On average, only about 8-10% of the heat generated during a forest fire is radiated downward to the forest floor (Wells et al. 1978, DeBano et al. 1976, Raison et al. 1985, Steward 1989, Hungerford 1989). Yet this heating is responsible for all of the direct changes in soil properties caused by forest fire (Neary et al. 1999).
Fire directly influences the soil's ability to absorb rainfall and snowmelt and to support plants and other life (Wells et al. 1978, Hungerford 1990, Hungerford et al. 1991, DeBano et al. 1998). The precise nature of these effects depends on both the temperatures reached at different soil depths and the degree of heating that different soil components can withstand before being irreversibly altered.
The degree of soil heating depends on both the magnitude and duration of energy transferred from fire into the forest soil (Wells et al. 1978). While aboveground temperatures spike rapidly during a burn (Robichaud et al. 2000), belowground temperatures rise slowly because soil water must be boiled off before temperatures can exceed about 176ºF at any depth and because dry soil is a very good insulator (Agee 1973, Frandsen and Ryan 1986, DeBano et al. 1998).
Because soil temperature does not spike the minute that flames pass over an area, a rapidly moving fire may release tremendous heat but effect only a modest heat pulse. For this reason, the average length of flames reaching into the forest vegetation, which is a good indicator of the aboveground heat release during a fire (or fire intensity), is a poor indicator of the heat transferred into the soil (Hungerford 1989, Hartford and Frandsen 1991). The downward heat pulse into the soil depends much more on duration than intensity of fire (Hungerford 1989, Neary et al. 1999).
Extensive soil heating takes time and is most likely to occur beneath heavy fuels, like large-diameter tree stumps and logs, which can smolder for days or weeks. In general, an enduring, low-intensity fire in logging slash will have more severe effects on forest soil than a fire that burns more intensely but rapidly (e.g., 15 mph) through tree crowns (Hungerford et al. 1991, Neary et al. 1999). Therefore, fire severity on soil and fire severity on aboveground vegetation (i.e., the mature trees) should always be distinguished when describing fire effects.
The most extreme temperatures generated in forest soils during fire are largely restricted to the uppermost soil (Macadam 1989). Yet, because the bulk of nutrients and the activity of soil organisms are concentrated in the one- to four-inch surface organic layer and the upper six inches of mineral soil, any heating of this soil region can have serious repercussions on post-fire forest productivity (Raison 1979, Macadam 1989, Bitterroot National Forest 2000). These effects may be fleeting or may linger for years.
After a low-severity burn, for example, recovery to pre-fire conditions may take only as long as necessary for soil biota to reestablish and surface organic matter to reaccumulate. In the meantime, the physical, chemical, and biological changes in forest soil will be manifest in post-fire plant establishment and growth as well as post-fire water yield and soil loss via erosion. We detail these domino-effects in the next section - After the Fire.
Agee, J. K. 1973. Prescribed fire effects on physical and hydrologic properties of mixed- conifer forest floor and soil. University of California Water Resources Center Contribution 143.
Bitterroot National Forest. 2000. Bitterroot Fires 2000: An assessment of post-fire conditions with recovery recommendations. USDA Forest Service, Bitterroot National Forest. Unpublished report available 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.
Frandsen, W. H., and K. C. Ryan. 1986. Soil moisture reduces belowground heat flux and soil temperatures under a burning fuel pile. Canadian Journal of Forest Research 16:244-248.
Hartford, R. A., and W. H. Frandsen. 1991. When it's hot, it's hot . . . or maybe it's not! (Surface flaming may not portend extensive soil heating). International Journal of Wildland Fire 2:139-144.
Hungerford, R. D. 1989. Modeling the downward heat pulse from fire in soils and in plant tissue. Pp. 148-154 in Proceedings of the 10th Conference on Fire and Forest Meteorology, Ottowa, Canada.
Hungerford, R. D. 1990. Describing downward heat flow for predicting fire effects. Fire effects: prescribed and wildfire. USDA Forest Service, Intermountain Fire Sciences Laboratory, Problem Analysis, Problem No. 1, Addendum.
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 A.E. Harvey and L. F. Neuenschwander, editors, Proceeding of the management and productivity of western Montane forest soils. USDA Forest Service, Intermountain Research Station, General Technical Report INT-280.
Macadam, A. 1989. Effects of prescribed fire on forest soils. B.C. Min. For., Victoria, B.C. Research Rep. 89001-PR.
Neary, D.G., C.C. Klopatek, L.F. DeBano, and P.F. Ffolliott. 1999. Fire effects on belowground sustainability: a review and synthesis. Forest Ecology and Management 122:51-71.
Raison, R. J. 1979. Modification of the soil environment by vegetation fires, with particular reference to nitrogen transformations: a review. Plant and Soil 51:73-108.
Raison, R. J., P. K. Khanna, and P. V. Woods. 1985. Mechanisms of element transfer to the atmosphere during vegetation fires. Canadian Journal of Forest Research 15:132-140.
Robichaud, P. R., J. L. Beyers, and D. G. Neary. 2000. Evaluating the effectiveness of post-fire rehabilitation treatments. USDA Forest Service, Rocky Mountain Research Station, General Technical Report, RMRS-GTR-63.
Steward, F. R. 1989. Heat penetration in soils beneath a spreading fire. Unpublished paper on file at: USDA Forest Service, Intermountain Forest and Range Experiment Station, Fire Sciences Laboratory, Missoula, Montana.
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.
ADDITIONAL AVAILABLE LITERATURE:
Borchers, J. G., and D. A. Perry. 1990. Effects of prescribed fire on soil organisms in natural and prescribed fire in Pacific Northwest forests. Pp. 143-158 in J. D. Walstad, S. R. Radosevich, and D. V. Sandberg, editors, Natural and prescribed fire in Pacific Northwest forests. Oregon State University Press, Corvallis, Oregon.
Chorover, J., P. Visousek, D. Everson, A. Esperanza, and D. Turner. 1994. Solution chemistry profiles of mixed-conifer forests before and after fire. Biogeochemistry 26:115-144.
DeBano, L. F. 1991. The effect of fire on soil properties. Pp. 151-156 in Harvey, A. C., and L. F. Neuenschwander, compilers, Proceedings - Management and Productivity of Western-Montane Forest Soils. USDA Forest Service, Intermountain Forest and Range Experiment Station, General Technical Report INT-280.
DeLuca, T. H. 2000. Soils and nutrient considerations. Pp. 23-25 in H. Y. Smith, ed., The Bitterroot Ecosystem Management Research Project: What have we learned? USDA Forest Service, Rocky Mountain Research Station, Symposium Proceedings RMRS-P-17.
DeLuca, T. H. 2001. Assessment of the USFS Soil Quality Standards and the application of those standards to the Pink Stone Environmental Impact Statement. A report to The Ecology Center, Inc.
Fisher, R. F. and D. Binkley. 2000. Ecology and management of forest soils. Wiley, New York.489 pp.
Frandsen, W. H., and R. A. Hartford. 1990. Ground fire temperatures. USDA Forest Service, Intermountain Research Station, Editorial draft.
Harvey, A. E., M. F. Jurgensen, and R.T. Graham. 1989. Fire-soil interactions governing site productivity in the Northern Rocky Mountains. Pp. 9-18 in D. M. Baumgartner, L. F. Neuenschwander, R. H. Wakimoto, eds. Prescribed fire in the intermountain region: forest site preparation and range improvement. Pullman, WA: Washington State University.
Harvey, A. E., M. F. Jurgensen, and M. J. Larsen. 1981. Organic reserves: importance to ectomycorrhizae in forest soils of western Montana. Forest Science 27:442-445.
Hungerford, R. D., W. H. Frandsen, and K. C. Ryan. Heat transfer into the duff and organic soil. Available online at http://fire.r9.fws.gov/ifcc/research/doiheat.pdf.
McNabb, D. H., and K. Cromack, Jr. 1990. Effects of prescribed fire on nutrients and soil productivity. Pp. 125-141 in J. D. Walstad, S. R. Radosevich, and D. V. Sandberg, editors, Natural and prescribed fire in Pacific Northwest forests. Oregon State University Press, Corvallis, Oregon.
Robichaud, Peter R.; Beyers, Jan L.; Neary, Daniel G. 2000. Evaluating the effectiveness of post-fire rehabilitation treatments. Fort Collins: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. General Technical Report RMRS-GTR-63. Available online at http://www.fs.fed.us/rm/pubs/rmrs_gtr63.pdf.
Stark, N. M. 1977. Fire and nutrient cycling in a Douglas-fir/larch forest. Ecology 58:16-30.
Swanson, F. J. 1981. Fire and geomorphic processes. Pp. 401-420 in Proceedings of the Conference on Fire Regimes and Ecosystem Properties. USDA Forest Service, Washington Office, WO-26.
Tiedemann, A. R. 1987. Combustion and losses of sulfur from forest foliage and litter. Forest Science 33:216-233.