BERKELEY - The age of vegetation in California's shrublands does not strongly influence the probability of wildfires, finds a new study led by a researcher at the University of California, Berkeley. The findings challenge a basic assumption underlying fire management strategies used to prevent wildfires like the ones that swept through southern California in October 2003.
"If the goal is to save people's homes and avoid loss of life, then treating extensive portions of the landscape to create a mixture of young and old vegetation is not money well spent," said lead author Max A. Moritz, wildland fire specialist at the UC Center for Forestry's Fire Program. The center is based at UC Berkeley's College of Natural Resources.
Conventional wisdom has been that older shrubs have a higher percentage of dry, dead biomass that can more easily fuel intense wildfires, said Moritz. In addition, decades of fire suppression are believed to have allowed extensive stands of older vegetation to accumulate. As a result, fire management strategies have focused on various techniques to create an "age-patch mosaic" on the landscape because it is assumed that fire will not spread through younger stands of regenerating shrublands.
The findings of the study, reported in the March issue of Frontiers in Ecology and the Environment, break from a school of thought in fire management that incorporates treatments of extensive portions of the landscape, such as prescribed burns, to reduce fire risk.
In forested ecosystems that prehistorically experienced frequent, low-intensity surface fires, decades of successful fire suppression are typically blamed for allowing the accumulation of underbrush and medium-sized trees, particularly in the ponderosa pine forests of the southwestern United States. Many researchers believe that fire suppression has led to the formation of "ladder fuels" that now allow high-intensity fires to climb into the canopies of taller trees.
This view of a direct link in an ecosystem between fire hazard and the time since the last fire appears to have been adopted in shrubland fire management without really being tested, said Moritz. The rationale for shrublands was that suppressing smaller fires promoted the accumulation of older, contiguous stands of more flammable vegetation. Fire management strategies therefore included methods such as small, prescribed fires as a way to initiate the growth of new vegetation that, theoretically, is less prone to burning.
"One problem is that the model of fire hazard for some forests is not necessarily appropriate for the shrublands of southern California, which are characterized by periodic, high-intensity fires," said Moritz. "There has been ongoing debate in fire management about whether the age and spatial patterns of fuels are really an important constraint on the development of large shrubland fires. Our study provides evidence that different strategies should be used for the different ecosystems."
The authors analyzed data from several hundred wildfires over the past century in an area from northern Baja, just south of the United States-Mexico border, up to Monterey Country. Areas of the study include Los Padres National Forest and the Santa Monica Mountains National Recreation Area. They used various sources of data to determine how frequently the fires occurred and how much vegetation burned in different age classes.
"If age had a strong effect on the frequency and extent of wildfires in shrublands, we would have seen minimal burning in younger vegetation and a significant increase in burning with older vegetation," said Moritz. "Instead, we found that for almost all of the regions studied, vegetation age was not a clear driver of fire hazard."
The authors conclude that an accumulation of older shrublands due to fire suppression would not play a major role in causing large wildfires. Rather, the impact of the hot, dry Santa Ana winds, which blow through California's southern and central coast regions every fall, is the more dominant factor.
The one exception where the age of the vegetation was linked to a lower hazard of burning was in the western end of the Santa Ynez Mountains, a region in the backcountry behind Santa Barbara. The authors note, however, that the area is relatively sheltered from the effects of the Santa Ana winds.
"It's not that the age of the vegetation is irrelevant," said Moritz. "But for fires that occur during extreme weather conditions of southern California, vegetation age simply becomes much less important."
Moritz said the findings from this study indicate that treatments of extensive portions of the landscape in California's shrublands to create a mixture of young and old vegetation patches will not have the fire hazard reduction impact people are expecting. The authors note that large wildfires occurred throughout the region's history, even before modern fire suppression methods were introduced, and that we may need to accept them as natural and inevitable events for the area.
"Fire management strategies should focus on more effective use of resources, like creating defensible space immediately around people's homes and communities, attempting to fire-proof structures, and developing better evacuation procedures," said Moritz. "We also have to ask ourselves whether it makes sense to build homes in areas at risk for fire or natural hazards in the first place. Doing so is inherently dangerous, and it is at least in part an urban planning problem."
Co-authors of the paper are Jon Keeley of the U.S. Geological Survey's Western Ecological Research Center; Edward Johnson of the University of Calgary's Department of Biological Sciences; and Andrew Schaffner of Cal Poly State University's Statistics Department.