n. The effect of a fire on ecosystem properties, usually defined by the degree of soil heating or mortality of vegetation.
The severity of a fire depends on the fire intensity and the degree to which ecosystem properties are fire resistant. For example, a fire of exactly the same fireline intensity might kill thin-barked trees but have little effect on thick-barked trees. Therefore, fire severity is, in part, a function of the ecosystem being burned and is not simply indexed from fireline intensity. If a fire has a long residence time, fire severity will usually increase.
Forest ecologists define severity by the degree of overstory plant mortality. Although the thresholds are subjective, in general, overstory mortality below approximately 30 percent is considered low severity, 30 to 70 percent is considered moderate severity, and greater than 70 percent is considered high severity. See Agee (1993) for further discussion. Soil burn severity is a category of fire effects related to the change in soil properties and is defined separately.