A dead grove of trembling aspen in Colorado’s Grand Mesa National Forest. William R.L. Anderegg

By JUSTIN GILLIS
12 December 2011 Scientists trying to understand the future of forests on a warming planet have a strange problem: They do not know how to kill trees. I don’t mean the trees in their backyards. I would bet that the average climate scientist, especially one who studies forests, is better with a chain saw than I would be. What I mean is that researchers don’t have a particularly good idea of how trees die in the wild, and as a consequence, they have a hard time constructing computer analyses that can accurately predict the future of forests. These programs are fine at running equations that describe the growth of trees and other plants as a function of climate in a particular area. But when it comes to understanding what causes large scale die-backs, the analyses are rudimentary at best. I encountered this problem in my long article a few months ago on the future of the world’s forests, in which I talked about forests’ displaying disturbing signs of stress, probably as a result of global warming. This is not a problem climate scientists saw coming, which illustrates the importance of getting a better handle on the life and death of trees. So it comes as good news to learn that one of the researchers I quoted in that article, William R.L. Anderegg of Stanford University, has offered substantial new insight into how trees die. His paper was written with five co-authors and released Monday by a journal, Proceedings of the National Academy of Sciences. In it, Mr. Anderegg and his colleagues focused on the phenomenon known as “sudden aspen decline” that has killed broad swaths of aspen forest in Colorado and other states in recent years. At least 15 percent of Colorado’s aspen forests have died, and Mr. Anderegg, a native of that state, was as shocked as other Coloradans. “I played as a kid in some of those aspen groves,” he said in an interview. As his research project for a doctorate at Stanford, he resolved to learn what he could about the situation. Scientists already knew that sudden aspen decline was a delayed response to a severe drought and heat wave in Colorado in the first half of the decade, but they did not know at a systemic level how the trees were dying. In fact, the question has almost never been studied in any forest during a massive die-back. Mr. Anderegg and his group spent long hours cutting limbs from dying trees to investigate their physiology, imposing a drought on nursery saplings and wild trees, and taking other steps to pin down exactly what was happening. As they did so, they were weighing the two main hypotheses about how trees die. One of these posits that in a drought or other stress situation, trees close the pores that allow carbon dioxide to enter their leaves and water to evaporate. That would conserve water, but it would also slam the brakes on photosynthesis, and the tree might essentially starve to death for lack of the carbohydrates it usually creates in its leaves to build and maintain its tissues. The second idea is that in a drought, trees undergo a kind of hydraulic failure, with the fine tubes that take water from a tree’s roots to its leaves filling up with tiny air bubbles. The idea is that this damages the tubes and can lead to problems for the tree even after rainfall resumes. It turns out that Mr. Anderegg’s findings strongly support the second idea: that trees die of thirst, not hunger. He found plenty of carbohydrates in the roots of dying aspens, but also clear evidence that their water distribution systems had been damaged and could not recover even after the drought ended in 2004. In my interview with him, Mr. Anderegg compared trees to people: we die of thirst a lot more easily than we starve to death. […]

Learning How to Kill Trees via Wit’s End