Fire's most significant indirect effects on soil are caused by the alteration of standing vegetation and the consumption of organic matter within and beneath the forest floor (DeBano et al. 1998, Neary et al. 1999). Organic matter at or above the soil surface includes the live and dead vegetation as well as the partially decomposed plant litter, or duff, that rests on the soil surface. Organic matter is also mixed into the mineral soil in the form of decayed and disintegrated plant and animal materials, living roots, charred wood, and charcoal.
The organic matter associated with forest soil is nature's compost, akin to the partially decomposed mixture of leaves, grass clippings, and household refuse used to enrich garden soil. In the forest, this material provides much the same service as your garden-variety mulch and compost. Partially decomposed organic matter can hold five times its weight in water (Cohen 2003), and, as a top-dressing, it effectively slows evaporative water loss from and moderates the temperature of forest soil. As insects and microbes further digest this organic matter, they release nutrients and substances that glue together individual mineral soil particles. These aggregated particles enhance soil structure by increasing pore space, which, in turn, increases air and water availability.
Consumption of organic matter begins at 212ºF and is complete at about 932ºF. Nearly all fires consume organic matter at and above the soil surface. Subsoil organic matter is much less likely to be affected directly by fire. In general, the greater the heat pulse onto the forest floor, the greater the consumption of litter and partially decayed organic matter and the more severe the fire's effects on forest soil. By consuming organic matter, fire influences soil temperatures, nutrient availability, and the flow of air and water through and over soil, which collectively influence forest productivity and soil stability. These indirect effects are detailed in the following sections:
The loss of shade from forest vegetation, the loss of insulating organic matter, and the accumulation of charred and blackened residues can all influence the temperature of forest soil long after fire has passed.
Both the tree canopy and the blanket of organic matter at the forest floor help to prevent heat loss from forest soil. Removal of a substantial amount of these shading and insulating materials by fire will invariably heighten daily and seasonal soil temperature extremes. The greater the losses, the more dramatic these effects will be. In other words, the higher the fire severity at the forest floor and in standing vegetation, the more striking the indirect effects on soil temperature are likely to be. As a rule of thumb, a fire that leaves at least four inches of insulating organic matter on the forest floor will have little to no residual influence on temperatures in the underlying mineral soil.
In forests with blackened soils and little or no shade from overstory vegetation, the temperature at the soil surface can exceed 150ºF on a hot, summer day (Neary et al. 1999). Without insulating materials, however, this heat dissipates rapidly from the soil surface at night. In one Northern Rockies study, average soil surface temperatures in severely burned forest were found to be 65ºF higher at midday and 17ºF lower at night than those in comparable unburned sites (Hungerford and Babbitt 1987). At 2 inches beneath the surface, soil temperatures within the same burned forest averaged 13ºF warmer than those within the unburned comparison site (Hungerford and Babbitt 1987). Because the soil itself is a good insulator, temperature differences in burned versus unburned forest always lessen with increasing depth below ground. Nevertheless, temperature increases attributable to fire have been detected as deep as 16 inches into forest soil (Hungerford et al. 1991).
Changes in soil temperature can have important repercussions for post-fire forest development. For example, elevated soil temperatures tend to heighten the activity of soil microbes, further enhancing decomposition and nutrient release from burned sites (Borchers and Perry 1990). While this nutrient release may facilitate post-fire plant growth, water-soluble nutrients (e.g. nitrate-nitrogen) released in excess of that which can be taken up by plants are often leached from the rooting zone and can wind up in nearby streams. We will discuss this phenomenon more thoroughly in the next section, which details fire's influence on nutrient availability in forest soils.
Substantial loss of soil insulation can also increase the frequency of freeze-thaw events in northern Rocky Mountain forests. Repeated freezing and thawing of forest soil tends to compromise soil structure and may uproot plants (McNabb and Swanson 1990). Furthermore, loss of soil insulation is apt to influence the dates of both the first and last frosts and freezes of forest soils and understory vegetation (Fisher and Binkley 2000). These events can damage tender plant parts, including any early spring growth stimulated by diminished shade and unseasonable temperatures. Plants that escape damage from late frosts or freezes, however, may reap the rewards of a head start on the growing season, including unfettered access to newly available nutrients. Such a head start would be especially beneficial to vegetation in high-elevation forests, for example, in which low temperatures tend to limit the length of the growing season (Barnes et al. 1998).
Fire-induced changes in soil temperature may linger for months or even years, depending on fire severity and the rate at which vegetation reestablishes and the forest floor re-accumulates (Hungerford et al. 1991). As a rule, the more dramatic the changes in temperature regime, the more likely that a new suite of plants and animals will dominate the recently burned forest (Barnes et al. 1998).
A forest fire is one big, dramatic chemical reaction - producing heat and light and liberating all sorts of chemical elements from forest materials. Most of the chemicals liberated by fire are essentially the same elements released during the microbial decomposition processes that are ongoing in soils, albeit at much slower rates. During a forest fire, chemical changes that require years of microbial activity can occur within a matter of seconds (DeBano et al. 1998).
Fire, then, can be thought of as an extremely rapid decomposition process (Harvey et al. 1989, Hungerford et al. 1991). This process, like microbial decomposition, breaks down plant and animal tissues into constituent elements, or nutrients, that can be recycled into new, living tissues after fire. Fire, however, generates enough heat to rapidly convert a substantial amount of these elements into other forms, including gases, that are otherwise uncommon byproducts of typical day-to-day decay processes that take place in our forests.
Fire-caused changes in soil nutrient availability can have long-lasting influences on plant growth and, by extension, the overall nature of burned forests (Neary et al. 1999). Plants have minimum daily nutrient requirements, just like we do. For optimum development, plants need relatively large inputs of nitrogen, phosphorus, sulfur, potassium, iron, calcium, and magnesium. Although indispensable, zinc, manganese, cobalt, molybdenum, and nickel are needed only in trace amounts. Rates of survival, growth, and reproduction of forest plants are dictated largely by the availability of these vital elements in soil.
Some soils are naturally more nutrient-rich than others. Northern Rocky Mountain forest soils are relatively nutrient-poor. New plant growth is almost wholly dependent on the continuous recycling of a limited pool of nutrients that has accumulated slowly via atmospheric deposition and rock weathering since the last glacial period ended about 10,000 years ago.
As plants grow and reproduce, they take up nutrients from the soil through their roots, and as they die or shed dead tissues, organic matter accumulates on the forest floor. These materials are first broken down by insects and earthworms. Indeed, much of the dark-colored, crumbly, well-decomposed organic matter in topsoil is earthworm, millipede, and insect poop (Fisher and Binkley 2000). This "frass" and any other animal remains, together with recalcitrant plant materials, are then fed upon and further decomposed by soil bacteria and fungi. Excess nutrients not needed for microbe growth and maintenance are released into the soil as inorganic ions, which are then available for uptake by plant roots once again. Nutrients are recycled in this way from the animal tissues and wastes, including the insect and worm carcasses and feces.
Because most plants can only take up inorganic nutrients that have been released from decomposing organic matter, this decay process often limits new plant growth. And because this process can be very slow - especially in cold, dry forests - the demand for these vital elements by plants likely always exceeds their supply in forest soil. Therefore, forest productivity can be greatly affected by any changes in nutrient availability.
Fire affects nutrient availability directly by chemically altering these vital elements and indirectly by altering soil temperature, pH, and water flow, with the magnitude of these changes depending largely on the degree of soil heating and organic matter consumption.
The following fates await nutrients contained in plants and soil during and following fire:
(1) Nutrients are released into the soil as inorganic ions. During fire's "decay" process, organic material is broken down into its constituent elements via combustion or simply from heating (Fisher and Binkley 2000). The oxidized, inorganic forms of these essential nutrients are readily available for plant use.
(2) Nutrients are converted into gases and lost to the atmosphere. Different elements have different critical temperatures at which they are converted to gaseous forms. Of all nutrients vital to plant growth, nitrogen is most readily oxidized and lost to the atmosphere. A substantial amount of nitrogen gas will be released from organic matter at approximately 400ºF. A substantial amount of sulfur is lost at the next highest temperature (about 707ºF), followed by potassium and phosphorus (at 1425ºF), magnesium (2025ºF), calcium (2703ºF), and manganese (3564ºF) (Raison et al. 1985a, 1985b; Tiedemann 1987). Because nitrogen is converted into a gas at a relatively low temperature, the total amount of this nutrient lost to the atmosphere during burning tends to be directly proportional to the amount of organic matter consumed by fire (Wan et al. 2001). Because oxides of magnesium, calcium, and manganese are vaporized at only very high temperatures, gaseous release of these elements during forest fire are usually trivial.
(3) Nutrients are lost as particulates in smoke. Nutrient-rich ash and charred materials can carried aloft in the form of smoke. The relatively heat-tolerant nutrients like potassium, phosphorus, magnesium, and calcium tend to be concentrated in ash. The total amounts of these nutrients carried from a site as particulates are, however, usually small (Macadam 1989). Most of these materials are deposited in nearby forests. During massive conflagrations, however, fire-generated convection currents may transport ash in towering smoke columns and deposit it hundreds of miles from its origin (McNabb and Cromack 1990).
(4) Nutrients remain on site in unburned and incompletely burned vegetation and organic debris. However, fire tends to create environmental conditions that encourage microbial activity, and the decomposition of partially decayed organic matter by microbes is usually accelerated after burning. In particular, the high pH (alkalinity) of ash tends to neutralize the normally acidic forest soils. This chemical change, along with increased soil temperatures after fire, stimulates microbial activity, resulting in higher rates of decomposition and release of readily available nutrients into the soil.
(5) Nutrients are washed, or leached, into and through forest soil by rainfall. Rainfall can dissolve ash and carry nutrients into the soil (DeBano et al. 1998). Soluble forms of nutrients in excess of that which can be used by plants and microbes in recently burned forest can be flushed by additional rains beyond the reach of plant roots and associated microbes. These excess water-soluble nutrients may then drain into nearby streams and bodies of water (McNabb and Cromack 1990). Nitrate-nitrogen is particularly prone to leaching from soil. Other nutrients, like phosphorus, calcium, and magnesium, are more strongly attracted to soil particles than water, and are therefore more tightly bound in soil. In general, leaching losses increase with increasing fire severity: the more plants and microbes killed by fire, the lower the immediate post-fire demand for (and greater the excess of) newly available soil nutrients will be. Rapid uptake of nutrients by plants and microbes that survive fire or quickly occupy recently burned areas help forests hold onto these essential elements.
(6) Nutrients are lost via high winds or storm runoff. Variable amounts of ash, particulates, and other debris can be carried off-site by wind or surface runoff following fire. The extent of such losses will depend on post-fire weather patterns and the magnitude of surface runoff and erosion during any storm events (Macadam 1989).
In summary, burning generally reduces the total nutrient load within a forest in the short-term due to volatilization, particulate loss in smoke, and post-fire leaching and erosion. However, fire tends to increase the short-term availability of nutrients for plant growth by converting inaccessible, organic forms into usable, inorganic forms directly via the combustion of organic matter and indirectly by creating environmental conditions that speed microbial decay processes (Ecological Society of America 2002). The magnitude of the post-fire flush of available nutrients depends on the temperature and duration of the burn and the amount of organic matter burnt. Levels of nutrients available for plant growth tend to increase with the severity of fire's effects on forest soils, up to a point at which the elements are readily volatilized and lost to the atmosphere. Of all nutrients, nitrogen is most easily vaporized and lost from the forest during fire, followed by sulfur and phosphorus (McNabb and Cromack 1990). The duration of enhanced nutrient availability for plant growth depends on post-fire weather, which influences rates of ash loss to wind and water erosion and, for water soluble nutrients like nitrogen, rates of loss via leaching through soil.
NITROGEN: A CLOSER LOOK
Fire-caused changes in nutrient availability can have enormous effects on subsequent forest development. Fire's influence on nitrogen levels is especially important, as nitrogen availability is believed to limit plant growth in northern Rocky Mountain forests.
While some mineral nutrients like phosphorus occur naturally in the parent rocks of forest soil, all nitrogen inputs to forest ecosystems come from the atmosphere, which is 78% molecular nitrogen (N2) by volume. Molecular nitrogen, however, must be converted to ammonium (NH4+) or nitrate (NO3-) before it can be taken up by plants. Plants rely largely on soil microbes for this conversion, a process also known as nitrogen fixation.
Once taken up by plants, the inorganic, or mineralized, nitrogen (ammonium and nitrate) is integrated into plant tissues. Once in plant and animal tissues nitrogen is considered to be in organic form. When these tissues are broken down via the microbial decomposition pathway, the organic nitrogen is once again converted to inorganic forms available for plant uptake. This release of plant-available nitrogen by soil microbes is known as nitrogen mineralization.
As it breaks down plant and animal tissues, fire likewise converts organic nitrogen into inorganic forms available for plant growth. The amount of inorganic nitrogen released by fire depends on how much biomass is consumed, which is a measure of the fire's severity. Some of this nitrogen is lost back to the atmosphere in gaseous form (N2), a process known as volatilization. Due to this and other means of nitrogen loss during fire, including the offsite transport of nitrogen in ash and smoke particulates, the total amount of nitrogen in a forest stand is invariably decreased to some degree by burning (Wan et al. 2001). Nitrogen available for plant growth in the form of inorganic pools of ammonium and nitrate in forest soil, however, almost always increases after burning.
The post-fire flush of inorganic nitrogen is not solely due to the physical breakdown of plant and animal tissues by fire. It is also a function of the enhanced activity of microbes in the warmer and more alkaline soil of a recently burned forest.
The timing and magnitude of the post-fire flush of inorganic, or plant-available, nitrogen depends on its form, either ammonium or nitrate. In general, the soil ammonium pool can increase twofold immediately after fire, gradually declining to the pre-fire level within one year (Wan et al. 2001). The pulse wanes largely due to plant uptake and the conversion of ammonium-nitrogen to nitrate-nitrogen by soil microbes, a process known as nitrification (Jurgensen et al. 1981, Smith and Fischer 1997, Wan et al. 2001). The nitrate pool, on the other hand, tends to increase relatively little (i.e., by about 25%) immediately after fire (Wan et al. 2001). It usually peaks at about threefold the pre-fire level within 0.5-1 year after burning (Wan et al. 2001). This pulse wanes again due to plant uptake, but also because the nitrate form of nitrogen does not adhere as readily to soil particles as ammonium does and is readily leached, or washed, from the soil (Jurgensen et al. 1981, Smith and Fischer 1997, Wan et al. 2001).
In sum, a short-lived pulse of plant-available nitrogen can be expected after forest fire. This pulse will encourage the development of nitrogen-rich plant growth for at least a year post-fire. Any excess nitrogen, however, is likely to be leached from the rooting zone and may wind up in nearby rivers and streams. By taking nitrogen up into their tissues, plants that establish in recently burned forests help prevent these leaching losses.
Once the short-lived forbs and grasses or woody plants that capitalized on the flush of post-fire nitrogen die or shed parts, their nutrient-rich tissues are broken down in the microbial decomposition pathway. Once again the nitrogen becomes available to support new plant growth, though at a much slower rate than right after fire.
Some of the forest plants that increase in abundance shortly after wildfire have very close relationships with certain soil microbes that fix nitrogen (i.e., convert gaseous nitrogen in soil air spaces to plant-available forms). In fact, these plants, like lupine and ceanothus, house nitrogen-fixing microbes right in their roots. By favoring populations of nitrogen-fixing plants, fire can enhance long-term nitrogen availability in forest ecosystems (Youngberg and Wollum 1976, Wells et al. 1978, DeLuca 2000, Newland and DeLuca 2000).
For more about the nitrogen cycle and its relationship to fire in forest ecosystems, check out the University of Alberta's forest ecology course web page. There you'll find a nice overview relevant to many forest systems, including our own.
AIR AND WATER MOVEMENT THROUGH SOIL
Life beneath the forest floor depends on a constant supply of air and water plus any essential nutrients. These elements are available to plant roots and soil microbes through a network of soil pores. Air constantly diffuses into soil pores from the atmosphere, while water percolates into these voids during rainfall or snowmelt events.
Fire can reduce soil porosity by: (1) ashing organic matter, (2) increasing rainfall impact (via the removal of protective organic layers), and (3) releasing hydrocarbons from organic matter (Wells et al. 1978, McNabb and Swanson 1990, Wondzell et al. 2003). Any reduction in the porosity of forest soil can compromise its productivity and its sponge-like ability to absorb moisture. Diminished water infiltration into forest soils and its consequences will be exacerbated by any fire-caused reduction of plant cover, which intercepts precipitation during rain and snowfall events (Wondzell et al. 2003).
In general, soil damage from post-fire rainfall impact is maximized when fire is moderate to severe in both forest vegetation and soils. Plants and organic matter at the forest floor dampen raindrop impact; therefore, raindrops will slam harder into bare, post-fire soil (McNabb and Swanson 1990). Increased raindrop impact on mineral soil can reduce the size and number of pores in the uppermost soil by breaking up soil particles and clogging pores with fine particles, including ash (Wondzell et al. 2003). Reduction in pore size and number caused by ashing organic matter and by raindrop impact will decrease the soil's ability to take in and hold air and water, increasing surface runoff during rains (Wells et al. 1978, Wondzell et al. 2003).
The release of hydrocarbons during the combustion of organic matter can likewise reduce the absorptive capacity of the uppermost forest soil by creating a water-repellent layer just below the soil surface. The release of chemicals that create water-repellent conditions in upper soil is a by-product of the constant decomposition of organic matter within our forests. Fire simply speeds the decay process that produces non-wettable molecules. As organic matter is burned, hydrocarbons are distilled. These materials move downward into the soil and condense on cooler soil particles. The waxy coating that they leave on soil particles is water repellent. These coated particles coalesce into a layer impermeable to water in the forest soil (Wondzell et al. 2003).
It is important to note that, when dry and unheated, the soils of many northern Rocky Mountain forests have a weak water-repellent layer above mineral soil (caused by natural decay processes) (Brady et al. 2001, Wondzell et al. 2003). In forests on such ash-cap soils, water repellency may be no worse after fire than before burning. Fire may simply expose already impermeable soils, with the only change in the depth of the water repellent layer - moving it from the soil surface to 3-5 cm below (Brady et al. 2001).
Fire is most likely responsible for post-burn water repellency in certain soil types - specifically those formed from weathered rhyolite. In rhyolite soils, effective coating of mineral soil particles occurs at relatively low temperatures and after relatively short periods of heating. Longer periods of heating at higher temperatures will destroy the organic substances that cause water repellency.
In general, soils must be heated between 349-550ºF for water repellency to occur - a rather narrow window (DeBano 2000, DeBano et al. 1976). Soils are affected little when heated below 349ºF. The most extreme water repellency is apt to result after soils are heated within the tight range of 349-399ºF (Wells et al. 1978). If the surface soil is heated within this range, then a water-repellent layer may form near the surface. If surface temperatures exceed those that tend to effect water repellency, a non-wettable layer is likely to develop deeper in the soil, leaving the uppermost soil wettable. Typically, water repellent layers develop at depths of less than one inch in mineral soil (Robichaud 2000a).
Fire-caused water-repellency will diminish over time as the hydrophobic substances dissolve (Robichaud 2000a). It may take as little as 10 minutes to break a water-repellent layer with heavy rain (Robichaud 2000a). Fire-caused hydrophobic conditions seldom persist longer than two years, even after a severe burn (Bitterroot National Forest 2000, Robichaud 2000a) (Wondzell et al. 2003).
A severe fire may also increase the rate at which the soil becomes saturated with the water that does infiltrate (Bitterroot National Forest 2000). This happens for two reasons: first, the loss of soil organic matter, which can store five times its weight in water (Cohen 2003), reduces the soil's total water-holding capacity, and second, extensive plant mortality can diminish water uptake from forest soils (Wondzell et al. 2003).
If the absorptive capacity of forest soil is compromised, a greater volume of water will be relegated to overland flow during rains and snowmelt events (McNabb and Swanson 1990). Increased surface flow is likely to force additional ash into soil pores, making the soil even less absorptive. The magnitude of all these effects will be directly proportional to the severity of fire at the forest floor and in the forest vegetation (Wondzell et al. 2003).
In summary, fire that is severe in the forest overstory, at the forest floor, or both, can impede the movement of air and water into soil. Reduced water-absorbing capacity of soil generally leads to greater surface runoff, or overland flow, during subsequent rains or snowmelt events (Wondzell et al. 2003).
RUNOFF AND EROSION
Overland water flow is usually trivial and soils are generally stable in mature, undisturbed forest (Wondzell et al. 2003). After a severe wildfire, however, increased surface runoff during storms can effect substantial surface erosion and shallow landslides, especially within areas in which roots and other organic structures that hold loose material on slopes are consumed or killed (Robichaud 2000b, Wondzell et al. 2003). On slopes with little protective vegetation or debris, soil can erode even without heavy rains, due to the sheer force of gravity (Wondzell et al. 2003). Eroded soil often winds up as sediment in nearby bodies of water and streams.
The risk of soil loss from erosion after a forest fire depends on both the size and severity of the fire, as well as the amount of precipitation that falls in the recently burned area (Robichaud 2000b). Based on computer models designed for northern Idaho forests dominated by Douglas-fir, Potts and others (1985) suggest that stands receiving less than 20 inches of rainfall per year are likely to have little or no increase in runoff after a severe burn. Stands receiving more than 20 inches of annual rainfall, however, could yield 29% more runoff in the first year post-fire.
In general, erosion risk wanes with increasing cover of post-fire vegetation, litter, and debris, all of which help protect and stabilize forest soil (Wondzell et al. 2003). According to Robichaud and others (2000), when more than 75% of the ground is covered by vegetation or plant litter only about 2% of the precipitation from a given storm event is apt to become runoff, and the potential for erosion is low. In contrast, when less that 10% of the soil surface is covered by plants and litter, which can be the case shortly after a severe wildfire, more than 70% of any precipitation may spill off the soil surface, increasing the erosion potential by up to three orders of magnitude. Some areas, like steep slopes, are simply more erosive than others (Wondzell et al. 2003). Some storms, too, are more likely to enhance surface runoff and erosion than others (Wondzell et al. 2003). Summer storms, for example, are much more erosive than is the overland flow from snowmelt. Of summer storms, heavy, enduring rainfall events are particularly erosive.
Any increase in runoff tends to fully wane within the first 1-2 years after severe fire, as fire-induced water-repellency diminishes, soil pores are rid of ash and other fine sediments by overland flow, and plant cover rebounds (Wondzell et al. 2003). Yet because the roots of fire-killed trees and shrubs that so effectively anchor soil onto slopes may deteriorate very slowly, a severe burn may compromise the mechanical cohesion of soil in some forest stands for up to 10 years (Wondzell et al. 2003). In other words, fire-induced landslides may occur long after the potential for runoff-initiated erosion events has subsided.
For more on post-fire runoff and erosion and to see photos of water-repellent soil and equipment used to quantify infiltration rates, erodibility, and sediment yield in forest ecosystems, check out Peter Robichaud's online report, Forest fire effects on hillslope erosion: what we know.
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ADDITIONAL AVAILABLE LITERATURE:
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Choromanska, U., and T.H. DeLuca. 2002. Microbial activity and nitrogen mineralization in forest mineral soils following heating: evaluation of post fire effects. Soil Biology and Biochemistry 34:263-271. 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.
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