Regional warming hurting NW forests, study says

The Northwest's oldest, richest forests are losing more trees in recent years, and scientists point to regional warming as the likely cause, according to a new study released today.

If more trees continue to die each year 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.

A paper in the journal Science says mortality rates for trees in Pacific Northwest forests have more than doubled over the past several decades, threatening to eventually change forest habitat and deplete timber resources.

"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 to the study. "It has to raise questions about how these forests will function in the future."

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

"But over time a lot of small numbers can add up," he said in a statement.

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 deliberately 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 Washington 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 basic data that tracked 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.

The scientists ruled out overarching causes such as trees overcrowding and competing with each other, or air pollution, and determined the probable factor affecting every forest is warmer temperatures.

"The important message is wherever we looked, mortality rates are increasing," said co-author Nathan Stephenson with the U.S. Geological Survey. "In more pristine places — — (such as) 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.

"These long-term data sets are the currency of environmental science," Franklin said. "If you've got them, you've got gold."

In general, average yearly temperatures have increased across the West, producing less snowpack in the mountains, quicker spring melting and longer summer droughts.

Warmer temperatures and limited access to water can affect trees differently. In some forests, destructive insects that thrive in warm temperatures may attack trees, or fires can sweep through and burn entires forests. If warming continues, it's very likely the death rate of trees in healthy forests will also rise, Stephenson said.

The study only looked at western forests, but scientists said given the trend they discovered, temperature changes probably are affecting trees in forests across the continent.

Michelle Ma: 206-464-2303 or mma@seattletimes.com