A mysterious malady has hit Aspen’s namesake trees hard in recent years, killing off nearly one-fifth of Colorado’s aspens, according to statistics from the U.S. Forest Service.
Forest ecologists have struggled to explain the widespread die-off, known as Sudden Aspen Decline (SAD), as it’s taken hold.
A new study from researchers at Stanford University and the University of Utah may provide a breakthrough in understanding SAD and how it kills trees.
The research, led by Stanford Ph.D. candidate William Anderegg, found that aspens have essentially dehydrated due to a drought that took hold of Colorado from 2000 to 2004. In a delayed reaction to the drought, they found, the systems that carry water through aspen stands broke down.
“If they can’t transport water, they’re kind of screwed,” said Duncan Smith of the University of Utah, who worked on Anderegg’s project and co-authored a paper on it released this month in the Proceedings of the National Academy of Sciences. It is available online at www.pnas.org.
Researchers pruned dying aspens and studied them in the lab, and listened to their inner workings with microphones as they died.
But explaining the widespread tree fatality has eluded researchers.
“Just as there are a number of ways people can die and you can’t always pinpoint it, it’s the same with trees,” Anderegg said.
He tested two main hypotheses. One was that the trees were essentially starving and had stopped photosynthesizing, having closed the pores in their leaves due to the stress from drought. Two was that they were dying of dehydration.
The researchers also considered whether SAD might be a disease in the traditional sense, and whether the trees were infected with pathogens. They found they weren’t.
Their conclusion, that drought caused widespread failure of water transport systems in the trees, is a foreboding sign for Aspen’s signature trees in the age of global warming.
Anderegg’s team studied climate records in 51 different aspen-filled areas in Western Colorado from 1900 to 2009. The period from 2000 to 2004 marked the most severe drought in the entire period.
Courtesy photoThe effect of Sudden Aspen Decline (SAD) is apparent in a grove of aspen trees.
“For aspens, hot temperatures tend to be really stressful,” Anderegg said. “Climate change and global warming will be a real problem for aspen trees anywhere.”
Utah researcher John Sperry developed a method for measuring the water transport system in trees. Anderegg brought him on board to study aspen’s water systems, which proved to be the key finding.
Sperry found that in response to drought, the trees were developing embolisms — much like the common human blood vessel blockages — which break down their ability to move water.
He found that in SAD trees, an average of 70 percent of the vascular system was blocked. That’s up from an average of 17 percent in healthy aspens.
The trees, researchers found, fought against dehydration for a few years after the drought but lost and eventually died.
“That said, ‘Wow, we have a kind of slow, years-long decline,’” Sperry said. “They couldn’t overcome that 70 percent blockage.”
For Anderegg, the SAD research served as a sad sort of homecoming. The Cortez native embarked on his research after returning to an aspen grove where he and his family went camping when he was a child. They had all died because of SAD.
“The original impetus was coming back to these mountains and seeing that,” Anderegg explained.
His work is focused on drought-induced forest health issues. Along with aspens, the pinion junipers around Colorado have had similar struggles as a result of drought.
While the findings are key in understanding forest life and aspen health, it doesn’t provide a solution to SAD. If the cause is drought — and by extension, global warming — the problem will continue to affect aspens.
Anderegg said he hopes to continue studying SAD, and following how the affected forest areas change in coming years. He said he’s hopeful that by studying their post-SAD trajectory, further research can help forest managers understand how to trigger healthier growth.
But prevention of SAD in Colorado’s aspens is not a likely outcome.
“Unless we can prevent droughts, we can’t prevent it in the future,” said Smith.
andrew@aspendailynews.com
