Fire characteristics

Fire behavior is characterized using one or more metrics of fire intensity, which are defined by the physical characteristics of the fire itself. These metrics include spread rate, flame length, fireline intensity (heat production per unit length of the flaming front per second), and heat per unit area (total heat produced during the residence time of the flaming zone). As spread rate, flame length, and fireline intensity increase, fire suppression becomes increasingly difficult, and the potential for extreme fire behavior such as spotting, fire whorls, and crown fire increases. Fire severity, defined as the effects of fire on vegetation, soils, and other ecosystem properties, is a function of both fire intensity and the physical and ecological characteristics of the site. Longer flame lengths emit heat higher in the forest canopy and increase the potential for crown scorch and crown fire initiation, whereas greater heat per unit areas results in a larger heat pulse and greater impact on below-ground properties. These elements of fire behavior will not always respond in a similar fashion to changes in fuels. For example, a fuelbed composed of dead grasses may have a relatively high spread rate but release only a small amount of heat per unit area. In contrast, a fire burning under similar weather conditions in fuels dominated by large dead wood will have a slower spread rate, but longer flame lengths and greater heat output per unit area (Pyne et al. 1996).

Predicting the effects of fuel treatments on fire behavior is challenging in part because the influence of any single fuel variable depends other fuelbed characteristics. For example, the effects of reducing fuel loading are contingent upon changes in fuelbed depth. Each fuelbed has an optimum packing ratio that is a function of the fuel size distribution (Burgan and Rothermel 1984). If depth remains relatively constant and packing ratio decreases below the optimum level as a result lower fuel loads, reductions in the rate of fuel consumption and the preheating of adjacent fuel particles will lead to lower spread rates, flame lengths, and fireline intensities (Burgan 1987). In contrast, reduced loading of live fuels and large woody fuels may eliminate a significant heat sink and lead to increased fire intensity in some situations. Decreasing fuel particle size increases the surface to volume ratio of fuels, which increases the rate of combustion, decreases the need for preheating, and generally leads to higher spread rates, flame length, and fireline intensity. However, fine particles are more easily compacted than large particles, and fire intensity may be reduced if the packing ratio increases above the optimum level for a particular fuelbed.

Another challenge in understanding the effects of fuel treatments on fire behavior is that the behavior observed in a particular fuelbed will vary as a function of weather. At any given time, only a portion of the total fuel load will be available fuels that can influence the behavior and effects of a fire. The amount of available fuel is influenced by fuel size, spatial arrangement, and fuel moisture, which vary over time with precipitation and evaporation. Different types of fuels (large versus small, live versus dead) respond to the environment at different temporal scales. Thus, it is important to understand how fuel treatments influence fire behavior over the full range of weather conditions likely to be observed at a site, ranging from moderate conditions suitable for prescribed burning to extreme conditions where the potential for large, destructive wildfires is highest. For example, when live and dead fuel moistures are relatively low, a shrub-dominated fuelbed will have much higher spread rates than compacted hardwood litter. When fuel moisture is high, hardwood litter has a higher spread rate, although spread rates in both fuel types are relatively low (Pyne et al. 1996). It is also important to recognize that vegetation also influences microclimate within a stand. Thus, treatments that modify fuels can also affect patterns of wind and fuel moisture within the fuelbed.

Literature cited
Burgan, R.E. 1987. Concepts and interpreted examples in advanced fuel modeling. Gen. Tech. Rep. INT-238. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 40 p.

Burgan, R.E.; Rothermel, R.C. 1984. BEHAVE: Fire behavior prediction and fuel modeling system—FUEL subsystem. Gen. Tech. Rep. INT-167. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station.

Pyne, S.J.; Andrews, P.L.; Laven, R.D. 1996. Introduction to wildland fire. New York: John Wiley & Sons. 769 p.

This web page was last updated on 14 March 2007 by D. Marshall, UGA

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