SCIENCE

What Was Killing the Aspens?

Scientists determine it was lack of water, not food, that was responsible for sudden aspen decline

December 19, 2011

Aspen trees in Colorado Courtesy of flickr user Bill Dayton

Three years ago, Michelle Nijhuis wrote about the phenomenon of sudden aspen decline (SAD) in her story “What’s Killing the Aspen?”

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.

At the time, scientists suspected that extreme drought and high temperatures in the West, probably due to climate change, were weakening the trees.

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.

But a new study in PNAS has found that it was lack of water, not food, that led to the aspen deaths. Nijhuis explained the findings on the blog The Last Word on Nothing:

When the researchers studied dying aspen in the field in Colorado, and induced drought stress in both potted aspen and full-grown trees, they found that the aspen hung on to plenty of carbohydrates. The problem was that the water-delivery systems in the trees’ roots and branches were blocked with air bubbles, like straws trying to pull water from too-shallow pools. … When trees lose 50 percent of their water-delivery capacity, they start to drop their leaves, no matter the season; the dying aspen in the study had lost 70 to 80 percent. And the more root blockage, the researchers found, the more root death. Aspen are a clonal species, and without healthy roots, they’re slow to resprout and recover.

The weakened trees are more vulnerable to other threats, such as insects and fungal infections, Nijhuis noted both in 2008 and in her recent post.

“Our study provides a snapshot of what future droughts could hold for the emblematic tree of the American West,” says the study’s lead author, William Anderegg of Stanford University. The study holds an even greater lesson, though, when it comes to climate change. As we pump more and more greenhouse gases into the atmosphere, the American West and many other places are expected to get drier. And that lack of water may hurt other tree species, animal species and humans, too.