FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

of duff and organic layers, and the moisture content of heavier fuels and duff. We
cannot presently predict the duration of time during which these combustion
phases will occur.
c. Total heat pulse to the site. Total heat pulse considers not only the heat
released in flames but also that released by smoldering and glowing combustion.
Heat per unit area only includes the amount of heat that is released in the flaming
front. Extensive studies in physics modelling is currently underway at the
Intermountain Fire Sciences Lab which may provide means to calculate the total
heat pulse to the site.
d. Soil heating. Most heat produced by the flaming front moves upward.
Downward movement of heat from flames cannot presently be predicted, but it is
not believed to be a significant source of subsurface heat. Most soil heating
results from long term fuel, duff, and organic layer burnout. Neither this heat, nor
its penetration into soil layers, has been modelled.
e. Burn severity. Burn severity is a term that qualitatively describes classes of
surface fuel and duff consumption. Large diameter down, dead woody fuels and
organic soil horizons are consumed during long-term, smoldering and glowing
combustion. The amount of duff or organic layer reduction is also called depth of
burn, or ground char (Ryan and Noste 1985). Because the amount and duration
of subsurface heating can be inferred from burn severity, this variable can be
related to fire effects on plants and soils. Factors regulating fuel and duff
consumption, and thus burn severity, are discussed in Chapter III.B.2. and 3. The
relationship between burn severity and its effects on plants is described in
Chapter VI.B.1.c. and VI.B.2.c.
(1) Descriptive classes. An example of a set of burn severity classes is given
below. Agency specific guidelines for assessing burn severity are described in
USDI-NPS (1992).
(a) Unburned.
(b) Scorched. Foliage is yellow; litter and surface vegetation are barely burned or
singed.
(c) Low severity. Small diameter woody debris is consumed; some small twigs
may remain. Leaf litter may be charred or consumed, and the surface of the duff
may be charred. Original forms of surface materials, such as needle litter or
lichens may be visible; essentially no soil heating occurs.
(d) Moderate severity. Foliage, twigs, and the litter layer are consumed. The duff
layer, rotten wood, and larger diameter woody debris is partially consumed; logs
may be deeply charred; shallow ash layer and burned roots and rhizomes are
present. Some heating of mineral soil may occur if the soil organic layer was thin.