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."
The big, old trees in long-established stands are particularly good at storing carbon. If they yield to younger, smaller trees, carbon storage would decline. Moreover, the researchers found that in the research plots, the establishment of replacement trees was not keeping pace with mortality, suggesting that old forests could become thinner.
It is even possible, Van Mantgem said, that Western forests could eventually become "net sources of carbon dioxide in the atmosphere -- further speeding up the pace of global warming."
The research team of 11 federal and university scientists reviewed data from undisturbed forest areas in California, the Pacific Northwest and the interior West.
Rising mortality was evident across a spectrum of plots and tree types in all three regions, leading the team to rule out other possible causes of tree deaths such as air pollution or overgrown conditions.
The findings were in sync with other recent studies that have linked rising temperatures to increasing wildfire activity in the West and massive bark beetle outbreaks.
"That may be our biggest concern," Stephenson said. "Is the trend we're seeing a prelude to bigger, more abrupt changes to our forests?"
Veblen argued that "society needs to discuss policies that will help adapt to the changes that are well underway."
For example, he said it may be better to deal with the growing wildfire risk by limiting development in fire-prone areas than by stepping up firefighting or forest-thinning efforts.
Hugh Safford, a U.S. Forest Service regional ecologist in California not involved in the study, said the paper's linkage of tree death and warming seemed sound.
But he added that the picture was much gloomier in many of the West's forests, which are overgrown as a result of decades of fire suppression and are experiencing much higher mortality rates than those documented in the study.
If death rates are climbing in undisturbed old forest, Safford said, "that's extremely bad news" for areas where tree density is increasing.
"The ante is going up constantly, and when you add a highly dense stand and increasing fire and insect beetle issues, it's alarming."