n. A fire of low to moderate fireline intensity that has high-severity effects.
The phrase lethal underburn emphasizes the differences between fireline intensity and fire severity. Fireline intensity is a physical characteristic of a fire, expressed as energy yield per unit length of fireline per unit time. Flame length can be mathematically related to fireline intensity. An underburn implies that the flame length is relatively low, as the fire remains under the canopy of the forest. The effect of fire on an ecosystem depends on other factors in addition to fireline intensity: the vegetation that is burning, the amount of fuel consumption, and the residence time of the fire, for example.
A lethal underburn is one where, although flame length is relatively low, the severity of the fire is high. This could be the result of a long-smoldering fire in deep forest litter (exhibiting low fireline intensity but with a long residence time) or a vegetation type that is poorly adapted to even low-intensity fire, such as thin-barked species or immature trees of species that have thick bark when mature.