Pain in the Aspen

The aspen die-off in the West leaves researchers stumped. By Sarah Parsons

The aspen is one of the most widespread trees in the Western U.S., its trademark white bark visible on acres ranging from Arizona to Alberta, Canada.

But for the past several years, researchers have noticed aspens dying at a greater rate and faster pace than usual. And in June, scientists came to another startling realization: Not only are the trees dying, in some cases, they’re not regenerating. Dale Bartos, an aspen ecologist with the Rocky Mountain Research Station in Logan, UT, estimates that the die-off affects about 10 percent of the West’s aspens, with as many as 40 to 50 percent affected in certain areas, such as southern Wyoming.

But despite the problem’s wide range, researchers are stumped as to why aspens are dying and not regenerating. The puzzling situation is indicative of the complex nature of tree diseases.

Determining the cause of the die-off is challenging because it isn’t one menace killing all the aspens—or any tree for that matter. Rather, several factors interact to trigger the die-off, and they could vary across locations.

“These are different for every species of tree in every different location,” says John Castello, a forest pathologist at State University of New York’s College of Environmental Science and Forestry. “It’s site-specific.”

Wayne Shepperd, a research forester at the Rocky Mountain Research Station’s headquarters in Fort Collins, CO, believes the aspens are stressed, meaning there’s an interruption in the normal flow of water and nutrients. Stress renders aspens susceptible to insects, fungi, and other common plagues that they can normally fend off. But determining the source of stress is tough, particularly if trees show no outside evidence of wear and tear, such as wounding.

“When a tree is stressed, it winds up using more of its reserves to exist and gets out of balance, much like when we get sick,” Shepperd says. “Since the diseases and insects that are killing the trees are associated with stressed trees, we assume an underlying stress is present. That’s something that’s very hard to put your finger on because you have to catch it in the act.”

Shepperd says some possible stress triggers could be drought, a dry or cold area in the earth, or frozen ground on a warm day, which prohibits trees from sucking nutrients from the frozen soil. While human-induced conditions like global warming could be contributing to aspen stress, Shepperd says this is just one of many possibilities. Because trees are dying in several different ecosystems and terrain, there could be a variety of stressors that affect different regions, adding to the complexity of the issue.

In addition to dying stands, the roots of some aspens seem to be dead or damaged. This is especially problematic since new aspens, or “clones,” sprout directly from the roots of parent trees, rather than from seeds. 

“Dying of mature stems is routine and normal,” Shepperd says. “But normally sprouts come up and everything’s fine. Even if it happens all at once (as in a forest fire), you immediately see a sea of aspen sprouts coming up. But we’re just not seeing that and that’s why we’re concerned.”

The reason for root death is tricky to research for obvious reasons: Roots are underground, and in aspens, they exist in expansive and intricate networks. 

“We need to get to the bottom of it and get some answers,” Shepperd says. “It’s going to take some time because it’s such a complicated system we’re working with.” 

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