Thinning Oregon forests develops spotted owl habitat, chases away flying squirrels -- the owls' chief prey

By Eric Mortenson, The Oregonian

Call it a deep-forest case of can't win for losing.

A new study by Oregon State University researchers indicates that thinning Douglas firs, which gives them more room to grow and develop the old forest characteristics favored by northern spotted owls, is bad news for the threatened bird's primary prey.

A study of four tracts in the Willamette National Forest showed the number of flying squirrels declined in areas that were commercially thinned. Not died, necessarily, but departed, vamoosed or otherwise didn't live there anymore.

The more thinning that took place, the fewer squirrels there were, the researchers found. It's clear, they concluded, that "densities of northern flying squirrels are particularly sensitive to thinning in young Douglas fir forests, for at least 12 years after treatment."

Squirrels evenutally may come back to thinned areas, but the findings "argue for caution" in thinning across large sections of the landscape, they said. "Especially if one eventual goal is to sustain the primary prey of the northern spotted owl."

Researchers caught and counted flying squirrels by setting live-capture traps in thinned areas of the Willamette's Oakridge and McKenzie Bridge ranger districts, east and southeast of Eugene-Springfield. Each of the four study areas had a control section that wasn't thinned and sections on which light to heavy thinning took place.

The report was written by Tom Manning and Brenda McComb, with OSU's Department of Forest Ecosystems & Society, and Joan Hagar, a U.S. Geological Survey employee who collaborates with OSU researchers and advises grad students.

Commercial thinning is done for multiple reasons: to reduce fire danger, provide small logs for mills, make room for remaining trees to grow larger and to aid eventual development of old-growth habitat characteristics.

Hagar said it's fairly common for such "restoration ecology" work to result in "winners and losers." Foresters should maintain connected areas of "dense, closed canopy" forest for flying squirrels when thinning is done, she said.

"It's good to leave some areas unthinned; we don't know how much," she said.

"The good thing to take away is that conundrum idea," Hagar said. "Everything we do out there affects some organisms positively and some adversely."

The lesson, she said, is "Don't do the same thing everywhere across the landscape."

--Eric Mortenson