PAONIA, COLO. — From the hillsides of extinct volcanoes in Arizona to the jagged peaks of Idaho, aspen trees are falling by the tens of thousands, the latest example of how climate change is dramatically altering the American West.
Starting seven years ago, foresters noticed massive aspen die-offs caused by parasitical insects, one of them so rare it is hardly even written about in scientific literature. But with warming temperatures and the effects of a brutal drought still lingering, the parasites are flourishing at the expense of the tree, beloved for its slender branches and heart-shaped leaves that turn a brilliant yellow in autumn.
What foresters have termed Sudden Aspen Decline affects more than just aesthetics. Aspen trees provide a rich habitat for birds, elk, deer and other animals. The grasses that sprout under them -- up to 2,000 pounds per acre -- hold water that is needed by metropolitan areas. The trees do not burn easily and create natural firebreaks in forests already ravaged by the pine bark beetle -- another parasite that is thriving because of global warming.
"It's just rolling through the forests," Wayne Shepperd, an aspen specialist at Colorado State University, said of SAD.
Noting the number of other changes to Western vegetation due to warmer, drier temperatures, he added: "Everything's happening all at once. We're living in interesting times here."
The decline of the tree is most visible in Colorado, which has seen nearly 500,000 acres afflicted by SAD -- nearly a fifth of its aspen groves.
Hillsides that used to draw tourists in the fall to gaze at the flickering aspen leaves are now populated only by the trees' pale skeletons.
Remote Paonia sits in the midst of the state's aspen belt, with the lush, heavily forested West Elk Mountains to the east and the enormous plateau known as Grand Mesa due west.
But the flaming yellows that normally paint these landmasses have been replaced this year by the gray of bare branches.
Neal Schwieterman, Paonia's mayor, said people have been abuzz over conditions in Kebler Pass, home to the biggest stand of aspens in the state.
"Only the lower levels had any aspen changing," he said. "There was nothing to look at high. . . . This is the least color year I can remember."
The most vulnerable trees grow on sunny, south-facing lower elevations, where warmer temperatures wear down the trees' resistance to pests. And though the largest number of deaths is in Colorado -- which has the most aspens -- the effect can be even more severe in places like Nevada, Arizona or California that have small bands of aspen.
Even before aspen trees began their abrupt die-off, the tree was under duress in the West.
Scientists estimate that the trees covered 10 million acres in the 19th century; now the number is 4 million.
That's mainly because humans have been so successful at preventing wildfires. Regular blazes would have knocked down old aspen trunks -- freeing up new stems to emerge from the roots, where aspens regenerate. The lack of fires has also led to a sharp rise in big conifers, which crowd out aspens.
Finally, animal grazing has killed some of the younger aspens trying to survive.
Dale Bartos, a Utah-based scientist with the federal Rocky Mountain Research Station, described the one stand in Lassen National Forest in Northern California, where fencing protects nine tiny aspen bulbs from foraging animals.
"We're seeing these really extreme situations, where that's all that's left," he said.
Now SAD is accelerating that loss.
The syndrome was discovered in Colorado when rangers in two national forests in the southwestern corner of the state noticed disturbing die-offs in formerly lush stands.
Researchers concluded that warmer temperatures stressed trees, especially older ones that hadn't been swept away by fire. They identified four parasites and one wasting disease that are now flourishing in the stands.
One of the parasites, the aspen bark beetle, is so rare that it had only been mentioned once before in an academic paper, scientists say.
"If the aspen were vigorous, these things would just be taking a minor role," said Jim Worrall, a plant pathologist for the U.S. Forest Service.
The Terror Creek drainage west of Paonia, one area where SAD was first identified, remains the focus of an intensive research project on whether the syndrome can be halted. Researchers are testing whether cutting down older aspen trees in affected areas can combat the syndrome.
The remaining, younger shoots are less susceptible to SAD.
Shepperd has worked on the project and has been discouraged by the results so far. In stands with little SAD, the cutting approach -- which he likens to amputation -- has shown some success. But the syndrome and its accompanying parasites and disease have moved so fast that stands that once had low levels of exposure are now almost entirely affected.
"We looked at the data and said, 'Oh my God,' " Shepperd said.