This is the way the ponderosa pine forests of the American Southwest used to look, says Covington, director of the Ecological Restoration Institute at Northern Arizona University, and it is the way they could look again if thinned of an unnatural density of trees. But time is running out, he fears, for owing to more than a century of mismanagement, these once magnificent forests along with the communities expanding around their fringes are threatened by the elemental force that at one time sustained them fire.
Every year, it seems, the threat posed by fire looms larger. Three years ago, for example, the Cerro Grande fire in New Mexico, driven by winds that gusted up to 50 m.p.h., burned out of control over an area that totaled nearly 50,000 acres, forcing the evacuation of some 18,000 people and the closure of the Los Alamos National Laboratory. Last year brought some of the largest, most intense wildfires in U.S. history, including the Hayman fire, which overran nearly 138,000 forested acres near Denver, and the Rodeo-Chediski fire, not far from Flagstaff, which blazed for weeks across 466,000 acres. These epic conflagrations helped make 2002 the third worst fire year on record (after 1988 and 2000) with nearly 7 million acres burned, 21 fire fighters dead and 2,000 structures destroyed.
This summer, with blazes erupting once again across the West from Arizona to Montana, Idaho, the Pacific Northwest and Canada the concerns long raised by Covington and others are fueling an intense debate. Should the U.S. Forest Service, in the name of protecting communities and restoring ecological balance, authorize tree thinning on a massive scale? If it does, what size trees ought to be thinned and in what sorts of forests? And if it does not, what are the alternatives?
These questions are currently smoldering in the U.S. Senate, the U.S. House having stampeded through a bill last May that seemed aimed more at currying favor with the timber industry than at establishing a broadly acceptable framework for action. Fearful that thinning might become Orwellspeak for logging, environmental groups have taken the position that cutting down small trees is appropriate only as a protective measure in the forested strips that abut human settlements. Uncomfortably wedged in the middle are Covington and his allies, who see thinning, undertaken responsibly, as perhaps our last chance to restore ecological health to an increasingly dysfunctional landscape.
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.