Fire severity is a measure of the physical change in an area caused by burning (Sousa 1984). Although fire intensity is a key component of burn severity, these are two distinct features of fire; the terms are often incorrectly interchanged.
Used correctly, fire intensity refers to the rate at which a fire produces heat at the flaming front and should be expressed in terms of temperature or heat yield. Fire severity, on the other hand, describes the immediate effects of fire on vegetation, litter, or soils. It is most commonly used to describe fire's effects on the primary tree cover. Unlike fire intensity, fire severity "cannot be expressed as a single quantitative measure that relates to resource impact" (Robichaud et al. 2000). Instead, fires are typically ranked from low to high severity based on the postfire appearance of soil, litter, vegetation, or other resource of interest (Robichaud et al. 2000).
Burn severity depends not only on the amount of heat generated along the flaming front of a fire (i.e., intensity) but also on the duration of the burn. Duration is a function of the fire's rate of spread and subsequent smoldering time. Both depend on weather conditions and the nature of the forest fuels. Rate of spread is additionally influenced by topography and wind speed. A ground fire smoldering in level terrain, for instance, may travel only one foot in a week. At the other extreme, a wind-driven crown fire can race through 15 miles of forest in just one hour (Pyne 1982).
While a fast-moving, wind-driven fire may be intense, a long-lasting fire that just creeps along in the forest underbrush could transfer more total heat to plant tissue or soil. In this way, a slow-moving, low-intensity fire could have much more severe and complex effects on something like forest soil than a faster-moving, higher-intensity fire in the same vegetation. For this reason, the terms fire intensity and fire severity are not synonymous and interchangeable (Hartford and Frandsen 1991).
Hartford, R. A., and W. H. Frandsen. 1991. When it's hot, it's hot . . . or maybe it's not! (Surface flaming may not portend extensive soil heating). International Journal of Wildland Fire 2:139-144.
Pyne, S. J. 1982. Fire in America: a cultural history of wildland and rural fire. Princeton University Press, Princeton, New Jersey, USA.
Robichaud, P. R., J. L. Beyers, and D. G. Neary. 2000. Evaluating the effectiveness of postfire rehabilitation treatments. USDA Forest Service, Rocky Mountain Research Station, General Technical Report, RMRS-GTR-63. Available online at http://www.fs.fed.us/rm/pubs/rmrs_gtr63.pdf.
Sousa, W. P. 1984. The role of disturbance in natural communities. Annual Review of Ecology and Systematics 15:353-391.