Rapid loss of aspen forests prompting research

MONTROSE — What first appeared a mysterious, rapid loss of local aspen forests is coming into focus as scientists delve into causes of the decline.

Their studies coincide with recent aerial surveys that uncovered a sharp, statewide increase in aspen deaths.

Sudden aspen decline or SAD has shown up in large areas in the San Juan, Grand Mesa, Uncompahgre and Gunnison National Forests. A fairly extensive outbreak is visible in the Terror Creek area near Paonia, said Lee Ann Loupe, GMUG public affairs officer.

Severe drought, especially in 2002, is suspected of triggering the sudden aspen decline. These drought conditions leave trees vulnerable to insects and diseases.

“That’s our best guess at this point. It was triggered by the drought, (and) these other organisms responded in a big way,” said entomologist Roy Mask, USDA Forest Service Forest Health Management in Gunnison. “We’re still trying to identify what impact there was on the root system. Those roots might have been in some declining condition already and they may have been even more severely impacted by drought.”

Aerial surveys in 2007 showed a 130 percent increase in aspen mortality over 2006 levels, the USDA Forest Service reported. Surveys conducted in 2006 show that 138,000 acres of aspen forest have been affected. In some stands, more than 60 percent of the mature trees died between 2003 and 2006.

These surveys also revealed that for the most part, the most significant SAD occurred on south-facing slopes and at lower elevations, which led to the drought hypothesis, said Mask. Researchers also found five organisms that consistently occurred in various combinations on these sites: two bark beetles, two wood-boring beetles and a canker fungus.

For the most part, aspen mortality happens when trees reach 80 to 100 years old, Mask said. What concerns scientists and managers is the sudden and expansive nature of recent declines, he said. Large amounts of aspen have been dying over short periods.

Researchers initiated a study they hope to continue in 2008 on whether affected aspen forests are going to regenerate and if so, whether that regeneration is adequate enough to stop decline, said Mask.

Aspen are among the largest living organisms in the world because they share the same root system. Although aspen sometimes reproduce sexually, the most common way for them to regenerate is by sprouting from the root, said Mask.

Aspen in the Grand Mesa, Uncompahgre and Gunnison forests are extremely important resources for wildlife, and also have commercial and aesthetics value, said Carmine Lockwood, GMUG renewable resources staff officer.

“We’re very concerned about the situation with the sudden aspen decline because we don’t know what it might mean to the long-term persistence of this forest cover type on the forest,” said Lockwood. “It’s unclear now whether or not the affected stands will regenerate.”

In areas with organic soil, aspen has a tremendous moisture-capturing capacity, said Mask. Aspen forests create biologically diverse habitats. They provide good cover and feed for deer and elk, said Colorado Division of Wildlife spokesman Joe Lewandowski. Elk eat aspen bark and deer eat the leaves and young sprouts on the branches. The animals also calve among aspen in the spring.

“They are important (to wildlife) but the long-term effects of (sudden aspen decline) are difficult to say,” said Lewandowski.

There are approximately 3.6 million acres of aspen in Colorado. Aspen comprise roughly 17 percent of the state’s forested lands, according to the Forest Service. Aspen comprise 22 percent of the vegetation of the GMUG, with additional aspen occurring in mixed stands of other forest species.

Contact Lisa Huynh via email at lisah@montrosepress.com