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Thinning mature trees ups fire risk

By Chad Hanson - Sat, Aug 25, 2001

The Union Grass Valley/Nevada City, CA

In his recent column, Steve Eubanks, supervisor of the Tahoe National Forest, states that commercial thinning of mature trees from 10-30 inches in diameter reduces the risk of wildfires, though he cites no scientific authority to support this claim. He would be hard pressed to do so, since the Forest Service's own science clearly takes the opposite view.

For example, the recently released Sierra Nevada Forest Plan states that reduction of forest canopy cover actually causes more severe fires by increasing the velocity of "mid-flame winds." Further, the Fire and Fuels section of the Sierra Plan acknowledges that "in areas where the larger trees have been removed, stand replacing fires are more likely to occur." This same section, for the purposes of fire and fuels management, defines "large trees" as those over 12 inches in diameter.

In a nutshell, commercial thinning reduces forest canopy cover, eliminating the moist, cool, shaded conditions associated with mature forests. The result is hotter, drier conditions on the forest floor - a situation ripe for severe wildland fires. In addition, logging leaves behind extremely flammable slash debris consisting of dry twigs and branches.

The Forest Service's own National Fire Plan warns that the agency's wildland fire policy should "not rely on commercial logging or new road building to reduce fire risks" because "the removal of large, merchantable trees from forests does not reduce fire risk and may, in fact, increase such risk." The National Fire Plan also finds that "logging and clearcutting can cause rapid regeneration of shrubs and trees that can create highly flammable fuel conditions within a few years of cutting."

Even the Forest Service's chief fire specialist, Denny Truesdale, repeatedly stated in an Aug. 10, 2000 interview on the C-SPAN program "Washington Journal" that the material that needs to be reduced to prevent severe fires is undergrowth less than 3 or 4 inches in diameter - not mature trees. In fact, the Forest Service's BEHAVE model, which measures potential for fire spread, doesn't even consider material larger than 3 inches in diameter.

Mr. Eubanks is correct that in some areas prescribed burning alone cannot effectively prevent severe wildland fires. In some cases, undergrowth comprised of saplings, shrubs and twigs should be manually reduced prior to prescribed burns. In fact, dozens of such projects are being implemented right now on Sierra Nevada national forests through service contracts. None of them are being challenged by environmentalists.

Challenges occur only when the Forest Service removes mature trees through commercial logging operations or through timber sales embedded in service contracts. Such activities not only seriously degrade habitat for imperiled forest species such as the California spotted owl and Pacific fisher, but they also increase the incidence and severity of wildland fires.

Finally, Mr. Eubanks states that he encourages commercial thinning of mature trees on the Tahoe National Forest supposedly in order to protect homes on adjacent private lands from fires. Yet the Forest Service's own scientific expert on this issue, Jack Cohen, has published recent studies which conclude that the only way to effectively protect homes is to reduce the flammability of the home itself and its immediate surroundings within at most 40 meters. The commercial thinning operations on Sierra Nevada national forests, including the Tahoe, are typically miles from the nearest home.

The Forest Service's credibility would be greatly enhanced if the agency would simply admit that it continues to cut down mature trees for their timber commodity value, and nothing more. If the commercial logging program on national forests were ended, the agency would be able to focus on real fire risk reduction efforts, rather than continue to waste taxpayer money on timber sales that destroy habitat and cause severe fires.

Chad Hanson is the executive director of the John Muir Project and a national director of the Sierra Club.