First observed in the state’s southwest region in 2004, Sudden Aspen Decline has overtaken 140,000 acres of Colorado’s 3.6 million acres of aspen – a number expected to increase after researchers analyze statewide summer surveys. The phenomenon is most widespread on the Western Slope, in the San Juan National Forest, where large brown patches of dead and dying aspen have replaced the shimmering green and gold slopes of healthy aspen, especially on low-elevation slopes. The loss could hurt tourism as well as destroy habitat for deer, elk and nesting birds like American robins and tree swallows. “Wildlife will be forced to adapt and move to higher elevations, while ponderosa pine and other conifers will move down into the lower elevations,” says Mark Krabath, San Juan reforestation forester, “and more meadows will develop.”
Healthy aspen groves experience die-off, but the process normally takes decades. Sudden Aspen Decline’s rapid large-scale onset distinguishes it from those natural declines. From 2005 to 2006, its statewide presence more than tripled, from 38,000 to 140,000 acres. Normally, when trees in an aspen stand die, the grove regenerates from the roots. But another distinguishing aspect of SAD – and the one of most concern to scientists – is that many of the affected stands’ roots appear to be weakened or dead. Once a stand’s roots die, the trees can’t come back.
A series of stress factors is thought to be responsible for SAD. Scientists believe that extended drought has weakened aspen stands and made the thin-barked species overly susceptible to secondary disease and infestation. Insect borers, beetles and canker fungus are naturally present at low levels, but now researchers believe that their populations have multiplied and ravaged the vulnerable stands.
Sudden Aspen Decline is directly related to exposure and elevation. Western and southwestern-facing stands below 8,200 feet have been hit the hardest; scientists aren’t sure why higher elevations haven’t been significantly affected. Tree wounding is also believed to be a catalyst. Caused by elk and cattle grazing, logging, sunscald and human vandalism, openings in the bark provide easy access for insects and pathogens.
The larger issue is tree age variation. Damaged stands that sport varied ages of trees can successfully regenerate. However, due to decades of fire suppression, Colorado’s aspen forests generally lack age variability and young trees. Statewide, 98 percent of aspen trees are over 80 years old, while 70 percent are over 110 years old. As aspen mature, they become more susceptible to injury and less responsive to regeneration.
To treat affected stands, researchers in the San Juan’s Mancos-Dolores district plan to use timber harvests and prescribed burns. But because the cause of root death is unknown, it is difficult to determine which damaged trees have enough vigor to regenerate. The foresters believe that stands that appear to be up to 60 percent dead may regenerate, either naturally or through treatment.
But recovery isn’t cut-and-dried. Due to the limited aspen timber market and the difficulty of large-scale prescribed burns, the district can treat only 10 to 20 percent of the projected 20,000 damaged acres. And treatment doesn’t necessarily equal success. Even in areas where saplings regenerate, there’s no guarantee of survival. “Around the Western Slope, it’s really variable whether they will grow successfully, be eaten by elk or trampled by cattle,” says Dan Binkley, professor of forest, rangeland and watershed stewardship at Colorado State University.
Aside from the Western Slope, the die-off has stricken aspen stands in south-central Colorado near the Spanish Peaks and in the north-central region near Craig and Steamboat Springs. And recently it’s appeared in southern Wyoming. With further study, researchers hope to find answers: the cause of root death, the best regeneration options, the duration of the phenomenon, how to slow down Sudden Aspen Death. In the meantime, SAD remains a gloomy situation.
“The situation is not getting better – canker fungus and borers don’t just go away,” says Roy Mask, entomologist for the Gunnison Forest Health Management Service Center. “And as of right now, there’s no speculation that SAD will just go away either.”