December 16th, 2011 by

Aspen trees (Credit: Flickr/jcookfisher)

Aspens are particularly striking trees. Their pale bark and bright fall colors have made them the subject of nature photographers, painters and even poets. But for the last 10 years, aspens have been disappearing.

Sudden Aspen Decline — or the aptly acronym-ed SAD — has been a thorn in the side of foresters, arborists and nature enthusiasts for the better part of a decade. The phenomenon is exactly what it sounds like: otherwise healthy aspen trees suddenly and quickly sicken and die, leaving swaths of forest decimated. SAD has killed hundreds of thousands of acres of trees across much of their native range in the high elevations of our western mountains.

Was it a disease? A fungus? An insect? Scientists seemed to find each of these and more in the dead stands, but only as symptoms of a deeper, undiscovered problem. Eventually, the phenomenon was determined to be a delayed response to the drought that struck the region in the early 2000s. But even this didn’t solve the problem because though they knew what was killing the trees, they didn’t know how.

So the search was on to discover exactly how a decade-old drought kills a tree. There are two main theories. The first is that drought affects a tree’s ability to photosynthesize, which forces it to use up its stored carbon reserves until there’s nothing left — effectively starving itself. The second theory is that drought interferes with a tree’s ability to conduct water internally, and when the roots are no longer able to provide water throughout the tree, it dies. So which one is it: death by hunger or thirst? No one knew — until this week.

Scientists with the Carnegie Institution for Science believe they may have found the answer. William Anderegg and his team of researchers found that while trees affected by SAD showed some signs of impeded photosynthesis, there was overwhelming evidence of damage to their water conduction systems. Their findings, published in the Proceedings of the National Academy of Sciences, found that trees affected by SAD actually lost around 70 percent of their ability to conduct water, suffering crippling losses to their root system. Thirst, not hunger, was the culprit.

It may sound like depressing news, but this one detail could help scientists and foresters predict future areas at risk of SAD, an important step in restoring our aspen forests to health. And since the bulk of the damage caused by SAD has been in the Rocky Mountains, a region where forests are already decimated by the mountain pine beetle, even a little hope goes a long way.