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, such as insect attack, fire suppression, overcrowding of forests, forest fragmentation and air pollution may all play more limited and temporary roles in the numbers of living trees. But these mechanisms were all considered and ruled out as a dominant cause of the long term, pervasive level of tree death found in this study.
“Forest fires or major insect epidemics that kill a lot of trees all at once tend to get most of the headlines,” Harmon said. “What we’re studying here are changes that are much slower and difficult to identify, but in the long run extremely important.”
The areas studied in this report were forest stands 200 or more years old, with trees of all ages and sizes. There were sites in Oregon, Washington, California, Arizona, Colorado, New Mexico and southwestern British Columbia.
The data for the research was literally gathered by generations of scientists over more than a 50-year period at multiple sites, doing some of the most rudimentary science of all – counting trees. Funding was provided by the National Science Foundation, USDA Forest Service, U.S. Geological Survey and other sources.
Using various approaches, the researchers examined mechanisms that can cause localized or temporary tree deaths, such as bark beetle epidemics, forest fire, increasing competition, fire exclusion, even old trees falling and crushing younger ones. But the only causative factor that consistently explained widespread and sustained mortality was regional warming and the associated drought stress, the scientists said.
An increase in temperature in the American West of less than 1 degree has been documented during this period, which the researchers said was enough to cause widespread hydrologic changes – less precipitation falling as snow, declining snowpack water content, earlier spring snowmelt and runoff, and longer summer drought.
This could lead to sustained drought stress on trees and enhance the growth and reproduction of insects and pathogens that attack trees, the scientists said.
“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,” Harmon said. “Over long periods of time that can change the whole composition of the forest.”
Ultimately, higher tree mortality may lead to significant shifts in forest structure and function, the report concluded. The study also demonstrated that global changes in tree mortality are not confined to the tropics, as has been documented in other studies, and may be as or more serious in temperate zones.
Collaborators on the study included scientists 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.