From introduction: Western forests have burned for millennia with a wide range of frequencies, intensities, and extents. Some forests have fire-resistant trees and others do not. The combination of the phyiscal character of the fires themselves, together with the adaptations of the tree species to fire, have resulted in forest types that can be classified into natural fire regimes (Agee 1993). Three broad categories of fire severity may be defined, based on the physical characteristics of the fire and the fire adaptations of vegetation: low, mixed or moderate, and high. A low severity fire regime is one where the effect of the typical historical fire is benign. Fires are frequent (often <20 years), of low intensity, and the ecosystems have dominant vegetation well-adapted to survive fire. At the other end of the spectrum is the high severity fire regime, where fires are usually infrequent (often >100 years) but may be of high intensity: most of the vegetation is at least top-killed. In the middle is the mixed or moderate severity fire regime, where fires are intermediate frequency (25-100 years), range from low to high intensity, and have vegetation with a wide range of adaptations.

[This publication is referenced in the "Synthesis of knowledge of extreme fire behavior: volume I for fire managers" (Werth et al 2011).]