FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

fires typically result in soil surface temperatures from 212 to 730 F (100 to 388 C)
although extremes to 1260 F (682 C) have been reported. The highest surface
temperatures are probably associated with local accumulations of loosely arranged
litter and intense winds created by the fire (Wright and Bailey 1982). The greatest
subsurface heating likely occurs where thick, dry litter layers are consumed
beneath shrubs and isolated trees. The soil heat pulse, including both amount and
duration (DeBano 1979), is instrumental in eventual effects of fire on plants (see
Chapter VI.B.1.c., and VI.B.2.c., this Guide) and in physical, chemical, and
biological effects on soils.
Less is known about heat effects on wetland soils. Due to the high water content of
wetland soils, penetration of heat generated by a surface fire can be significantly
less than in mineral soils. Since many wetland soils are composed of significant
amounts of organic materials, and organic matter has a lower thermal diffusivity
than mineral soil, penetration of heat can be further reduced. However, organic soil
layers can become dry enough to burn. Significant amounts of heat can be
generated when organic soils burn, particularly in drought situations when the fire
burns deeply into organic layers.
(2) Postfire temperature increases. Soil temperature may increase after a fire
because of the removal of vegetative cover, consumption of fuels, thinning or
removal of the litter and/or duff layer, and the enhanced "black body" thermal
characteristics of charred material on the surface. This is of great significance in
Alaska where permafrost (permanently frozen soil) is present. The soil layer above
the permafrost thaws each summer, and is called the "active layer." Soil
temperatures usually increase after a fire because fire removes the overstory
vegetation, blackens the surface, and consumes some of the layer of moss,
lichens, and semi-decomposed organic matter that insulated the soil from summer
warmth. Soil temperatures were 9 to 11 F (5 to 6 C) greater at depths of 4 to 20
inches (10 to 51 centimeters) after fire in a black spruce/feathermoss stand in
interior Alaska (Viereck and Foote 1979). Eight years after this fire, the depth of
the active layer had increased from about 18 inches to 72 inches (46 to 183
centimeters) (Viereck and Schandelmeier 1980). The depth of the active layer
eventually stabilizes, and then decreases to its original thickness. The length of
time before this occurs depends upon the rate at which new vegetation grows and
shades the soil surface, and how long it takes for a soil organic layer to develop
that has the same insulating properties as the organic layer that was removed by
the fire.
Under similar moisture regimes, warmer soils increase the rate of decomposition,
and nutrient availability to postfire vegetation. Within physiological limits, higher
soil temperatures also improve growing conditions for plants. In Alaska, deeper
annual soil thawing increases the depth of soil available for rooting. This makes
additional nutrients, especially nitrogen, available to plants, simply because they
are not in frozen soils (Heilman 1966; 1968). Postfire vegetation productivity
generally increases significantly after fire on permafrost sites (Viereck and
Schandelmeier 1980), although the duration of this effect is undocumented.