Changing climate blamed for massive decline of aspens

As insects thrive, thousands of trees are wiped out

By Nicholas Riccardi
Los Angeles Times / November 1, 2009

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PAONIA, Colo. - From the hillsides of extinct volcanoes in Arizona to the jagged peaks of Idaho, aspen trees are falling by the tens of thousands, the latest example of how climate change is dramatically altering the American West.

Starting seven years ago, foresters noticed massive aspen losses caused by parasitical insects, one of which is so rare it is hardly even written about in the scientific literature. But with warming temperatures and the aftereffects of a brutal drought lingering, the parasites are flourishing at the expense of the tree, beloved for its skinny branches and heart-shaped leaves that turn a brilliant yellow in autumn.

What foresters have termed Sudden Aspen Decline has more than just aesthetic consequences. Aspen trees provide a rich habitat for birds, elk, deer, and other animals.

The grasses that sprout under them - as much as 2,000 pounds per acre - hold water that is needed by metropolitan areas. The trees do not burn easily and create natural fire breaks in forests already ravaged by the pine bark beetle - another parasite that is thriving due to global warming.

“It’s just rolling through the forests,’’ said Wayne Shepperd, an aspen specialist at Colorado State University, said of the aspen decline. Noting the number of other changes to western vegetation due to warmer, drier temperatures, he added: “Everything’s happening all at once.’’

The decline of the tree is most visible in Colorado, which has seen nearly 500,000 acres - nearly one-fifth of its aspen groves - afflicted by Sudden Aspen Decline. Hillsides that used to draw tourists in the fall to gaze at the flickering aspen leaves are now populated only by the trees’ pale skeletons.

The most vulnerable trees grow on sunny, south-facing lower elevations - where warmer temperatures wear down the trees’ resistance to pests. And though the largest number of deaths is in Colorado, this is also the state with the greatest amount of aspen. The effect can be even more severe in other places, such as Nevada, Arizona, or California, which have only small bands of aspen habitat.

Even before aspen trees began their abrupt depletion, the tree was under duress in the West. Scientists estimate that the trees covered 10 million acres in the 19th century; now there are 4 million left.

The main culprit is fire prevention. Aspen trees regenerate from their roots. Regular wildfires that would knock down old aspens’ trunks - and free up new stems to emerge from the roots - have been snuffed out by human intervention.

Additionally, the lack of fires has led to a sharp rise in big conifer trees, which crowd out aspens. And livestock and wild animal grazing has killed some of the younger aspens trying to survive.

Researchers concluded that warmer temperatures stressed trees, especially older ones that hadn’t been replaced by fire. They identified four parasites and one wasting disease that are now flourishing in the stands. One of the parasites, the aspen bark beetle, is so rare that it had only been mentioned once before in an academic paper.

“If the aspen were vigorous, these things would just be taking a minor role,’’ said Jim Worrall, a plant pathologist for the US Forest Service.