Decline of arctic-alpine plants at the southern margin of their range following a decade of climatic warming

Question: Have recent increases in temperature caused a decline in arctic-alpine plants at the southern margin of their range? Location: Above tree line; Glacier National Park, Montana, western USA. Methods: We monitored the abundance of seven arctic-alpine vascular plants at or near the southern limits of their ranges at three sites in Glacier National Park, Montana from 1989 through 2002. In addition we recorded canopy cover of all plant species in sample plots once at the beginning and again at the end of the study. Results: Mean summer temperature during this period averaged 0.6 degreesC higher than the previous four decades. Results of ordinations with non-metric multidimensional scaling suggested that vegetation moved toward the dry end of a moisture gradient at two sites during the course of the study. At the same time four of the peripheral arctic-alpine indicator species demonstrated 31-65% declines in abundance, while none increased. Conclusions: We cannot rigorously infer causality from our descriptive study; however, changes in both indicator species and the vegetation matrix were consistent with predictions of climate-induced extirpation of high-elevation species and the northern migration of floras. Our results also suggest that species responded to the decade of warming individualistically with little relationship to growth form.