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Forest Changes in Alaska Reveal Changing Climate

External Reference/Copyright
Issue date: 
August 25, 2010
Publisher Name: 
Science of America
Lauren Morello and Climatewire
Author e-Mail: 
http://www.scientificamerican.com/author.cfm?id=2047 http://www.scientificamerican.com/author.cfm?id=2312
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CAMBRIDGE, Md. -- Evidence is mounting that climate change is transforming Alaska's boreal forest, an expert said yesterday.

"A biome shift is now occurring," University of Alaska, Fairbanks, forest ecologist Glenn Juday said. "You don't have to wait for the effects. They're happening."

The state's white spruce stands, which according to one recent study contain half of the genetic diversity of all white spruce in North America, are suffering.

Empirical studies of forests across Alaska show that North America's white spruce require at least 280 millimeters (11 inches) of precipitation each year, a number that rises if mean summer temperatures are higher than 15.5 degrees Celsius (roughly 60 degrees Fahrenheit).

When the mercury hits a mean temperature of 21.1 degrees Celsius, or just shy of 70 degrees Fahrenheit, the trees can't survive.

In Fairbanks, conditions are hovering right on that edge. Annual precipitation is about 280 millimeters, and July temperatures have exceeded 15.5 degrees Celsius several times in the last couple of decades.

White spruce that grow around Fairbanks are "probably not viable on a long-term basis under such conditions," Juday said. "With each additional degree of warming, their growth goes down."

Part of the problem is that the region is drying, the scientist said. The growing season in Fairbanks now stands at 120 days, 50 percent longer in 2007 than it was in 1905.

Birch trees not performing as expected
That means a shorter period for snow to accumulate in winter and a bigger gap between snowmelt and summer rains, two factors that work together to produce more intense dry spells.

"We had one of those this year," Juday said. "We had a near-record warm April-May period, prolonging the effects of July [2009] drought and a low snow year."

Meanwhile, the birch trees that scientists once suspected would "pick up the slack" when climatic conditions began to overwhelm white spruce appear to be similarly susceptible to the environmental changes.

Juday said he can see as much when he looks out the window of his office on the University of Alaska's Fairbanks campus, where birch trees show symptoms of leaf scorch. It's the last symptom before the death of the tree.

"It's the result of the 2004-2005 drought ... followed by root dieback, followed by a warm 2007," he said.

Trees in several research forests Juday works in across interior Alaska showed similar symptoms this summer. "We found incredibly stressed birch on a widespread basis," he said.

The behavior of a wildfire now burning uninhabited military lands near Fairbanks is also a concern, the scientist said. The Willow Creek blaze, which began June 10, was caused by lightning. It was still smoldering yesterday, the Bureau of Land Management's Alaska Fire Service said, reporting that the wildfire has burned through 13,766 acres of military land.

Planning for the future
To the scientist, the Willow Creek fire is notable because "high-intensity" rainstorms this summer slowed the blaze but could not provide enough help to allow firefighters to stop it entirely. The wildfire has persisted in a fire season that was not expected to be severe.

"The cumulative drying effect is enormous," Juday said. "Fuels are so dry they are sustaining combustion. That's stored carbon, and it's being mobilized, and we can't -- apparently -- stop it."


Extpub | by Dr. Radut