Historical fire regimes were important disturbance processes in western forest ecosystems. While quite variable over space and time, they served to alter species composition, nutrient cycling, and other ecosystem structure and function attributes. Accompanying the variable environments of western forests were a similar variety of fire regimes, defined by different combinations of fire frequency, intensity, extent, season, and synergism. For simplicity, these fire regimes are classified into three types: low (nonlethal), mixed or moderate, and high (lethal). Each has characteristic signatures on landscape composition and pattern.

The low-severity fire regime was characterized by very frequent but relatively benign individual fires. These fires occurred every 5-15 years, killed trees when they were small and had little effect on larger, fire-tolerant trees

The moderate-severity fire regime was characterized by a complex mix of low- moderate-, and high-severity fire, Average fire return interval ranges of 25-75 years were common. The result was a complex set of stands.

The high-severity fire regime was characterized by large patches of stand replacement burning, occurring usually at intervals exceeding 75-100 years. Large even-aged stands resulted from this fire regime.

Photo credits: Unthinned, dense forest ,above, by Washington State DNR, Below, Wildland fire, is curtesey of the Yakima Indian Reservation.

Implications of changes in historical fire regimes

In the 20th century fire potential has changed in three definable ways: surface fire intensity has increased due to increased fuels; torching potential, or the ability of a fire to move into the crowns, has increased due to vertical "fuel ladders" and low height to live crown; and the ability of fire to move through the crown has increased due to higher crown bulk densities.

The solutions to these problems depend on (a) what forest types that need treatment the most, and (b) what type of treatment should be applied in a high priority forest type. In general, the low-severity fire regimes are highest priority. Not only are the fuel buildups and changes due to fire suppression most significant here, but the result of treatment will be a forest with higher resistance to stand replacement fire.

The key to successful forest restoration for fire purposes is to reduce potential fire behavior while leaving fire-resistant vegetation. The order of treatment should be to (a) reduce surface fire behavior, (b) reduce torching potential, and (c) reduce crown fire spread potential. This can be accomplished through a variety of treatments, with prescribed fire and mechanical thinning the most likely tools of choice. Reduction in surface fuels and increases in height to live crown should be a first priority, with reductions in canopy closure as a second priority. Leaving large trees and removing smaller ones is the means to achieving the fire hazard goals: essentially a low thinning.

Fire will be in our future whether we like it or not. We can choose the type of fire we want to deal with, and significantly reduce wildfire damage in low-severity fire regimes by targeting restoration efforts to change potential fire behavior and effects.