Sprawl gets the blame for shrub-land fires

The Bel-Mar fire races through Santa Monica Mountains chaparral in June 1988

June 14, 1999
Web posted at: 4:00 PM EDT

Urban sprawl, not fire suppression, is responsible for the catastrophic wildfires that occur in the shrub lands of southern and central-coastal California, according to a U.S. Geological Survey study.

Catastrophic shrub-land wildfires have raced across this region in recent years, taking lives, destroying property and pushing Californians to seek a solution to the problem.

It was believed that fire suppression was increasing the intensity of the shrub-land fires in the same manner that it had increased the intensity of forest fires, by allowing the buildup of fuel. According to the study authors this is not true.

"In contrast to coniferous forests, where fire suppression has indeed led to hazardous accumulation of fuel, and the potential for unnatural catastrophic fires, fire suppression in the brush lands of southern and central-coastal California has not altered the natural fire cycle," said USGS scientist Jon Keeley.

Keeley and colleages C.J. Fotheringham of California State University, Los Angeles, and Marco Morais, formerly of Santa Monica Mountains National recreation Area, explained their study in a recent issue of the journal Science.

To determine the role of fire suppression in shrub-land wildfires, Keeley and colleagues investigated historical changes in fire regimes from the 19th century onward, by using the recently available California Statewide Fire History Database. The database contains records from the California Department of Forestry, U.S. Forest Service and other county records.

They analyzed counties dominated by shrub lands subject to periodic high-intensity wildfires from north to south, Monterey, San Luis Obispo, Santa Barbara, Ventura, Los Angeles, San Bernardino, Riverside, Orange and San Diego counties.

There probe revealed several important facts:

• Not only has the number of fires per decade increased, but also during the period of the study, no significant decline has occurred in the area burned.
• The number of fires and area burned increases as population density increases.
• Very large fires have been reported since the start of record keeping in 1878 and that there has been no increase in the average size of wildfires. ("Indeed," said Keeley, "the average wildfire size has significantly declined in four counties.")

Most wildfire in this region are carried by the Santa Ana winds, said Keeley, which occur in the fall during periods of low humidity. The winds often exceed 60 mph. Such wildfires burn through both young and old age classes of shrub land. Attempts to alter vegetation age structure across large landscapes in the hope of managing these wind-driven fires is unlikely to stop catastrophic fires, he said.

Keeley and his colleagues conclude that wildfire management should focus on strategic locations instead of on the chaparral landscape at large.

Intensive management, said Keeley and his colleagues, should occur at buffer zones where urban lands and wildlands meet. They said that buffer zones should be selected based on the landscape features that the worst wildfires predictably follow.

They warn, however, that even with such management, ecological impacts may be enormous because of the already-extensive size of the still-growing urban-wildland buffer zones.

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