CORVALLIS, Ore. Regional warming and drought stress are the "dominant contributors" to a rapid increase of tree mortality in old growth forests across the West during the past 50 years, a new report concludes, with the Pacific Northwest the hardest hit of all areas studied.
The findings, to be published Friday in the journal Science, suggest that a persistent increase in the mortality rate would ultimately cause a 50 percent reduction in the average tree age in forests, a potential reduction in average tree size and make many forests vulnerable to abrupt dieback.
A doubling of tree mortality has been occurring as fast as every 17 years in the Pacific Northwest in recent decades, and at slower rates in California and Rocky Mountain states. In one of the first studies of this type ever done in temperate zones, this disturbing phenomenon was found to be occurring at every elevation, in trees of different sizes and various species.
"We may only be talking about an annual tree mortality rate changing from 1 percent a year to 2 percent a year, an extra tree here and there," said Mark Harmon, professor of forest ecology at Oregon State University. "But over time a lot of small numbers can add up. The ultimate implications for our forests and environment are huge."
Another significant part of the concern, Harmon said, is that a "feedback loop" appears to be developing. As regional warming causes some trees to die, the diminished forests will absorb less carbon dioxide and then inject more greenhouse gases back to the atmosphere. This in turn could cause even higher levels of atmospheric warming.
"In ecology there's a bias toward understanding how things grow," Harmon said. "But my studies are mostly on how things die and decompose, and that's what's happening here. When trees across the West appear to be dying at twice the rate they used to, that's not a good sign."
Other possible causes of tree mortality,
|Contact: Mark Harmon|
Oregon State University