Climate change stress killing forests, and why it matters
By Michael D. Lemonick
9 September 2012 Forests cover some 30 percent of Earth’s surface, and it’s hard to overestimate how crucial they are to the functioning of the planet. Forests provide shelter for uncountable numbers of species, hold soil in place that would otherwise wash away, pull excess carbon out of the atmosphere, absorb and re-emit water at such a rate that they literally control the weather, and serve as an economically vital natural resource. All of those functions have long been endangered by human activities such as excessive logging and clear-cutting to open new agricultural land. But another factor, increasingly, is the stress of climate change — in particular, the higher temperatures and more frequent and intense droughts that human-generated greenhouse gases have begun to trigger. Now a new paper, released Sunday in Nature Climate Change, has attempted to lay out just how climate stress affects forests, and how serious the consequences of could be. “This is the first snapshot of how these things fit together,” said lead author William Anderegg, “So we don’t have a lot of final answers yet.” What they do know is that tree deaths go up during periods of excessive heat and drought. Some of the biggest of these die-offs have happened in the western U.S. in recent years, among piñon pines forests in the Southwest, for example, and trembling aspens in both the U.S. and Canada, and lodgepole pines and spruces in the Northwest — but they’ve also been documented on every continent except Antarctica. “It’s a very important type of forest mortality,” Anderegg said, “And we expect it to become more common.” Until recently, however, ecologists hadn’t really focused on drought-induced die-offs as a discrete category of forest trauma. That began to change, Anderegg said, after a meeting in Austin, Texas, last year. “Several of us decided to sit down, put our heads together and begin to look at the possible effects.” They looked at dozens of individual studies, and found plenty. The loss of a forest’s dominant tree species has a ripple effect on all the other species that live there, by changing the amount of sunlight that reaches the forest floor; changing the mix of nutrients that enter the soil as leaves or needles decompose; allowing soil to wash away, especially on steep slopes, and — in some cases, at least — encouraging more fires. Forest die-offs also impose an economic hit on loggers and those who depend on income from hikers, campers, and others who use forests for recreation. “One of the most interesting studies we looked at,” Anderegg said, “looked at the negative effect on real estate values in areas with forest loss.” Forests are not only affected by climate; they also affect it. A living tree absorbs carbon from the air; kill the tree, and the carbon stays in circulation to trap heat. Not only that, as the tree decays, the carbon locked inside is gradually released back into the atmosphere. “It’s a double whammy,” Anderegg said. […]
