Fire Behavior
General strategies for promoting resilience to wildfire
Early successional habitat following the 2000 Storrie Fire on the Plumas and Lassen National Forests. Snags and shrubs dominate the landscape after high severity fires.
Ecosystems that are resilient to wildfire are those that can burn and then return to their pre-fire condition prior to the next fire.
Fire-adapted ecosystems are characterized by a "fire regime" which describes the frequency at which fires in a given forest type typically burn, the season(s) in which they burn, and the amount of vegetation killed.
Low frequency fire regimes
When forests are usually wet year-round, vegetation that accumulates during the fire-free period can fuel high-severity fires that result in nearly complete mortality of the forest. The climate makes another fire unlikely for perhaps hundreds of years, allowing the forest to regenerate and return to its pre-fire condition.
Moderately frequent fire regimes
In shrublands or forests with seeds that require fire to germinate, moderately frequent fires allow the growth from those fire-dependent seeds to eventually return the vegetation to the pre-fire condition. These ecosystems are also resilient to fire, but if fire returns before the new vegetation matures and produces more seeds, the vegetation could be replaced by a different type, such as grass.
Fires in these systems could increase in frequency for a variety of reasons, including more human activity in the area leading to accidental fire starts, or invasive plants which dry out and burn readily before the re-growing vegetation can reach maturity. Public awareness, invasive species management, or strategic fire suppression strategies can reduce the frequency of fire and increase resilience in such systems.
Frequent fire regimes
Fire occurs frequently in some ecosystems, such as Ponderosa pine or mixed-conifer forests, that lack fire-dependent seeds. When fire burns through the understory, the resilience of these forests depends on the survival of the adult trees with thick bark (a fire-adapted trait). If fire is suppressed in these forests, fuel in the understory can accumulate to significant amounts. When fire eventually returns, it will likely be so severe that it will kill the mature trees.
Promoting resilience in these forests after an extended period without fire generally involves reducing the fuel load through thinning, prescribed fire under suitable weather conditions, and other means. Once much of the fuel is removed, fire is more likely to be less severe and most adult trees will not be killed. Because these forests are adapted to frequent fire, long-term resilience can only be maintained by allowing fire to burn frequently once fuels have been reduced.