As noted in the wildfire questions and answers discussion, fire is a natural process to which most ecosystems in the Southwest have adapted. However, due to the increasing population in rural areas, combined with recent decades of determined fire suppression, the threat of wildfire is one that cannot be ignored. The fires in New Mexico and Arizona in May of 2000 are proof of how destructive and dangerous this phenomenon can be. This discussion is meant to serve as an introduction to the behavior of wildfire and its control.

Fire managers speak of the “fire triangle” depicted to the right. The three main factors affecting fire behavior are the weather, the topography, and the fuels; and the interactions among these three components can affect fire behavior in both obvious and subtle ways. In this depiction of the fire triangle, fuel is placed at the top, because without fuel, fire will not happen. Moreover, when it comes to trying to control wildfire, the only factor over which we have any real influence is the fuel.

The factors interact in ways depicted. Fuel types are determined, in part, by the aspect of the topography, that is, which way a slope faces. North-facing slopes tend to hold more moisture, and ecosystems requiring more moisture can survive there; while south-facing slopes favor dry-land ecosystems to a greater degree.

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The interactions between fuels and the weather occur through a variety of processes. On time scales of years, of course, the local climate determines the ecosystem, to a large degree, and therefore the fuel type and density (that is, how much fuel there is on a given piece of land). On shorter time scales, weeks to months, the fuel moisture is determined by the weather. Dry seasons lead to dry fuel, which is more combustible than moist fuel.

Not included explicitly in the fire triangle is the way that a fire starts, that is, the ignition. This can be divided into two categories, natural ignitions (mostly lightning) and man-made ignitions, which include careless tending of campfires and cigarettes as well as arson. What is important about this, however, is understanding that wildfires will happen without man-made ignitions. Their number may be decreased by public awareness campaigns such as the decades-long advocacy of Smokey Bear, but lightning will still ignite fires. With this in mind, it becomes relevant to think about minimizing the damage wildfires can cause.

©1999, Corel Corporation

It may be too ambitious to discuss “controlling” wildfire, because there always have been and will continue to be fires that get out of control. Insofar as it is a natural process, putting fire to use in preserving ecosystem health would seem to be a good strategy in resource management. And we know from experience that knee-jerk fire suppression is not a wise strategy. Almost all recent wildfires, from the famous ones in Yellowstone National Park to the recent ones in the Southwest, have been the result of unnatural fuels build-up over the years of attempting full fire suppression as a management policy.

An important factor in this unnatural build-up of fuel is the concept of ladder fuels. When there is a continuum of fuel—from grasses on the ground, to small bushes, to small trees, to medium-sized trees, to the mature trees of the ecosystem—a ground fire can climb to the canopy of the mature trees and create a crown fire. Because the winds are stronger at the top of the canopy, a crown fire can spread faster than a ground fire. Further, the process of spotting, in which embers are carried downwind to create new spot fires ahead of the main fire front, is more effective in a crown fire.

When there are no ladder fuels, wildfires tend to stay on the ground, and the mature trees, such as ponderosa pines with their fire-resistant bark, can survive. Grasses grow back quickly because ground fires tend not to be too hot. But a crown fire is destructive to just about everything, from the grasses to the mature trees—this is called a stand-relacement fire. Of course, even unmanaged ecosystems will have the occasional stand-replacement fire in dense stands of mature trees. But because these dense stands are relatively isolated, such fire will not spread widely. The trees that are destroyed will give way to new species that occur earlier in the natural succession of species in the forest.

Because of decades of fire suppression, however, there are dense stands of mature trees with ladder fuels below them in forests across wide areas of the Southwest. Crown fires are easily started and, once started, can be hot enough to destroy everything in their path. It’s for this reason that most large fires are contained as much by changes in the weather as they are by firefighting efforts.

If it’s not entirely possible to control wildfire, it may be possible to manage it to some degree. The experience gained in both the Oso Fire (1998) to the north of Los Alamos, New Mexico, and the Cerro Grande Fire (2000) on the Los Alamos National Laboratory site suggests that removing ladder fuels can keep fire on the ground and in a more “natural” mode of burning. Unfortunately, removing these ladder fuels can be an expensive and labor-intensive process, especially when the extensive acreage that needs such treatment is considered.

Fuels treatment is also controversial from a political perspective. Decades of logging in the National Forest system have sensitized the public to the process of removing trees, and, even if such removal is for fire prevention, it is not always politically possible.

It is important to address the issue of the “state of the forest,” however. People are moving to the Southwest in increasing numbers and they like to live in rural areas, including forests. And to complicate matters, the Southwest’s climate may be changing to a more fire-prone mode in the future.

©2000, Reuters Corp.

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