Northwest tree die-off likely due to warming trend, study says

Trees in the Northwest's oldest, richest forests are dying at an increasing rate, and scientists point to regional warming as the probable cause, according to a new study published today.

If the mortality rate continues to rise in Washington and across other Western states, it's likely that forests will be made up of smaller, younger trees that are more susceptible to fires and massive die-offs, the study concludes.

The paper in the journal Science said the death rate for trees in Northwest forests remains low, but has nearly doubled over the past few decades.

"It just might be a harbinger of more to come," said Mark Harmon, a professor at Oregon State University's Department of Forest Ecosystems and Society and a collaborator on the study. "It has to raise questions about how these forests will function in the future."

Harmon said the mortality rate in some areas may be changing from 1 to 2 percent a year, "an extra tree here and there."

"It doesn't look like a lot of trees, but the concept is it's happening in many areas," he said.

The study's authors compiled data from more than 75 plots in old-growth forests throughout the Western United States and Canada. Many of these plots were set up four decades ago as long-term study sites for researchers at various universities and federal agencies. In Washington, a dozen sites are near Mount Rainier, and a handful are on the Olympic Peninsula and close to Mount St. Helens.

The study looked at forest sites that hadn't recently been through fires or insect infestations. The plots in Washington come from healthy, stable forests with trees up to 7 feet in diameter, said Andrew Larson, a doctoral candidate in University of Washington's College of Forest Resources who helped monitor the sites.

"On the surface, there are no clues — if you just look casually at the forest — that things have really changed," Larson said. "We normally think of them as being quite healthy and normal."

And yet by looking at data that track the number of trees in each plot, researchers have found that trees of every size and species are dying in greater numbers, and for a variety of reasons, no matter the elevation or climate of their particular forest.

While the death rate was increasing, the rate of new trees sprouting and surviving was not, the study found.

Scientists looked at factors that can cause trees to die, including competition from overcrowding, tall trees falling on smaller ones and exposure to air pollution. But data analysis ruled out each factor as being a dominant reason for the long-term trend.

Warming temperatures emerged as the probable reason for more tree deaths, the study says. Higher average temperatures can cause greater stress on trees from lack of water, leaving them vulnerable to disease and insects.

Across the West, average temperatures have increased by nearly 2 degrees F over the past 30 years or so, said Nathan Stephenson with the U.S. Geological Survey.

"The important message is wherever we looked, mortality rates are increasing," said Stephenson, a co-author of the study. "In more pristine places — the Olympics in Washington — their mortality rates were skyrocketing also, and that's an area that's not polluted."

Still, forests in the Pacific Northwest are incredibly resilient, and the finding that more trees are dying in otherwise healthy forests shouldn't signal panic, said Jerry Franklin, a UW professor in the College of Forest Resources who set up and has monitored many of the sites used in the study.

The report should drive home the need for more of the most basic research — counting trees and keeping track of what's dead and alive year-to-year, he said.

Michelle Ma: 206-464-2303 or mma@seattletimes.com. The Associated Press contributed to this story.