"Wherever we looked, mortality rates are increasing," said Nathan Stephenson, a study co-author and USGS research ecologist.

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.

"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, co-author of a paper being published in the Jan. 23 issue of the journal Science and released today.

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.

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.

Although the data were from long-established stands at least 200 years old, the authors said regional warming was probably affecting younger forests as well.

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.

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 gradually transform the region’s old-growth forests, eroding their ecological value. They would be sparser, have younger, smaller trees that provide less suitable wildlife habitat and have diminished capacity to store carbon.

"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 co-author Mark Harmon, a forest ecology professor at Oregon State University. "Over long periods of time, that can change the whole composition of the forest."

As decaying trees release carbon into the atmosphere, Van Mantgem said, it is possible Western forests could even 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, old-growth 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 linked rising temperatures to massive bark beetle outbreaks and increasing wildfire activity in the West.

In forests growing in borderline areas, Stephenson said, increasing temperatures and tree deaths could result in wholesale changes, converting the landscape to grass or shrub lands.

"Our society needs to discuss policies that will help adapt to the changes that are well underway," said co-author Thomas Veblen, a University of Colorado geography professor.

For example, he said, it might 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 it was "hard to assail" the paper’s logic.

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 areas, Safford said, "that’s extremely bad news" for other forests 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."

-- Bettina Boxall

Photo: Old-growth Douglas fir trees stand along the Salmon River Trail at Mt. Hood National Forest outside Zigzag, Ore. Credit: Rick Bowmer / Associated Press