Study: NW forests getting denser, more vulnerable to fire

While large fires have had dramatic impacts in some Pacific Northwest forests, only about 10 percent of the forested lands in the eastern Cascades have burned in the last 30 years, and young trees and dense forests are continuing to grow at a rate that outstrips losses from disturbance, a recently published study says.

As a result, many forests across this region are becoming denser. Efforts to reduce their vulnerability to future high-severity fires — through tree thinning, prescribed burning and harvesting — have had little overall effect on forest structural conditions across the region as whole.

Those are among the results of a comprehensive analysis of forest structure and biodiversity based on satellite imagery and on-the-ground field work in the eastern Cascades of Washington, Oregon and Northern California from 1985 to 2010. Matthew Reilly, a former Ph.D. student in the College of Forestry at Oregon State University led the study, which was published in the journal Ecological Applications. Reilly is now a post-doctoral scientist at Humboldt State University.

Large fires in highly visible places such as the Columbia Gorge, Santiam Pass and the Northern California wine region capture public attention, said Reilly, but are not representative of what’s happening across the whole region. “We become fixated on losses associated with tree mortality and fire, but the gains from new growth are really important. They have the potential to offset those losses. This study zooms out beyond the perimeter of recent fires and considers them in the context of what else is going on in the forests and woodlands east of the Cascades.”

For forest managers, the findings emphasize the tradeoffs between two goals: dense forests that provide habitat for threatened species such as the northern spotted owl, and more open-canopy forest ecosystems that scientists call early seral (the youngest stage in forest development) with large, thick-barked trees that are considered less vulnerable to high-severity fire.

Reilly and his team analyzed patterns of landscape change and low-, medium- and high-intensity fire in three zones — the warm, dry zone dominated by ponderosa pine; areas comprised of mixed-conifer species such as ponderosa pine, Douglas fir and white and grand fir; and high-elevation subalpine forests with extensive areas of lodgepole pine. The study covered private as well as public lands, including the network set aside in 1994 by the Northwest Forest Plan, known as “late successional reserves.”

Forests in the reserves comprise about 8 percent of the 10.5-million-acre study area and burned at a higher rate than did forests outside that system. About 15 percent of the network burned during the study period.

Subalpine forests grow at the highest elevations and comprise only 9 percent of the study area. But among the three vegetation zones, these areas took a disproportionally large hit from fire. About 30 percent of that zone burned during the study period. Fires in Washington in 2006 accounted for most of the total area burned, about half of which was high-severity. However, these sorts of events may be similar to historical fire activity, and some high-elevation species, especially lodgepole pine, are very resilient to fire, said Reilly.

In contrast, less than 10 percent of the mixed conifer and ponderosa pine forests burned. Although a third of the area burned was classified as high severity, one of the major threats to biodiversity in these zones continues to be a deficit of low- and moderate-severity fire, which historically maintained open, mature forest habitats and resilience to drought and high-severity fire.

“When you pull fire out of the picture, the forests are getting denser,” said Reilly. “There’s a push for restoration activities such as thinning and prescribed fire to make the forests more resilient. And there has been some really good work done on the ground, but it’s a drop in the bucket. It’s hardly enough to nudge things in the direction of the way we think things were historically and towards landscape-wide conditions that are likely to be more resilient to drought and fire.”

In the future, he added, scientists expect climate change to produce warmer and drier conditions, leading to decreased growth, a longer fire season and an increased risk of fire. “It’s very uncertain how much more, but as droughts increase in frequency and length, we’ll need to reassess the capacity of forests in the region to absorb these disturbances,” he said. “Continued monitoring will be essential to understand the impacts of these disturbances on our landscapes.”

Reilly worked with scientists in the Pacific Northwest Research Station of the U.S. Forest Service, Michigan State University and University of Bari Aldo Moro in Italy. Funding was provided by the U.S. Forest Service Region 6 Inventory and Analysis program and the Pacific Northwest Research Station.