Summit County Citizens Voice

A grove of healthy aspens in the Straight Creek drainage, near Dillon, Colorado

Sudden mortality of aspens linked to intense 2002 drought; aspen habitat in the state could shrink significantly under a global warming regime

By Bob Berwyn

SUMMIT COUNTY — Colorado’s iconic aspen trees aren’t going to die off any time soon, says a Forest Service researcher who studied a puzzling and unprecedented decline in aspen health that was first noticed in 2004. Sudden aspen decline ultimately affected about 17 percent of the state’s aspen forests, mostly in the southwestern part of the state.

“It was a pretty severe event. We haven’t ever seen anything like it,” said Jim Worrall. “There’s continuing damage in the affected areas but it does look like the spread has stopped.”

The hot, dry weather interfered with the trees’ ability to process water and also made them more susceptible to other pathogens, the researchers concluded.

“Within affected areas, mortality and crown loss have resulted in loss of about half the canopy on average, with crowns continuing to deteriorate and die,” Worrall wrote in a paper on the aspen decline that was recently published in the journal Forest Ecology. “The damage, especially at low elevations, is severe and unprecedented in our experience, but most of the cover type in Colorado is still relatively healthy. Even in declining stands, complete mortality is still uncommon.”

The researchers traced the decline with aerial surveys and then zeroed in on some of the aspen stands to study tree physiology. In the end, they linked the dramatic die-off to moisture stress from the 2002 drought, when hot weather prevailed across much of Colorado for most of the summer.

Worrall said some impacts to some of the hardest-hit aspen stands were so intense that they may never recover and may convert to some other type of vegetation, especially where the trees are heavily browsed by elk.

The research will help Forest Service planners decide where regeneration efforts could be effective. Those management efforts should be directed toward stands with intermediate levels of crown loss and higher levels of pre-existing regeneration, indicating that roots are still abundant and vigorous enough to respond to disturbance.

If foresters want to try and improve aspen stand resiliency, as the trees face potentially longer and hotter droughts resulting from climate change, they should try and increase the proportion of regenerating stands, Worrall wrote in the conclusion of the paper.

“These are all episodic events” said Susan Gray, a forest health expert with the agency’s regional office. “The stands that were affected are still experiencing decline, but we’re not anticipating an expansion of sudden aspen decline,: she said.

“We’ve learned a lot,” she said, explaining that the research helped show that long, hot dry spells during the growing season most likely triggered the sudden decline. The areas most affected were marginal aspen habitat to begin with — dry sites with shallow soil, characterized by an aspen-sagebrush mix. Older and younger stands were equally affected, and the link between drought and the die-off is strongly supported by the research.

“Other scientists have referred to it as a global climate-change type of event,” Worrall said, explaining that, if you compare maps of the areas affected by sudden aspen decline with climate change projections, there is a significant overlap.

In other words, areas expected to see significant warming and drying under a global warming regime are the same areas where the aspens took a major hit from a one-year drought.

Based on the climate profile for aspens, the areas suitable for the trees would shrink quite a bit, Worrall said. The mapping suggests that large areas of the landscape in southwestern Colorado would become unsuitable habitat for aspens as soon as 2060, he added.

A warming climate would push aspens uphill by as much as 250 vertical meters by 2030 and 750 meters by 2060, he said, explaining how the climate change predictions might play out, and adding that the changes would likely come in fits and starts, and not in a smooth progression.

Whether the sudden aspen decline that began in 2002 is part of that change, or just a one-time hiccup remains to be seen.

Filed under: Environment, forests, Summit County Colorado Tagged: | Aspen, Forest health, forests, global warming, sudden aspen decline, Summit County, Summit County News