Wildfire Risk and Ecological Restoration in Mixed-Severity Fire Regimes

Abstract

Wildfire risk to ecosystems and human communities varies considerably depending on the severity of fire behavior that occurs during burning. Fire severity, in turn, is dependent on fuel amount, arrangement, and horizontal and vertical continuity, which – in forests – is often broken into broad categories of surface and canopy fuels. Fire scars – areas of cambial mortality caused by low-severity surface burning – recorded in cross-dated tree-ring sequences have proven extremely useful for documenting that frequent surface fires occurred in many mid-elevation forests in western North America, especially those dominated by ponderosa pine (Pinus ponderosa). These fires burned mainly in surface fuels and rarely killed mature trees because of their thick bark and high crowns. Fire cessation in these forests began in the middle to late nineteenth century because of Euro-American settlement, initially as a result of widespread livestock grazing that removed surface fuel biomass and followed later in the twentieth century by active fire suppression by land management agencies. Fire exclusion has resulted in unchecked tree establishment, increased stand densities, a lowering of stand-level canopy base heights, increased numbers of small trees, deeper layers of needle litter, and an overall increase in crown fire potential in current forests. Conversely, canopy fuels have always dominated in upper-elevation subalpine forests, where fires are generally less frequent but much higher severity during which large areas of forest are killed.