Colorado’s aspens dying off
October 8, 2009
Sometime in the next few weeks, federal land managers will try to stimulate moribund aspen groves by intentionally set fire to about 500 acres of aspen stands in western Colorado, outside of Battlement Mesa.
Biologists and fire experts want to know if they can treat aspen stands that have experienced widespread, severe, rapid dieback and mortality in recent years. Researchers are calling the phenomenon “sudden aspen decline.” In 2008, they recorded unusual damage across 553,000 acres of aspen groves in Colorado. More than 17 percent of the state’s aspen stands have been affected.
Prescribed fire may be a potential tool for regeneration in some areas and under certain conditions. Monitoring plots have been identified to help determine the effects of this treatment and to monitor the long-term effects of re-introducing fire into this ecosystem.
Researchers aren’t completely sure why Colorado’s signature trees have experienced this sudden decline, but they suspect fire suppression is a contributing factor, along with over-grazing of young aspen trees by elk. Drought, and a changing climate may also play a role.
The controlled burns are part of a wider effort to explore options for treatment. Aspen forests are important for wildlife habitat, scenic values, water storage and quality.
The trees, with their twirling leaves and brilliant fall colors, are dying at an unprecedented rate, especially in the southern part of the state. Natural regeneration is not keeping pace with the die-off.
Recommended Stories For You
In parts of the San Juan National Forest in southwestern Colorado, aspen mortality increased 58 percent from 2005 to 2006, with a five-fold increase in the incidence of mortality over a three- to four-year period. Nearly 10 percent of the aspen stands in the San Juan National Forest have been affected, with mortality increasing at a rapid rate.
The intense drought in the early 2000s likely triggered the startling decline in the health and vigor of the trees, said James Worrall, one of the primary authors of a recent study.
“I didn’t feel comfortable making the direct link between climate change and aspen decline,” Worrall said. “But it’s safe to say, if the climate change predictions turn out to be true, we’re going to see more aspen problems,” Worrall said.
The tree’s range could shrink significantly, especially on south-facing slopes. It’s conceivable, but not likely, that aspens could spread to new areas in response to climate change, Worrall said. Limiting factors would be soil conditions, as well as the presence of existing aspen stands. The trees rarely sprout from seeds. Almost all reproduction comes from new shoots growing from health root clusters, so any spread would be very slow.
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 prairie 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.
As the scientists studied aspen stands in four southern Colorado national forests, they also found there is very little regeneration of aspen growth.
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.
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-change factors could be involved.