Shrub invasion of alpine areas. An uphill battle?

Background/Question/Methods

Shifting range distributions observed worldwide provide some of the best evidence of species responses to increasing global temperatures over the past century. Many predictions of species range shifts are based on the climate envelope approach, with the null prediction that species ranges will shift poleward and upward in elevation to track suitable climate. However, observed rates of range alteration vary widely among species, potentially due to both differential dispersal rates and species interactions. In 2010 we conducted a re-survey of plant species distribution and abundance in the White Mountains of Eastern-California, in areas that were originally surveyed by Harold Mooney in 1961. Species presence and abundance data were collected along 50 m line transects at 150 m elevation intervals between elevations of 2,900 m and 4,100 m in alpine and sub-alpine habitats. With these data we asked two focal questions: Did all species display similar elevational range shifts? And, was there evidence of shifting species interactions (i.e. competition or facilitation)? 

Results/Conclusions

Since 1951, mean annual temperatures at this site have increased 0.93 ± 0.2 °C. Differential shifts in abundances were observed across the elevational gradient. One species, Artemisia arbuscula, a woody shrub, increased its elevational range limit 150 m (30 m/decade) from the original 1961 survey. Corresponding to this upward range expansion were significant declines in abundances of three alpine cushion plants Trifolium andersonii, Phlox condensata, and Eriogonum ovalifolium in the elevational belt of A. arbuscula invasion. Above the current range limit of A. arbuscula, these three forb species displayed similar abundances to those observed in 1961. These data suggest that the rapid elevational range expansion of A. arbuscula could lead to the competitive exclusion of T. andersonii, P. condensata, and E. ovalifolium, highlighting the importance of species interactions as community assembly processes are influenced by changing climate.