A mysterious malady has killed off nearly one-fifth of Colorado's aspens. But forest ecologists have struggled to explain the widespread die-off, known as Sudden Aspen Decline. The Aspen Daily News reports that a new study from researchers at Stanford University and the University of Utah may provide a breakthrough in understanding the decline and how it kills trees. The research 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, 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 Researchers pruned dying aspens and studied them in the lab, and listened to their inner workings with microphones as they died. "Just as there are a number of ways people can die and you can't always pinpoint it, it's the same with trees," said Stanford Ph.D. candidate William Anderegg, who led the research. Researchers found that in response to drought, the trees were developing embolisms -- much like common human blood vessel blockages -- which hampered their ability to move water. In trees affected by the decline, 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. The researchers concluded that drought caused widespread failure of water transport systems in the trees. Their conclusion 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. "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."Read more on the National Academy of Sciences website.