Fireproofing The Forests

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Not all forests are good candidates for thinning. Among the prime examples are the lodgepole pine forests that occupy higher elevations across the mountain West. Lodgepole pines, which are thin-barked, flourish only in areas where sufficient moisture and cool temperatures keep fires at bay for long periods of time. There they grow quite densely together--so densely, in fact, that numerous trees are shaded out by more vigorous competitors. These dead and dying trees, intermingled with low-limbed spruce and fir, add a vertical dimension to the fuels structure that one day will carry fire into the canopy--as happened across a third of Yellowstone National Park in 1988.

Yet attempting to thin lodgepole pine forests to prevent such blowups would be ludicrous, say scientists, for these seemingly catastrophic blazes serve important ecological functions. Among other things, lodgepole pine saplings do not flourish beneath the shade of mature trees and thus are dependent on fires to clear sun-filled openings. Moreover, many lodgepole pines package their seeds in resin-sealed cones that can be opened only by intense heat. "What you have to keep asking yourself is what range of fire frequency and severity a particular forest has experienced," says Tania Schoennagel, a University of Colorado researcher who studies postfire recovery. "Using forestry practices to mimic these fires is O.K., but if you mimic fires outside that forest's experience, then I think you're doing the forest harm."

Thinning also seems of dubious merit in many mixed-severity fire regimes, except as a protective measure around the perimeter of communities. Consider, for example, the Biscuit fire that hopped and skipped across 500,000 acres in southern Oregon's Siskiyou National Forest last year. Slightly more than 15% of this rugged, geologically complex region was so seriously burned that virtually all the trees died. Around 65%, however, experienced fires of light and moderate severity, while some 20% escaped unscathed. Seed from areas where vegetation survived is already drifting into areas where vegetation was lost, and many important species--knobcone pine trees, flowering kalmiopsis bushes and carnivorous cobra lilies--are taking root in the ashes. "This is not a catastrophe," says World Wildlife Fund ecologist Dominick DellaSala, "but a process that drives biodiversity."

Even in forests where frequent, low-severity fires are the rule, the possibility that thinning may have unintended consequences merits careful consideration. Among other things, thinning can open forests to drying winds, making branches and needles even more flammable. It can expose pristine areas to vehicle and foot traffic that compacts soil and facilitates the spread of exotic grasses and weeds. And then there are all the other considerations, ranging from the aesthetic (what a forest should look like after it's thinned) to the practical (what to do with all the small-diameter trees a massive thinning program would generate).