Effects of Wildfire on Soils and Watershed Processes

16 Journal of Forestry • September 2004 organics, and water repellency. The loss or transformation of surface litter and soil organic matter depends on local burn temperatures, which are controlled by such factors as fuel loads, types, and moisture contents, and fire weather. At higher temperatures, much of the mass of organic matter can be transformed into carbon dioxide and water vapor, with nutrients lost as gases or converted (mineralized) into forms more readily transported by surface runoff or drainage water. The nutrient most vulnerable to gaseous losses is nitrogen, which can be volatilized at relatively low temperatures (e.g., 200–500° C or 392–932° F). Phosphorus can be volatilized at high burn temperatures (e.g., 770° C or 1,418° F), whereas other mineral nutrients such as calcium, magnesium, and potassium are typically converted to oxides (often a major component of the light-colored ash remaining after fire) that are relatively soluble. Water repellency results when volatilized organic compounds condense on cooler soil particles associated with steep temperature gradients below the soil surface. This results in negatively charged layers that repel water, thereby reducing infiltration (water movement into the soil surface) or percolation (water drainage within the soil). Reductions in the infiltration rate can be dramatic (e.g., one to two orders of magnitude), and sometimes create a “parking lot” or “tin roof” effect with very rapid surface runoff. “Any mineral soil containing more than a couple percent of organic matter is likely to become water repellent to some degree when heated” (DeBano 1981). However, coarse textured soils are more prone to water repellency than fine soils (Neary et al. 2004), as are fuels of certain species (e.g., chaparral vegetation). Light fires over moist soils tend to produce less water repellency than intense fires over dry soils. Defining Burn Severity Because resource effects are influenced so much by fire severity, systems for classifying burn areas have been developed. Confusion over existing assessment terminology led Debano et al. (1998) and Parsons (2003) to argue that burn intensity should simply describe the rate of burning (heat per area per time unit), whereas severity should characterize results of the burn, thus integrating burn intensity, duration, and site conditions. Parson’s classification is aimed largely at “soil burn severity” to assess how soil changes from fire can impact hydrologic functions, as shown by some key excerpts closely adapted but truncated from those provided by Parsons (2003). Soil burn severity. A term that qualitatively describes classes of fire-caused changes to soil hydrologic function, as evidenced by soil characteristics and surface fuel and duff consumption. Large diameter down, woody fuels, and organic soil horizons are consumed during long-term, smoldering, and glowing combustion. The amount of duff or organic layer reduction is also called depth of burn, or ground char. The amount and duration of subsurface heating determine the degree of soil burn severity, and can be inferred from fire effects on ground fuels (plants and other organic matter) and soils. Descriptive Classes—Soil Burn Severity These are guidelines to visual indicators only, and the boundaries between the classes often become “blurred” in real world situations. Unburned to very low. Fire has not entered the area, or has very lightly charred only the litter and fine fuels on the ground; soil organic matter, structure, and infiltration unchanged. Low. Low soil heating or light ground char occurs; mineral soil is not changed; leaf litter may be charred or partially consumed, and the surface of the duff may be lightly charred; original forms of surface materials, such as needle litter or lichens may be visible; very little to no change in runoff response. Moderate. Moderate soil heating with moderate ground char; soil structure is usually not altered; decreased infiltration due to fire-induced water repellency may be observed; litter and duff are deeply charred or consumed; shallow light colored ash layer and burned roots and rhizomes are usually present; increase in runoff response may be moderate to high. High. High soil heating, or deep ground char occurs; duff is completely consumed; soil structure is often destroyed; decreased infiltration due to fire-induced water repellency is often observed; top layer of mineral soil may be changed in color (but not always) and consistence and the layer below may be blackened; deep, fine ash layer is present, often gray or white; all or most organic matter is removed; essentially all plant parts in the duff layer are consumed; increase in runoff response is usually high. High soil burn severity areas are primary treatment candidate sites if there are downstream values at risk. This classification scheme assumes that soil conditions are the primary influences on hydrologic functions after wildfire. Weather events (e.g., antecedent conditions, storm size, rainfall intensity) and vegetation conditions also influence hydrologic (e.g., interception, evapotranspiration) and water quality (e.g., root strength, nutrient cycling, shade) functions. A soils focus may be suitable for screening for immediate site rehabilitation needs, but it may not capture more subtle or complex watershed responses, such as changes in stream temperature or nutrient concentrations. Specific burn locations, patterns, and extents are also important in determining watershed responses. If riparian areas remain intact, for example, key Figure 1. Headwater reach scour following wildfire on Boise National Forest . 17 September 2004 • Journal of Forestry C ou rt es y of J oh n T ho rn to n, B oi se N at io na l F or es t