FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

structures.
(3) The amount of heating that results from combustion in the flaming front of a
prescribed fire can be regulated. Ignition methods and techniques must be selected
with consideration for fuel conditions, weather, and slope steepness and concavity.
(a) The width of the flaming zone can be manipulated by controlling the number of lines
of strip headfires that are ignited at once (Norum 1987), and the spacing between
them.
(b) Regulating the interval between lines of strip headfires controls flame length,
because the shorter the interval between lines, the shorter the flames (ibid.)
(c) Use of rapid ignition techniques can greatly increase the rate of heat release and
decrease the duration.
b. Crown scorch height.
(1) The height to which tree crowns are being scorched is often not obvious during
ignition of a prescribed fire.
(2) Scorch height can be estimated from current weather, and observed flame lengths,
using the graphs in Albini (1976, pages 63 to 66).
(3) If scorch height is too high, then ignition can be altered to lower flame lengths, or
the fire may be curtailed until more moderate burning conditions occur.
(4) Too high a scorch height can indicate that the fire prescription may require
modification to reduce scorch heights, such as by prescribing increased fuel moistures,
or lower air temperatures when the fire is ignited.
c. Mortality of crowns.
(1) Dormant buds have varying degrees of sensitivity to fire heat. Sensitivity relates to
size, the presence of protective bud scales or needles, and whether they are
physiologically active or dormant.
(2) Foliage flammability and sensitivity to scorching temperatures varies seasonally,
especially because of changes in foliar moisture content.
(3) Foliage flammability varies by species according to branch density, the presence of
lichens, presence of flammable compounds, retention of ephemeral or evergreen
leaves or needles, and the proximity of the crown base to the surface of the ground.
d. Mortality of tree stems and cambium.
(1) Thick barked species are more resistant to fire heat than thin barked species.