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1 4 Effects of Fire on Soil ErosionDavid H. McNabb and Frederick J. Swanson

Executive SummaryErosion is the product of complex interactions among geomorphic process-es, climate, vegetation, soils, and landforms. The energy derived from fall-ing or flowing water and gravity dominates erosion processes in PacificNorthwest forests. Rainfall causes splash erosion of exposed soil, and flow-ing water may cause sheet, rill, and gully erosion. Freezing water occasion-ally loosens soil by frost heaving for subsequent transport by water. Gravitycauses soil and rocks to move down steep slopes as ravel, the slow creep ofthe soil mantle downslope; and mass wasting includes various types oflandslides. Channel erosion adds material directly to streams.

Disturbance of Pacific Northwest forest ecosystems generally accelerateserosion processes, alters the transport and storage of sediment within water-sheds, and increases export of material from watersheds relative to undis-turbed forested lands. The frequency and severity of wildfire, a major forestdisturbance, affects the magnitude of accelerated erosion. Prescribed fire inmanaged forests can also accelerate erosion and alter the dominant forms oferosion.

Fire increases the potential for accelerated erosion primarily through itseffects on vegetation and soil. As fire increases in severity, more vegetationis killed, more forest floor is consumed, and it becomes more likely that thephysical properties of the soil and watershed are changed. These changesincrease the potential for erosion by exposing mineral soil to erosion proc-esses.

The Potential for prescribed fire to increase erosion increases with fireseverity, soil erodibility, steepness of slope, and intensity or amount of pre-cipitation. The magnitude of fire-accelerated soil loss from forest land in thePacific Northwest is usually minor because the times and situations whenthese four factors occur concurrently are rare.

Hydrologic and other soil physical properties are particularly importantfactors affecting the potential for surface erosion. Coarse-textured soils lowin organic matter are most susceptible to surface erosion; these soils aremuch less common in the Pacific Northwest than elsewhere in the westernUnited States. Most undisturbed forest soils in the region have a high porosi-ty which, coupled with the low intensity of most rainfall events, seldomresult in overland flow. Prescribed fire can increase soil movement by ravelon steep slopes, but has a negligible effect on mass wasting.

Accelerated erosion from prescribed fire usually has a minor effect onlong-term forest productivity in the Pacific Northwest. The potential for

159

SOILAND

HYDROLOGY

(1 IVEGETATION

GEOMORPHICPROCESSES

CLIMATE . LANDFORM

V

160 Natural and Prescribed Fire in Pacific Northwest Forests

prescribed fire to affect productivity, however, increases if fires are severe,soils are highly erodible, and prescribed fires are more frequent than pastwildfires. The potential for prescribed fire to cause accelerated erosion de-creases with the severity and frequency of its use.

IntroductionFire changes forest ecosystems and interacts

with geomorphic processes, climate, and land-form in a variety of ways to alter the landscape andtemporarily increase the potential for erosion (Fig.14-1). Soil erosion in Pacific Northwest forests istypically low when the soil is protected by litterand the site is covered by a closed forest canopy.Disturbance of forests by wildfire, harvesting,road construction, and site preparation (includingprescribed burning) increases the potential forerosion.

Fire affects erosion processes by exposing read-ily erodible material and in some cases, increasinghydrologic energy available to move it. Exposureof erodible material depends on how the severityof fire affects vegetative cover, the organic forestfloor, and soil. Following a fire, landform, soilproperties, and climate interact to determine thedominant geomorphic processes that move mate-rial downslope and into stream channels. The hy-drologic characteristics of the soil and drainagenetwork control many erosional processes, but onsteeper slopes gravitational processes become in-creasingly important.

In presettlement time, wildfire was an importantfactor affecting geomorphic processes and forestcommunities in the Pacific Northwest (Chapter 3;Swanson 1981). In many respects, the effects ofprescribed burning on geomorphic processes aresimilar to wildfire, although the severity of pre-scribed fires is often less than that of wildfires.However, the effect of harvesting and road con-struction either supercede or dominate many ero-sion processes in managed forests, particularlywhen prescribed burns are not severe.

This chapter begins with a discussion of fire-in-duced changes in vegetation, soil physical proper-ties, and hydrology that may alter geomorphicprocesses and accelerate erosion. The direct ef-fects of fire are followed by a discussion of thethree types of accelerated erosion which may fol-low—surface erosion, mass wasting, and streamchannel erosion. Where appropriate, changes fol-lowing prescribed burning are contrasted with

EROSION(SEDIMENT PRODUCTION)

AND SOIL LOSS

Figure 14-1. Fire and other disturbances interact with cli-mate, landform, and vegetation to shape the landscape fra variety of geomorphic processes. The interaction othese factors determines the type and rate of soil loss frerosion.

those following a wildfire or the interactions oprescribed burning with other management practices. Finally, the potential for erosion from pre-scribed burning to affect forest productivity is addressed.

Fire Effects on VegetationA combination of harvesting and prescribe(

burning in managed forests temporarily leave,most sites with little, if any, vegetative cover. Thttemporary removal of vegetation detrimental ureforestation efforts is an important objective ofusing prescribed burning for site preparation(Chapter 6); however, several species that aggressively invade newly burned sites or require fire fo

Effects of Fire on Soil Erosion 161

germination may ultimately become more compet-itive with newly planted seedlings than existingvegetation (Chapter 4). In contrast to prescribedfire, the effect of a wildfire on vegetation is oftenmore dependent on fire severity. Low-severitywildfires generally burn slowly through the under-story and have little direct effect on overstory veg-etation. At higher fire severities, trees may bekilled by heat. Under the most severe conditions,entire crowns and portions of the stem may beburned. If the crowns do not burn, dead foliageoften falls to the ground and covers some of theexposed soil (Megahan and Molitor 1975).

Pioneer species or sprouts of residual speciesthat have adapted to disturbance by fire rapidlyoccupy most burned sites (Chapter 4; Dymess1973, Gholz et al. 1985). Although the cover pro-vided by vegetation is not as effective at protectingthe soil as that provided by the forest floor, newlyestablished vegetation produces litter to replacethat burned.

Fire Effects on SoilsDepending on severity, fire may change several

forest floor and soil properties which affect themovement of water into or over the soil surfaceand the susceptibility of exposed soil to erosionprocesses. The forest floor is the organic horizonscovering soil and comprises litter, duff (partiallyand completely decomposed organic material),and woody debris. Retention of a portion of theforest floor generally protects the soil from the ad-verse effects of fire.

Forest floorThe forest floor buffers the soil from extremes in

temperature and moisture and protects it from sur-face erosion. The bulk density of forest floors istypically 20 percent or less of the underlying soilhorizons (Wooldridge 1970). The low bulk densityresults from the lower particle density of organicparticles; the high porosity of these layers causesthe forest floor to have much higher hydraulic con-ductivities than soil.

The thickness and natural variation in density offorest floors depend on the age and type of forestecosystem (Gessel and Balci 1965). Forest floorsare often less than an inch thick in drier climatesand young forests (Amaranthus and McNabb1984, Edmonds and Hsiang 1987), but are much

thicker in more mesic or older forests (Little andOhmann 1988).

The thickness and density of forest floors alsoaffect physical properties, such as temperatureand moisture gradients, that limit or slow theirconsumption by fire (Sandberg 1980). Thin forestfloors dry quickly and are most susceptible to con-sumption by fire (Amaranthus and McNabb 1984).Forest floors greater than about 2 inches in thick-ness typically dry from the surface downward, re-sulting in a dry surface layer over a much wetterlayer (Little and Ohmann 1988). While the dry sur-face layer may be readily consumed by a fire, thewet layer is much less susceptible to consumption(Sandberg 1980, Little and Ohmann 1988). Asmore of the surface layer dries in late summer,more of the forest floor may be consumed by a fire.

Moisture content of the forest floor also affectsits ability to change temperature; high moisturecontents slow changes in temperature and helplimit the consumption of forest floor materials.Forest floors with a high moisture content are diffi-cult to burn without an external source of heat,such as the burning of woody fuels (Sandberg1980). Dry forest floors, however, are often con-sumed during a fire because less water must beevaporated before the temperature for combus-tion is reached; this condition is most likely to oc-cur during a severe late summer wildfire or an ear-ly fall prescribed fire.

Water repellency and infiltration capacityWhen forest floors burn, some of the organic

compounds may be partially volatilized (Chapter16). Most of these are lost to the atmosphere insmoke, but some move downward into the soil byconvection where they condense on the surfacesof cooler soil particles. Compounds of long-chainaliphatic hydrocarbons are believed to cause wa-ter repellency when they condense on soil parti-cles (DeBano 1981). The development of a water-repellent layer reduces the infiltration capacity ofthe soil and increases the potential for overlandflow. The infiltration capacity is the potential max-imum rate at which water enters soil. The infiltra-tion capacity of soil is not constant but decreasesas soil becomes wetter and the wetting frontmoves deeper into the soil. Some water repellencyof soil is a natural phenomenon in unburned soilsof the Pacific Northwest (Singer and Ugolini 1976,Johnson and Beschta 1980, McNabb et al. 1989).

162 Natural and Prescribed Fire in Pacific Northwest Forests

An increase in water repellency of soil followingprescribed burning or wildfire has been reportedfor several locations in the Pacific Northwest (Me-gahan and Molitor 1975, Dymess 1976, Johnsonand Beschta 1980, McNabb et al. 1989). Water re-pellency induced by a low-to-moderate-severityprescribed fire is usually of short duration. Insouthwest Oregon, repellency resulting from a latespring prescribed burn returned to near naturallevels soon after the fall rains began (McNabbet al. 1989). Following a late summer wildfire inthe Oregon Cascade mountains, however, repel-lency of volcanic ash soils did not return to non-burn levels for about 6 years (Dyrness 1976).

Water repellency and its persistence are affect-ed by differences in soil moisture, soil texture, se-verity of the fire, and quantity and composition ofthe litter (DeBano 1981). Water repellency is morecommon in coarse-textured soils because the soiltemperature gradients that affect volatilizationand condensation during burning are often greaterand the soil has less surface area on which volati-lized compounds may condense. Coarse-texturedsoils include soils derived from pumice and othervolcanic ash, glacial till, and granite. Fortunately,these soils are generally less common in the high-precipitation zones of western Oregon and Wash-ington, have high porosities, or tend to weather tofiner-textured soils.

Water repellency at the soil surface is most like-ly to occur when fire severity and duration causemoderate increases in soil temperatures (DeBano1981). Severe fires producing high soil tempera-tures force the water-repellent layer to form deep-er in the soil. Reducing the severity of prescribedburns, retaining forest floor, and burning when soilmoisture is high are effective techniques for keep-ing soil temperatures low and minimizing water re-pellency.

Water repellency is less likely to cause surfaceerosion if the reduced infiltration capacity of thesoil is higher than precipitation intensity. In south-west Oregon, an increase in water repellency re-duced the infiltration capacity of the surface soilfrom greater than 4 inches per hour to an averageof 3.5 inches per hour following a late spring pre-scribed burn; the lowest infiltration capacitymeasured was 2 inches per hour (McNabb et al.1989). Despite this decrease, the lowest capacitystill exceeded, by a factor of two, the maximumstorm intensity expected in a 25-year period.

A water-repellent layer forming below the soilsurface is likely to cause more erosion than such alayer at the surface. The infiltration capacity of thesoil over a water-repellent layer is much higherthan the hydraulic conductivity of the underlyingwater-repellent layer. As a consequence, the sur-face soil is easily saturated by even low-intensityrainfall events, which can cause overland flow orloss of the surface soil in a shallow debris flowabove the water-repellent layer (DeBano 1981).

Water repellency has been measured in numer-ous studies, but only a few have also measured theinfiltration capacity of the soil (Megahan and Moli-tor 1975, McNabb et al. 1989). The high porosityof most forest soils in the Pacific Northwest (Dyr-ness 1969, Han 1977), coupled with the low tomoderate intensity of most rain events, normallypreclude overland flow except immediately after asevere fire or other site disturbance. Overlandflow is more likely to occur because the lower hy-draulic conductivity of subsoil horizons and un-derlying rock causes the upper soil horizons to be-come saturated during large storms; this can causeoverland flow unrelated to fire (Dyrness 1969,Beasley 1976). The hydrologic properties of thesubsoil are not affected by fire.

Porosity and soil structureOnly the most severe fires are likely to alter soil

properties sufficiently to directly affect soil poros-ity and structure (Dymess and Youngberg 1957).These conditions may occur where large concen-trations of dry fuel are burned over a dry forestfloor and soil.

Soil structure improves the macroporosity ofsoil responsible for the high infiltration capacitytypical of forest soils of the Pacific Northwest(Dyrness 1969, Harr 1977). It is altered when fireburns the organic matter that helps bind soil parti-cles together (Dymess and Youngberg 1957). Thebreakdown of soil structure frees individual soilparticles, making them more susceptible to trans-port by raindrop splash and overland flow. Smallersoil particles may also move downward into themacropores of the underlying aggregated soils, ef-fectively blocking the macropores, sealing the soilsurface, and reducing the infiltration capacity.

Following a severe fall prescribed burn, soilstructure was affected over less than 8 percent of aseverely burned site in the Oregon Coast Range(Dymess and Youngberg 1957). But further de-

Effects of Fire on Soil Erosion 163

struction of soil aggregates may occur from rain-drops falling on exposed soil (Packer and Williams1973).

The most severe fires may occasionally raisesoil temperatures enough to alter clay mineralogyand fuse silicate minerals together into cinder-likerocks (Dyrness and Youngberg 1957). Fusion ofsoil particles is less common than the loss of soilstructure and is generally confined to areas withthe highest concentration of woody debris, such asnear landings. Current burning prescriptions,more complete utilization, and removal of otherwoody debris generally prevent the high fireseverities that cause fusion of soil particles.

Soil temperatureAll fire may cause some changes in the thermal

properties of soil, but large changes in soil temper-ature usually result from the loss of vegetation andforest floor. The forest floor is a heat sink whenwet and acts as an insulator when dry; both at-tributes help minimize soil temperature fluctua-tions.

Burning can alter soil color and the ability of thesoil to absorb heat. Charred or blackened surfaces

absorb more heat than unburned litter layers orlighter colored soil. As a consequence, burning of-ten results in higher soil temperatures, and greaterdiurnal temperature fluctuations and extremes(Isaac and Hopkins 1937, Neal et al. 1965). Thesechanges may slow the rate at which vegetation isreestablished and increase the period when soil ismore susceptible to erosion processes.

Increased frequencies of low temperatures cancause repeated freezing and thawing of soil ex-posed following burning of the forest floor. Ice lay-ers forming in soil may destroy soil structure, sep-arate soil aggregates, and uproot shallow-rootedplants as the surface soil is heaved upward. A sin-gle frost heave cycle may temporarily raise thesurface soil by several inches. Heaving of soil isaccomplished by the transfer of soil water fromdeeper in the soil profile to the freezing front(Chalmers and Jackson 1970, Heidmann 1976).Maximum heaving occurs when the heat lost fromfreezing water is balanced by the transfer of warm-er water deeper in the soil profile to the freezingfront. Frost heaving is most severe in moist, medi-um-textured soils with relatively high rates of un-saturated hydraulic conductivities.

Figure 14-2. Vegetation and the forest floor reduce the energy of water flowing through the litter and reaching the soil.Length of the shaft on the arrow suggests the relative energy of the raindrop or flowing water. A severe burn increases theenergy of rain striking the ground and of flow across exposed soil. The flow of water into and through the surface mayalso be reduced.

164 Natural and Prescribed Fire in Pacific Northwest Forests

Fire Effects on HydrologyFire-induced changes in vegetation, forest

floor, and soil properties may alter the movementof water over as well as into the soil. The reductionor loss of the forest floor has the greatest potentialfor altering hillslope hydrology but is often con-founded by the concurrent loss of vegetation byfire and harvesting. The largest changes inhillslope hydrology are likely to occur followingchanges in soil properties but are generally limitedto small areas where fire was severe. Changes inhillslope hydrology will ultimately cause somechanges in channel hydrology and erosion proc-esses.

Hillslope hydrologyVegetation intercepts precipitation and alters

the energy of falling raindrops striking the ground(Fig. 14-2). Although some intercepted precipita-tion may evaporate or flow down the stems ofplants to the soil surface, the changes in the energyof droplets striking the ground have a major im-pact on erosion of surface soil.

Vegetation killed by fire no longer transpiressoil water, resulting in reduced on-site storage ofwater during subsequent precipitation events.Changes in soil moisture are least during the win-ter when transpirational rates are low but low tran-spiration causes soil moisture to be higher for alonger period of the year and increases the annualyield of water (Chapter 17; Han 1976, Klock andHelvey 1976, McNabb et al. 1989).

The loss of tree cover following a severe wildfiremay increase snow depths and affect melt rates.Changes are similar to those following clearcutharvesting. Snowpacks in openings will melt fast-er as a result of direct radiation and contact with amore turbulent, warm air mass. In the PacificNorthwest, heat exchange and air turbulence dur-ing rain events accelerate the melt of thin snow-packs; this is most likely to occur on middle eleva-tion sites with intermittent snowpacks (Han 1981,Berris and Han 1987). Increased melt rate in-creases the potential for surface erosion, masswasting, and stream channel changes.

The loss of vegetation will have minimal effecton erosion of surface soil if the forest floor remainsrelatively undisturbed, because the forest floor ismore effective at adsorbing the impact of fallingraindrops. In addition, the forest floor stores wa-ter, slows the flow of water over the soil surface,

helps maintain the porosity of the surface soil, andreduces the transport of sediment by surface wa-ter.

Depending on thickness, the forest floor mayhold up to 2 inches of water (Gessel and Balci1965); however, the amount of water that can bestored in the forest floor during a precipitationevent will depend on the initial water content. Fol-lowing a severe slash fire in the Oregon CoastRange, Dyrness et al. (1957) estimated that the de-struction of a 2-inch-thick forest floor reduced soilwater storage by 0.75 inch of water. More impor-tantly, the forest floor temporarily detains precipi-tation for later infiltration into the soil or slows theflow of water over the surface (Dyrness 1969). Theforest floor protects the soil from raindrop splashthat decreases porosity and increases the potentialfor overland flow (Packer and Williams 1973).

Fire has a minor effect on the ability of the soilprofile to hold water. The water storage capacityof the surface 2 inches of soil was reduced by 0.25inch following a severe broadcast burn in the Ore-gon Coast Range but the loss was observed to oc-cur only over a small percentage of the site (Dyr-ness et al. 1957). The importance of this loss isminor compared to the overall water-holding ca-pacity of soils capable of holding 6 or more inchesof water and temporarily detaining nearly an equalamount (Dyrness 1969).

Deeper soil horizons, with lower permeabili-ties, can cause a temporary rise in the water tableduring rainfall/snowmelt (Han 1977). The watertable may eventually reach the soil surface duringstorms of moderate to high intensity and long du-ration; this condition may produce overland flow,increasing the potential for surface erosion. Firemay cause overland flow on sites where it normal-ly does not occur by reducing the permeability ofthe surface soil that would normally be adequateto transport water throughout the slope (Packerand Williams 1973). This is the most likely cause ofoverland flow on sites having thin surface soil hori-zons, well-developed subsurface horizons of lowpermeability, high percentage of coarse frag-ments, or a shallow soil over bedrock.

Overland flow is also less likely on undisturbedforest slopes of the Pacific Northwest coast be-cause most winter storms are of low intensity.Summer thunderstorms over the high mountainsof the region, however, may produce higher-inten-sity storm events of shorter duration (1.5 inches in

Effects of Fire on Soil Erosion 165

30 minutes) than winter storms (Helvey 1973).These storms are generally local events whose fre-quency of occurrence has not been measured. Incontrast, much higher rates of sustained precipita-tion occur in Midwest and Eastern forests wherelarge thunderstorms are common (Orr 1973, Patric1981).

The potential for overland flow also increasesfor a specific rainfall intensity if the soil profile isnearly saturated from a previous storm event orresults in rapid melt of snowpack (Meeuwig 1971,Han 1981, Berris and Harr 1987). This is an impor-tant factor increasing runoff from small water-sheds in the Pacific Northwest where rain-free pe-riods are often short during the wettest months ofthe year (Istok and Boersma 1987).

Channel hydrologyThe reduction in transpiration increases annual

water yield from a few inches in forests east of theCascade crest to approximately 20 inches in west-ern Oregon forests (Chapter 17; Helvey 1972, Han1976, Klock and Helvey 1976). Increases in floware most noticeable in early fall, although flow alsoremains higher through the summer months; theseincreases have little effect on channel erosion westof the Cascade crest because the flows generallyremain below those necessary for significant sedi-ment transport. Changes in spring snowmelt fol-lowing wildfire in high elevation and eastside for-ests, however, may result in increased peak flows(Helvey et al. 1976). Winter peak flows in westsideforests seldom are affected by fire unless it chang-es soil physical properties sufficiently to causeoverland flow (Chapter 17; Harr 1976). Althoughoverland flow can speed the movement of4surfacewater to streams, thereby increasing peak flows, itis not known to what extent higher sediment yieldsmay be attributed to such changes in flow.

Fire Effects on Erosion ProcessesErosion is an important factor shaping the land-

scape of the Pacific Northwest and, historically,disturbance of forest ecosystems by fire has been amajor factor affecting geomorphic processes. As aresult of disturbance, geomorphic processes andaccompanying erosion vary both temporally andspatially (Swanson 1981). In addition to the com-plex factors affecting natural rates of erosion (Fig.14-1), forest operations such as road construction,

harvesting, and site preparation also influence ero-sion, making it more difficult to allocate erosion toa specific type of disturbance, such as prescribedfire.

Although the literature on the direct effects offire on vegetation and soil is voluminous (Chapters4, 12, and 13; Wells et al. 1979, Feller 1982), a con-ceptual framework for estimating erosion has notbeen formulated for all types of erosion processesaffected by fire. Several watershed studies havebeen installed in the past few decades that providevaluable information as to the direct effects of fireon larger areas, but the complexity of the erosionprocesses involved requires careful review beforeextrapolating to other sites. Studies of erosion atthe level of forest ecosystems and on the timescaleof repeated disturbances are extremely rare be-cause of their greater complexity and the longerperiods of time necessary to observe changes(Swanson 1981).

The following is an overview of soil erosionprocesses—surface erosion, mass wasting, andchannel erosion. Changes in hillslope and channelhydrology dominate most forms of erosion butgravity becomes increasingly important onsteeper slopes. Erosion processes and transport ofsediment are seldom constant but typically accel-erate in response to ecosystem disturbance.

Surface erosionVegetation and soil properties altered by fire af-

fect surface erosion in a variety of ways. Splash,sheet, rill, and gully erosion caused by changes inhillslope hydrology, frost heaving caused bychanges in the soil temperature regime, and ravelfrom the gravitational movement of materialdownslope all contribute to surface erosion. Theinteraction of geomorphic processes, soil and hy-drologic properties, climate, and landform deter-mine the relative importance and magnitude ofthese processes on a specific site.

Splash erosion occurs when raindrops strike ex-posed mineral soil with sufficient force to dislodgesoil particles and small aggregates. Vegetation andthe forest floor generally protect the soil fromsplash erosion. Splash erosion on exposed soil isleast under low shrubs, forbs, and grasses becauseof the short distance intercepted raindrops musttravel to the soil. Interception of raindrops by foli-age and stems of tall trees, including fire-killedtrees without needles or leaves, generally in-

166 Natural and Prescribed Fire in Pacific Northwest Forests

creases drop size and subsequent splash erosion(Herwitz 1987).

Precipitation intensity and slope steepness af-fect splash erosion less than the size of the soilparticles or aggregates (Farmer 1973, Yamamotoand Anderson 1973). Fine sand-sized particles(less than 0.004 to 0.01 inch diameter) are mosteasily transported in droplet splash; larger parti-cles of single-grained soils are more easily dis-placed than those in clayey soils (Farmer 1973).

When soils are saturated, splash erosion in-creases markedly; part of the increase is from a 2-fold increase in the size of material susceptible todetachment by raindrop impact (Farmer 1973). Inaddition, when raindrops strike saturated soil theycause positive hydrostatic pressures in the surfacesoil from the soil deforming under their impact.These positive pressures are transmitted outwardand upward from the point of impact, aiding parti-cle detachment (Al-Durrah and Bradford 1982).This process is an important factor responsible fora significant increase in splash erosion of finer-tex-tured, single-grained soils.

Splash erosion is an underrated and often misdi-agnosed surface erosion process in the PacificNorthwest. It has not been measured in this regionas it has elsewhere (Farmer and Van Haveren1971). It is often confused with sheet erosion be-cause the saturated soil conditions most condu-cive to splash erosion often cause the overlandflow responsible for sheet erosion. Splash erosionis generally uniform across a slope but sheet ero-sion results in more variable loss of soil and pro-duces deposits of sediments in depressions and be-hind obstructions.

Splash erosion generally occurs whenever soilis exposed; however, exposure of rock fragmentsin some soils will eventually protect the underly-ing soil from raindrop impact. Partial retention ofthe forest floor during prescribed burning is criti-cal to reducing splash erosion. The relative impor-tance of splash erosion as a geomorphic processincreases if prescribed fires that expose soil be-come more frequent than past wildfires.

Sheet and rill erosion. Once soil particles aredetached by splash erosion, they are more easilytransported in overland flow. The hydraulic ener-gy of water flowing over the soil also has the abilityto detach soil particles. Transport is often as sheeterosion where water flow is not concentrated intosmall channels (Meeuwig 1970).

Sheet erosion increases exponentially with in-creasing slope steepness and as the clay content ofthe soil increases (Meeuwig 1970). Organic matterhas a variable effect on sheet erosion; coarse-tex-tured soils become more erodible as organic mat-ter increases while fine-textured soils become lesserodible. In general, soils with a relatively highpercentage of sand particles (0.002 to 0.08 inchesin diameter) are the most erodible. These includemany soils derived from granite, sandstone, andvolcanic ash.

Sheet and splash erosion are most severe imme-diately following exposure of the soil to rain orsnowmelt. These forms of erosion decrease rapid-ly after the first year, primarily because of reestab-lishment of vegetative cover, but also because of"armoring" of the surface by larger particles andaggregates (Megahan 1974). Armoring may be thedominant process reducing erosion of gravellysurface soils (soils with greater than 35 percentrock fragments). Reports of sheet erosion follow-ing prescribed burning are rare. Some transport ofsediment in overland flow on burned sites oc-curred during snowmelt in western Montana, butthe rate was less than 200 pounds per acre per yearand only lasted 2 years (DeByle and Packer 1972).

Rill and gully erosion are less common in forestsoils than in agricultural soils because of greatersurface roughness, more rock fragments, and ab-sence of tillage that regularly mixes and loosenssoil horizons. Rill erosion has been reported onerodible soils following a severe wildfire in un-burned logging slash, although tilling was not evi-dent in adjacent uncut timber killed by the wildfire(Megahan and Molitor 1975). Gullies are uncom-mon in undisturbed forest ecosystems (Heede1975).

Frost heaving increases the downslope move-ment of soil when the ice lenses supporting soilparticles melt, allowing the soil to drop verticallyto the surface. Rapid warming or rain also cancause frost-heaved material to slide or flow downthe slope, particularly if the underlying soil re-mains frozen. Furthermore, the loosening of soilparticles by frost heaving makes the individualparticles more susceptible to transport by othersurface erosion processes.

Fire increases the potential for accelerated ero-sion by frost heaving when it consumes the forestfloor and exposes mineral soil. The contribution offrost heaving to accelerated surface erosion, how-

Effects of Fire on Soil Erosion 167

ever, depends on the texture of the soil, landform,and climate. Daily, or periodic, freeze-thaw cyclesin temperate climates are most likely to increaseerosion from frost heaving. Frost heaving is gener-ally less in cold climates because the freezing frontmoves progressively deeper into the soil, andthawing is infrequent or snow cover insulates thesoil.

Ravel. The movement of soil particles and or-ganic debris down steep slopes in response togravity is a geomorphic process accelerated byfire. This process is often referred to as "dryravel," but movement may occur during any sea-son (Anderson et al. 1959); referring to surfaceerosion by gravity as ravel is a more encompassingterm. The material moving may include soil, grav-el, cobbles, boulders, and organic debris (Figure14-3). Detachment can occur by drying and shrink-ing of the soil particles, frost heaving, animal dis-turbance, and decomposition or burning of sup-porting organic debris. Because transport is bygravity, substantial ravel occurs only on slopes ex-ceeding the angle of repose—approximately 35degrees (Mersereau and Dyrness 1972, Bennett1982).

Ravel is a natural process occurring on steepforest slopes, but rates are often low because veg-etation, forest floor, and other woody debris slowor stop the movement. Burning initially disturbs oreliminates many of the organic structures holdingloose material on the slope. As a result, large in-creases in ravel are observed during and immedi-

Figure 14-3. Ravel is the downslope movement of surfacesoil, rock fragments, and organic debris. Rates increaseexponentially as slopes exceed 60 percent.

ately following a burn. In the Oregon Coast Range,two-thirds of the ravel measured the first year fol-lowing prescribed burning occurred in the first 24hours (Bennett 1982). Part of the accelerated ravelfrom burning of harvested sites, however, may bematerial initially dislodged by movement of logsand equipment over the soil surface during har-vesting (McNabb and Crawford 1984). Slash andother woody debris remaining after harvestingtrap loose material that is released by burning.

Locally, ravel is an important geomorphic proc-ess that is responsible for talus slopes and scree(gravel layers) that may bury soil horizons onsteep slopes. Rates of ravel following prescribedburning are several hundred times greater onslopes greater than 60 percent than on less steepslopes (Mersereau and Dymess 1972, Bennett1982). Also, vegetation is established more slowlyon slopes with high ravel; consequently, ravel con-tinues at elevated rates for a longer time (morethan a decade). Ravel deposited directly instreams is readily transported from the watershedby fluvial processes (DeBano and Conrad 1976).

Mass wastingRain, snow, and rain-on-snow events may trig-

ger the downslope movement of one or more soilhorizons, parent material, and sometimes the un-derlying rock, by mass wasting. Mass wastingincludes debris flows, debris slides, debris ava-lanches, earthflows, slumps, and creep, depend-ing on the landform, and the depth, rate, and prop-erties of the material moving (Burroughs et al.1976). In the Pacific Northwest, steep slopes, highrainfall, a history of tectonic uplift, and rapidweathering of weak rocks combine to make masswasting a dominant erosion process.

Fire leads to an apparent reduction in soilstrength following the decay of root systems offire-killed vegetation (Burroughs and Thomas1977). Live roots increase the stability of shallowsoils on steep slopes by binding the soil mantleacross potential failure surfaces (Ziemer andSwanston 1977). The root component of soilstrength is less significant in deeper soils where theroot zone occupies a smaller percentage of a land-slide zone of failure.

The potential for shallow mass wasting in-creases for several years during the period whendead roots decay and before the roots of new vege-tation become fully established (Burroughs and

168 Natural and Prescribed Fire in Pacific Northwest Forests

Thomas 1977). Soil strength is reduced more whenconifers are killed because most conifers cannotsprout and maintain a viable root system, as domany hardwood tree and shrub species.

Soil creep is generally a slow process in whichthe soil mantle may move downslope only a fewhundredths of an inch to a few inches annually.The rate is thought to be affected by the length oftime a soil remains wet (Gray 1973, Harr et al.1979). Soil creep contributes to accelerated ero-sion along stream banks as the banks encroach onstream channels. Devegetation lengthens the sea-sonal period when soil moisture is high (Chapter17; Rothacher 1973), which may increase the an-nual rate of soil creep.

On steep slopes at high elevations, fires may killvegetation and consume organic debris that helpanchor snowpacks and prevent or limit the extentof snow avalanches (Swanson 1981). The loss ofvegetation may also increase snow accumulationand hence the risk of avalanches. Avalanches mayentrain soil and rocks by scour and uproot trees,including unburned vegetation in the runout area.Avalanche tracks can be slow to revegetate be-cause of repeated avalanches.

Of the several forestry operations that can af-fect mass wasting, the effect of prescribed burningis usually minor. Most of the potential for the rootsystems of trees to reduce the risk of mass wastingis lost when the trees are harvested. Road con-struction and harvesting, rather than prescribedburning, are the dominant factors contributing toincreased mass wasting in managed forests(Swanston and Swanson 1976). Wildfires that killvegetation can have an equivalent or greater effecton mass wasting than harvesting, but the amountof road constructed to harvest fire-killed timberwill have an important effect on the overall rate oferosion.

Channel erosionFluvial transport is the main mechanism moving

soil and nutrients from watersheds. This process isdiscussed in Chapter 17 but part of the fluvialtransport process involves channel erosion, whichis relevant here.

Streambank cutting is primarily aided by en-croachment of streambanks into channels fromcreep and other mass wasting processes (Fig. 14-4). Of lesser importance are effects due to in-creases in peak flow. Changes in hillslope hydrolo-

Figure 14-4. Material which encroaches upon streamchannels by surface erosion, creep, or mass wasting maybecome unstable when supporting vegetation or debris isconsumed by fire.

gy often result in an expanded network ofperennial and intermittent streams which trans-port water only following major forest disturb-ances or extreme peak flows (Fig. 14-5). Seldomused and intermittent stream channels are suscep-tible to erosion because they are less likely to bearmored with rock (Helvey et al. 1985).

Fire increases channel erosion as a result of al-tered hydrology and sediment availability (Chap-ter 17; Helvey et al. 1985, Berris and Harr 1987).These conditions may occur when loss of coveraffects snow accumulation or melt, water yield is asmall percentage of the total precipitation, or lossof transpiring vegetation temporarily increasesstream flow.

Wildfires are far more likely than prescribedburning to increase channel erosion. Prescribedfires, in addition to typically being less severe, aregenerally separated from larger channels by an un-cut, unburned buffer strip. Wildfires generallyconsume all the vegetation and fuels alongstreams, and the topography along intermittentstreams is likely to cause the most severe fire.

Acce lerated erosionErosion is the consequence of numerous

geomorphic processes. Much of the material mov-ing on hillslopes in Pacific Northwest forests istemporarily stored on slopes or in stream chan-nels. Some material is stored for very long period,of time, sometimes centuries in small watershed,

UNBURNED

BURNED

Figure 14-S. Severe fires which remove vegeta ion andalter soil properties may increase runoff and expand thestream network into smaller, less frequently used chan-nels. Fluvial transport of sediment from the new channelsmay be high because organic debris which stored sedi-ment was consumed by fire, and the channels generallyare not armored.

and longer in larger basins. Noticeable movementgenerally awaits severe disturbances. The fre-quency and severity of disturbance such as wild-fire, windthrow, road construction, harvesting,and site preparation can affect the balance be-tween the relatively low baseline erosion of theundisturbed forest and the accelerated erosiontriggered by disturbance of vegetation (Swanson1981).

Although the baseline rate of erosion provides areference for measuring accelerated erosion from

Effects of Fire on Soil Erosion 169

natural forest disturbances, the average long-termrate of erosion is an integration of the baseline andaccelerated rates of erosion. The long-term rate oferosion is higher than baseline and can only be es-timated over a timescale of multiple disturbances.In many forest ecosystems of the Pacific North-west, the historical frequency of vegetation dis-turbance resulting in accelerated erosion is pre-sumed to be closely associated with the frequencyof wildfire. In regions with a long interval betweenfires, accelerated erosion is a smaller percentageof the long-term average rate of erosion than inregions where natural disturbance is more fre-quent (Swanson 1981).

Forest management affects both the frequencyand severity of disturbance that, in turn, may alterthe accelerated, baseline, and average rates ofsediment production. The effects of prescribedburning on these rates can only be assessed byknowing how forest management practices affectthe frequency of fire and how fires in managed for-ests affect erosion. In general, prescribed fire in-creases erosion less than associated forest prac-tices or severe wildfire in the Pacific Northwest,particularly when burning results in partial reten-tion of the forest floor. The risk of detrimental ero-sion increases when burning fails to leave some ofthe forest floor.

Soil Loss Following FireIn a hypothetical analysis, Swanson (1981) sug-

gests that accelerated erosion following a majorforest disturbance in a small watershed in thewestern Oregon Cascades may persist as long astwo to three decades and account for 25 percent ofthe total long-term sediment yield, assuming ma-jor disturbances occur at an average interval of 200years. Accelerated erosion accounts for a higherpercentage of the total long-term erosion whendisturbances are more frequent. More frequentdisturbances are likely to increase surface erosionand mass wasting because mineral soil is exposedand root contribution to soil strength is reducedfor a larger percentage of the time. Increased ero-sion is likely because of the greater probabilitythat a site may be susceptible when a storm capa-ble of causing major erosion occurs.

Estimates of accelerated erosion are highly var-iable because of the complex interaction of vari-ous factors that affect erosion processes (Fig. 14-

10

a)

a>

E

z0

0ctUJ

10

170 Natural and Prescribed Fire in Pacific Northwest Forests

Erosion ProcessMajor Factors Affecting

Erosion Processes

Amplitude of Peak Timing of Peak

-4,1-

Splash Soil exposureSoil textureSlopeAmount of rainfallIntensity of rainfallLoss of vegetation

Formation of forest floorRate of revegetation

Sheet

.÷....e---

Soil exposureInfiltration capacityArmoring of soil surfaceAmount of rainfallIntensity of rainfallAntecedent precipitation

Armoring of soil surfaceFormation of new forest

floorTiming of major storm

events

Ravel Soil exposureSlope steepnessType of material exposedPosition on slopeRemoval of supportingorganic debris

Other disturbances

Rate of revegetationOther disturbances

Mass Wasting SlopeAmount of rainfallAntecedent precipitationLoss and importance of

root strengthRoad construction/

harvesting

Major storm eventsRevegetation (root

strength)

Channel Size and stability ofchannelArmoring of channelAmount of rainfallAntecedent precipitationRain or snowRemoval of organic

debris

Major storm eventsMass wastingRevegetation

I1 10 100411—YEARS—►

Figure 14-6. The interaction of numerous factors affects erosion processes. Important factors which affect both theamplitude and timing of specific types of erosion are listed in conjunction with the relative effect on erosion rate:inherent to all factors is the effect of fire.

01

DAY

FIRE

Effects of Fire on Soil Erosion 171

6). For example, ravel is high immediatelyfollowing a prescribed burn (Bennett 1982), but al-so includes ravel dislodged by harvesting andstored in slash (McNabb and Crawford 1984), andelevated rates of ravel can persist for years onsteep slopes if revegetation is slow (Mersereauand Dymess 1972). Other forms of surface erosionwill most likely be greatest during the first fewyears following a fire, depending on how quicklythe site is revegetated and the surface becomes ar-mored. The potential for severe mass wasting isgenerally highest 4 to 7 years after disturbance(Burroughs and Thomas 1977, Ziemer and Swans-ton 1977), but can occur sooner or later dependingon precipitation events and rates of decompositionand reestablishment of root systems.

Erosion directly attributable to prescribedburning is rarely considered a serious problem inthe Pacific Northwest (Swanston and Swanson1976, Wells et al. 1979, Helvey et al. 1985). Wherefire-related erosion has been observed, the pre-scribed fires were locally severe or were con-founded by harvesting disturbance (Fredriksenet al. 1975, Beschta 1978, Bennett 1982). Directmeasurements of erosion specifically resultingfrom prescribed fires are invariably confoundedby erosion from road construction and harvesting(Swanston and Swanson 1976).

The potential for increased erosion from pre-scribed fire in a managed forest is primarily a func-tion of the frequency and severity of prescribedfire relative to that of past wildfires. The greaterthe difference in frequency or severity, the morelikely that rates of erosion will differ between nat-ural and managed forest conditions. Surface ero-sion will likely increase in managed forests finlessthe intensity of prescribed burning is such that aportion of the forest floor is retained. On the otherhand, managed forests will most likely have lowerfuel loads which should result in less severe burns.

Erosion following prescribed burning in the Pa-cific Northwest is different from other regions inthe United States where burning may be as fre-quent as every other year, soils are more erodible,or burning vegetation releases compounds whichcause the soil to be water repellent (Ralston andHatchell 1971, Megahan 1974, DeBano and Con-rad 1976). A combination of thick forest floors,high soil infiltration capacities and hydraulic con-ductivities, complex slope configurations, andlow-intensity precipitation events results in fewer

opportunities for serious erosion in the PacificNorthwest.

Effects of Soil Loss on Forest ProductivitySoil erosion decreases forest productivity when

the rate at which that soil is lost exceeds the rate ofsoil formation (Swanson et al. 1989), but forestproductivity may be affected by other fire-inducedchanges in soil than erosion (Chapter 12 and 13). Anet soil loss can reduce the volume of soil availablefor root occupancy, soil moisture storage, andavailability of nutrients. The ratio of soil loss tosoil formation by weathering provides a relativeindex of the sensitivity of forest ecosystems to theeffects of forestry practices on productivity. For anumber of reasons, such tolerance indices (theamount of soil loss a site can withstand withouthaving productivity impaired) either have notbeen developed or are highly speculative esti-mates for Pacific Northwest forest ecosystems(Wischmeier 1974, Pierce et al. 1984). Further-more, existing tolerance indices are based on soillost by sheet and rill erosion, which are relativelyminor forms of erosion in forests of the PacificNorthwest, except on severely disturbed sites(Swanson et al. 1989).

Loss of soil from some forest ecosystems ismore critical than others (Klock 1982, Swansonet al. 1989), and the history of past site disturbanceis important to interpreting these differencesamong sites (McNabb 1988). Losses of forest floorand soil are more critical on sites where the poten-tial for transpiration by vegetation is high, precipi-tation is low, and evaporation of water from ex-posed soil is significant (Flint and Childs 1987).Increased plant moisture stress caused by exces-sive evaporative and transpirational losses gener-ally have greater influence on revegetation andplant growth than the direct loss of soil waterholding capacity.

Locally, mass wasting may substantially reducethe productivity of the failed portion of a site(Miles et al. 1984), but prescribed burning has aminor effect on mass wasting. Reduced forest pro-ductivity from soil loss following sheet or rill ero-sion has not been measured; but where soil hasbeen removed mechanically, forest growth hasbeen reduced significantly (Glass 1976). Surfaceerosion rarely causes a critical loss of soil in Pacif-ic Northwest forests because of high infiltration

172 Natural and Prescribed Fire in Pacific Northwest Forests

rates and relatively low precipitation intensities(Wells et al. 1979). With the possible exception ofhighly erodible soils such as granitic soils, or thepossible formation of a water-repellent layer be-low the soil surface following an unusually severefire, surface erosion is unlikely to reduce forestproductivity in the region (Swanson et al. 1989).Prescribed burns which leave a portion of the for-est floor covering the soil can minimize all forms ofsurface erosion.

The loss of nutrients during a fire is much morelikely to reduce the productivity of Pacific North-west forests than is erosion of surface soil (Chap-ter 12). Significant reductions in site nutrients andthe associated forest floor and woody debris, how-ever, can reduce soil organic matter and affect soilbiology over an extended period of time (severalrotations). Reduced soil organic matter will causedetrimental changes in soil physical propertiesthat could substantially increase surface erosion.Thus, using prescribed burning techniques whichminimize the loss of nutrients will lessen the risk offuture soil erosion.

Managing Prescribed Burning to ReduceErosion

Based on the few studies of wildfires and severeprescribed burns, accelerated erosion is greateston sites with steep slopes and highly erodible soilsprone to overland flow (Megahan and Molitor1975, Helvey et al. 1985). Retention of some forestfloor over all soil until postfire vegetation begins toreplace lost forest floor materials, particularly inthe riparian zone, is an important method of reduc-ing surface erosion (Brown and Krygier 1971). Thethickness of forest floor which should be left toachieve this objective varies by forest ecosystembecause of differences in decomposition rate andhydrology. In addition, maintaining some vegeta-tion in intermittent stream courses with poorly de-fined channels may also be an effective techniquefor reducing erosion when the stream network ex-pands during major storm events.

Using prescribed burning techniques whichleave a portion of the forest floor covering the soil(Chapter 22) prevents detrimental changes in soilproperties. Of the many burning techniques avail-able, burning when the forest floor is moist is par-ticularly effective for retaining some forest floor.Retention of a portion of the forest floor is least

likely to occur during late summer and fall burnsbecause the forest floor remains dry even afterseveral inches of precipitation.

Grass seeding sometimes speeds revegetationof severely burned sites after a late summer wild-fire. Seeding generally has minimal impact on de-terring surface erosion in the Pacific Northwestbecause most soils are generally well aggregated,seldom subject to overland flow, and revegetatequickly (Dyrness et al. 1957, Helvey and Fowler1979, Wells et al. 1979). Grass seeding may suc-cessfully reduce the immediate risk of surface ero-sion in riparian zones, soil disturbed by construc-tion of fire lines, and areas with particularly steepslopes and erodible soils. Grass seeding for ero-sion should be used judiciously, however, to avoidjeopardizing the reforestation and natural revege-tation of burned sites. Grass seeding may hinderthe establishment of native vegetation on burnedsites (Taskey et al. 1989) and increase the risk ofmass failures on steep slopes in later years.

All prescribed burning practices that minimizenutrient losses and protect soil biota are compati-ble with reducing the potential for soil erosion.Practices that burn fuels when the forest floor isdry are incompatible with minimizing impacts onsoil erosion and forest productivity.

ConclusionSevere wildfires have been, and potentially re-

main, a dominant factor that temporarily acceler-ates several erosion processes, including masswasting, surface erosion, and ravel, in the forestecosystems of the Pacific Northwest. Wildfires in-crease erosion by removing part or all of the vege-tation canopy, consuming the forest floor, andcausing detrimental changes in surface soil prop-erties. In contrast, prescribed fire alone is lesslikely to cause as much erosion, because harvest-ing rather than fire removes most of the vegeta-tion, and the intensity of the burn can be con-trolled to reduce its severity. It is possible, andcommon, to prescribe burn a site while leaving aportion of the forest floor covering the soil and notto affect soil physical properties.

Retention of some forest floor covering the min-eral soil and rapid revegetation of burned sites ef-fectively prevent or limit most accelerated surfaceerosion. Therefore, the thickness and areal cover-age of forest floor remaining after a prescribed

Effects of Fire on Soil Erosion 173

burn is the most important measure of the poten-tial for surface erosion. Protection of the forestfloor is more difficult in the managed forests ofnorthwestern Oregon and western Washington be-cause the planned rotation length in these forestsis less than the natural fire frequency. Increasedfrequency of burning results in thinner forestfloors that will require greater adherence to burn-ing prescriptions designed to minimize consump-tion of the forest floor. Elsewhere in the region,retention of the forest floor is no less important.Although rotations may be longer than the intervalbetween wildfires, slower forest growth and slow-er production of a new forest floor will requiregreater retention of forest floor to sustain its pro-tective role.

Surface erosion caused by prescribed burninghas seldom been measured in the Pacific North-west because it is not perceived to be a majorcause of erosion from forested lands. Road con-struction and harvesting are more serious causesof erosion, particularly mass wasting on steeperslopes. Furthermore, erosion by these practicesoften obscures erosion caused by prescribed burn-ing. Measurement of surface erosion processes,however, will become more important in the fu-ture because forest floors in managed forests willbe thinner and more difficult to protect from burn-ing, particularly if other constraints such as con-cern for air quality or fire hazard reduction resultin burning when fuels and forest floor are dry.

The judicious use of prescribed burning in mostforest ecosystems of the Pacific Northwest is notanticipated to cause sufficient erosion to adverselyaffect long-term forest productivity, although theproductivity of a few highly erodible soils may bereduced if burned frequently or severely.

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