What’s Killing the Aspen?

It's a relentlessly sunny day in the Rocky Mountains, and here at 9,000 feet, on the Grand Mesa in western Colorado, the aspen trees should be casting a shadow. But something is wrong in this stand: the treetops are nearly bare, their branches twisting starkly into the blue sky. Sarah Tharp, a wiry biologist for the U.S. Forest Service, hoists a small ax, takes aim and delivers an angled blow to an aspen trunk, peeling off a sample of diseased bark.

"Sometimes," she says, "I feel like a coroner."

Aspen, one of the few broad-leaved trees to grow at high altitude in Western mountains, are emblems of the Rockies. Their lean, chalky trunks are instantly recognizable on an alpine slope, their blazing-yellow fall displays part of the region's seasonal clockwork. The characteristic flutter of their heart-shaped leaves in the breeze gives them their nickname—"quakies"—and fills their stands with an unmistakable shhhhh.

In 2004, foresters noticed that aspen in western Colorado were falling silent. While the trees have always been susceptible to disease and insect attacks, especially in old age, "this was totally different from anything we'd seen before," says forester Wayne Shepperd. "In the past, you'd maybe see rapid die-off of one stand out of an entire landscape—it wasn't really a big deal. But now, we're seeing whole portions of the landscape go."

By 2006, close to 150,000 acres of Colorado aspen were dead or damaged, according to aerial surveys. By the following year, the grim phenomenon had a name—"sudden aspen decline," or SAD—and the devastated acreage had more than doubled, with some 13 percent of the state's aspen showing declines. In many places, patches of bare and dying treetops are as noticeable as missing teeth, and some sickly areas stretch for miles. Aspen declines are also underway in Wyoming, Utah and elsewhere in the Rockies. Surveys of two national forests in Arizona showed that from 2000 to 2007, lower-elevation areas lost 90 percent of their aspen.

Aspen grow in "clones," or groups of genetically identical trunks. Some clones are thousands of years old, although individual trees live 150 years at most. One especially large stand in Utah, known as "Pando" after the Latin for "I spread," was recently confirmed by geneticists to cover 108 acres. It is variously said to be the world's heaviest, largest or oldest organism. Disturbances such as wildfires or disease usually prompt clones to send up a slew of fresh sprouts, but new growth is rare in SAD-affected stands.

Tharp and three other young Forest Service biologists—under the genial supervision of veteran plant pathologist Jim Worrall—are chasing down the causes of the decline. They walk among the aspen trunks and divvy up their tasks for the day.

"You want me to dig? Is that where this is heading?" Worrall teases the crew members, who are outfitted in hard hats and orange vests and sport the occasional nose piercing.

A tiny mark on the bark of one trunk prompts Angel Watkins to probe underneath with a knife, where she finds the wood is decorated by the convoluted track of a bronze poplar borer larva. While the inch-long larvae don't usually kill aspen outright, their trails can weaken the trees and open new portals to fungal infections, which in turn form oozing bruises under the bark. On another tree, Worrall finds small cracks like those on the surface of a cookie, a clue that tunneling underneath has dried out the bark. Closer inspection turns up a bark beetle, no more than one-twelfth-inch long but capable, en masse, of cutting off the tree's nutrient supply.

"These beetles are the biggest mystery," says Worrall. Before SAD, aspen bark beetles were known to science, but "most entomologists who worked on aspen had never heard of them," he says. His crew now finds bark beetles in almost every damaged stand. They've also observed that some fungi, borers and other insects and diseases are proliferating.

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.