Trees dying in the West at record rate

(01-22) 11:30 PST SAN FRANCISCO -- Trees are dying faster than ever in the old-growth forests of California and the mountains of the West, a phenomenon scientists say is linked to rising regional temperatures and the destructive forces of early snowmelt, drought, forest fires and deadly insect infestations brought on by global warming.

Over the past 17 years in some regions and 25 years in other regions, the death rates of trees have doubled, the scientists say, raising concerns that the problem goes well beyond trees: As the forests shrink, their capacity to absorb carbon dioxide from industrial lowlands are diminished, meaning more greenhouse gases are being added to the warming planet's atmosphere.

"The ultimate implications for our forests and the environment are huge," said Mark Harmon of Oregon State University who helped write the report which appears today in the journal Science.

While no trees are immune, the scientists say, the victims are primarily the conifers whose abundance throughout California's Sierra Nevada range makes the forests famed throughout the world. Varied species of pines, firs and hemlocks are most at risk, they say.

The unique study involved nearly a dozen leading forest ecologists who studied mortality rates of trees in 76 forest plots located primarily in California, Oregon, Washington and southwestern British Columbia. They also looked at trees in a few interior states: Idaho, Montana, Utah, and Arizona.

The increase in death rates for the trees has been "pervasive," said Phillip van Mantgem, a forest expert with the U.S. Geological Survey's Western Ecological Research Center in Arcata (Humboldt County) and a leader of the research team.

The most likely cause of the increasing deaths, van Mantgem said, is the widespread increase in average temperatures over the past 30 years throughout the region - an increase of a full degree Fahrenheit and an amount consistent with the global warming measurements and models reported by world's experts of the Intergovernmental Panel on Climate Change.

Van Mantgem led his own team tracking the fates of 20,000 individual trees in Yosemite and Sequoia National Parks, and found that their death rates had doubled in 25 years. Colleagues did a similar job for the study of old-growth forests throughout the Pacific Northwest.

E-mail David Perlman at dperlman@sfchronicle.com.