Fireproofing The Forests

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And yet, as Covington acknowledges, the science that undergirds thinning is still evolving, and the danger of inaction is counterbalanced by the danger of inappropriate action. That's because dense stands of young trees are not necessarily signs of poor forest health, and intense fires that kill off big, forested tracts are not necessarily ecological catastrophes. Thanks to variations in climate, topography and elevation, different types of forests have evolved under different fire regimes. Prior to embarking on thinning on a massive scale, it is necessary to distinguish between forests in which fire continues to play a positive role and those in which it does not.

THE CASE FOR THINNING

For centuries fires swept through the ponderosa pine forests of Arizona and New Mexico on average once or twice a decade, killing off saplings but not larger trees. Scientists know this because these fires left a succession of healed-over burn scars in the trees' cambium, the living tissue that lies just beneath the bark. By dating the scars left in tree rings, University of Arizona dendrochronologist Tom Swetnam and his colleagues have reconstructed a fire history of Southwestern forests that extends back to the 14th century. And the most striking feature in the graphs they have produced is a marked drop-off in the number of fires beginning in the late 1800s.

What happened? First, sheep and cattle were allowed to overgraze the grasses and broad-leafed plants that used to carry low-intensity ground fires through the understory, consuming litter, releasing nutrients and thinning out saplings. Then came decades of logging, coupled with increasingly effective fire suppression. The structure of the forest changed so that hundreds of small trees now crowd into acre-size plots that used to support a few dozen large ones. The result: millions of acres of Southwestern forest land are packed with enough woody tinder to power wildfires of unprecedented severity.

Indeed, the situation has reached the point at which some experts are convinced that even low-intensity fires--including so-called prescribed fires set to clear out the understory--pose grave dangers to large, mature trees. A quarter-century ago, in fact, Covington and two Forest Service researchers experimented with the use of prescribed fire in the Coconino National Forest. Their reasoning seemed sound: since fire exclusion had created the problem, the solution must lie in bringing fire back. Alas, the results were contrary to expectations. The dog-hair thickets of young trees the scientists hoped to kill survived, and the old-growth trees they hoped to save died.

Why? In the absence of fire, too much fuel, in the form of dropped needles and branches, had accumulated at the bases of the largest trees. Yet not enough time had elapsed to allow a similar buildup of fuel beneath the crowns of the smaller trees. As a consequence, flames traveled quickly through the thickets of new growth but smoldered long enough at the feet of the giant trees to girdle and kill them.