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USGS Research Indicates Fire Suppression and Fuel Buildup are Not Responsible for Shrubland Fires in Southern California
Released: 10/30/2003

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Gloria Maender 1-click interview
Phone: 520-670-5596

Jon Keeley
Phone: 559-565-3170



With the loss of life and property being experienced in the fires that have burned out of control in four Southern California counties, research by the U.S. Geological Survey on fire in the region reveals that to effectively manage fires to help prevent loss of life and property in Southern California shrublands, it is essential to understand the natural role of fire in chaparral ecosystems.

Large, high-intensity fires sweep the landscape in this region each year, threatening lives and homes, as is occurring with such devastation in this area. Ecologists have long known that chaparral ecosystems burn extensively and often, and that much of the dominant vegetation in these systems is highly adapted to a fire-prone environment. Many native plants here have seeds that require fire to germinate, or need the kind of disturbed habitat fires leave behind to grow. It was long thought that fire suppression played the same role in chaparral shrublands as it has in forests, creating a build-up of fuels that can eventually lead to more destructive fires.

"Past fire suppression is not to blame for causing large shrubland wildfires, nor has it proven effective in halting them," said Dr. Jon Keeley, a USGS fire researcher who studies both southern California shrublands and Sierra Nevada forests. "Under Santa Ana conditions, fires carry through all chaparral regardless of age class. Therefore, prescribed burning programs over large areas to remove old stands and maintain young growth as bands of firebreaks resistant to ignition are futile at stopping these wildfires."

In recent studies Keeley and his colleague, C. J. Fotheringham of the University of California, Los Angeles, analyzed historical records for counties dominated by shrublands subject to periodic high-intensity wildfires, from Monterey County in the north to San Diego County in the south. They found that although fire suppression is critical to protect homes, buildings and other structures, fire suppression does not prevent large wildland fires in southern California shrublands because these fires usually occur with powerful Santa Ana winds that blow at high speeds from the desert to the coast. In the present fire, hot Santa Ana winds of over 60 mph greatly increased the intensity and the movement of the fire. These winds occur each autumn, at the time when natural fuels are driest.

A close analysis of state fire records reveals the real story, said Keeley. Since 1910, chaparral fires have become more frequent as the human population has grown but fire size has not increased. The researchers found that large, intense fires were equally common in the years before widespread fire suppression as today, and do not appear to be the result of fuels build-up. In this highly fire-prone ecosystem, suppression efforts appear not to have greatly altered patterns of fire incidence. Keeley notes that the greater financial cost of fires today is most likely the result of constant urban expansion into areas subject to frequent burning.

For example, written documents reveal that during the 19th century human settlement of southern California altered the fire regime of coastal California by increasing the fire frequency. This was an era of very limited fire suppression, and yet like today, large crown fires covering tens of thousands of acres were not uncommon. One of the largest fires in Los Angeles County (60,000 acres) occurred in 1878, and the largest fire in Orange County’s history, in 1889, was over half a million acres.

The main ignition source of chaparral wildfires under natural conditions is lightning, but lightning-ignited fires are of an order of magnitude fewer in coastal ranges than in interior ranges of California and much of the western United States, said Keeley. Keeley hypothesized that before the arrival of humans, the majority of area burned occurred at overlaps of summer and autumn weather events. Small lightning-ignited fires of summer occasionally persisted until the arrival of autumn Santa Ana conditions. Such fires then rapidly increased in size and might continue to burn until winter rains finally doused them.

Most fires in California shrublands are human-caused, and the beginnings of human influence on the natural fire regime date to pre-Columbian peoples, who used fire to convert the dense shrubland to a more open mosaic of shrubland and grassland, long before the arrival of Euro-Americans, said Keeley.

Fotheringham and Keeley noted that that throughout much of the shrubland landscape humans play a dominant role in promoting fires beyond what was likely the natural fire cycle. Future fire management, they said, needs to take a strategic approach to prefire fuel manipulations and move beyond evaluating effectiveness strictly in terms of area treated. Fire management should consider designing strategies tailored to different regions, as there are marked differences between the central coastal region and southern California in source of ignition, season of burning, and historical patterns of population growth and burning.

In terms of management implications, the fire researchers note that:

  • The contemporary fire regime in these shrublands mirrors the natural crown fire regime far more than is generally accepted and that catastrophic crown fires may be an inevitable feature of this landscape.
  • There may be little justification for using fire for resource benefit, since vast portions of shrubland landscape currently experience a higher-than-normal fire frequency.
  • While landscapes managed by rotational prescription burning may contribute to easier containment of fires burning under moderate weather conditions, they are of limited value during severe weather such as the Santa Ana winds causing such destruction to life and property now.
  • Limited and strategically placed prescription burns are the most cost-effective way to help prevent large catastrophic wildfires in southern California chaparral habitat.


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