Burning Issues

By evaluating fire scars on mature trees, scientists have been able to determine both the intensity and frequency of wildfire cycles across much of the West. Prior to European settlement, which in terms of natural history wasn't long ago, seasonal fires burned through forests just as they do today. Many of these were surface fires; small and fairly short-lived, they appeared in cycles ranging from 2 to 30 years, depending on the area. Catastrophic fires happened, too—enormous, intensely hot blazes that burned most every standing tree—but these were infrequent, appearing every 50 to 200 years, again depending on the area.

A wildfire is chaos made real. The typical fire is triggered by lightning, and may smolder on the ground or in the limb of a tree for days or weeks. It may go nowhere, or it may erupt. Once it begins to move, a wildfire rarely advances in a uniform line or path. The irregularities of terrain, availability of fuel, and presence of wind cause fires to skip some areas altogether, while others burn. Fires of low to moderate intensity tend to remain on or close to the ground. Only if enough “ladder” fuel is present—such as deadfall, immature trees, brushy thickets, and low-hanging limbs—can a fire climb upward into mature trees, where, if conditions are right, it may become a “crown fire.” Burning in the canopies of the oldest, tallest trees, this is the kind of fire capable of remaking a forest.

When Fire Works
In a stable forest system, fire returns such nutrients to the soil as it burns decayed leaves, bark, and other material on the forest floor—in the process reducing fuel buildup for the next season. Fire serves as a regulator of insect- or disease-stricken trees, which often are in a weakened state and readily submit to the flames; by eliminating these “host” trees, the odds of a widespread disease or insect outbreak are reduced. Fire may burn through a large stand of young trees, all of them the same age and growing in close proximity. For that stand to remain healthy, a sizable percentage of trees must be eliminated, and fire does the job. By incinerating brushy undergrowth and allowing sunlight to reach the ground, fire prepares a nursery for new generations of trees. After a fire, seeds are dispersed by surviving mature trees in the area. The soil is recharged, shrubs and other competition are set back, and the odds are maximized that a seed will germinate.