GRANTS PASS -- A new study suggests warming temperatures predicted over the next century could boost tree growth on Northwest forests, but less so at lower elevations where most of the timber is and temperatures are already warm.
Researchers from Oregon State University and the U.S. Forest Service calculated an increase in forest growth rates in Oregon and Washington of between 2 percent and 12 percent by the end of the century, when climate models predict temperatures to be between 0.9 degrees and 6.7 degrees higher than they are now.
The greatest increases are likely at higher elevations, where temperatures are cold and growth rates have been low.
The lowest increases in growth, and even decreases, would be at lower elevations, where temperatures are already warm and growth rates have been high.
"There's a lot of variability here, depending on which climate scenario turns out to be most accurate and what policy changes are made as a result," co-author Darius Adams, a professor of forest economics at OSU, said in a statement. "And there are dramatic differences in forest regions and elevations."
Maps in the study showed the lowest growth rates in southwestern Oregon's Klamath Mountains and the east side of the Cascade Range, where temperatures are warm and rain is limited. The highest growth rates showed in the Olympic Mountains and Northern Cascades of Washington and the Blue Mountains of Oregon.
The study focused on predicted changes in temperature, and did not take into account factors such as changes in the amount of carbon dioxide in the atmosphere and wildfire, said lead author Gregory Latta, a forest economist at Oregon State.
Researchers plan to expand the study to coastal Alaska.
The authors noted that legislation to control global warming includes a role for forests as an important means of storing carbon to limit the growth of greenhouse gases.
In the study, the authors said that private forests, which account for 45 percent of timberlands and 83 percent of the Northwest's timber harvest, will have a tough time maintaining current production levels, because they are generally at lower elevations. Harvest there is already at 104 percent of current growth rates.
Federal forests tend to be at higher elevations, where they account for 47 percent of the timber base and just 6 percent of the harvest, and will tend to increase the amount of carbon they store, the study said.
The scientists used four different computer models of changing climate conditions, a national inventory of forest growth, and past temperature and rainfall data to project growth at five-year intervals through the end of the 21st century.
The study was published in the journal Forest Ecology and Management and was funded by the U.S. Forest Service.
The General Accounting Office this week reported that the U.S. Forest Service devotes $9 million to researching ways of helping forests and grasslands adapt to a changing climate, and the agency feels it needs to make global warming a more prominent part of its decision-making process.
Oregon State ecology professor Mark Harmon, who was not part of the study, said while it is difficult to consider everything, he had reservations about the approach, which left out factors such as the increasing amount of carbon dioxide in the atmosphere.
"I have more confidence (in a study's results) when those mechanisms are really addressed," Harmon said.
"There are people who do suspect that lower elevation forests have problems with water, even if precipitation stays constant, because it will effectively be drier," Harmon added. "But there is this issue that trees do respond to (increases in) carbon dioxide. One of the things that happens is they become better at using water.
"Those kinds of mechanisms aren't in their model at all."
Steven W. Running, professor of ecology at the University of Montana and a lead author of the Intergovernmental Panel on Climate Change's latest report on global warming, said in an e-mail that the study "gives results consistent with what theory would suggest," adding that the climate models project similar temperature trends but a wide range of rainfall.
Running, who was not part of the study, also noted that it was important to recognize that it does not take into account factors such as the increase in wildfires and insect infestations.