Other instances of aspen mortality, for example in the Great Lakes region, have also been tied to warm and dry conditions, according to the report.
Another large-scale die-back was documented in the 1980s and 1990s in the prarie provinces of Alberta and Saskatchewan, associated with the dual stresses of drought and insect defoliation, followed by secondary wood-boring insects and diseases.
And in the early 1970s, a similar trend was observed in Utah and Wyoming, at the time linked to fire suppression and overgrazing by deer and elk.
But the rapid spread of the current aspen decline in southern Colorado appears to be unprecedented. Aspen mortality in one part of the San Juan National Forest increased 58 percent from 2005 to 2006, with a five-fold increase in the incidence of mortality over a three to four year period.
As the scientists studied aspen stands in four southern Colorado national forests, they also found that there is very little regeneration of aspen growth. The trees generally spread via suckers, or shoots of the roots of healthy trees. After several years of drought, the aspens are basically starving to death, Worrall said.
Worrall explained the physiology of the die-off: “The stress we’re seeing to the overstory is an energy drain that lead to poor root conditions. They don’t have the energy to re-grow suckers.”
When the trees are stressed by heat and dry conditions, the stomatae (small openings in the leaves) close. That slows the loss of water in response to drought but also slows photosynthesis, the process by which plants create energy.
“After years of drought, they’re basically starving to death,” Worrall said.
The researchers also found that agents that typically attack and kill healthy aspen stands in Colorado were not significant factors in the recent dramatic die-back.
The recent mortality in southwestern Colorado had “ a sudden onset and was very rapid,” in contrast to previous documented episodes of “aspen decline,” according to the report. And the mortality agents appear to be different, suggesting that climate factors are involved.