Soil Hydrology Under Fire
Soil type is a function of five factors: parent material, climate, topography, biota, and time (Singer and Munns, 1996). Fire's effect on soil can be described in the same manner and, interestingly enough, is almost identical to the five factors of soil formation with only a couple of minor modifications. First, the parent material for a burned soil is the original soil itself plus the fuel consumed in the fire. Second, climate includes not only weather-related variables but also the burning temperature of the fire. This paper focuses specifically on one aspect of the climate factor: the effect of water. It investigates both how soil moisture influences fire severity, and conversely, how fire severity influences the hydrology of a soil. Based on this discussion a land management recommendation states that prescribed fires should be utilized during wetter times of the year to maximize the positive effects for vegetation and to minimize the negative effect of soil erosion.
Rapid plant regrowth is essential to the rehabilitation of a burned area, for plants greatly influence the hydrology of a soil. But for plants to grow back on a burned area, they require several nutrients whose concentrations are modified by fires. The degree of these modifications are determined by a fire's temperature, but there are a few general trends. Levels of phosphorus (Kutiel and Shaviv, 1993 and Marion et al., 1991) and pH (Kutiel and Shaviv, 1993) both increase during a fire. Conversely, nitrogen decreases during a fire (Kutiel and Shaviv, 1993, Marion et al., 1991).
Chemical concentrations in burned soils are greatly affected by a fire's intensity. Ulery and Graham (1993) classify fire intensity into three major classes. A low intensity fire is characterized by temperatures of 100-250° C and the burned soil color is black (from charred organic material). A medium intensity fire has temperatures ranging from 300-400° C and most of the organic material is consumed, which exposes the mineral soil. A high intensity fire has temperatures in excess of 500° C and leaves a white ash over a reddened mineral soil.
Several published studies show how minerals essential for plant growth in the soil are affected by fire intensity. Low intensity fires tend to increase levels of ammonium (Kutiel and Shaviv, 1989 and 1993), calcium, magnesium, and potassium (Marion et al., and Kutiel and Shaviv, 1989), while high intensity fires tend to decrease them (Kutiel and Shaviv, 1989). Kutiel and Shaviv also noted in their study that pH increased with fire intensity and that the highest concentrations of sodium, potassium, and magnesium occurred at a fire temperature of 250° C.
Several factors, such as the type of plants growing on a soil (Scott and Van Wyk, 1990 and Kutiel and Shaviv, 1991) and the amount of available fuel in the soil (Scott and Van Wyk, 1990), influence fire intensity, but by far the most important is fuel moisture. If soil...