Fire Intensity and Severity

Fire intensity describes the energy released from the fire or characteristics of the fire behavior such as flame length and rate of spread. Fire severity refers to the ecosystem impacts of a fire such as mortality of trees or loss in biodiversity.

A widely used measure of fire intensity is fireline intensity, which is the rate of heat transfer per unit length of the fire line (measured in kWm-1) and represents the radiant energy release in the flaming front. Fireline intensity is a good measure of how likely the fire is to propagate and how difficult it will be to stop, and thus it is a critical component of fire behavior models used to inform fire-suppression activities. Very seldom is fireline intensity actually measured, rather it is generally inferred based on flame length, which has been found to correlate with fireline intensity; however, this correlation has not been widely tested.

Fire severity measurements vary depending on the ecosystem. In forests, fire severity is measured in terms of tree mortality, canopy loss, or bole and crown scorch. These measures of fire severity are often interpreted as surrogate measures of fire intensity. It is assumed that these reflect the impact of the fire on ecosystem properties such as its ability to recover. Fire severity measures used in forests are not generally relevant to shrublands, where all aboveground biomass is killed in crown fires. In such crown fires ecosystem indicators of fire severity that may be useful measures of ecosystem impact are resprout-ing success and seed bank survivorship. However, the linkage between fire intensity, fire severity measures, and ecosystem impact are complex and largely unexplored. A number of studies have found that in California chaparral shrublands indicators of fire intensity are related to diminished cover in the first post-fire growing season; however, these effects were are short-lived. Also, while high fire severity may inhibit regeneration of some species, it is actually correlated with improved regeneration in other species. Fire severity is often a measure used in making assessments of watershed stability after fire, but the relationship between these parameters is not well established for many ecosystems.

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