Comparison of Fire Regime Classification Systems

“Fire regime” refers to the nature of fire occurring over long periods and the prominent immediate effects of fire that generally characterize an ecosystem. Descriptions of fire regimes are general and broad because of the enormous variability of fire over time and space (Whelan 1995). Classification of fire regimes into distinct categories faces the same difficulties and a dilemma that underlie any ecological classification. One difficulty is that putting boundaries around segments of biological processes that vary continuously involves some degree of arbitrariness. The dilemma is that for a classification to be useful to managers it must be practical and easily communicated, thus free of complexity. Yet to accurately reflect the nature of a biological process, such as response to fire, it must account for a complexity of interacting variables. A tradeoff between practicality and accuracy or between simplicity and complexity is required. The fire regime concept brings a degree of order to a complicated body of fire behavior and fire ecology knowledge. It provides a simplifying means of communicating about the role of fire among technical as well as nontechnical audiences.

Classifications of fire regimes can be based on the characteristics of the fire itself or on the effects produced by the fire (Agee 1993). Fire regimes have been described by factors such as fire frequency, fire periodicity, fire intensity, size of fire, pattern on the landscape, season of burn, and depth of burn (Kilgore 1981). The detail of a classification determines its best use. The more detailed classifications are primarily useful to ecologists and fire specialists attempting to describe and understand the more intricate aspects of fire. The simpler classifications are more useful for broadscale assessments and for explaining the role of fire to nontechnical audiences.

Early Fire Regime Classification Systems

Heinselman (1978) and Kilgore (1981) produced the first classifications of fire regimes directed at forests. Two factors, fire frequency and intensity, formed the basis for their commonly referenced fire regime classifications (Figure: Fire regime classification systems). A difficulty with fire intensity is that a wide range of intensities, including crown fire and surface fire, can cause stand-replacement because mortality to aboveground vegetation is complete or nearly complete. Fire intensity relates only generally to fire severity. Severity of fire reflects

1. the immediate or primary effects of fire that result from intensity of the propagating fire front and
2. heat released during total fuel consumption.

Plant mortality and removal of organic matter are the primary fire effects. Kilgore emphasized fire severity in his modification of Heinselman’s fire regimes by referring to mortality of the primary tree cover as stand-replacement.

Recent Fire Regime Classification Systems

Two recent fire regime classifications have proven useful for mapping extensive areas of forest, shrubland, and grassland vegetation at 1 km resolution. Morgan and others (1998) mapped historical and current fire regimes in the Interior Columbia River Basin based on four fire severity and five fire frequency classes. Hardy and others (1998) mapped fire regimes of the Western United States using fire severity and fire frequency combined into five classes. They keyed the fire regime classes to spectral images and biophysical data including elevation, hydrologic units, Kuchler’s vegetation types, and Bailey’s (1995) sections. Results were used to prioritize allocation of funds and resources as part of a national strategy for prescribed fire. For example, high priority for restoration using prescribed fire was assigned to areas where current and historical fire regimes have departed significantly such as in the ponderosa pine type.

Readers who wish to view a more complex ecological classification are referred to the detailed classification of fire regimes developed by Frost (1998). It incorporates periodicity of the fire cycle, primary season of burn, fire frequency, and fire effects on vegetation. It is very sensitive to fire frequency and its effects on understory herbaceous species. Sensitivity to frequency is provided by recognizing four frequency classes between 0 and 25 years. This separates some eastern and western vegetation into different fire regime types.

Fire Regime Classification Used in Encyclopedia of Southern Fire Science

The fire regime classification used in this encyclopedia follows that of Brown and Smith (2000) and is based on fire severity. Characteristic fire frequencies are reported but not combined with fire severity into classes. Use of fire severity as the key component for describing fire regimes is appealing because it relates directly to the effects of disturbance, especially on survival and structure of the dominant vegetation. It is intended for broadscale applications and for communication about fire’s role among resource managers and others interested in natural resources.

Detailed information available about past fire regimes is based mostly on biophysical evidence, written records, and oral reports that encompass the period from about 1500 to late 1800, a time before extensive settlement by European-Americans in many parts of North America, before intense conversion of wildlands for agricultural and other purposes, and before fire suppression effectively reduced fire frequency in many areas. In this volume, we refer to the fire regimes of the past several centuries as “presettlement” fire regimes.

The following describes the fire regime types are used in Brown and Smith (2000) and in this encyclopedia:

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

Using this system, all ecosystem types other than forest and woodland are considered to have stand-replacement fire regimes because most fires in those ecosystem types either kill or remove most of the aboveground dominant vegetation, altering the aboveground structure substantially. Most belowground plant parts survive, allowing species that sprout to recover rapidly. This is true of tundra, grasslands, and many shrubland ecosystems. Morgan and others (1998) consider grasslands to have “nonlethal” fire regimes based on the criterion that structure and composition of vegetation is similar to the preburn condition within 3 years after a burn (Figure: Fire regime classification systems). Because fire radically alters the structure of the dominant vegetation for at least a short time, however, we consider grassland ecosystems to have stand-replacement fire regimes. Because grassland, tundra, and many shrublands are stand-replacement fire regime types, a more interesting aspect of fire regimes in these ecosystems is fire frequency, which can vary substantially and have a major influence on vegetation composition and structure.

The understory and mixed severity fire regimes apply only to forest and woodland vegetation types. The mixed severity fire regime can arise in three ways:

1. Many trees are killed by mostly surface fire but many survive, usually of fire resistant species and relatively large size. This type of fire regime was described as the “moderate severity” regime by Agee (1993) and Heyerdal (1997).
2. Severity within individual fires varies between understory burning and stand-replacement, which creates a fine-grained pattern of young and older trees. This kind of fire regime has not been recognized in previous classifications. It probably occurs because of fluctuations in weather during fires, diurnal changes in burning conditions, and variation in topography, fuels, and stand structure within burns. Highly dissected terrain is conducive to this fire regime. In actuality, a blend of these two mixed severity types probably occurs.
3. Fire severity varies over time with individual fires alternating between understory burns and stand-replacement. Kilgore (1981) described this as the “variable” regime and applied it to redwood forests. It also fits red pine forests.

Comparison of Classification Systems

The fire regime types used in Brown and Smith (2000) and in this encyclopedia were simplified from the classifications reported by Heinselman (1978) and Kilgore (1981). They are identical to the fire severity component utilized by Hardy and others (1998) except “understory” is used instead of “nonlethal” to depict that fire regime. The term understory is chosen as a fire regime name because the term nonlethal is more easily misinterpreted when considering forest and nonforest ecosystems. This fire regime classification is also similar to that reported by Morgan and others (1998). To show how all of these classifications are related, equivalent or similar fire regime types are connected by lines in Figure: Fire regime classification systems. The primary ecological knowledge imparted by fire regime types is whether fires leave the dominant aboveground vegetation standing and alive or result in stand-replacement. To reflect this, the fire regime types used in this encyclopedia are characterized as nonlethal understory fire, stand-replacement fire, and mixed severity fire.

Fire severity is defined by what happens on areas that actually burned. In reality, unburned islands and patches of variable size and shape occur within the perimeter of fires. In studies of historical fire, it is difficult to separate burned from unburned patches. Thus, in applying the classifications, some nonlethal effects of fire can be attributed to unburned patches.

Forests of all types can be grouped into the understory, mixed, or stand-replacement fire regimes, which correspond to low, moderate, and high fire severity types described by Agee (1993). Some forest types occurring over a wide range of environmental conditions can fall into two fire regime classes. For example, most lodgepole pine and jack pine forests were characterized by stand-replacement fire. But some of the forests, typically on drier sites, reflect a mixed fire regime history. Evidence (Arno and others [in press]; Frost 1998) indicates that the mixed fire regime type was more prevalent than previously thought especially in coniferous forests. As fire moves across the landscape its behavior and effects can change dramatically due to variability in stand structure, fuels, topography, and changing weather elements. This can result in highly variable tree mortality and survival patterns within a fire’s boundary. Generally, the severity and intensity of fire are inversely related to fire frequency (Swetnam 1993). For example, stand-replacement fires tend to occur in forests with low frequency, and understory to mixed severity fires tend to occur in forests with high fire frequency. Considerable variability exists within this generalization.

The system used in this encyclopedia considers grasslands and tundra fire regimes to be essentially all stand-replacement because the aboveground dominant vegetation is either killed or removed by fire. Also, many shrubland ecosystems are stand-replacement fire regime types because the dominant shrub layer is usually killed back to growing points in or near the ground. Stand-replacement fire in grass and sedge dominated ecosystems may be either lethal or nonlethal to aboveground vegetation. It is nonlethal if vegetative parts have already cured and exist as dead fuel, which is often the case in Western United States. But it is lethal if some of the aboveground grasses and sedges are living and are killed by fire, as is commonly the case in marshes of eastern North America and in tundra. Fire is usually nonlethal to belowground plant parts allowing species that sprout to recover rapidly.