DENVER, Sept. 25 — The aspen, an emblematic tree of the West and the most widely distributed tree in North America, is rapidly and mysteriously dying.
Its rapid decline is bewildering scientists and forest ecologists, who say they cannot pinpoint a cause.
“What’s causing the aspen to die?” asked Wayne Shepperd, a veteran researcher at the Rocky Mountain Research Center of the United States Forest Service. “We don’t know. Maybe this has been there all along, and we haven’t noticed it before, or maybe it’s something new.”
There is no shortage of suspects. Forest experts, who met this month at a conference in Utah to discuss the problem and look for solutions, say it may be insects, drought or climatic stress in general or overgrazing by animals like elk and cattle. Or it may be none or all of the above.
The aspen dieback is particularly baffling in that it seems to be occurring just in some Western states and is not affecting any neighboring trees, many of which already suffer from a plague of mountain pine beetles that has been devastating the West.
Since word of the aspen dieback began spreading last fall, Dr. Shepperd’s office began receiving reports of similar losses throughout the West — not just Colorado, where the problem is most predominant, but also in Utah, Idaho, Nevada, Montana, Arizona and southern Wyoming.
But because there has been no concerted investigation, it is not clear how many aspen have died, much less why. “Quite honestly we just don’t have any answers,” Dr. Shepperd said.
The die-off is particularly worrisome because of the special nature of the aspen. The tree reproduces through vegetative regeneration. Genetically identical suckers sprout from the root of one tree and become clones. If the root of an aspen dies, it is unlikely to reproduce.
“We actually dug in a couple of instances and looked for live roots, and we couldn’t find any,” Dr. Shepperd said.
The dieback may go back as far as 1996, at the beginning of the recurrent drought, according to Mary Lou Fairweather, a plant pathologist for the Southwestern region of the Forest Service, who said she first noted the loss of trees in Arizona.
“I started monitoring the dieback on how the trees were growing, when and if it was continuing,” Ms. Fairweather said.
She added that she thought that the first deaths were caused by seasonal climate changes like spring freezes and then accelerated by other factors, including stress affecting the trees, although she noted that those were just theories.
In Utah, Dale Bartos, an aspen ecologist for the Forest Service, agreed that the lack of precipitation had contributed more stress to the aspen than initially thought.
“We’ve been in a major drought over the last few years, and I’m sure this has exacerbated the problem,” Mr. Bartos said.
Dr. Shepperd is not so sure. Although the current drought is probably a factor, he said, extensive long-term studies are needed.
“There’s no real pattern,” he said.
But he has noticed one intriguing clue. Some younger groves of aspen appear to be more resilient than the mature ones.
“Generally, younger stands seem to be healthier,” he said. But he added that it would be discouraging if these younger stands did not sprout. “We’re seeing it across the spectrum of site-conditioned aspen,” he said, meaning that no aspen is safe.
Mr. Bartos estimates that at the current rate 10 percent of the aspens in the West could die within several years. Some of his colleagues call that a conservative estimate.
“We’ve seen in southern Utah, over a period of 12 years, where we have very healthy-looking clones with dark green leaves go to sites where there aren’t any trees left at all,” Mr. Bartos said, adding that in other cases researchers had observed an even quicker rate of decline. “To me, 12 years is fairly rapid when we’re talking about trees that have been on site for 100 to 125 years.” Nevertheless, the scientists who met in Utah to look for a solution came away somewhat hopeful, said John Guyon, a forest pathologist for the Intermountain Regional Office of the Forest Service.
Mr. Guyon said he thought that grazing elk and cattle might be eating away the regenerating aspen. And because the trees thrive after disruptions like avalanches and fires, the lack of such disturbances may be contributing to the dieback.
Jim Worrall, another Forest Service pathologist, said the question was fraught with uncertainty.
“What we don’t know is what the future holds,” Mr. Worrall said. “It’s an unprecedented event.”