In the 1850s, “gold fever” lured hopeful prospectors to Colorado, but left most disappointed. Now people still flock to the state in search of gold, but instead of precious metal, they look for bright yellow aspen leaves. This autumn, however, those gold seekers, like the disenchanted souls before them, may find a depressing development: Colorado's aspen trees are dying on a large scale. The U.S. Forest Service calls it SAD – Sudden Aspen Decline.
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
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
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.
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
“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