The postfire appearance of the forest floor can help us gauge the amount of heat radiated downward during a burn and, by extension, the severity of a fire's effect on the soil (Wells et al. 1978). Soil color, for example, is a good indication of the amount and condition of the organic residues that remain after burning. Slightly decomposed organic matter will be a brownish color, while more thoroughly decomposed material will be black (Fisher and Binkley 2000). In areas with little or no organic material remaining post-fire, the soil may appear gray, white, or red, depending on both the type of rock from which the soil is derived and on the temperatures reached during fire.
In general, the more litter and soil organic material consumed by fire, the more severe the burn. In the sections that follow, we describe the immediate appearances and short-term repercussions of low-, moderate-, and high-severity fire in forest soils.
In general, a blackened soil surface immediately after fire indicates that the heat generated while burning was insufficient to completely consume the litter and other organic materials at the soil surface. For charring to occur, temperatures must have reached at least 212ºF, but if the materials were only partially consumed, it is unlikely that temperatures exceeded 480ºF at the soil surface for very long, if at all. Incomplete consumption of surface organic material furthermore indicates that temperatures probably did not exceed 250ºF in the mineral soil below. Some portion of the nutrients contained in the affected organic matter were probably converted to inorganic forms, which can be readily taken up by plants (DeBano et al. 1998). Materials heated to 400ºF or greater, however, probably lost nitrogen to the atmosphere. In addition, areas of extensive surface char are sure to support fewer microbes near the soil surface immediately after burning because few can survive soil temperatures greater than about 150ºF. Fires that cause these sorts of changes in the uppermost soil are considered low-severity burns.
Moderate-severity fires consume most of the organic matter at the soil surface, leaving mineral soil exposed but not visibly altered. These fires typically raise surface temperatures into the range 482-932oF. (The consumption of organic matter is complete at approximately 932oF). At temperatures above about 572oF, over half of the nitrogen within the affected organic material may be lost in gaseous form to the atmosphere. At temperatures above about 707oF, more than 25% of the sulfur (another important plant nutrient and soil acidifier) within the affected organic material may be lost in gaseous form to the atmosphere. Despite the high surface heating, temperatures are unlikely to exceed 390oF one inch into mineral soil. However, temperatures between 349-399oF can effectively distill organic matter into hydrocarbons that condense around soil particles, altering the physical structure of soil and diminishing soil permeability (DeBano 1976, Bitterroot National Forest 2000). Temperatures lethal to soil organisms can be reached as deep as two inches into mineral soil.
High-severity fires leave only white ash atop mineral soil that may appear red in color. To this effect, soil surface temperatures must exceed about 950 oF (Wells et al. 1978, Ulery and Graham 1993). In general, these fires cause changes similar to or more extreme than those caused by low and moderate severity fires, but at greater depths in the soil. Nearly all surface organic material is apt to be consumed, with a great deal of its constituent nitrogen and sulfur lost to the atmosphere (McNabb and Cromack 1990). Subsurface materials may be altered and microbes killed six inches into mineral soil (Wells et al. 1978, Bitterroot National Forest 2000). In the uppermost soil, phosphorus, potassium, magnesium, calcium, and manganese - all of which are important plant nutrients - are likely to be oxidized and thus made more readily available for plant use. Where temperatures exceed 1500oF, however, these fires can volatilize, or vaporize, many of these nutrient oxides, weld individual soil particles together, and cause clay particles in soil to disintegrate altogether.
Classifying large areas
The preceding criteria can be used to classify affected forest stands and can then be extrapolated to an entire fire complex. To account for spatial variation in soil heating within the area of interest, such classifications are based on the percentage of the total area that falls within each severity category (Hungerford 1991). For example, Wells and others (1978) suggest that an area be classed as having experienced a high-severity burn with respect to fire's effects on soils if more than 10% of the area merits a "high-severity" rating, 80% merits a "high-" or "moderate-severity" rating, and the remainder deserves a "low-severity" or "unburned" designation. An area should be classified as having experienced a moderate- or mixed-severity burn if less than 10% of the soil surface appears to merit a "high-severity" rating, with another 15% or more deserving a "moderate-severity" rating. And finally, an area should be classed as having experienced a low-severity burn if less than 2% merits a "high-severity" rating, less than 15% deserves a "moderate-severity" rating, and the rest rates as low-severity or is unburned.
Bitterroot National Forest. 2000. Bitterroot Fires 2000: An assessment of postfire conditions with recovery recommendations. USDA Forest Service, Bitterroot National Forest. Unpublished report online at http://www.fs.fed.us/r1/bitterroot/recovery/fires_2000-screen.pdf.
DeBano, L. F., S. M. Savage, and D. M. Hamilton. 1976. The transfer of heat and hydrophobic substances during burning. Soil Science Society of America Journal 40:779-782.
DeBano, L.F., D. G. Neary, and P. F. Ffolliott. 1998. Fire's Effects On Ecosystems. John Wiley & Sons, New York, New York, USA.
Fisher, R.F. and D. Binkley. 2000. Ecology and management of forest soils. Wiley, New York.489 pp.
Hungerford, R.D., M.G. Harrington, W.H. Frandsen, K.C. Ryan, and G.J. Niehoff. 1991. Influence of fire on factors that affect site productivity. In Proceeding of the management and productivity of western Montane forest soils. A.E. Harvey and L. F. Neuenschwander, editors. US Forest Service, Intermountain Research Station General Technical Report INT-280.
McNabb, D. H., and K. Cromack, Jr. 1990. Effects of prescribed fire on nutrients and soil productivity. Pp. 125-141 in Natural and prescribed fire in Pacific Northwest forests. Walstad, John D., ed., et al. Corvallis, OR: Oregon State University Press.
Ulery, A. L. and R. C. Graham. 1993. Forest fire effects on soils color and texture. Soil Science Society of America Journal 57:135-140.
Wells, C. G., R. E. Campbell, L. F. DeBano, C. E. Lewis, R. L. Fredriksen, E. C. Franklin, R. C., Froelich, and P. H. Dunn. 1979. Effects of fire on soil, a state-of-knowledge review. USDA Forest Service, Washington Office, General Technical Report WO-7.