lived plant species as a group. Total vegetation cover, as well as cover of graminoids, forbs, and shrubs, did not differ between control and warmed plots, nor did species richness or species' distributions along a small elevational gradient within each plot. Shrub cover tended to increase more, and forb cover to decrease more, in warmed than in control plots during one summer season, but not significantly so. This lack of detectable plant community response contrasts with pronounced responses to warming in some arctic and alpine ecosystems over similar time spans. Warming in these ecosystems is thought to act indirectly via increased mobilization of soil nutrients. One possible reason for the lack of response in our system is that drying of soil limits microbial activity, photosynthesis, and plant growth sooner in the season in warmed plots, canceling out effects of earlier snowmelt. If this is correct, and if summer precipitation patterns are unchanged under global warming, then vegetation in arid high-elevation ecosystems may change only slowly.