Colorado: Research suggests last summer’s heat wave is likely to result in another wave of aspen mortality

‘It’s never just drought, and boom, you’re dead’

Is there more trouble ahead for Colorado’s aspen forests?

By Bob Berwyn

SUMMIT COUNTY — Colorado’s aspens could take another big hit in the next few years as the after-effects of last summer’s heat wave take a toll on the state’s iconic trees. Recent research suggests that aspens damaged in previous droughts are more likely to die during subsequent heat waves.

Overall, Colorado reported one of its hottest summers on record in 2012, and even though researchers didn’t see excessive aspen mortality last year, it may take a few years before the full impact becomes apparent.

“We had a two or three year lag after the last drought,” said Forest Service aspen expert Jim Worrall, who helped analyze and describe the massive wave of aspen mortality that started in about 2005 and lasted for several years, leaving aspen ecologists puzzled and worried.

At its peak, sudden aspen decline affected as many as 17 percent of Colorado’s aspen forests, as well as stands in Utah and Nevada. The die-off has waned again in the last couple of years, with only about 3,000 acres of mortality reported last year, Worrall said.

“That’s very minimal compared to what we were seeing compared to five or six years ago,” he said. “They’re not dying on a landscape scale like they were before.”

But that could change in the next few years, according to research done by Colorado natives William and Leander Anderegg. After carefully monitoring numerous aspens to measure their physiological processes, the pair concluded that high summer temperatures are the key factor in mortality.

Perhaps most importantly, they found that trees that are damaged once by hot and dry conditions become much more susceptible to future damage due to a process called cavitation fatigue —  the breaking of the water pipe vessel that conducts water from roots up to leaves.

“If that gets interrupted the system doesn’t work any more … if the tree goes through an episode of cavitation, severe water stress, the next time they undergo water stress it happens more easily,” Worrall said, explaining that the Anderegg’s study provided a lot of physiological evidence for how drought affects the trees.

“It’s never just drought, and boom, you’re dead,” he said.

At this point, there’s no reason to believe that the aspens that survived the last drought will acclimate.

“They actually become more vulnerable,” said Leander Anderegg, describing how 30 years of temperature records in the Colorado high country show a steady increase that’s loading the climate dice and adding to chronic tree stress.

Acute episodes of drought and heat like in 2002 and 2012 could be the tipping point that pushes some aspens over the brink.

“We have found that trees that survived the last drought are more vulnerable to water stress, because their water-conducting machinery has already been damaged … and is weaker than normal, even after being repaired.  This means that a smaller drought could potentially have larger consequences,” Anderegg said.

“So even if the current drought ends up not being as severe as the 2002 drought, we could still see more death in the next few years. This last summer was super hot. We have found summer temperature to be the most important factor in determining whether aspens died during 2002 (and more generally the biggest factor distinguishing 2002 from previous droughts).  So 2012 may well have the same unique characteristics that made 2002 so devastating,” Anderegg said.

The detailed measurements enabled the Andereggs to measure exactly how much water the trees were using last summer. By measuring isotopes, they could even tell during which season the water was absorbed by the trees, helping differentiate between spring and summer drought stress.

“By the end of last summer, healthy trees were using almost as little water as dying trees,” he said, describing some as-yet unpublished preliminary data suggesting that healthy trees suffered serious damage this past summer summer.
“Unfortunately, we won’t be able to know whether these trees actually ‘died before our eyes’ or if this is just normal stress that they can recover from,” he said, adding that they plan to follow up my assessing the same trees during the next few years.
If there’s any silver lining to Colorado’s aspen story, it’s that the trees have managed to survive for thousands of years in something close to their present distribution. During that span, the West has experienced longer droughts than the relatively short-lived episodes of the past decade. In some cases, much drier conditions persisted for decades, but the wild card right now is the steady upward trend in temperatures.

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