Effects of Fire on Soil Physical Properties

Fire may alter several physical soil properties, such as soil structure, texture, porosity, wetability, infiltration rates, and water holding capacity.  The extent of fire effects on these soil physical properties varies considerably depending on fire intensity, fire severity, and fire frequency.  In general, most fires do not cause enough soil heating to produce significant changes to soil physical properties (Hungerford et al. 1990).  This is particularly true for low intensity prescribed fires. Even where fires do cause direct changes to soil physical properties, their indirect effects on soil hydrology and erosion will vary greatly depending on the condition of the soil, forest floor, topography, and climate.  In the southeastern U.S., these conditions are such that fires do not create serious erosion and surface-runoff problems, unlike areas in the western U.S. where rehabilitation and restoration efforts are often needed following wildfires. Nevertheless, it is still important for fire managers to assess the susceptibility of their particular site to soil damage and erosion and follow guidelines for mitigating these potential effects. In the sections that follow, the potential effects of intense fires on soil physical properties is reviewed and the practical implications of these effects in the southeastern U.S. is discussed.  

Potential effects of fire on soil physical properties

Intense burns may have detrimental effects on soil physical properties by consuming soil organic matter (see diagram: Temperature effects on soil).  Soil organic matter holds sand, silt, and clay particles into aggregates, therefore a loss of soil organic matter results in a loss of soil structure. By altering soil structure, severe fires can increase soil bulk density (DeByle 1981), and reduce soil porosity (e.g., Wells et al. 1979), mostly through the loss of macropores (>0.6 mm diameter).  Soil porosity can also be reduced by the loss of soil invertebrates that channel in the soil (Kettredge 1938).  When fire exposes mineral soils, the impact of raindrops on bare soil can disperse soil aggregates and clog pores, further reducing soil porosity (Ralston and Hatchell 1971).

Intense fires (> 400 C) may also permanently alter soil texture by aggregating clay particles into stable sand-sized particles (Dyrness and Youngberg 1957, Ulery and Graham 1993), making the soil texture more coarse and erodible (Chandler et al. 1983).  In some cases, increasing the coarseness of clays can make soils more permeable to air and water.

Intense burns may also induce the formation of a water repellent soil layer by forcing hydrophobic substances in litter downward through the soil profile (DeBano 1969).  These hydrophobic organic compounds coat soil aggregates or minerals creating a discrete layer of water repellent soil parallel to the surface.  Water repellent soil layers are reportedly formed at temperatures of 176-288^o C and destroyed at >288^o C (Neary et al. 1999). Extensive water repellent layers can block water infiltration and contribute to runoff and erosion.  While formation of water-repellent layers is an important concern in western shrublands, particularly chaparral (DeBano et al. 1977), it has not been documented in the South.

Fire induced changes in soil structure and texture can potentially impair soil hydrology.  Decreased soil porosity and the formation of water repellent layers decrease water infiltration rates (DeBano 1971). Loss of soil organic matter and increased bulk density can decrease the water storage capacity of soils. In flat terrain, this contributes to soil desiccation, particularly in the surface soil layer (Dyrness and Youngberg 1957).  However, in steep terrain it can significantly accelerate runoff, ash transport, erosion, and mass wasting (Neary et al. 1999).  Just exposing soil surfaces can also cause soil erosion. Without the mitigating effects of vegetation on the impact of raindrops, bare soil surfaces can form a sealed surface layer resulting in much higher rates of surface runoff.  Surface erosion by wind or gravity can also increase when ground cover, surface litter, and/or duff protecting the mineral soil are removed.  For this reason, re-establishment of ground cover naturally or by seeding is the most effective erosion control following fire (Wells et al. 1979).

Recent evidence suggests that charcoal can also affect soil hydrology. Fine charcoal particles enhance the water-retentive properties of a soil and can make a sandy soil behave like a clay (Moore 1996).  While this effect could be ecologically significant in bottomland sites where it could contribute to poor drainage and waterlogged conditions, it has not been investigated or reported in the southern U.S.

By altering soil physical properties and soil hydrology, fire can also have indirect effects on plants. Plant uptake of nutrients and water is slowed in structurally degraded soils through the combined effects of lower soil moisture and lower soil porosity (Nye and Tinker 1977).  Root growth can also be impeded by increased bulk density and soil strength (Gerard et al. 1982).

The long-term effects of fire on soil physical properties range from a single season to many decades, depending on the fire severity, rate of recovery as influenced by natural conditions, post-fire use, and restoration and rehabilitation actions.  Persistent soil degradation following fire is more common in the cold and/or arid climates typical of the western U.S.. In the eastern U.S., recovery is usually more rapid. For example, Heyward (1937) found that excluding fire for as little as 10 years in longleaf pine forests resulted in more porous penetrable soil.

Effects of fire on soil physical properties in the southern U.S.

Single low-intensity prescribed burns in the southern U.S. typically do not cause dramatic changes to soil structure and texture.  Elevated soil temperatures during these fires are usually brief (Heyward 1938).  Clay minerals are not changed to a great extent during these fires because of their low content in surface soil horizons and the high temperatures required for aggregation into sand-sized particles (> 400 C; Ulery and Graham 1993).  Moreover, changes in soil pore space and infiltration rates are usually slight if organic soil layers are not completely consumed. 

While single prescribed burns may not have significant effects on soil, the high frequency of fires in the south can have cumulative effects on soil physical properties.  Soil organic matter is usually lower in soils that are repeatedly burned, and early researchers noted that burned soils were harder, denser, and less permeable than unburned soils (Garren 1943, Wahlenberg et al. 1939).  Decreased infiltration rates have also been reported at several burned sites in the south (Ozarks: Arend 1941, Mississippi: Meginnis 1935). 

By altering soil physical properties and removing protective surface covers, fire does lead to increased erosion rates in areas throughout the south.  However, in contrast to western states where fire-induced soil degradation and erosion is a serious issue, large soil losses after burns are rarely documented in the south.  Nevertheless, land managers should still assess the susceptibility of their particular site to soil damage and erosion and follow guidelines for mitigating these potential effects.