n. A fire that has strong ecosystem effects, such as complete canopy mortality or extensive soil heating.
Fire severity is a continuous variable ranging from low to high. It can be expressed at various scales, from fire affecting the individual tree to fires ranging up to 500,000 acres or more, such as the 2002 Biscuit Fire in Oregon and California. A high-severity fire occurs at the extreme end of this continuum and is expressed by complete fuel consumption and extensive soil heating, and usually more than 70 percent top-kill of vegetation. When patches of vegetation that have experienced high-severity fire are interspersed with patches that have experienced low severity in a single fire, the event may instead be considered a mixed-severity event.
High-severity fire is a common historical fire regime pattern in cold, dry forests such as lodgepole pine in the Rocky Mountains and wet coastal forests such as western hemlock. Fire return intervals in high-severity forest fire regimes often exceed several hundred years, but in chaparral ecosystems, high-severity fires have return intervals of 20 to 40 years. See DeBano and others 1998, Agee 1993, and Agee 1998 for further discussion.