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Trees and Shrubs at Their Poleward Limits in a Warming World
Capers, R.S. and Stone, A.D. 2011. After 33 years, trees more frequent and shrubs more abundant in northeast U.S. alpine community. Arctic, Antarctic, and Alpine Research 43: 495-502.

The authors write that "woody species are predicted to increase in tundra ecosystems as a result of global climate change (Chapin et al., 1996; Epstein et al., 2000)," and, in fact, it has been found that in response to rising air temperatures, "trees have established where they did not previously occur, both in alpine areas (Wardle and Coleman, 1992; Peterson, 1994; Kullman, 2001, 2002), and in arctic tundra (Lescop-Sinclair and Payette, 1995; Danby and Hik, 2007)," while "increasing shrub abundance also has been reported in alpine (Klanderud and Birks, 2003) and arctic locations (Sturm et al., 2001; Tape et al., 2006; Wilson and Nilsson, 2009)."

What was done
In an attempt to learn more about this phenomenon, Capers and Stone "studied a community in western Maine, comparing the frequency and abundance of alpine plants in 2009 with frequency and abundance recorded in 1976," while noting that "the 2009 survey was designed to provide a fair comparison with that of 1976," which was conducted and described by Stone (1980).

What was learned
The two researchers found that the 2009 survey "provided evidence of the increasing importance of woody plants - both trees and shrubs - in the alpine community," commenting that "the most widespread tree species increased dramatically." In addition, they say they "recorded an increase in total species richness of the community with the addition of four lower montane species that had not been recorded previously." And in another important positive finding, they say they "found no evidence that species with high-arctic distributions had declined more than other species."

What it means
In the words of Capers and Stone, the changes they recorded "are consistent with those reported in tundra communities around the world." And although there is some concern that the observed increase in species richness could ultimately turn out to be temporary if alpine species were to disappear because of competition from the new species appearing on the scene, they state that "species losses resulting from competition have not typically been found with rising richness in high alpine areas, possibly because newly arriving species occupy different microhabitats," citing the work of Walther et al. (2005).

Thus, it looks like the planet itself has decided to Go Green! ... as ice makes way for life.

Chapin III, F.S., Bret-Harte, M.S., Hobbie, S.E. and Zhong, H. 1996. Plant functional types as predictors of transient responses of arctic vegetation to global change. Journal of Vegetation Science 7: 347-358.

Danby, R.K. and Hik, D.S. 2007. Variability, contingency and rapid change in recent subarctic alpine tree line dynamics. Journal of Ecology 95: 352-363.

Epstein, H.E., Walker, M.D., Chapin III, F.S. and Starfield, A.M. 2000. A transient, nutrient-based model of arctic plant community response to climatic warming. Ecological Applications 10: 824-841.

Klanderud, K. and Birks, J.H.B. 2003. Recent increases in species richness and shifts in altitudinal distributions of Norwegian mountain plants. The Holocene 13: 1-6.

Kullman, L. 2001. 20th century climate warming and tree-limit rise in the southern Scandes of Sweden. Ambio 30: 72-80.

Kullman, L. 2002. Rapid recent range-margin rise of tree and shrub species in the Swedish Scandes. Journal of Ecology 90: 68-77.

Lescop-Sinclair, K. and Payette, S. 1995. Recent advance of the arctic treeline along the eastern coast of Hudson Bay. Journal of Ecology 83: 929-936.

Peterson, D.L. 1994. Recent changes in the growth and establishment of subalpine conifers in western North America. In: Beniston, M. (Ed.). Mountain Environments in Changing Climates. Routeledge, London, United Kingdom, pp. 234-243.

Stone, A. 1980. Avery Peak on Bigelow Mountain, Maine: The Flora and Vegetation Ecology of a Subalpine Heathland. M.S. Thesis. University of Vermont, Burlington, Vermont, USA.

Sturm, M., Racine, C. and Tape, K. 2001. Climate change - increasing shrub abundance in the Arctic. Nature 411: 546-547.

Tape, K., Sturm, M. and Racine, C. 2006. The evidence for shrub expansion in Northern Alaska and the Pan-Arctic. Global Change Biology 12: 686-702.

Walther, G.-R., Beissner, S. and Burga, C.A. 2005. Trends in the upward shift of alpine plants. Journal of Vegetation Science 16: 541-548.

Wardle, P. and Coleman, M.C. 1992. Evidence for rising upper limits of four native New Zealand forest trees. New Zealand Journal of Botany 30: 303-314.

Wilson, S.D. and Nilsson, C. 2009. Arctic alpine vegetation change over 20 years. Global Change Biology 15: 1676-1684.

Reviewed 16 May 2012