More trees are dying in the West's forests as the region warms, a trend that could ultimately spell widespread change for mountain landscapes from the Sierra Nevada to the Rockies.
Scientists who examined decades of tree mortality data from research plots around the West found the death rate had risen as average temperatures in the region increased by more than 1 degree Fahrenheit.
"Tree death rates have more than doubled over the last few decades in old-growth forests across the Western United States," said U.S. Geological Survey scientist Phillip van Mantgem, coauthor of a paper published in today's issue of the journal Science and released Thursday.
The researchers found rising death rates across a wide variety of forest types, at different elevations, in trees of all sizes and among major species, including pine, fir and hemlock.
"Wherever we looked, mortality rates are increasing," said Nathan Stephenson, a study coauthor and USGS research ecologist.
Tree death rates had risen the most rapidly in the U.S. Pacific Northwest and southern British Columbia, Canada, doubling in 17 years. But the highest mortality -- more than 1.5% a year -- showed up in California.
If temperatures continue to rise, as many climate models predict, "it's very likely that mortality rates will continue to rise," Stephenson said.
That could eventually alter not just the face of Western woodlands, but the quality of wildlife habitat and forests' ability to store carbon. Extensive tree die-back could lead to wholesale landscape changes, converting forests in borderline areas to grass and shrublands.
Described as the first large-scale analysis of mortality rates in temperate forests, the study examined data from tree stands at least 200 years old. But the authors said the same dynamics were probably at work in younger forests as well.
"If it's affecting the old-growth stands, it's likely to affect the young stands too," said coauthor Thomas Veblen, a University of Colorado geography professor.
Rising temperatures favor insects and pathogens that attack trees. Warming also reduces the winter snowpack and lengthens the summer dry season, placing trees under greater drought stress.
"One degree warmer may not seem like a lot, but the effects can be cumulative and put many more trees under stress, and cause a few more trees to die than used to," said study coauthor Mark Harmon, a forest ecology professor at Oregon State University. "Over long periods of time, that can change the whole composition of the forest."
