Associated Press
Colorado State University researchers examine a stand of dead and dying aspen trees in southwestern Colorado in 2006. Now researchers say a severe drought damaged their water systems.

SALT LAKE CITY — As drought tightened its grip across much of the West in the early 2000s, it hit aspen stands particularly hard, killing off upwards of a fifth of the trees in some areas.

Researchers who scrambled to figure out what might explain "sudden aspen decline" now think they've got an answer.

"What was behind this die off was a severe drought," said Stanford researcher and doctoral student William Anderegg. "But most importantly severe drought with really hot springs and summers." That, he said, dealt a one-two punch that proved too stressful for many aspen to survive.

The trees are an iconic element of the West's world-class landscapes, providing benefits for tourism, timber, wildlife habitat, purifying water and air.

Anderegg, who grew up camping, hiking and hunting in southwestern Colorado, said part of the motivation for his project "was coming back (to that part of the state) as a researcher and realizing there's something profound going on and that all of these forests that I grew up in as a kid are now mostly dead."

He had two theories for the tree's decline. One, that the trees starved by not being able to photosynthesize for long periods. Or that they "died of thirst," Anderegg said.

"That their water systems, that move water from the roots up to the leaves, get compromised by drought."

He compared healthy trees with dying aspens, joining with biologists at the University of Utah in testing branches and roots, which have countless vessels to carry water, like human veins and arteries carry blood.

They found dying trees, afflicted with "sudden aspen decline," or SAD, had suffered "hydraulic failure."

"So the healthy ones can move a lot more water than the SAD-affected ones, because the SAD-affected ones have the embolism in their vessels," said Duncan Smith, a University of Utah graduate student who worked on the study.

The trees stressed by drought developed the equivalent of blood clots, which U. biologist John Sperry calls a "huge signal there that these trees were having trouble transporting water."

"What we found is that drought in concurrence with high temperatures was really responsible for driving this aspen die off," said Anderegg.

As the globe warms, parts of the West are predicted to get drier. That's a challenge for aspen trees, some of which are among the world's oldest living things.

"If we get more of these drought episodes, we're going to lose more of these aspen forests," said Sperry.

Anderegg agreed, saying "this is an early signal of what we're likely to see a lot more of."

He said a new project will attempt to answer whether the aspen trees will grow back or if this is a permanent shift in the forests of the West.

This study is not the first to show drought causing decline in aspen, but it helps explain the mechanisms underlying the problem.