The once-mysterious Sudden Aspen Decline, or SAD, seems to have reached its peak and is on the wane, says Mark Krabath, supervisory forester with the Dolores District of the San Juan National Forest.
Krabath said staff first noticed SAD on his district around 2005 in lower-elevation aspen stands. In 2009, the Forest Service conducted an analysis of 30,000 acres of aspen country from Mancos to Dolores. At the time, 30 percent of the aspen in the study were showing the effects of SAD – about 17 percent of all the aspen in the district. Statewide, about 15 percent of aspen succumbed, almost always in the lowest-elevation stands.
Because research suggested that SAD would inhibit regeneration of young aspen, San Juan National Forest officials prescribed 3,000 acres of timber harvest to remove the affected trees. They’re still tackling the thinning projects, which appear to be working.
“After a few years, we’re getting pretty successful sprouting in some of those stands, although it’s less than it would be with a green stand,” Krabath says. He adds that in his view, the main threat from SAD has passed. Ultimately, the mystery ailment appears to have been caused by a combination of hits to aspen stands, especially at low elevations.
“The drought and high temperatures in 2002 set up a lot of our lower-elevation stands to decline and to be stressed,” he said, adding that “a host of insects and diseases moved in because those trees were compromised.”
Now, most of the stands that would have been affected have been. “Some we treated, and some regenerated on their own,” he said. “Nature kind of fixed itself; these trees died and it triggered a response.”
Still, Krabath worries that sustained hotter temperatures – as predicted by climatec h a n g e models – could continue to hammer aspen at lower elevations.
“If we continue to get hotter and drier climates, aspen will be moved off of lower-elevation slopes,” he said. “We’ve gotten some wetter summers; the monsoons have come back. I think we’re looking OK. But there are some climate models that predict that aspen may move out of Colorado within the next 100 years, which is a little disturbing.”
Krabath said such models predict that as aspen disappear, they’ll be replaced by ponderosa pines in the lower elevations, and Engelmann spruce, Douglas fir and alpine fir in the higher zones. “We see that natural progression already at the higher elevations,” he said.