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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.