What's Killing the Aspen?

The signature tree of the Rockies is in trouble

  • By Michelle Nijhuis
  • Smithsonian magazine, December 2008, Subscribe
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Aspen trees
Angel Watkins and co-workers in Colorado blame many culprits in the decline of the Aspen. (JT Thomas)

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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.


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.


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I can tell you the phenomenon what could be killing the aspen trees. It's human-made and it is called: CHEMTRAILS. Do your own research, Google and Youtube "chemtrails" spend days watching videos etc. Watch these professional videos by finding them on Google: "What in the world are they spraying?" "Why in the world are they spraying?"

Posted by John Morton on September 30,2012 | 08:12 PM

Bees and wsps in Aspen are most ofen a sign of aphids. The aphids secrete 'hone dew' s part of the digestive process

Posted by on July 8,2012 | 08:20 PM

Not to be picky, but shouldn't "Aspen," as you have it capitalized in the headline, be "aspen?" As far as I can tell, the name of the tree is lower case, while the name of town is upper case. Of course, there is plenty of room to discuss the decline of Aspen, but I think that's a different article.

Posted by Brent Gardner-Smith on December 19,2011 | 12:34 PM

The aspens in my neighborhood in Fort Collins, CO (5,000') were quite diverse in health. Some died young, some were big and healthy. It didn't seem to matter if the yards were watered well or ignored, there was an abundance of root sprouts in the yards. So with some thriving at a lower, warmer environment than what is native, it would be foolish to blame massive die-offs in native areas on global warming. I now live in Divide, CO (9,100') and the aspens here are in fine shape, even after the driest season in ten years. However, root sprouts and young trees are practically non-existent. The very obvious browse lines from the deer would explain why there is no new growth. Years ago in California the forestry folks wised up when they realized that their "burn ban" practices were preventing the new growth of Giant Sequoias, which require a bare mineral soil to grow. In Montana de Oro State Park, CA, a natural grove of Monterey Pines was suffering from old age until a fire came through and opened up the cones to let the seeds out so new growth could occur. Let's stop using the mythical global warming scare tactic to manage everything. We need to admit that there are natural cycles and fire is one of them. And when we interfere with the fire cycle we allow for devastating fires to occur, and insects, fungi, and diseases to overproduce unchecked. I'm glad I got my degree in Natural Resources Mgt when science was still unpolluted by global warming mythology.

Posted by Bill on September 21,2011 | 12:43 AM

I live in Idaho, in the Treasure Valley. I have what I believe are several Aspens in my backyard. They are sparse of leaves and in several places such as the middle of the tree, those branches appear dead. On the trunk from base to top, I see slash marks as if an animal clawed the trunk, (horizontally). A neighbor mentioned a "cutter worm" could be the issue. I have no idea. You are welcome to investigate.I believe I am 2300ft elevation. Any comments?

Posted by Leah Niacaris-Harper on June 26,2011 | 08:28 AM

Strange they have found a cause with no recommendation to fix the problem. Of course a lot will blame global warming, but come on.
Does global warming breed beetles?

Posted by Lee on April 22,2011 | 03:35 PM

We live in the Keweenaw Peninsula of the UP of Michigan. About five years ago our birches began to die (most are now gone) but the aspen seemed alive and well. This year they, too, began to weaken, lose their leaves, and fall over. In the summer they sent out scores of new suckers a lot of which had blackened leaves.
Does this mean that we, like the western states, are destined to lose our aspens?
According to Public Radio this AM. it is because of climate change. Could this be because we are so far north?
If so, what can be done?

Posted by Elizabeth Haro on September 26,2009 | 01:06 PM

I have been concerned about the aspen trees where I live near Eagle CO for the past several years. This year has been the worst by far with whole stands of aspen being affected by whatever this disease is. Has anyone heard of any relationship between the trees and bees or wasps? I've noticed that in the past few years hundreds of bees and wasps will be in the aspens. Normal or not? I usually notice things like this but it seems to be more so lately.
I've read where bees and wasps will eat fungus. Is this what is going on? Thank you for your coverage of this disturbing event. I realize that nature has her cycles it's just different when you experience such a profound change in a huge landscape.

Posted by Brian on September 11,2009 | 08:33 PM

If drought is truly weakening the trees by pulling the reserved energy from the roots, then it would make sense that either selective cutting, clear cutting, or fire would reduce the amount of above ground growth. The resulting smaller/newer shoots would have lower water transpiration rates in comparison to the larger older trunks. This could potentially give the overall organism a chance to survive until water resources were more plentiful.

There are several examples from nature where organisms have undergone a reduction in size in light of diminished resources. Perhaps we should take this into consideration and use natures examples in developing a management plan

Posted by Eyvind on July 8,2009 | 05:51 AM

I will be visiting the area with SEEDS this coming late spring. Thank you for the article it is astounding to make some of the connections of global warming and forest loss. It is a sad realization to know and accept that human demand and industrialization is responsible for these wonderful Aspens.

Posted by Kellen Marshall on April 21,2009 | 06:22 PM

I worked as a wildlife manager in Michigan, where there is also a lot of aspen. Aspen is important for many game and non-game species, as is the young, regenerating forest type that it represents. With a sparse canopy that allows sunlight to penetrate to the forest floor, aspen forest also have an abundance of ground cover and understory that benefits many species. Now living in Colorado, I see most of our aspen forests are of an older age class. Aspen is aggressively managed in the upper midwest for economic reasons but also for wildlife reasons. It seems as though it is hardly managed at all in the Rocky Mountain region. I see so many stands in a declining condition with bracket fungus and conch all over; I am not surprised that aspen is in trouble in the west. It is important to know that aspens grow in clones, and all trees in a clone are part of the same living organism. You can create forest openings in aspen stands by clearcutting MOST of the trees in a clone while leaving several per acre standing. The standing trees will inhibit the auxin (like a hormone to humans) that stimulates root suckering or sprouting, almost as if the standing trees say to the root system "don't worry, there's damage to us but we're up here photosynthesizing", and the energy to regenerate is reserved until total destruction of the stand occurs. If this does not occur, aspen will most likely be lost from the site over time. So, "let it burn" is a great thing! However, if we don't let it burn, we really need to start clearcutting some aspen in the west to maintain it.

Posted by Tim on February 16,2009 | 09:55 AM

The aspen found in Vail are Populus tremuloides. It is THE aspen species of the rockies. There are some good comments here. Fire is very important to the establishment of new aspen stands. Young stands of aspen are still thriving in the midst of their neighboring older dying stands at mid elevations. There are also some amazing older aspen stands (~150 years old)in the best of sites. Not every old tree is a bad tree. I don't want to see a massive clear cut or fire program put in place because people think it will save the aspen - patchiness is good for these forests. Also, burning low elevation stands will probably speed up the process of aspen being replaced by sagebrush. There is also a lot of evidence that aspen established in low elevation during wetter times 100 years ago. These trees seem to be fading out with the last hot droughty decade. If our climate continues to heat up and dry out, we can expect the mid elevation aspen to die (and not regenerate) and the population to shift to higher elevations where they can. If all the climate models are dead wrong - cool. I like aspen.

Posted by Debbie on January 28,2009 | 05:26 PM

Let it burn. Fire cleanses the forest, not blaming everything under the sun on global warming. These trees will be around long after humans have killed themselves off.

Posted by Flouncer on January 14,2009 | 11:50 PM

I am from Slovenia (graduated forestry studies) an on a work on Vail mountain. There is a lot of Aspen trees but cant find which species is most common. It is winter now and your help would be helpful. please post me on my e-mail. Martin

Posted by ilco on December 24,2008 | 01:36 AM



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