CORVALLIS, Ore. - Thinning of young Douglas-fir forests in the Pacific Northwest, done in part to help them return to a structure more similar to old-growth forests and aid recovery of the threatened northern spotted owl, also reduces the populations of flying squirrels that form an important part of the owl's diet.
A recent study by scientists from Oregon State University and the U.S. Geological Survey found that this unwanted impact illustrates the complexity of trying to restore old-growth characteristics in forests that for decades were managed primarily for Douglas-fir timber production.
In the long run, researchers said, a restoration of old-growth structure should be a positive force for both spotted owl recovery and the northern flying squirrel - but in the near term, forest stands that have been thinned support significantly lower densities of flying squirrels than unthinned stands.
"Some of the stands being thinned were probably not great spotted owl habitat to begin with, and the impact on flying squirrel populations may not be permanent," said Joan Hagar, a researcher with the U.S. Geological Survey and courtesy faculty member in the OSU Department of Forest Ecosystems and Society.
"This is a fairly common problem in restoration ecology, in which there are always winners and losers," she said. "What this really suggests is that we may not want to thin all of these forest types, we need to preserve some as a refuge that would allow flying squirrel populations to recover in the future."
Flying squirrels, Hagar said, are a major part of the diet of the northern spotted owl and also help disperse fungi important to tree health. There are millions of acres of even-aged, Douglas-fir dominated forests being considered for thinning, for both spotted owl recovery and other goals, both economic and ecological.
The northern flying squirrel is considered a "keystone species" by ecologists and an indicator of forest health. They do best in forests with many large live trees and well-developed understories, characteristics that are now largely lacking in many young forests.
This research studied various types of thinning treatments, and found that the heavier the thinning, the heavier the impact on flying squirrel populations. It was one of the longer-term studies done on this issue, for up to 13 years after thinning.
The findings "would seem to argue for caution in carrying out commercial thinning across large portions of the Pacific Northwest landscape, especially if one eventual goal is to sustain the primary prey of the northern spotted owl," the researchers said in their conclusion.
Continued monitoring of northern flying squirrels and their habitat will help determine when flying squirrel populations begin to recover in thinned stands, in which treatment levels this occurs most quickly and which habitat features are most important, the scientists said.
The study was supported by the USDA Forest Service. It was published in Forest Ecology and Management, a professional journal.
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Joan Hagar, 541-737-6574