encyclopedia of inland waters || fires

FiresE Prepas, N Serediak, G Putz, and D W Smith, Lakehead University, Thunder Bay, ON, Canada

ã 2009 Elsevier Inc. All rights reserved.

Introduction

Fire is a periodic occurrence in vegetated landscapes,with variable impacts dependent on both biotic andabiotic factors. It has a distinct impact on freshwatersystems compared with other landscape disturbances.Stable watersheds exhibit chemical (mass concentra-tion) and hydrological (discharge and velocity) fluxeswithin a predictable range of variability. This can bealtered dramatically by wildfire, which is essentially anoxidative chemical reaction of varying duration andintensity. The heat generated by wildfires of sufficientsize can shift nutrient composition, redistribute cations,and alter pH balances in burned watersheds and subse-quently in the waters that drain them. Fire effects canalso induce soil hydrophobicity, decrease infiltration,and enhance lateral subsurface flow, thus increasingrunoff and the risk of sedimentation and landslip.Fire is a natural disturbance in most instances, and

some forests have beenmanaged in attempts to emulatethe physical changes fire exerts within watersheds. Incontrast to a disturbance like forest harvesting, how-ever, wildfire is first a chemical process, which is thencompounded by mechanical disruption. This is a fun-damental difference from commercial harvesting,where the mechanical disturbance (i.e., tree removal,soil disruption, and compaction) initiates the resultingchemical response. This is a central factor in the effectsof wildfire on freshwater systems.

The Structure of Wildfire

To understand the basic effects of fire on aquaticsystems, it is helpful to first qualify what constitutesa wildfire and to differentiate between wildfire andprescribed burns. Although both types of fires can becaused by human activity, the components of a pre-scribed burn are held under tight control to achieve adesired land management objective. Generally, thisoutcome is to reduce fine fuel loading (accumulationof shed needles, leaves, bark, and other organic mate-rials), manipulate the understorey structure, controlsusceptible tree pathogens, and condition sites fortree regeneration. In contrast to prescribed burns,wildfires behave randomly and have varying degreesof impact relative to the particular physical descrip-tors that constitute a watershed, and to the size andstructure of the fire itself.Fire originates where an available fuel source coin-

cides with a successful ignition event. Ignition can be

74

anthropogenic (accidental or prescribed) or natural(lightening strike). In general, fires can be classified asone of three types: surface (lowest impact), intermit-tent crown (moderate impact), and crown fire (high-est impact). Several countries now use the FireWeather Index (FWI) to provide a numerical assess-ment of fire type (Figure 1).

Wildfires generally move in an elliptical patternfrom the ignition source, driven by prevailing winds.In forested regions, combustible understorey oftenprovides the initial fuel, after which fires may developprogressively into the overstorey or crown. Crownfires can produce frontal fire intensities sufficient toignite vegetation too humid to serve as an initial fuelsource. Although wildfires often have a single ignitionsource, wind transport of hot ash and embers, espe-cially from crown fires, can result in multiple ignitionevents attributable to the same fire. After successfulignition, fires are self-propagating until the fuelsource is exhausted, oxygen is consumed, or untilfuel is rendered incombustible (e.g., through wettingfrom rainfall or active suppression).

The characteristics of wildfires can be divided intofour primary components: intensity, severity, size, andfrequency.

Fire Intensity

The intensity of an active fire is a measure of verticalheat transfer above ground, and the degree to whichvegetation mortality has occurred. Although tempera-ture extremes are an integral part of any fire, tempera-ture itself is not a good indicator of fire intensity.Woodyfuels ignite at roughly 350 �C and maximum tempera-tures in forest fire flames can reach 1000 �C. However,flames 5 cm high have a markedly different measure ofintensity relative to flames 5m high, despite having thesame core temperature. Therefore, standardized mea-sures of fire intensity have been developed. One exam-ple is the Canadian Forest Fire Behaviour PredictionSystem, which quantifies fire intensity as energyreleased per unit length of fire front in kilowatts permeter (kWm�1) (Table 1). Fire intensities becomeenhanced as flammable vapors are released and them-selves ignited as fuel combustion progresses.

Fire Severity

The vertical transfer of heat from surface to belowground, as well as general biological impacts, are a

Fireweatherobservations

Fuelmoisturecodes

Firebehaviorindices

Temperaturerelative humidity

wind speedrain

Windspeed

Temperaturerelative humidity

rain

Temperaturerain

Fine fuelmoisture code

(FFMC)

Duff moisturecode

(DMC)

Droughtcode(DC)

Initial spreadindex(ISI)

Fire weatherindex(FWI)

Buildupindex(BUI)

Figure 1 Variables involved in quantifying the FWI rating. Reproduced with permission from Canadian Forest Service, Natural

Resources Canada.

Table 1 Ranges of fire severity and intensity with associated

fire types

Severity(�C)

Intensity(kW h�1)

Associated firetype

Low <180 100–2000 Surface

Moderate 180–300 2000–10 000 Intermittent crownHigh >300 >10 000 Continuous crown

Pollution and Remediation _ Fires 75

measure of fire severity. Crown fires are often farmore severe than grass fires, even though they typicallymove at slower speeds. For example, fast movinggrass fires have typical spread rates ranging from 25to 80mmin�1 and have been clocked at 300mmin�1.In comparison, crown fires advance at a rate of25–50mmin�1 and reach 100mmin�1 at top speed.Forested watersheds can experience intense downwardheat transfer during crown fires, enough to completelyash fine fuels and organic layers down to mineral soils(Figure 2). Extreme heat during wildfire can sterilizethe soil microbial community, volatilize soluble inor-ganic and organic compounds in plants, soil and water,and liberate labile mono- and divalent dominant ions.

Fire Size

The area and volume of materials consumed by wild-fire constitute the overall fire size.While human estab-lishment in many areas has increased the number offires, this may not have had as large an impact on totalarea burned per year as once thought, because fire sizeis often affected by the ignition source. Lightening-induced fires that occur in remote areas can developinto large wildfires before detection. Human-causedfires often occur in or near populated areas where theyare reported quickly and their effects are minimized

before sizeable areas are burned. These generaliza-tions do not take into account global regions wheretraditional burning for agricultural purposes is morefrequent and can occasionally develop into uncon-trolled bush or crown fires.

Fire Frequency

Naturally occurring wildfires are now seen as more ofan integral component rather than strictly a disrup-tion of watershed processes, thus the practice of firesuppression has begun to shift focus from a mandateof complete elimination to one of controlling progres-sion (where feasible). More effort has been exerted indetermining the periodicity of wildfires in presettle-ment forests through pollen counts, dendrochro-nolgy, and carbon deposition in lacustrine systems,and comparing these results with what occurs whenfire suppression becomes a part of watershed man-agement. There is a growing belief that climate hasthe greatest impact on the frequency of large, severefires. It is the large fires that cause most of the damageassociated with wildfire (Figure 3). Periodicity of nat-urally occurring wildfire ranges from 5 to 20 years invulnerable areas, up to estimates of 500 years in lessfire-prone regions.

Wildfires as an Ecological Impact onAquatic Systems

The effects of wildfire are felt differently within andamong watersheds, and are dependent on many fac-tors. Watersheds with a predominance of dry, resin-ous, and easily ignitable plant matter have a higherprobability of impact from fire, as well as those withbroad tracts of uninterrupted vegetation with few

(a)

(b) (c)

Figure 2 Examples of fire intensity in a burning jack pine forest based on the FWI rating system: (a) low intensity surface fire (FWI 9),

(b) moderate intensity intermittent crown fire (FWI 20), (c) high intensity continuous crown fire (FWI 34). Reproduced from Alexander, MJand De Groot, WJ (1988) Fire behavior in jack pine stands as related to the Canadian Forest Fire Weather Index (FWI) System, with

permission from the Canadian Forestry Service, Northern Forestry Centre, Edmonton, Alberta.

0%

20%

40%

60%

80%

100%

120%

Proportion of totalnumber of fires

Proportion of totalarea burned

Small firesLarge fires

Figure 3 Average fire size relative to number of reported fire

starts (from North American estimates). Large fires are defined as

burning over 200ha of land area.

76 Pollution and Remediation _ Fires

naturally occurring firebreaks, which serve to limitspread (firebreaks include features such as ridges,surface waters, and areas with little or poor qualityfuel). Fires burn more readily uphill because of theinherent upward movement of hot air, which resultsin differential burn patterns dependent on topography.

The presence of a tinder source is key: watershedswhere sufficient fine fuel sources exist with concurrentlow moisture levels are prone to more frequent andsevere fires than humid watersheds or areas where thelitter layer remains thin and soil organic content is low.Seasonal timing plays an important role in ignition,intensity, and severity. Wildfires that occur in earlyspring before adequate precipitation and prior to leaf-out can be much larger in size compared with late-spring fires that ignite after plants are foliated and thewatershed is sufficiently wet.

Globally, the effects of wildfire are more pro-nounced in regions where (1) hot, dry conditionsoccur regularly, (2) watersheds are primarily closedcanopy systems, and (3) fuel sources are readily com-bustible (Figure 4). Fire-prone regions possess manyfactors both pre- and post-fire that contribute tocumulative watershed impacts.

Pre-Fire Considerations

Conditions that exist prior to wildfire events have adirect impact on the structure of fires passing throughwatersheds. These components affect fire movement,intensity, severity, and duration, and post-fire res-ponses within the watershed.

Figure 4 Four 10day averages of global fire occurrences showing seasonal differences in fire locations over 2 years. Fire

intensities are represented by red (lowest), orange (moderate), and yellow (highest). Image courtesy of MODIS Rapid ResponseProject at NASA/GSFC.


Watershed Slope

Steeper, leeward slopes have a higher probability ofburning than windward areas of lower relief. Steepslopes are at risk for landslide after severe fires, espe-cially if the fire is followed by intense precipitation.Also, runoff and sediment loads are higher from steepslopes following fire.

Catchment Size

Burned watersheds of first- and second-order streamsize generally have a larger, faster response than thoseassociated with streams of increasing size, whereresponses may be dampened by proportionally moreunburned areas.

System Morphology

Watersheds with a high proportion of low-orderstreams draining high relief areas display more mea-surable shifts in chemistry post-fire in comparisonto regions of low topographic relief, where fire-related chemical changes may be ameliorated bybogs, fens, swamps, marshes, or open-water areas.The overall percentage of surface water in a drainagebasin will affect burn area, and the size of individualwater features will also affect fire movement (e.g.,the ability of the fire to cross a first-order versus afifth-order stream).

Soil Characteristics

The structure, porosity, existing hydrophobicity, andtype of soil underlying the drainage basin will affectchemical and infiltration responses post-fire. Thecondition and depth of the litter layer (alternatelyknown as duff or LFH horizon) contributes to fuelloading pre-fire, surface spread during fire, and pro-tection of mineral horizons post-fire.

Baseline Water Chemistry

The chemical composition of surface waters pre-firecan have a direct effect on post-fire response. Streamswith inherently low alkalinity are at increased risk forpH reduction from influxes of soil-bound, solubleacidic anions (e.g., sulfate, chloride) released duringprecipitation events following wildfire.

Weather Patterns

Ecoregions exist in concert with long-term weatherpatterns and there is a growing realization that periodicwildfires do as well. Areas with prolonged seasonaldrought are often at increased risk for shorter fire cyclesthat can include very largewildfires. In addition, globalphenomena, such as El Nino and La Nina events, canproduce localized drying cycles that create conditionsfor wildfire outside of expected regional normals, orexacerbate those which usually occur.


Timing

Wildfires that occur after very dry winters, in earlyspring before significant precipitation, or after dryingevents, are more severe than those that occur whenvegetation is actively growing. The importance oftiming can be diurnal as well as seasonal. Fires thatburn over several days tend to cycle, causing lessdamage at night, as available moisture condensesout of the cooler air. This reduces the severity of theburn and aids in creating unburned patches.

Fuel Load and Forest Age

Combustion for naturally occurring wildfires isdependent on the amount of fine fuel available foreither ignition or propagation. In early successionalforests, fine fuel inputs are primarily from the canopyin the form of shed needles, leaves, and bark. Thisgradually progresses to a composition includinglower growing vegetation (e.g., mosses, lichens, andshort grasses) as the forest matures. Successful igni-tion is less likely in younger forests or recently burnedareas where fuel loads have yet to reach an adequatedensity or quality. In harvested forests, fire risk maybe high if sufficient fine fuel remains as an exposedignition source in proximity to unharvested areas.

Canopy Type

In general, the greater the fire intensity, the greater isthe severity of impact. Frontal fire intensities reachtheir highest levels in closed canopy systems wherewildfires have the opportunity to burn upwards aswell as outwards. Watersheds consisting of open can-opy systems are more likely to experience lowerintensity surface fires that have decreased post-fireimpacts. Alternately, in grassland ecosystems whereurban settlement has created a shift in plant composi-tion approaching a closed canopy, crown fires mayoccur where they historically had not.

Post-Fire Impacts

Watershed characteristics affect the movement, pat-tern, and behavior of wildfire. Subsequently, fireimpacts watershed functions with immediate, inter-mediate, and long-term effects (Figure 5).

Immediate Effects: 0–5Years

What remains following wildfire is a fire legacy, themost visible impact of which is consumption of

aboveground biomass (Figure 6). In severe fires,much of the over- and understorey vegetation isreduced to standing dead and ash. If enough of theaboveground portion of trees and shrubs aredestroyed, root systems may die as well. Post-fireimpacts are influenced by the patchiness of the burn(burned versus unburned areas within the limits of thefire’s spread), mean distance to undisturbed areas,depth of ash, coarse woody debris inputs, and surviv-ing cover and root systems.

Nutrient cycling Nutrient losses are variable post-fire depending on the ecosystem and where the major-ity of nutrients were distributed before the fire (i.e.,soil or vegetation). In general, combustion of over-storey plants releases a more homogenous mixture ofinorganic nutrients into the ash layer relative to whatexisted prior to fire. Nutrient loss is felt differentlydepending on location: tropical forests tend to bephosphorus (P) limited, while northern boreal foreststend to be nitrogen (N) limited. Tropical regrowthwill be negatively affected if heavy rains follow severeburns and remove forest floor P fractions already inshort supply. Northern forests may have a greater lagtime before regrowth occurs if volatilized N losses arehigh, followed by P-leaching precipitation events(Table 2).

Heat-induced volatilization greatly affects N stores.Organic N released during combustion is oxidized togaseous forms (NO, NH3), which is then lost to theatmosphere. Inorganic N (primarily NH4) remains inthe ash layer and may lead to a stimulation of nitrifica-tion in the surviving microbial community, resulting inhigh levels of nitrate (NO3) that can persist for severalyears. With little appreciable atmospheric loss otherthan particulate transport, P remains as an ash constit-uent along with base cations (calcium, magnesium,potassium) and acidic anions (sulfate, chloride). Car-bon exists post-fire in soluble (dissolved organic car-bon) and insoluble forms (charcoal). The presence ofcharcoal as a strong adsorbent is thought to inhibitcertain fire-liberated compounds from joining thepost-fire soil nutrient cycle and from moving into sur-face waters. These include both particulate P, and sec-ondary metabolites that can inhibit plant growth.

Water movement following fire results in leachingand transport of soluble materials. Receiving watersmay experience multiyear increases in nitrates, dis-solved P, potassium, sulfate, and chloride. Less mobiledivalent calcium and magnesium tend to remain insoil, temporarily increasing soil pH until regenerationbegins to consume stores. Particulate inputs into sur-face waters also increase post-fire, most notably par-ticulate P following heavy rain events. The movementof previously soil-bound acidic anions can decrease

Vegetation(fuels)

Direct effects

Biotic and physical environment

Post-fire weather

Aquatic systemTerrestrial system

Indirect effects

Climate

Fire

Biogeochemistry water temperatureVegetation mortality soils

Species composition Nutrient cyclingHabitat dynamicsVegetation pattern

Erosion patternHydrologic processesWater yieldWoddy debris

Physical environment(geology, tophography,

geomorphology)

accumulation

Figure 5 The interaction of fire, biota, and the physical environment. Adapted from Figure 1 in Gresswell RE (1999) Fire and

aquatic ecosystems in forested biomes of North America. Transactions of the American Fisheries Society 128: 193–221, withpermission from the American Fisheries Society.


pH levels in stream waters, which is exacerbated incatchments draining regions with a high percentageof peatlands or with a history of acid deposition (i.e.,low pre-fire alkalinity). As well, rapidly developingregrowth is more likely to uptake basic cations overacidic anions, creating an additional relative increasein acidic export. Post-fire revegetation is often fromNO2-tolerant plants, such as fireweed (Epilobiumangustifolium) and raspberry (Rubus spp.) in borealforest regions, which are gradually replaced by latersuccessional plants as ammonification increases rela-tive to nitrification as the primary mechanism forinorganic (bioavailable) N formation.

Infiltration and runoff The heat generated by fireresults in water repellent soil conditions of varyingdegrees: ground fires in grass or chaparral can destroynaturally occurring surface hydrophobicity but createhydrophobic layers at depth, and intense crown firesare capable of producing a highly water repellentsubsurface layer in forest soils. The burning of ever-green plants with naturally high levels of waxes,resins, or oils (e.g., eucalyptus, pine and fir) is proneto inducing or increasing hydrophobicity in soils afterfire. Combustion of overstorey vegetation liberateslarge numbers of long-chain organic compounds.Flammable portions of these organic compounds are

Figure 6 Standing dead trees and heavy ash layer on

scorched soil following a 2001 continuous crown fire

near Delorme Lake, Alberta, Canada. Photograph byMark Serediak.


volatilized during the active fire, and the remainingfractions migrate into the soil to a depth governed bythe temperature gradient. Condensation occurs sub-surface and the hydrophobic compounds are depos-ited in a much more concentrated and uniform layerthan would normally occur (Figure 7). Hydrophobicconditions are also thought to increase with increas-ing soil organic matter and litter depth.The establishment of post-fire hydrophobicity has

a direct impact on water infiltration. Water move-ment can shift from primarily downward migrationto increased lateral flow over the hydrophobic soillayer. Induced hydrophobicity degrades followingfire, attenuating in influence anywhere from directlyafter the first substantial rain event up to 6 years post-burn. Although temporary, water export can be dra-matically enhanced, even if hydrophobic conditionsexist for only one episode of peak flow. Increaseddischarge has been noted in many watersheds follow-ing fire, with corresponding rises in coarse woodydebris inputs to streams and element export fromwatersheds. This is especially true for particulate frac-tions, as post-fire runoff moves laterally throughunconsolidated ash and burned soil layers uninhib-ited by vegetation.

Sedimentation and erosion In burned watersheds,sediment movement is most noticeable in precipitationevents immediately following fire, but may be sus-tained for several years, especially during seasonal peakflows. The post-fire shift from infiltration-dominatedto overland flow-dominated processes tends to pro-portionally increase the sediment carrying capacity.Many watershed studies have found substantial risesin water export and associated sediment loading afterfire. This can substantially increase the level of sus-pended solids in receiving waters, which in turn canaffect light penetration and productivity. The resultsof sediment relocation include two major factors:increased deposition of upland sediments in streamsand ultimately lakes, and increased erosion and bankincision as soil layers normally covered are exposedand peak flows increase after fire.

Burned soil horizons are more vulnerable to detach-ment from rainsplash and other erosion processes ifhydrophobic conditions exist, the ash layer is thin,vegetation is absent, or relief is high. Any or all ofthese situations result in a decrease of a watershed’sability to reduce sediment transport to surface waters.The movement of ash and silt into receiving waters canleave mineral layers unprotected and open to furthererosive impacts. If relief is high, sediment export canreach landslide extremes either through surficial debrisflows or subsurface failures resulting in soil slip. In-stream effects range from an increase in fine-fractionsediments to massive shifts in channel morphology.

Biota Relative to terrestrial plants, animals, andbacteria, aquatic organisms are largely protectedfrom fire by virtue of location. However, intensefires can produce enough smoke and ash to causeimmediate chemical toxicity, even in aquatic systems.In areas with established fire regimes, animals such assalmonid fishes exhibit adaptive strategies to surviv-ing fires. Migration can follow heat-induced stress inlower-order streams where fires are capable of raisingwater temperatures to over 20 �C. However, mobilityand other response mechanisms may fail for localpopulations when fires are extreme and escape routesare blocked. Phytoplankton, zooplankton, and ben-thic macroinvertebrates are more likely to be affectedby in situ hydrological and chemical effects post-firethan by physical effects of the fire itself (e.g., hightemperature). These effects can also have largeimpacts on the re-establishment of fish populationsafter fire, delaying colonization until fluvial processeshave stabilized.

Lake zooplankton and phytoplankton communitycomposition can also shift in response to fire. Manyfactors affect lake biotic response post-fire, includinglake position in the landscape: shallow headwater

Table 2 Selected studies where response to fire was investigated in burned watersheds

Fire location Year of fire Area burned Sampling focus Dominant cover Fire source Fire type Time sincefire

Effects of fire Source

Maine, USA 1947 >4000ha Permanent streams Birch (Betula

papyrifera), striped

maple (Acer

pennsylvanicum)

Wildfire Continuous

crown

50 years Lower C and

N concentrations in forest

floor litter but higher C and

N concentrations in uppermineral soil layers.

12

NE

Minnesota,USA

1971 6000ha Watershed streams

and lakes

Black spruce (Picea

mariana), Pinusspp., maple (Acer

rubrum), aspen

(Populus

tremuloides), birch(B. papyrifera)

Wildfire NA 0–3 years Concentrations of P in runoff

elevated for 2 yearsfollowing fire and

decreased in third year. No

detectable effects on

P concentrations inreceiving waters.

9

South

Carolina,

USA

1973 NA Runoff Pinus species Experimental Surface 0–1month Solubility increased

following artificial leaching

for Ca (20�), Mg (10�), Na,and K (both 2.3�). No

change in biologically

available N and P in litter.

8

W Montana,USA

Mid-1970s 60 plots of0.3 ha

Soil water Douglas fir(Pseudotsuga

menziesii), larch

(Larix occidentalis)

Experimental Surface 0–2 years Net losses of Ca and Mgbelow the root zone when

soil surface

temperatures>300 �C.Highest nutrient release

from ash followed first

precipitation event,

tapering with successivewettings. If ash was initially

hydrophobic, nutrient

release was delayed.

16

SouthCarolina,

USA

1978–1979 60% ofwatershed

over 2.5 years

Groundwater andstreams

Pinus species Experimental Surface 0–3.5 years Low severity fires consumedless than 30% of forest

floor matter, resulting in

low ash production. Nosignificant nutrient effects

for groundwater or stream

water.

14

Continued

Pollu

tionandRemediatio

n_F

ires

81

Table 2 Continued



Experimental

Lakes Area,NWOntario,

Canada

1980 Most over- and

understorey inwatershed

Lake catchment Jack pine (Pinus

banksiana), blackspruce

(P. mariana), birch

(B. papyrifera),aspen

(P. tremuloides)

Wildfire Continuous

crown

0–3þ years ANC decreased 20%,

corresponding increase instream acidity. Reduced

ANC linked to increase in

SO4 produced by organicmatter oxidation. Recovery

generally within 3 years.

2

NW Montana,

USA

1988 15 000 ha Third-order streams Lodgepole pine

(Pinus contorta)

Wildfire Continuous

crown

Fire

activelyburning

5–60� increase in NH4–N

andOrtho-P, with�90%oftotal nutrient pool as

soluble portions. Return to

baseline levels in 2weeks

for P and up to 6weeks forNP increase attributed to

ash falling directly into the

water and N increaseattributed to diffusion from

smoke.

15

N Australia 1990–1994 670ha Intermittent streams Eucalyptus Experimental Intermittent

crown

1–4 years Increases in N, P, and TSS

�10� only after stormrunoff in most heavily

burned catchment.

17

NW Quebec,

Canada

1995 >50000 ha Forest floor Boreal forest

species

Wildfire Variable 1 year Forest floor dry weight

reduced. Total andexchangeable Ca

concentrations increased.

Exchangeable K increasedin 0–10 cm mineral layer.

Magnitudes of pH changes

proportional to fire

severity. Nutrientconcentrations reduced

only in severely burned

areas.

3

82

Pollu

tionandRemediatio

n_F

ires

Quebec,Canada

1995 Three separatefires,

30 000–50000ha

Lakecatchment

Transitionof Boreal

Mixedwood

and BorealConiferous

forests

Wildfire Variable 0–3 years Increased K (3–8�),TN (2–3�), TP (2�), Mg

(2–3�), and SO4 (3–4�)

export rates. Elementexport highest in the first

year and declined overall in

the remaining two years.

Mobile monovalent ionsand SO4 rapidly flushed,

declining 50% by the

third year.

5, 7

N Alberta,Canada

1995 50–100% treecover killed

per watershed

Headwater lakes Black spruce(P. mariana)

dominated

peatlands, aspen(P. tremuloides) in

upland areas

Wildfire Variable 2 yearsand

20–40

years

Recently impacted lakeshad >2� increases in P,

>1.2� increases in N and

1.5� increases indissolved organic carbon.

Mean pH 9% lower in burnt

catchments, including

lakes with longer timessince fire explained 74% of

the variance in P, using

time since disturbance and

percent disturbancecombined.

10

Thunder Bay,

Ontario,

Canada

1996 10ha blocks Forest floor Aspen (P.

tremuloides),

spruce (Piceaspp.), post-

infestation

standing deadbalsam fir (Abies

balsamifera)

Experimental Variable 24 h to

3months

Fire severity, as manipulated

through fuel loading,

reduced depth of forestfloor and was negatively

associated with nutrient

availability for regeneratingplants.

6

NW Alberta,

Canada

1998 >150 000ha Lake catchment Pinus species,

spruce (Piceaspp.), balsam fir

(A. balsamifera),

poplars (Populus

spp.)

Wildfire Variable 2 years Dissolved organic carbon

concentrations increased1.4-fold. Color increased

proportional to catchment

area burned divided by

lake volume.

1

Continued

Pollu

tionandRemediatio

n_F

ires

83

Table 2 Continued



NW Alberta,Canada

1998 84–89% ofwatersheds

burned

Thrid- and fourth-order watersheds

White spruce (Piceaglauca), lodgepole

pine (P. contorta),

poplars (Populus

spp.)

Wildfire Variable 0–4 years Runoff and particulateP fractions higher for

4 years, with average

increases of 1.6� and

3.7�, respectively.Discharge, with enhanced

responses to summer

storm events. IncreasedP exports attributed to

peak flow periods.

4, 13

Nevada, USA 1999 NA Soil water Native sagebrush

(Artemesiatridentata) v.

native and

invasive annuals

Wildfire Surficial

grass

2 years Conversion of cover to

annual species maystimulate water loss

immediately following

fires.

11

Sources

1. Allen EW, Prepas EE, Gabos S, Strachan W, and Chen W (2003) Surface water chemistry of burned and undisturbed watersheds on the Boreal Plain: an ecoregion approach. Journal of Environmental

Engineering and Science 2: S73–S86.

2. Bayley SE, Schindler DW, Parker BR, Stainton MP, and Beaty KG (1992) Effects of forest-fire and drought on acidity of a base-poor boreal forest stream: similarities between climatic warming and acidic

precipitation. Biogeochemistry 17: 191–204.

3. Brais S, Par D, and Ouimet R (2000) Impacts of wild fire severity and salvage harvesting on the nutrient balance of jack pine and black spruce boreal stands. Forest Ecology and Management 137: 231–243.

4. Burke JM, Prepas EE, and Pinder S (2005) Runoff and phosphorus export patterns in large forested watersheds on the western Canadian Boreal Plain before and for 4 years after wildfire. Journal of

Environmental Engineering and Science 4: 319–325.

5. Carignan R, D’Arcy P, and Lamontagne S (2000) Comparative impacts of fire and forest harvesting on water quality in Boreal Shield lakes. Canadian Journal of Fisheries and Aquatic Sciences 57(Supp.2),

105–117.

6. Johnston M and Elliott J (1998) The effect of fire severity on ash, and plant and soil nutrient levels following experimental burning in a boreal mixedwood stand. Canadian Journal of Soil Science 78: 35–44.

7. Lamontagne S, Carignan R, D’Arcy P, Prairie YT, and Par, D (2000) Element export in runoff from eastern Canadian Boreal Shield drainage basins following forest harvesting and wildfires. Canadian Journal of

Fisheries and Aquatic Sciences 57(Suppl. 2), 118–128.

8. Lewis Jr, WM (1974) Effects of fire on nutrient movement in a South Carolina pine forest. Ecology 55: 1120–1127.

9. McColl JG and Grigal DF (1975) Forest fire: effects on phosphorus movement to lakes. Science 188: 1109–1111.

10. McEachern P, Prepas EE, Gibson JJ, and Dinsmore WP (2000) Forest fire induced impacts on phosphorus, nitrogen, and chlorophyll a concentrations in boreal subarctic lakes of northern Alberta. Canadian

Journal of Fisheries and Aquatic Sciences 57(Suppl. 2), 73–81.

11. Obrist D, Delucia EH, and Arnone III JA (2003) Consequences of wildfire on ecosystem CO2 and water vapor fluxes in the Great Basin. Global Change Biology 9: 563–574.

12. Parker JL, Fernandez IJ, Rustad LE, and Norton SA (2001) Effects of nitrogen enrichment, wildfire, and harvesting on forest-soil carbon and nitrogen. Soil Science Society of America Journal 65: 1248–1255.

13. Prepas EE, Burke JM, Chanasyk DS et al. (2003) Impact of wildfire on discharge and phosphorus export from the Sakwatamau watershed in the Swan Hills, Alberta, during the first two years. Journal of

Environmental Engineering and Science 2: S63-S72.

14. Richter DD, Ralston CW, and Harms WR (1982) Prescribed fire: effects on water quality and forest nutrient cycling. Science 215: 661–663.

15. Spencer CN and Hauer FR (1991) Phosphorus and nitrogen dynamics in streams during a wildfire. Journal of the North American Benthological Society 10: 24–30.

16. Stark NM (1977) Fire and nutrient cycling in a Douglas fir/larch forest. Ecology 58: 16–30.

17. Townsend SA and Douglas MM (2000) The effect of three fire regimes on stream water quality, water yield and export coefficients in a tropical savanna (northern Australia). Journal of Hydrology 229, 118–137.

ANC – acid neutralizing capacity, TN – total nitrogen, TP – total phosphorus, TSS – total suspended solids.

84

Pollu

tionandRemediatio

n_F

ires

Figure 7 Schematic representation of fire-induced hydrophobicity in soil layers. Subsurface water repellency develops afterorganic fractions condense in soil, enhancing lateral flow. Surface runoff increases as the wettable zone above the hydrophobic layer

becomes saturated.


lakes have experienced increased cyanobacterialevels, whereas deeper lakes in large watershedshave experienced increased diatom biomass. Lakeeffects seem to be linked more to area burned withinthe watershed relative to lake surface area than toother indicators.

Intermediate Effects: up to 10Years

Nutrient cycling Following the initial pulse afterfire, chemical components in soil and receiving watersbegin to stabilize. The levels of readily available nutri-ents begin to decline as plant regeneration starts tooccur and litter depth accumulates and recovers.Nitrogen losses are ameliorated through inputs fromprecipitation and dry deposition, as well as N fixationfrom the recovering soil microbial community.Particulate P losses are reduced as runoff rates appro-ach pre-fire conditions. Nitrate and acidic anionexports from watersheds decrease and stream pHrecovers proportionally.

Infiltration and runoff Fire-induced hydrophobicityis gradually degraded and downward flow is re-established. Initial water losses can cause soil compres-sion, producing a runoff effect more persistent thanthat caused solely by hydrophobic soil. However, thedeath of root systems can increase available flowpathsto soil layers at depth, reducing lateral flow. In addi-tion, as revegetation occurs, compacted soil layers areloosened by new root growth, which also serves tostabilize watershed hillslopes, stream banks, and lake-shores. Coarse woody debris inputs associated withrunoff can continue for several years following fire asstanding dead vegetation topples to ground level.

Sedimentation Accumulation of in-stream sedimentgradually decreases following fire. Soils that are notrapidly revegetated are especially prone to erosionand bank incision, which can alter mid- to long-term channel morphology. Channel profiles graduallyreturn to pre-fire conditions, although widening anddeepening may persist for several years in areas ofgeomorphic sensitivity (i.e., higher relief, coarse tex-tured soils).

Biota Recovery of aboveground vegetation con-tinues as the microbial community is re-establishedand nitrate-tolerant plants are succeeded. Mostpost-fire monitoring for other biotic components(e.g., fish, benthic macroinvertebrates, phytoplankton,zooplankton) has occurred over the short-term, andintermediate to long-term responses across a varietyof fire-impacted watersheds are not known or poorlyunderstood. There is some evidence that functionalfeeding groups among stream macroinvertebratesshift in dominance from allochthonous (shredderand collector) to autochthonous (scraper and filterfeeder) communities in accordance with stream prod-uctivity for several years following fire (Figure 8).

Long-Term Effects: 10± Years Post-Fire

Most wildfire effects are immediate and short-lived,with a rapid return to pre-fire conditions for manyecosystem components. However, some long-termeffects have been noted. The decomposition of coarsewoody debris both in-stream and within the water-shed can provide a persistent source of nutrient inputsfor many years. Coarse woody debris loading can also

kg ha−1

kg ha−1

EQ ha−1

gC m−1

# m−2

# m−2

# m−2

F W S S

Normalyear

Post-fireyear

Years after fire

Collectors

Grazers

Stredders

Litteroutput

Primaryproduction

Incidentradiation

Nutrient output

Suspendedsediments

2 5 10 50 100 300F W S S

watts

Figure 8 Watershed component response relative to time since fire. Adapted from Minshall GW, Brock JT, and Varley JD (1989)

Wildfires and Yellowstone’s stream ecosystems. BioScience 39: 707–715, with permission from the American Institute of BiologicalSciences.


affect stream flow and water chemistry, creatingimpounded areas that did not exist prior to wildfire.There can be cumulative effects on channel mor-phology related to initial increased discharge post-fire, and to sediment pulses that move through thewatershed. Wildfires often reset the successionalstage of an ecosystem, the effects of which are feltfor many years following fire. Long-term monitoringis an area of increasing research, as the complexities ofliving in ecosystems governed by periodic fires becomebetter understood.

Summary

The effects of wildfire on freshwater systems areshaped not only by the fire but also by characteristicsof the system itself. The impacts of fire can begeneralized with the following major points:

1. Fire regimes are established through the interac-tion of physical, chemical, and biological ele-ments within a watershed in combination withclimate.

2. Climate affects fire in both the short-term (e.g.,localized drought) and the long-term (e.g., sea-sons, global weather patterns), and is a primarydriver for both fire frequency and occurrence.

3. The impacts of fire are governed both by intensity(vertical height) and severity (downward heattransfer).

4. Intense, severe fires are more likely to occur inclosed canopy ecosystems where crown fires arepossible.

5. Most ecosystem components recover rapidly afterfire, although persistent effects can be present formany years afterwards.

6. Fire is a naturally occurring, periodic componentof functioning watersheds.

What is known about wildfire is sure to be amendedas research continues into its effects on aquatic sys-tems. While much is already understood, there arestill many areas for increased focus. These include:

1. long-term monitoring at the watershed scale;2. response and recovery of lacustrine systems;3. modeling the effects of fire;4. quantifying watershed risk predictors.


Continued interest in wildfire and its impacts willonly strengthen the understanding of aquatic systemsand their capacity to respond to large-scaleperturbations.

Glossary

Continuous crown fire – Also referred to as a crownfire, bush fire, or brush fire.

Hydrophobicity – A condition of water repellency,either permanent or transient.

Infiltration – The movement of water through pores,gaps, or interstices, generally from surface to deepersoil layers.

kWh�1 – Unit of fire intensity equal to energy releaseper unit length of fire front.

Pyrolysis – Chemical change brought about by theaction of heat.

Surface fire – Often interchanged with grass fire,although a surface fire can occur under any covertype.

Volatilization – Causing to pass off in vapor.

See also: Acidification; Alkalinity; Carbon, UnifyingCurrency; Chemical Fluxes and Dynamics in River andStream Ecosystems; Chemical Properties of Water;Chloride; Coarse Woody Debris in Lakes and Streams;Fluvial Transport of Suspended Solids; GroundWater andSurfaceWater Interaction; Groundwater Chemistry; MajorCations (Ca, Mg, Na, K, Al); Natural Organic Matter;Nitrogen; Nitrogen Fixation; Nutrient Stoichiometry inAquatic Ecosystems; Organic Nitrogen; Phosphorus;Rivers.

Further Reading

Clark JS (1997) Sediment Records of Biomass Burning and GlobalChange. Berlin: Springer.

Doerr SH, Shakesby RA, and Walsh RPD (2000) Soil water repel-

lency: its causes, characteristics and hydro-geomorphological

significance. Earth-Sciences Reviews 51: 33–65.Goldammer JG and Jenkins MJ (Des) (1990) Fire in Ecosystem

Dynamics: Mediterranean and Northern Perspectives. The

Hague: SPB Academic Publishers.

Gresswell RE (1999) Fire and aquatic ecosystems in forestedbiomes of North America. Transactions of the American Fish-eries Society 128: 193–221.

Kasischke ES and Stocks BJ (2000) Fire, Climate Change, andCarbon Cycling in the Boreal Forest. New York: Springer.

Kozlowski TT and Ahlgren CE (1974) Fire and Ecosystems. New

York: Academic Press.

Likens GE (1989) Long-Term Studies in Ecology: Approaches andAlternatives. New York: Springer-Verlag.

Mooney HA, Bonnicksen TM, Christensen NT, Lotan JE, and

Reiners WA (eds.) (1981) Fire Regimes and Ecosystem Proper-ties. U.S. Forest Service General Technical Report WO-26.

Pickett STA and White PS (eds.) (1985) The Ecology of NaturalDisturbance and Patch Dynamics. San Diego: Academic Press.

Sala M and Rubio JL (1994) Soil Erosion and Degradation as aConsequence of Forest Fires. Logrono, Spain: Geoforma Edi-ciones.

Strauss D, Bednar L, and Mees R (1989) Do one percent of

forest fires cause ninety-nine percent of the damage? ForestScience 35: 319–328.

Swanson FJ (1981) Fire and geomorphic processes. U.S. Forest

Service General Technical Report WO-26, pp. 401–420.

Turner MG and RommeWH (1994) Landscape dynamics in crownfire ecosystems. Landscape Ecology 9: 59–77.

Wein RW and MacLean D (1983) The Role of Fire in Northern,Circumpolar Ecosystems. New York: Wiley.

Wright HA and Bailey AW (1982) Fire Ecology. New York: Wiley.

Relevant Websites

http://www.fire.uni-freiburg.de/welcome.html – Global Fire Moni-

toring Centre.

http://rapidfire.sci.gsfc.nasa.gov/ –MODIS Rapid Response System.

http://www.idrc.ca/imfn/ – International Model Forest Network.http://www.ciffc.ca/ – Canadian Interagency Forest Fire Centre.

http://www.fire.uni-freiburg.de/welcome.html

http://rapidfire.sci.gsfc.nasa.gov/

http://www.idrc.ca/imfn/

http://www.ciffc.ca/

encyclopedia of inland waters || fires

Documents