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"Greening" of the Arctic May Not Be Caused by Warming, But by Lemmings

Reference
Johnson, D.R., Lara, M.J., Shaver, G.R., Batzli, G.O., Shaw., J.D. and Tweedie, C.E. 2011. Exclusion of brown lemmings reduces vascular plant cover and biomass in Arctic coastal tundra: resampling of a 50+ year herbivore exclosure experiment near Barrow, Alaska. Environmental Research Lettters 6: 045507 doi:10.1088/1748-9326/6/4/045507.
Recently, researchers reviewing satellite remote sensing data over the period 1982-2008 detected a widespread 'greening' in many coastal Arctic regions and concluded that this enhanced growth of tundra plants was due to warming from sea ice declines offshore (Bhatt et al., 2010). On the ground in one such coastal location-Barrow, Alaska -- Johnson and colleagues (2011) set out to document what effects herbivorous brown lemmings (Lemmus trimucronatus) had on the growth of such tundra plants (including grasses, sedges, lichens and moss).

In 2002 and 2010, Johnson et al. sampled 2 m x 2m plots: twelve control plots and twelve "exclosure" plots (which excluded brown lemmings, the area's primary herbivore). The sampling periods occurred two years after documented lemming population outbreaks that occurred in 2000 and 2008 (when lemming populations grew exponentially). Then, for each of these time periods the authors "examined the effects of sustained herbivory on plant community composition and the degree to which lemming herbivory may have contributed to the regional greening signal detected by Bhatt et al. (2010)."

Results indicated there were more sedges and grasses in the control plots than those that excluded lemmings. This suggests that the kind of sustained lemming activity that occurs during lemming population outbreaks promotes greater growth of grasses and sedges afterward -- or in the authors' words, "brown lemmings facilitate the production of their preferred forage." Some of this increased plant growth after high lemming years may be due to their waste products fertilizing the soil.

Because grasses and sedges were found to respond favorably to periodic lemming activity as well as to increased temperature, the conclusion that "greening" of coastal Arctic tundra is the direct result of warming due to reduced sea ice may well be premature.

Additional Reference
Bhatt, U.S., Walker, D.A., Raynolds, M.K., Comiso, J.C., Epstein, H.E., Jia, G., Gens, R., Pinzon, J.E., Tucker, C.J., Tweedie, C.E. and Webber, P.J. 2010. Circumpolar Arctic tundra vegetation change is linked to sea ice decline. Earth Interactions 14: 1-21.

Archived 6 December 2011