Terminal problem: The red, recently dead tree is an Abies grandis or Grand Fir native to the Pacific Northwest of the USA. Other trees surrounding it in this grove in Wallowa County, Oregon, include Douglas fir and ponderosa pine.
Credit: Andrew J. Larson
OREGON, USA: Old growth trees are dying at twice the rate they were in 1955, scientists say, and they're pointing the finger of blame at climate change and resulting drought.
In a study to published today in the U.S. journal Science a team of researchers led by the U.S. Geological Survey tracked 76 plots, each about 0.8 hectares in size, in Washington, Oregon, California, Arizona, New Mexico, Colorado, and southwestern British Columbia.
The study compared trees' death rates on these tracts from 1955 to 2007, finding that the rate had doubled at a range of elevations and was hitting many species, including firs, pines, and hemlocks.
"Huge" implications
"We may only be talking about an annual tree mortality rate changing from one per cent a year to two per cent a year, an extra tree here and there," said Mark Harmon, professor of forest ecology at Oregon State University and study co-author. "But over time a lot of small numbers can add up. The ultimate implications for our forests and environment are huge."
Other studies have shown a similar phenomenon in tropical forests, but this research is the first of its kind in temperate zones. Because the study plots were limited to western North America, the scientists aren't yet sure of the degree to which it represents a global trend.
"Change is occurring in temperate forests throughout the world, but it might not be the same kind of change," said co-author Jerry Franklin, an ecologist at the University of Washington in Seattle. In particular, Franklin said, increasing summer drought in the Western U.S. may play a significant role.
Primary culprit
But global warming still appears to be a primary culprit. Other factors, such as forest fires and insect infestations, can kill trees on any given site, but when these were taken into account, the scientists found that only temperature increases consistently explain the widespread increase in tree mortality.
Even if it's only the North American forests that are dying at this rate, the finding is important globally, the scientists said, because the smaller, thinner forests would take less carbon dioxide out of the atmosphere, increasing the rate of global warming.
Franklin likened the process to dollars in the bank: Old forests have a very large accumulation of carbon in them and, therefore, a high level of capital. "If old forests are cut down [or die], we'll lose that capital."
In fact, he warned, forests could become carbon emitters instead of carbon sinks.
Other experts agree. "This study's important because it shows that systems are already responding to a warmer climate," commented Dave Breshears, who studies natural resources at the University of Arizona in Tucson and was not involved in the study. "These are changes that may not be visibly obvious just walking into the woods."
"This study is clear evidence that our water-limited western forests are
now reacting in a very fundamental way to the changing climate," said Steven Running of the University of Montana's College of Forestry. "I see the western forests of 100 years from now being dramatically different then today."
