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Angel Watkins and co-workers in Colorado blame many culprits in the decline of the Aspen. (JT Thomas)

What's Killing the Aspen?

The signature tree of the Rockies is in trouble

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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.

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