Mixed severity fire regime continued
low intensity surface fires, to stand replacing conflagrations can occur within a single fire event, demonstrated locally by the many mixed severity fires burning in the Klamath-Siskiyou Mountains in recent years.
The severity of a fire in the Klamath-Siskiyou is heavily influenced by past fire history, weather patterns, climatic conditions, fuel loads, steepness of terrain, exposure and slope position. The variable fire history of the Klamath-Siskiyou Mountain’s mixed conifer forests has prevented generalization of fire and its effects. Such complex fire histories have created equally complex and diverse forests in terms of both composition and structure. The mosaic of these forests can be varied and patchy to the extreme, ranging from dense canopied forests to regenerating brush fields.
High severity fire regime
The high severity fire regime is typical of brushfields and fire dependent pine stands at both high and low elevations in the Klamath-Siskiyou Mountains. Although influenced by the same fire regime, shrub communities found in the foothills have a significantly different species composition and life history than those found at higher elevations in the montane chaparral belt.
Contrary to the popular—and convenient—belief of many fuel/fire specialists and timber managers, historic fire regimes did not exclusively create open habitat conditions. Hot fires, harsh exposure, summer heat, wind driven fires,
and steep, rocky slopes with shallow, unproductive soil types are all factors that could have favored brushfields, dense hardwood regeneration, and fire dependent conifer species such as knobcone pine, lodgepole pine and the rare Baker’s cypress. The high severity fire regime tends to be highly episodic, including long fire return intervals and high severity fire effects. Long, fire-free periods allow sufficient fuels to build in both brushfields and fire dependent conifer stands that high severity fire effects tend to perpetuate themselves over time.
In high elevation forests adapted to high severity fire, such as white fir, red fir, mountain hemlock, Pacific silver fir, and Brewer spruce, often the forest is simply too moist and fuels too discontinuous for fires to sufficiently spread and burn with intensity. Yet during the occasional drought year fires can grow large and burn with intensity. Rather than developing adaptations of fire resilience—as the forest at low and mid elevations do—these forests are adapted to fire by colonizing sites that are less likely to burn such as high elevation snow forests, north slopes, cirque walls and rocky outcrops. These species tend to avoid fire by growing where fire is less prevalent. In many cases these stands appear to be transitory in nature, rotating around the landscape as the occasional fire burns, creating high levels of mortality. These transitory stands tend to mingle with wetlands, meadows, high mountain balds, rock outcrops and high elevation brushfields created by past fires.