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The Pan-Arctic Shrub Expansion
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.

What was done
The authors analyzed northern Alaska repeat photography data from the Colville River photo study conducted there between 1945 and 1953, together with 202 new photos of the same sites that were obtained between 1999 and 2002, to determine the nature of shrub expansion in the region over the past half-century. Then, for Canada, Scandinavia and parts of Russia, as well as the same region of northern Alaska, they analyzed both previously published plot studies and satellite remote sensing data for evidence of shrub expansion over the broader pan-Arctic region.

What was learned
Tape et al. report that "repeat photography from northern Alaska shows that large shrubs have increased in size and abundance over the past 50 years, colonizing areas where previously there were no large shrubs." In addition, they say their review of plot and remote sensing studies confirms that "shrubs in Alaska have expanded their range and grown in size" and that "a population of smaller, intertussock shrubs not generally sampled by the repeat photography, is also expanding and growing." Taken together, they conclude that "these three lines of evidence allow us to infer a general increase in tundra shrubs across northern Alaska."

The plot and remote sensing studies outside of Alaska also indicate, in the researchers words, that shrubs are "expanding across much of arctic Canada and in Scandinavia, and possibly Russia and Siberia." Based on these results they further conclude that "a pan-Arctic expansion of shrubs is underway."

So what is the cause of the shrub expansion? ... and when did it begin? Tape et al. are inclined to attribute it to large-scale pan-Arctic warming; and from analyses of logistic growth curves, they estimate that the expansion began about 1900, "well before the current warming in Alaska (which started about 1970)." Hence, they conclude that "the expansion predates the most recent warming trend and is perhaps associated with the general warming since the Little Ice Age."

What it means
Tape et al.'s findings and inferences appear reasonable, although we would add that the increase in the air's CO2 concentration since 1900 (a gain of some 80 ppm) likely played a role in the shrub expansion as well. If continued, they say "this transition will alter the fundamental architecture and function of this ecosystem with important ramifications," the great bulk of which, in our opinion, will be positive, as the greening of the earth continues.

Yes, never forget, for man and nature alike, green is good, and whatever produces it is also good.

Reviewed 19 July 2006