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Trends in Wildland Fire

Trends in Wildland Fire

Adobe PDF document Pre-settlement fire regimes and modern-era fire trends (11 pages) (6,076 KB)
Adobe PDF document Findings on predicting future fire threats (12 pages) (5,364 KB)

October 2003

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Over millennia, fire has played an integral role in regulating the spatial pattern, composition, and structure of California’s natural resources. With its Mediterranean climate, productive soils, and frequent ignitions from lightning and Native American peoples, fire has been an endemic force shaping the landscapes of the State. Many areas of the State have evolved under the natural selection pressure of frequent and relatively low intensity fires. Other areas have been subjected to less frequent, but higher intensity fires. From coastal grasslands to sub-alpine forests to the Mojave Desert, fire has been an active ecological agent in almost all vegetated areas.

The role of fires and vegetation is one part of the focus of Criterion 3 of the Montreal Process, “Maintenance of forest and range ecosystem health and vitality.” Indicator 15 specifically focuses on identifying and measuring agents, such as fire, that have affected ecosystems beyond the historic range of variability. To help measure this indicator this section reviews several components of wildfire in California including:

  • Pre-settlement fire regimes; and

  • Findings on modern-era fire trends including trends in area burned and future wildland fire threats.

In another section we compare these findings and integrate them into a measure of modern-era fire divergence from the natural range of variation. This assessment, termed "Condition Class", assigns categories of broad ecosystem-level sensitivity to fire based on habitat types (see Wildfire Risks to Assets).

Photo of CDF firefighter

Forest and range area and percent affected by processes or agents beyond the range of historic variation (Montreal Process indicator 15)
  • Historically, fire has shaped ecosystems in California. This can be seen by interpreting fire occurrence and effects during discrete periods where human influences have managed both fire and fire environments differently. There are three periods: 1) prior to European settlement (before 1700); 2) the settlement period (1700-1920); and 3) the suppression era (1920-present).

  • In fire-adapted ecosystems, natural (pre-settlement) fire regimes provide long-term ecological stability that annually involved millions of acres of wildfire. Many California ecosystems depend on a particular fire regime for long-term resilience. Disruption of these natural cycles often has significant ecological ramifications regarding vegetation stability and ecosystem health.

  • While fire is often described as a destructive agent, the ecological role that fire plays on vegetation is often better characterized as fire-maintained or fire-recycled, rather than fire-destroyed. In areas where the regime indicates severe stand-replacing types of fires, often these fires served as forces of renewal for mature vegetation that required fire to restore vegetation life cycles.

  • Natural fire regimes that existed prior to European settlement in California involved a wide range of fire frequencies and effects on ecosystems; roughly one-third of the State supported fires every 35 years or less. Pre-European settlement fire patterns resulted in many millions of acres burning each year, with fire acting as a major cause of ecosystem change.

  • Eighty-seven percent of the State's wildlands supported mixed or low-severity fire regimes; only 13 percent supported high severity fires that would typically kill all the dominant vegetation present.

  • The settlement period was marked by increasing influence of changing land use, first from Spanish missionaries then by miners, which brought widespread changes to the fire environment. Subsequently, ranching, open range grazing, farming, timber/fuelwood harvesting, and residential and commercial land development all placed increased demands on land and resources, and led to significant changes in ignition patterns and to the vegetation landscape (i.e., fuels) with which fire interacts.

  • The modern-era has seen continued modification of land use with the added influence of active and highly effective fire suppression systems.

  • In the modern-era, statewide fire frequency is much lower than before the period of European settlement. Over the last two decades, California has averaged 250,000 acres burned annually, only a fraction of the several millions of acres that burned under the pre-settlement regimes. Land-uses such as agriculture and urbanization have reduced the amount of flammable vegetation, and most fires are effectively suppressed to protect resources, commodities, and people.

  • Area burned in wildfires varies greatly year to year, with climate driving much of the variability. When viewed statewide, the temporal variation masks any possible trends in total acreage, although there has been an apparent increase in high fire years (total area burned greater than 500,000 acres) since 1985 (see figure).

  • Annual area burned*, statewide, 1950-2000

    chart showing area burned

    *fires over 300 acres in area

  • Modern era fire frequency differs greatly across the State. Of the 80 million acres (80 percent) of California where the Fire and Resource Assessment Program has sufficient data to calculate fire frequency, eight percent has a very high frequency of fire, 22 percent has a high frequency, and the majority (70 percent) has a moderate frequency of fire.

  • Trends in wildfire across vegetation types indicate the heavy influence of vegetation characteristics on expected fire frequency. Using the decadal averages of percentage of area burned in each broad vegetation life form, brushlands have burned the most frequently, remaining consistent over the last five decades.

  • When all vegetation types are combined and percent of area burned annually are compared between broad ownership groups, lands in public ownership began burning more frequently than private lands around 1970, and this has continued to the present. The trend over time for percent of area burned on public lands appears to be increasing.

  • Expected fire frequency using the fire rotation calculation, varies dramatically across the State. While the majority of California (70 percent of the area mapped in the analysis) shows only a low expected fire frequency, where fire rotation is greater than 300 years, certain areas of the State, such as much of the southern California brush and woodland areas, and some north Sierra foothill zones are in the high fire rotation class, indicating these areas have the highest expected frequency of fire in the future (see table).

  • Area of fire rotation classes

    Rotation class Acres Percentage of Statewide Percentage of mapped area
    Low (>300 years) 54,242,542 54 70
    Medium (100-300 years) 17,137,175 17 22
    High (<100 years) 6,411,106 7 8
    Non wildland/not mapped 22,209,176 22 n/a

  • While modern fire frequency is much lower in most areas than prior to European settlement, much of California's wildlands support conditions of high or very high potential fire behavior if fires are not aggressively suppressed. Of the 85 million acres of vegetated areas statewide, 51 percent have fuel and slope conditions that would support high or very high fire behavior when burned under typical severe weather conditions (see table). Fires that burn in these areas under hot, dry, and windy conditions are difficult to control even by the world's most comprehensive wildland fire protection system.

  • Area of potential fire behavior

    Rank Acres Percentage of State Percentage of State
    (non-wildland excluded)
    Moderate 41,912,451 41 49
    High 31,475,139 31 36
    Very High 11,994,298 13 15
    Non-wildland 15,582,152 15 --

  • Extensive areas of very high potential fire behavior are adjacent to areas of population centers such as the Los Angeles basin, and the western flank of the Sierra Nevada forms a continuous belt of dangerous fuels.

  • A composite index of fire threat that reflects both the chance of wildfires (expected fire frequency) and the prevailing potential fire behavior finds 35 percent of the area mapped in the High threat class, 18 percent in the Very High class, and two percent in the Extreme threat class (see table). Many of the locations of greatest concern for fire threats (High, Very High or Extreme) are also located in the Los Angeles basin, and the western flank of the Sierra Nevada (see figure).

  • Statewide fire threat

    Fire threat Acres Percentage of State Percentage of area mapped
    (non-wildland excluded)
    Moderate 36,942,600 37 45
    High 30,370,766 30 35
    Very High 15,769,155 16 18
    Extreme 2,249,365 2 2
    Not mapped 15,582,151 15 --

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