Responses of subalpine meadow vegetation to four years of experimental warming

Ecosystems at high elevations may be especially sensitive to global warming , because productivity is limited to a snow-free growing season, and warmin g is expected to cause earlier snowmelt. Here we report on vegetation respo nses to experimental warming in a subalpine meadow in the Colorado Rocky Mo untains. We found no evidence that the plant community changed during four years of warming. Species composition in warmed plots did not change more t hrough time than in control plots, nor did warmed plots diverge from adjace nt control plots through time. Contrary to an earlier report, we found no e vidence that warming facilitated adults or seedlings of sagebrush, a shrub characteristic of lower elevation ecosystems; nor did it facilitate short-l ived plant species as a group. Total vegetation cover, as well as cover of graminoids, forbs, and shrubs, did not differ between control and warmed pl ots, nor did species richness or species' distributions along a small eleva tional gradient within each plot. Shrub cover tended to increase more, and forb cover to decrease more, in warmed than in control plots during one sum mer season, but not significantly so. This lack of detectable plant communi ty response contrasts with pronounced responses to warming in some arctic a nd alpine ecosystems over similar time spans. Warming in these ecosystems i s thought to act indirectly via increased mobilization of soil nutrients. O ne 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 g lobal warming, then vegetation in arid high-elevation ecosystems may change only slowly.