It’s not just the lodgepole pines that are dying in the Colorado mountains. Thirteen percent of aspen trees in the state are now affected by Sudden Aspen Decline, which is caused by disease and insects taking advantage of trees weakened by years of drought.
In 2006, there were 140,000 acres of aspen tree stands dying in Colorado. A year later, the number shot up to 338,000 acres of dead or dying aspen trees.
An expert on the topic said that 54,000 acres of aspen tree stands in the White River National Forest, which surrounds Aspen, are affected by SAD.
“In the last couple of years, significant areas of decline have occurred,” said Roy Mask, a forest health protection specialist with the U.S. Forest Service who is based at the Gunnison Service Center.
Mast gave a presentation on the increasing spread of SAD to the Pitkin County commissioners on Tuesday.
“We’re not losing all the aspens,” Mask said, but he also expects to see SAD continue to spread.
Large mature trees at low elevations on south- and west-facing aspects are getting hit the hardest. Trees in the southwestern and northwestern parts of the state are showing the worst signs of the condition.
While aspen trees are part of a constantly changing forest landscape, the change in the aspen trees is now being seen on a “landscape” scale and the mortality rate has increased dramatically over just a few years, Mask said.
Aspen trees in Colorado were weakened by the 2000-2005 drought and are now being killed by a fungus and four types of insects.
The Cytospora fungus, common to a number of trees, causes a Cytospora “canker” on aspen trees, which have the scientific, or binomial, name of “populus tremuloides.”
Meanwhile, the poplar borer, the bronze poplar borer and two aspen bark beetles are burrowing into trees and slowly killing them.
The Cytospora canker causes orange stripes on aspen trees in its early stages while the borers and beetles leave tell-tale trails behind them in the trees.
The disease and the insects causing SAD are different than the “weeping disease” that has plagued some aspen trees planted at lower elevations in the Roaring Fork River valley, Mask said.
One study in the San Juan National Forest found that in 2002-2003 only 8 to 9 percent of mature stands of aspen trees were afflicted. By 2006, 30 to 60 percent of the trees were being killed.
“That’s the kind of information that caught our eye,” Mask said, adding that open stands with large trees are more affected.
After being killed by wildfire or by cutting, aspen trees usually send out “suckers” from their roots to regenerate. But after being killed by disease or insects, the aspen trees regenerate at a much lower rate.
And, Mask said, aspen trees are in a “downward spiral” and there is really nothing that can be done to stop the spread of SAD.
“It won’t just go away in a year’s time,” Mask said.
The Forest Service is, however, studying the situation by conducting aerial surveys and field studies and is cooperating with other land management agencies to better understand the trend.
bgs@aspendailynews.com
