Colorful Colorado — named for golden-orange aspen panoramas capable of stopping RVs mid-highway to spill tourists gawking in amazement — is suddenly at risk of fading away.
Aspen are dying at an alarming rate across the Intermountain West, threatening biodiversity, tourism and timberbased economies in the region. But what exactly is causing Sudden Aspen Decline (SAD), as it is now glumly called, is still a mystery that has scientists and ecologists scrambling for answers.
“This is unprecedented and it is difficult to predict what will happen,” said Jim Worrall, a state forester. “Large areas continue to die off and it is rapidly increasing landscape-wide.”
In the southwest corner of the state, the San Juan Mountains have been hit particularly hard. Just within the last few years aerial surveys revealed a giant swath of dead aspen forests extending northwest from Durango, through the La Plata range, across Haycamp Mesa and continuing up the Dolores River Valley towards Stoner.
In four years an estimated 359,000 acres of aspen trees have perished statewide, representing 13 percent of the total aspen acreage, according to recent aerial surveys. When SAD was first documented by in 2004 by Phil Kemp, a forester with the Dolores Ranger District of the San Juan National Forest, an estimated 100,000 acres of the trees had died statewide. And there is nothing to suggest the dramatic decline will stop soon.
The demise of aspen, the most common tree in North America, is more noticeable here because there are more of them, but it is the far-reaching and synchronized mortality that has experts worried.
The statistics are troubling. Locally, for instance, in 2002- 03, aspens stands in four units north of Mancos called Turkey Knolls had 8 to 9 percent mortality, considered normal. In 2006, the same stands suffered between 30 and 60 percent mortality.
“The Mancos- Dolores district is considered ground zero because it was first detected here,” explained Worrall. “We’re seeing mortality die-back more than usual since 2004 and it has increased every year.”
The Grand Mesa, Uncompahgre and Gunnison national forests also are seeing a steep decline in aspen stands, as are the northern mountains near Craig.
At a seminar at Fort Lewis College in Durango recently, a panel of foresters, scientists, environmentalists and business owners gathered to tackle the problem and share ideas for a solution.
Explanations for the die-off range from climate change and wildfire management to tree disease, insects, browsing habits of elk and cattle and even the lack of wolves in the region.
Experts believe that large areas of aspen stands will continue to die in the near future. What triggered their demise is unclear, but managers are hopeful that adjusting forest and range management to promote ideal regrowth conditions may help save the state’s signature tree.
Aspen autopsy
To probe the causes, foresters first analyze the ecology of the aspen lifecycle over time, a science known as silviculture.
Test plots established since 2004 revealed the basic cause of Sudden Aspen Decline: dying root systems. For aspen this is especially detrimental because thousands of trees can be connected to the same roots, putting them all at risk.
Aspens use vegetative regeneration to reproduce clones, called suckers, that emerge from the collective root system of a grove. But once the root dies, reproduction is not likely.
Normally when an aspen dies or is cut down it releases growth hormones called cytokinins that trigger new sprouts. Why the roots are dying is the mystery confounding scientists.
“We’re conducting harvests and then observe how roots regenerate to answer the question,” Worrall said.
A key suspect is the drought earlier in the decade, which may have stressed the trees to the point where regeneration was permanently halted.
“Drought plays a large role in the decline and what we see is that when that [the drought] is relieved as it has been, the trees are still dying, so the stressed conditions are not quickly reversed,” Worrall explained.
One clue to a delayed drought-related death is that SAD is more severe on lower and middle slopes, where moisture levels in a drought are severely low. Meanwhile, higher-elevation stands have been spared somewhat because they receive relatively more moisture during drought.
The shallow root systems of aspen don’t fare well during dry years, especially on flatter terrain where roots don’t tend to go as deep compared to aspen groves on steeper terrain.
The die-off is more prevalent on south and southwestern aspects (which are also hit harder during drought) and occurs more often with older trees than younger ones, according to research.
Older trees have a higher demand for water and are therefore more stressed during hot, dry conditions.
Aspen are more susceptible to bark beetles and fungi called cankers because their bark is living tissue, compared to evergreens that have a thick protective bark of dead material.
“But the most striking observation was we did not see much of these cankers that we would typically see in aspen mortality,” Worrall said. “What we find instead are more insects related to stress conditions from drought.”
Bark beetles are “a major player” in the decline, he said, and have been heavily attacking trees that have very healthy crowns.
Wolves: A missing piece
Researchers say recovery of the dead or dying aspen groves does not look likely. Poor root conditions translate to slower and weaker sucker growth. And new sprouts must be abundant and vigorous to combat browsing by elk, deer and cattle.
Which animal is most responsible for munching aspen suckers has never been studied, but some kind of regulation, such as fencing, more livestockallotment rotation and increased elk tags during hunting season may be a solution.
“Over-browsing is critical where regeneration is not abundant; that is a key management issue,” said Wayne Shepperd, a veteran aspen ecologist, formerly of the Rocky Mountain Research Center in Fort Collins.
He cited an experimental test plot in Arizona, which is also experiencing the SAD syndrome. There, the only two aspen trees in a seven-mile radius were fenced off to prevent sucker grazing by cattle and elk. Within a few years, those aspen had flourished into a healthy grove.
Aspen ecologists have shown that cutting or burning healthy, mature aspen trees leads to vigorous suckering that can endure browsing by cattle and wildlife. But it is not known if cutting and burning weak and dying trees would have the same positive outcome.
Wildlife biologists also point out that the lack of wolves in Colorado creates an imbalance that has an effect on natural vegetation, including aspen. Currently, elk, deer and moose can graze their fill on aspen sprouts without fear in open meadows, which they wisely avoid when wolves are in the area, according to studies in Yellowstone National Park where wolves roam and natural vegetation has rebounded.
“Predators like wolves have a profound effect on browsing habits of elk,” said Ryan Bidwell of Colorado Wild, an environmental nonprofit.
But if aspen are affected by wildlife, their decline in turn has impacts on wildlife habitat.
Considered a keystone species, the sun-loving tree offers homes for small mammals and birds. Its understory provides shade and refuge, and the leaves it drops produce rich soil for grasses and plants depended on by bears and grouse. Elk, moose and deer depend on the tree for food as do beavers and porcupines.
Aspen ablaze
“Aspen are like Baby Boomers — we’re all old but won’t admit it,” Shepperd quipped. “The average age of stands is 120 years and that is because control of wildfires over the past 150 years has interrupted the natural processes in aspen ecosystems.”
Aspen are known as a disturbance-dependent species. Wildfire triggers reproduction of suckers, effectively regenerating older stands. The area of Missionary Ridge fire near Durango in 2002, for example, is showing vigorous regrowth of aspen stands.
“Aspen have adapted to fire in regeneration. The disturbance is rejuvenating,” Shepperd said.
Aspen are also beneficial as a barrier to wildfires, because they do not burn very well. When fires hit aspen groves they generally either go around them or drop to the ground, where they can be better controlled.
Prescribed burns within aspen groves are another tool that land managers use to stimulate growth of younger trees.
Logging as well creates good conditions for suckering to occur. But in both situations, subsequent monitoring for over-browsing is critical to allow new groves to get started, Shepperd emphasized.Logging vs. fire
Environmental groups, such as Colorado Wild, are concerned that SAD will prompt the Forest Service to increase aspen timber sales, a solution they oppose.
“Fire, insects and disease are all fundamental for diverse habitats,” said Ryan Bidwell, the group’s executive director. “Restoring fire is the highest priority, and logging should not be seen as a substitute for fire.”
But David Dallison, a San Juan Forest timber official, said it’s too early to say if timber sales will increase because of the dying aspen trees.
“The manufacturing market here is only so large, so they can only use so much,” he said. “We focus the program where it will do the most good.”
But business owners who depend on aspen for their products worry that letting dead aspen groves rot is a waste of a good resource.
“I can use dead aspen but there is only a two-year window to harvest it,” said Dewayne Findley of Aspen Wall Wood near Dolores. “After that it starts to crack and I can’t use it.”
The new, draft San Juan forest plan earmarks 53,000 acres for aspen harvest, which equals 6 million boardfeet. Regenerating the fast-growing aspen is critical for logging and has worked well in the past, Dallison said.
But without monitoring of overgrazing, regeneration fails, causing considerable damage, said Phil Miller of the Sheep Mountain Alliance, an environmental group based in Telluride.
“I’ve seen 2,000 acres of clearcut aspen with no regeneration due to cattle- overgrazing,” he said, “so we need range-management change to relieve browsing on aspen stands.”
Normally annual growth for aspen is 150 percent, so there is more growing than is being harvested for timber, Dallison said, “but in the future that remains to be seen,” he said. “That’s the $64,000 question: When is mortality too far advanced to harvest?”
One problem is that, currently, 68 percent of the aspen in the San Juan National Forest are in the mature stage, which is not ideal, Dallison said. (More than 95 percent of Colorado aspen forests are older than 80 years.) The draft revised forest plan, now under public review, calls for harvesting 8,000 to 10,000 acres of older trees in the San Juans to create younger age classes in the next 20 years.
For Findley, harvesting enough to produce his popular line of tongue-ingroove paneling can be a challenge.
“Each year we need to harvest two to three truckloads per acre (over 250 acres) but some years it is a struggle to get that much,” he said. “In the near future, Sudden Aspen Decline may give me more work, but if it does not arrest itself soon, long-term it is a catastrophic event.”
Another successful use of aspen is for erosion-control mats, created by Norm Birtcher of Western Excelsior in Mancos.
Because the aspen trees are shredded, they don’t have to be in as good condition as they do for wood paneling.
“We can use lower-quality logs,” Birtcher said. “But we can’t tolerate a lot of rot.”
The timber-products industry has a good foothold in the local economy. Aspen Wall Wood employs 20 people and brings in nearly $2 million in annual sales. Western Excelsior, which recently expanded its product line, employs 170 people and brings in $25 million in annual sales.
But while the timber industry has a role in forest management, there is debate about whether trees dying from natural causes should spur more logging sales.
“Logging causes erosion and removes nutrients that fire naturally recycles back into the soil,” Bidwell said. “Dead and dying trees are used for wildlife habitat. Disease and insects are agents that help sustain ecological diversity.”
Colorado Wild advocates keeping the timber industry small-scale so that it is sustainable over the long run rather than reverting back to the boom-andbust patterns of logging past.
“We’re concerned that increased harvests will inadvertently spur new industry we cannot support over time,” he said.
The prognosis is not good for the vast, shimmering aspen forests we are used to, but the recent loss does not mean Colorado’s emblematic tree will disappear.
Over time the population will adjust to a level nature allows for, observers say. Higher-elevation stands are hanging on well, perhaps an indication that as the climate re-adjusts to historically drier levels, the lower-elevation groves die back and retreat upward.
“We cannot predict how long SAD will continue,” states a January 2008 Forest Service memo. “A series of stresses often results in a downward spiral of tree health that may be irreversible and takes years to reach mortality.”
