GUNNISON, Colo. — Aspen trees, with their quivering, delicate foliage and the warm glow of color they spread across the high country of the Rocky Mountains this time of year, have an emotional appeal that their stolid, prickly evergreen cousins do not.
So tree lovers and scientists alike felt the impact when the aspen in the West started dying around 2004 — withering away in a broad band from here in southwest Colorado through the mountains of Utah, Arizona, and New Mexico and into Wyoming.
“There’s definitely something powerful about these trees,” said James Worrall, a forest pathologist for the United States Forest Service, gazing at a brilliant yellow swath of healthy aspen in a stand in the mountains here, about four hours southeast of Denver.
“It’s partly, I think, an emotional impression,” he said. “Partly a very real impression that the aspen is very important in our forests — hydrologically, biologically, to wildlife, every kind of way you can imagine.”
The good news is that the phenomenon known as sudden aspen decline, or SAD, appears to have stabilized, Dr. Worrall and other researchers say. Individual trees are still dying, since the process can take years to unfold, but many stands of trees are holding their ground against any new onset.
A sudden severe drought and heat wave early in the decade set off the decline, according to a paper co-authored by Dr. Worrall this year in the journal Forest Ecology and Management. Wetter, cooler seasons since then — more to the aspen’s liking — have halted SAD’s spread. Other evidence supports the weather as the cause. Although the aspen is the most widely distributed tree species in North America, the die-off struck mostly in the Southwest, where the drought beginning in 2002 was most severe. And lower elevations were affected more than upper ones, which tend to be cooler and wetter.
“It was a really large stressor to the trees, and that made them more susceptible to other things — there’s pretty good comfort among scientists that that was what was going on,” said Dan Binkley, a professor of forest ecology at Colorado State University, who was not involved in Dr. Worrall’s paper.
The new research delivers some bad news as well. It has shown how profoundly vulnerable aspen are to environmental events outside their niche. In keeping with their delicate image, they do not like sudden weather shifts.
And the 2002 drought was a doozy. The winter was dry, with snowpack about half the long-term average in much of the aspen heartland here in Colorado. Early heat then melted what snow there was weeks ahead of average, and June arrived with searing temperatures about six degrees above average, which fried the already weakened trees.
Long-term climate projections, Dr. Worrall and other scientists say, all point to more curveballs ahead — wider, more severe fluctuations and variations of hot, dry, wet and cold.
Gerald Rehfeldt and others at the forest service’s Rocky Mountain Research Station in Moscow, Idaho, using three climate models and carbon-dioxide projections, concluded that stable aspen climate could be lost in at least two-thirds of the tree’s habitat area in Colorado and southern Wyoming alone by 2060.
“It’s the extremes of variation that gets the aspen — not the average,” Dr. Worrall said.
The aspen is, of course, not the only tree affected by changes in weather and climate. But different species respond in different ways. Millions of lodgepole pines have also died in the West in recent years, killed by beetles that are natural predators and control agents for the trees — but which have gone out of control, many ecologists say, because the cold winters that once kept beetle populations in check have not been happening the way they used to. In other words, lodgepoles have always been killed by beetles; climate patterns simply tilted the relationship out of whack.
In contrast, aspen die-off of the magnitude and pace seen in the last few years had never been recorded, Dr. Worrall and others said. The weakened trees became vulnerable to a fungus and a tiny beetle that had in the past mostly fed on dead or dying aspen and had not posed a serious threat.
Because aspen are relatively short-lived by tree standards — 100 years is long in the tooth, 300 is Methuselah — tree ring studies that might show how the species fared in some of the devastating droughts in the past 1,000 years or so in the West are hard to come by.
But the West’s perennial anxiety over water makes the research into that question — the aspen’s response to drought — more crucial than ever.
Colorado has more aspen than any other state in the West, and it is also the fountainhead for major river systems that tens of millions of people depend on, including the Rio Grande, the Colorado and the Arkansas. Water generated in Colorado’s high mountains travels through 17 states and Mexico.
Aspen stands, acting like sponges or underground reservoirs, hold much of those headwaters in place, growing where things are wettest and coolest, and adding greatly to the water storage capacity of the mountains.
Aspen stands are also centers of biodiversity in a forest. Many insects and plants have evolved in conjunction with trees, which provide shelter for elk and other animals in the most severe winter weather.
But there is hope for the species — and perhaps an indicator that it has dealt with setbacks before and recovered — in the peculiar properties and strengths of its reproductive system.
Aspen, unlike evergreens, do not usually grow from seeds, but rather by vegetative reproduction, sending up suckers from a mother tree. (Aspen seeds — tiny and nutrient-poor — can become established only under unusual conditions.) For a tree solely dependent on seed reproduction, recovery from an event like SAD would be a much slower road.
In perhaps the oddest twist of all, aspen are in some places colonizing areas of the Rockies where they did not exist in recent years, filling in spaces with their quick-shot sucker system that were once home to hardier-looking lodgepoles, killed off by beetle attack.