Birnam Wood in the 21st Century: northern forest invading Arctic tundra as world warms

In Shakespeare's play Macbeth the forest of Birnam Wood fulfills a seemingly impossible prophecy by moving to surround the murderous king (the marching trees are helped, of course, by an army of axe-wielding camouflaged Scots). The Arctic tundra may soon feel much like the doomed Macbeth with an army of trees (and invading species) closing in. In a recent study, researchers found that climate change is likely to push the northern forests of the boreal into the Arctic tundra—a trend that is already being confirmed in Alaska.

Analyzing 16 global climate models starting in 1950 and ending at the close of the 21st Century, researchers found that the tundra of North America, Europe and Asia will be increasingly encroached upon by pine and deciduous forest.

"The response of vegetation usually lags changes in climate. The plants don't have legs, so it takes time for plant seed dispersal, germination and establishment of seedlings," Song Feng, research assistant professor at University of Nebraska-Lincoln's School of Natural Resource, said in a press release.

As temperatures rise regionally by 5.6 to 9.5 degrees Fahrenheit, a quarter to a half of the Arctic could be impacted, according to the study in Climate Dynamics, depending on future greenhouse gas emissions. Tundra vegetation is expected to lessen by 33-44% by 2099.

The ecosystem changes could impact climate as well, eplains Feng: "the expansion of forest may amplify global warming, because the newly forested areas can reduce the surface reflectivity, thereby further warming the Arctic. The shrinkage of tundra and expansion of forest may also impact the habitat for wildlife and local residents."

The models predicted that the tundra would vanish quickest in North America with boreal forest covering most of northern Canada and Alaska's tundra by mid-century.

Happening in real time in Alaska

While Feng's study employed computer models to predict the future, a recent study in Ecology Letter has shown that ecosystems are already shifting in Alaska.

Combining satellite images and tree-ring data, the study found increased growth of boreal forest at the edge of Alaska's tundra.

"The results provide evidence for the initiation of a biome shift in response to climate change, and indicate that some ecosystem models may be missing fundamental changes taking place in the circumpolar region," explained lead author Pieter Beck, a post-doctoral fellow at Wood's Hole Research in a press release.

In addition, the study found that tree growth had actually slowed throughout much of Alaska's forest.

"Recent temperature increases have reduced tree growth over most of central Alaska, and increased growth in places where the temperature used to be too low for optimum growth, such as the Western Alaska tundra margin. Summer temperatures in central Interior Alaska are now almost too warm for white spruce to survive," explained co-author Glenn Juday, professor of forest ecology at the University of Alaska Fairbanks.



CITATIONS: Feng S., C-H. Ho, Q. Hu, R. J. Oglesby and S-J. Jeong (2010).Evaluating observed and projected future climate changes for the Arctic using the Koppen-Trewartha climate classification. Climate Dynamics. DOI: 10.1007/s00382-011-1020-6.

Pieter S. A. Beck, Glenn P. Juday, Claire Alix, Valerie A. Barber, Stephen E. Winslow, Emily E. Sousa, Patricia Heiser, James D. Herriges, Scott J. Goetz. Changes in forest productivity across Alaska consistent with biome shift. Ecology Letters, 2011; DOI: 10.1111/j.1461-0248.2011.01598.x

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