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Fire Regimes

Authored By: L. J. Lyon, J. K. Brown, M. H. Huff

Fire varies in its frequency, season, size, and immediate effects, but general patterns occur over long periods. These patterns describe fire regimes. The practice of organizing biotic information around fire regimes originated in North America around 1980 (Heinselman 1978, 1981; Kilgore 1981; Sando 1978). Descriptions of fire regimes are general because of fire’s tremendous variability over time and space (Whelan 1995). Nevertheless, the fire regime is a useful concept because it brings a degree of order to a complicated body of knowledge.

The fire regimes that have influenced North American ecosystems in an evolutionary sense are those of pre-Columbian times (prior to 1500), before diseases introduced by European explorers began to decimate populations of Native Americans (see Kay 1995). While knowledge of pre-Columbian fire regimes would be useful for understanding ecosystem patterns and processes today, little information is available from that era. Detailed information available about past fire regimes is mostly based on biophysical evidence, written records, and oral reports that encompass the time from about 1500 to the mid-to late-1800s. This was a time before extensive settlement by European Americans in most parts of North America, before extensive conversion of wildlands for agricultural and other purposes, and before fire suppression effectively reduced fire frequency in many areas. In this encyclopedia, the fire regimes of the past several centuries are referred to as “pre- settlement ” fire regimes. Fire frequency and severity form the basis for the commonly referenced fire regime classifications described by Heinselman (1978) and Kilgore (1981).

Two concepts, fire return interval and fire rotation, describe the frequency with which fires occur on a landscape. Mean fire return interval is the average number of years between fires at given location. Fire rotation, called by some authors the fire cycle, is the number of years that would be required to completely burn over a given area. Fire severity describes the immediate effects of fire, which result from the rate of heat release in the fire’s flaming front and the total heat released during burning. Fire severity determines in large part the mortality of dominant vegetation and changes in the aboveground structure of the plant community, so Kilgore (1981) refers to severe fires in forests as “stand-replacement ” fires. The concept of stand-replacement by fire applies to non-forest as well as forest areas. Fires in vegetation types such as prairie, tundra, and savannah are essentially stand-replacing because the aboveground parts of dominant vegetation are killed (and often consumed) by fire. Most shrubland ecosystems also have stand-replacement fire regimes because fire usually kills the aboveground parts of shrubs.

The following four kinds of fire regime are used in this encyclopedia (following Brown and Smith 2000):

  1. Understory fire regime (applies to forest and woodland vegetation types)—Fires are generally not lethal to the dominant vegetation and do not substantially change the structure of the dominant vegetation. Approximately 80%or more of the aboveground dominant vegetation survives fires.
  2. Stand-replacement regime (applies to forests, shrublands, and grasslands)—Fires kill or top-kill aboveground parts of the dominant vegetation, changing the aboveground structure substantially. Approximately 80% or more of the aboveground dominant vegetation is either consumed or killed as a result of fires.
  3. Mixed-severity regime (applies to forests and woodlands)—Severity of fire either causes selective mortality in dominant vegetation, depending on different species’ susceptibility to fire, or varies between understory and stand-replacement.
  4. Nonfire regime —Little or no occurrence of natural fire (not discussed in this encyclopedia).

For more information on fire regimes and a comparison of this fire regime classification with others, see: Comparison of regime classification systems.


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