The most extensive SAD is in the hottest and driest areas—low-lying, south-facing slopes. The pattern suggests that the region's extreme drought and high temperatures—both possible symptoms of global warming—have weakened the trees, allowing more disease and insect attacks.
It seems that new stems aren't growing back after trees die because drought and heat have stressed the trees. During drought, aspen close off microscopic openings in their leaves, a survival measure that slows water loss but also slows the uptake of carbon dioxide, required for photosynthesis. As a result, the trees can't convert as much sunlight into sugar. Worrall speculates that the trees absorb stored energy from their own roots, eventually killing the roots and preventing the rise of new aspen sprouts. "They basically starve to death," he says.
The drought here has lasted nearly a decade, and climate scientists predict that severe droughts will strike even more often in parts of the West as greenhouse-gas levels continue to rise and contribute to global warming. "If we have more hot, dry periods as predicted, SAD will continue," says Worrall. Aspen at lower elevations will likely disappear, he says, and those at higher elevations will be weaker and sparser.
Aspen aren't the only trees in trouble in the Rockies. The needles of many spruce and pine trees in Colorado are tinged with red, a sign of bark beetle infestation. The outbreak began in 1996 and today 1.5 million acres are infected. Foresters recently projected that the state will lose most of its mature lodgepole pines to beetles within the next five years. Whitebark pines, whose fatty seeds provide meals for grizzly bears in the northern Rockies, have long been protected from insect attack because they thrive in high-mountain habitat, but invading beetles have now knocked out most of the mature trees. Biologists say several types of bark beetles are reproducing more quickly and expanding their range, thanks to warming trends that allow the insects to survive winters at higher elevations and more northern latitudes.
"We're seeing major ecological responses to warming," says Thomas Veblen, an ecologist at the University of Colorado at Boulder and a longtime student of Rocky Mountain forests. "That's the common theme that's hitting everybody in the face."
While Worrall and his crew of biologists investigate the damage done by SAD, the Forest Service is testing treatments for the decline. In some places, researchers find, logging and controlled burns encourage aspen stands to generate new trees. In northern Arizona, where the Coconino National Forest has fenced off several hundred acres of aspen, foresters hope the barriers will protect new growth from hungry elk and deer. But no one has found a cure.
In the fall, aspen's golden foliage creates a stunning contrast with the surrounding evergreens. These dramatic panoramas appear to be threatened. Future visitors to the Rockies are likely to find an altered forest, if, as experts foresee, aspen cede territory to evergreens or open meadows. Not that a forest is ever a static thing. "The forest of our grandparents' time wasn't the best of all possible forests, ours isn't the best of all possible forests, and the forest of the future won't be, either," says Dan Binkley of the Colorado Forest Restoration Institute at Colorado State University. Still, aspen's grandeur would be sorely missed.
Michelle Nijhuis wrote about Walden Pond in the October 2007 issue of Smithsonian.