OSU study outlines faster rate of tree death cycle in Northwest because of global warming

By KYLE ODEGARD

Gazette-Times reporter

The latest sign of global climate change? Trees in old-growth forests across the Western United States aren't living as long because of higher temperatures and drought stress, according to a new study out of Oregon State University.

"When trees across the West appear to be dying at twice the rate they used to, that's not a good sign," said Mark E. Harmon, a professor of forest ecology at OSU, and one of the authors of the paper. "The doubling of the mortality rate, if it's really widespread, that implies the lifespan could be halved. -. It may be one of many wake-up calls for us, that nature's responding."

The rapid increase in tree mortality has occurred over the past 50 years, according to the report, which will be published in the journal "Science" today.

During that same period, the temperature has increased by about 1 degree in the West, Harmon said. That might seem small but, "Averages are deceptive because they eliminate a lot of extremes," he added.

The increase in tree mortality, which was about 1 percent to about 2 percent in many areas, might not seem like much either, at least at first glance, but, Harmon said, "It has this big cumulative effect."

The study included hundreds of thousands of trees. Many of the species in the stands could live to be hundreds of years old or, in the case of Douglas fir trees, more than 1,000 years old.

The Pacific Northwest had the steepest increase of tree mortality, but other regions had higher rates of tree death overall.

In Oregon, old-growth forests were studied in the Cascades east of Eugene and in Central Oregon, near Sisters. There also were sites in the Olympic Peninsula and elsewhere in Washington, California, Arizona, Colorado, New Mexico and southwestern British Columbia.

A vicious cycle could develop as regional warming causes trees to die. The diminished forests will absorb less carbon dioxide and then inject more greenhouse gasses back into the atmosphere, which could in turn lead to even higher levels of warming.

Drought conditions also are related to warming, since higher temperatures result in less snowpack, earlier snowmelt and longer dry periods. That also could enhance the growth and reproduction of insects and pathogens that attack trees.

The impacts, Harmon said, could greatly impact what future forests might look like.

"This is actually the beginning of the change. It's the tip of the iceberg. It's not the iceberg. The changes in the future should be much more dramatic if scientists are right about their projections," Harmon said.

Other possible causes of tree mortality, such as insect attack, fire suppression and pollution, were ruled out as a dominant cause of the long-term tree death found in the research.

The study included researchers from OSU, the U.S. Geological Survey, U.S. Forest Service, University of British Columbia, University of Washington, Northern Arizona University, University of Colorado and Pennsylvania State University.

Kyle Odegard covers Oregon State University. He can be contacted at kyle.odegard@lee.net or 758-9523.