Fireproofing the Forests

WILLIAM CAMPBELL FOR TIME
In Arizona, a weary fire fighter walks away from a blaze he just set

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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.

For Covington the unexpected loss of so many old-growth specimens was a wake-up call. Before setting fire loose in the forest again, he concluded that the forest had to be made more fire tolerant, and that meant restoring it to its original structure. For guidance, he and his colleagues turned to old photographs and historic texts, all of which confirmed that prior to European settlement, the ponderosa pine forests of the Southwest looked very different, with "every foot...covered with the finest grass," wrote a traveler who passed through the area in the mid-1800s, "and unencumbered with brush wood."

An even more detailed guide to what these forests originally looked like came from records kept by early foresters, who in 1909 established a series of experimental plots across the Southwest. Among these was an unlogged eight-acre plot in the Coconino that was set aside as a long-term control. Covington and his colleagues made 1876 the reference year for this plot it was the year the last fire occurred and then proceeded to reconstruct the way the forest had looked at the time. The difference between then and now, they found, was dramatic. In 1876 the plot boasted just more than 20 trees an acre, compared with 1,250 some 120 years later.

This was the plot that Covington's team experimentally thinned in 1993 and 1994, taking care to preserve all old-growth trees. The area now boasts some 60 trees an acre, and as individual trees, they seem far healthier than before. For one thing, the outer coating of their needles has increased in toughness, which helps discourage foliage-eating insects. For another, their nonwoody tissues are producing greater quantities of resin, which affords protection against bark beetles. Best of all, there is no longer any need for mechanical thinning, as low-intensity prescribed fires can safely do the job.

Indeed, says Covington, thinning should not be viewed as a substitute for prescribed fire but as its prerequisite. Otherwise, the nation could spend as much as $700 an acre on it and, a decade or so later, wind up back where it started.

The Value of Patchiness
How a wildfire behaves is determined by many variables, but among the most important are wind speed, topography, air temperature, humidity and, last but not least, fuel load. Variations in fuel load create the equivalent of speed bumps in the landscape that serve to slow fire down, and the problem we have now is that this patchiness in many places has all but disappeared replaced by vast tracts of forest that are uniformly dense with unburned kindling.

Restoring this lost patchiness is critical. Unfortunately, there is no easy equation for doing so, as the optimal distribution of fuel varies widely from forest to forest.

The frequent-fire regime that prevailed in the ponderosa pine forests of Arizona and New Mexico, for example, kept fuels low over widespread areas. In the ponderosa pine forests of Colorado's Front Range, however, big burns were spaced farther apart, allowing flammable material to accumulate. These fires rolled through every few decades or so and occasionally burned extremely hot. Their legacy, says Merrill Kaufmann, a senior scientist with the U.S. Forest Service's Rocky Mountain Research Station, was a mosaic of forested areas that alternated with clearings ranging from 5 acres to 100 acres in size.

It was this scale of patchiness in the landscape that once minimized the danger of horrific conflagrations like the infamous Hayman fire of 2002. In a single day, it is sobering to recall, the Hayman fire flared across some 60,000 acres in Denver's watershed, torching the crowns of trees and cooking the soil. Among the casualties were most of the 300-to-600-year-old ponderosa pines on a 7,500-acre site that Kaufmann has closely studied. It was a beautiful site, he says, ungrazed and unlogged. The only problem was that fuel loads were off-scale because a good fire had not moved through in more than 120 years.