During the Fire: Direct Effects of Fire on Forest Soil

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 176F 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.


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