Winter biological processes may help spread shrubs in the Arctic

Shrubs have become more abundant in the Arctic over the past 30 years as air temperatures have increased, a change that is likely to affect the grazing of caribou and the communities that rely on them for food. According to an article in the January 2005 issue of BioScience, a variety of evidence now suggests that winter biological processes form a positive feedback mechanism that is contributing to the expansion of shrubs in the Arctic. The effect could have important implications for the global carbon budget, as the mechanism may liberate large stores of carbon that are currently frozen and not participating in the carbon cycle.

The article, by Matthew Sturm of the U.S. Army Cold Regions Research and Engineering Laboratory-Alaska and seven coauthors, notes that the evidence for increasing shrub abundance–including historical photographs– is most comprehensive in northern Alaska. Information from other arctic regions supports the idea, however. Sturm’s group argues that observations indicate that shrubs encourage deeper snowdrifts, which warm the soil below, preventing some subsurface water from freezing even during winter. This effect alters and boosts the winter activity of subsurface soil bacteria and fungi that provide accessible nutrients for shrubs, notably nitrogen. As a result, shrubs grow more rapidly, and so the spread continues.