Fires in U.S. Pacific Northwest change forest ecology

The ecology of some of the U.S. Pacific Northwest's forests is changing in unprecedented ways as the landscape, annually affected by forest fires, has slowly increased across the region over the last 30 years.

While an estimated less than one-half of one percent of the region's forest is subject to fire in any given year now, studies of fires prior to year 1900 suggest that severe fires occurred over smaller patches of forest and accounted for a much smaller proportion of the total burned area than they do today.

In a project using satellite imagery and ground-based tree inventories, researchers reported in a paper published in the journal Ecosphere that, in areas historically dominated by low- and mixed-severity fires, nearly a quarter of the burned landscape was subject to patches of high-severity fires that often exceeded 250 acres, or one square kilometer, in size.

From a regional biodiversity perspective, fires are enhancing diversity by creating early seral habitats, which is the first stage of forest development dominated by grasses, forbs and shrubs.

They provide important habitats for species that depend on open conditions and fire-killed trees or snags, said Matthew Reilly, lead author and a post-doctoral researcher in the College of Forestry at Oregon State University (OSU).

"Large fires can have significant social and economic costs, but they are also playing an important role in the ecology of our forests," said Reilly, who noted that about 98 percent of forest fires are put out before they have a chance to grow and his study is about the other 2 percent that tend to burn during the hottest, driest, windiest conditions.

By analyzing images taken with the Landsat program between 1985 and 2010, Reilly and his colleagues evaluated burned area and fire severity in seven different ecosystems, ranging from high-elevation subalpine forests to those dominated by western hemlock, ponderosa pine and Douglas fir. Since high-severity fire kills trees outright, they were able to link fire-related tree mortality to changes in images from year to year.

More high-severity fires occur in hotter, drier years. However, in dry areas east of the Cascades, a major mountain range of western North America, fires burn a smaller portion of the landscape than they did before 1900.

As a result, forests are becoming denser as vegetation accumulates, creating what scientists call a "fire deficit." "In the ponderosa pine forests in eastern Oregon, we estimated it would take about 380 years at the current rate for fire to cover the whole region," he said. "But historically, we know that those forests were subject to fire every 12 to 28 years."

Findings of the study, the first to document how recent fires vary in different ecosystems across the Pacific Northwest, are consistent with other studies that document a fire deficit in the forests of the western United States.

"There's no one out there who thinks that fire will play the historical role that it used to. We just can't really have that," Reilly was quoted as saying in a news release from OSU on Monday. "But we need to figure out how to let the low- to moderate-severity fires burn in forests where fire was frequent historically. There is growing consensus among scientists that use of managed wildfire may be one way to do this, especially in cool, wet years." Endit