SALT LAKE CITY -- Researchers from Stanford and the University of Utah think they've figured out what could be killing off the iconic aspen tree across the West, identifying the mechanism behind it. It's a troubling phenomenon called "sudden aspen decline." It affects multiple species and the trees are dying off in record numbers. Most alarming of all, it's getting worse.
Those researchers identified a powerful one-two punch as the cause: Warm and dry winters and springs with hot summers - a deadly combination that is too stressful for many aspen to survive.
"What was behind this die off was a severe drought, but most importantly, severe drought with really hot springs and summers," said Stanford researcher Bill Anderegg, whose study was published in the Proceedings of the National Academy of Sciences.
Aspen are a key ingredient in the West's world-class landscapes, providing priceless benefits for tourism, timber, wildlife habitat and purifying our water and air.
That's why more than a few people are troubled by the die-off of trembling aspen forests across the western states - people like Anderegg, who grew up camping and hiking and hunting in Southwestern Colorado.
"Part of the motivation for this project actually was coming back as a researcher and realizing there's something profound going on," Anderegg, "and that all of these forests that I grew up in as a kid are now mostly dead."
Nearly one-fifth of the aspen have died in some areas over the last decade. Anderegg had two theories at first to explain the die-off. One, that the trees starved by not being able to photosynthesize for long periods. Two, that they were dying of thirst.
"That their water systems that move water from the roots up to the leaves get compromised by drought," he said.
He compared healthy trees with dying aspens, and joined with biologists at the University of Utah, testing branches and roots, which have countless vessels to carry water like human veins carry blood.
They found that the dying trees, afflicted with "sudden aspen decline" or SAD, had suffered "hydraulic failure."
"The healthy ones can move a lot more water than the SAD-affected ones," said U. graduate student Duncan Smith, "because the SAD-affected ones have the embolisms in their vessels."
The trees stressed by drought developed the equivalent of blood clots, says U. biologist John Sperry.
"(There is a) huge signal there that these trees were having trouble transporting water," Sperry said.
"What we found is that drought in concurrence with high temperatures was really responsible for driving this aspen die off," Anderegg said.
As the globe warms, parts of the West are predicted to get drier, like the conditions Utah is seeing this winter. That's a challenge for aspen trees, some of which are among the world's oldest living things. Organisms like Pando, a grove of genetically identical quaking aspen in southern Utah, which is the heaviest organism in the world, and also 80,000 years old.
"If we get more of these drought episodes, we're going to lose more of these aspen forests," Sperry said.
Anderegg said that this was an early signal of what we could be seeing a lot more of. The question now is "Will the aspen trees grow back, or is this a permanent shift in the forests of the West?" Anderegg also said that a new project he's begun will attempt to answer that question.
This study is not the first to show drought causing decline in aspen, but it helps explain the mechanisms underlying the problem.