flame length. It can be determined from the product of biomass consumption (energy) and rate of spread of the fire. Fire severity is an ecological parameter that ...
The Influenceof ForestStructureon Fire Behavior JamesK. Agee Collegeof ForestResources,University of Washington,Seattle,Washington Westernforestshaveburnedfor millennia with a wide rangeof frequencies,intensities. andextents.Someforestshaveflre-resistant ffeesand othersdo not. The combinationof the physicalcharactersof the fires themselves,togetherwith the adaptationsof the tree speciesto fire, haveresultedin foresttypesthat can be classifiedinto naturalflre regimes (Agee 1993). Three broadcategoriesof fire severitymay be defined,basedon the physicalcharactersof fue and the lue adaptationsof vegetation:low, mixed or moderate, and high. A low severityfue regimeis one wherethe effect of the typical historicalflre is benign. Firesare frequent(often 100 years)but may be of high intensity;most of the vegetationis at leasttop-killed. In the middle is the mixed or moclerateseverityfire regime,wherefires are of intermediatefrequency(25-100years), rangefrom low to high intensity,and havevegetationwith a wide rangeof adaptations. Fire behavioris a function of fuels, weather,and topography,the "fire behaviortriangle". All threelegsof the trianglehavesignificanteffectson fire behavior,but the fuels leg is most relatedto forest structure,and is the only controllablefactor of the three. There are also interactionsbetweenthesefactorswhich will be discussedlater. Foreststructure consistsof flammablebiomass,whetherit is live or dead. Forestsfructurecan be interpretedas three-dimensional patchesof fuel, with differing amounts,sizeclasses, arrangements, and flammability. Somefuels, suchas large tree boles,riuely are consumed by fire, while others,suchas surfaceneedlelitter, are partially to wholly consumedin every fire. Others,suchas leavesin the treecrowns,are inconsequentialin surfacefires but a major sourceof energyin crown fires. Foreststructureaffectsfire behavior.and fire behaviorin turn affectsforeststructure. It is importantto separatefireline intensityfrorn fire severity. Intensityis the energy releaserate per unit lengthof fireline, and is a physicalparameterthat can be relatedto flame length. It can be determinedfrom the productof biomassconsumption(energy)and rate of spreadof the fire. Fire severityis an ecologicalparameterthat measures.albeit somewhatloosely,the effectsof the fire. Two fires of the samefireline intensitycan have quite differenteffectsbetweenan old-growthrnixed-coniferforest and a young plantation of similar speciesbecausethe smallerplantationrees will be more easily scorchedand havethinnerbark. The fire in the old-growthmay be of low severitywhile the plantation fire is of high severity. We generallyare more interestedin fire severity,but must approachseveriryfirst by estimatingfireline intensityand then using modelssuchas FOFEM (Keaneet al. 1995)to predict treemortality from fireline intensity. We havetraditionallyevaluatedfire and forest stmctureat the standlevel, and are beginningto utilize landscape-level tools to study larger-scaleissues.At the standlevel,
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thereare horizontaland vertical componentsto the fuel matrix, and at the landscapelevel, myriad patchesof foresteachwith a uniquefuel sffucturethat may carry fire along the surfaceor throughthe ffee crowns. The following discussionis organizedaroundsurface and crown fire behaviorratherthan all of the combinationsof fuel structuresthat could be irnagined. The intent is to summarizesomeof the structuralcharacteristicsof foreststhat fire behavior,or "fire-safe"forests. "Fire-safe"forestsare not leadto rnanageable fireproof,but will have: . . .
Surfacefuel conditionsthat limit surfacefireline intensity; Foreststandsthat arecornprisedof fire-toleranttrees,desoribedin termsof spccies. sizes,and structures. A low probabilitythatcrown fires will eitherinitiateor spreadthroughtheforest;
'fhese characters of one another.but it is possibleto definea rlatrix arenot independent conclitions at the standlevel that constitutea fire-safeforest. One irnportarrt of acceptable here:not everyparcelof landscape can or neeclbe treatedto caveatrnustbe ernphasized rnakeit "fire-safe".Therearecompetingobjectivesai.isociated with biodiversityandfish habitat landscape patches prioritized wildlife that suggest units or be for treatment,as and forestsalwaysburnedwith crown not all can or shouldbe treated.Somehigh-elevation probably so fire and to makesuchforests"fire-safe"are alwayswill, that attenrpts objectiveswill sor.netimes favor maintaininglatesomewhatfutile. Management or old-growthforests,and while thosepatchesr-naynot be treatecl, successional forestpatchesrnaywell needfuelstreatmentto help protectthe latesurrounding patchcs. successionul
MANA(; IN(; SURI,'ACIt l'UltLS T( ) LI MIT I,'IRIILINFI IN'I'IINSI't'Y oanlimit firelineintensityand lower potentialfire severity.It Surfacefuel management firelineintensityor increasefire severity,dependingon which fuelsare canalsoincrease rnanaged and how the operationis conducted.Forestentryfor fuelsmanagement pulposesusuallyresultsin alterednricroclimates, andalthoughlesstotal bionrassrnaybe present,moreof it rnay be in the deadfuel categoryand left lying on the forestfloor. The of surfacefuelsso thatpotentialfirelineintensityremainsbelow somecritical managenlent (discussed later)can be accomplished throughseveralstrategies level andtechniques. Arnongthe cornmonstrategies arefuel removalby thinningtrees.adjustingfuel produce less flammable to a fuelbed,and "introducing"live understory arrangernent vegetationto raiseaveragemoisturecontentof surfacefuels. interactwith one anotherto influencefirelineintensity. The varioussurfacefuel categories Althoughmorelitter andfine branchfuel on the forestfloor usuallyresultin higher intensities, that is not alwaysthe case. If additionalfuelsarepackedti-ehtly(low fuelbed porosity),they may resultin lower intensities.Largerfuels(>3 inches)are ignoredin fire spreadmodelsas theydo not usualll'affectthe spreadof the fire (unlessdecornposecl
lTth Forest Vcgetation Management Conl'erence
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over longer [Rothermel1991]).They may, however,resultin higherenergyreleases periodsof time and havesignificanteffectson fire severity. The effectof herband shrubfuelson firelineintensityis not simplypredicted.Firstof all, moreherband shrubfuelsusuallyimply moreopenconditions,which areassociated with lowerrelativehurniditiesandhigherwindspeeds.Deadfuelsrnaybe drier,and therateof spreadrnay be higher,becauseof the alteredmicroclirnatefror-nnroreclosedcanopyforest with lessunderstory.Secondly,shrubfuelsvary significantlyin heatcontent.Waxy or oily shrubslike snowbrtsh(Ceanothusvelutinus)or bearclover (Chamoehotia foliolosa) burn quite hot; othershavelower heatcontents.Perhapsmost important.though.is the effectof the live fuelson moisturecontentof the fuelbed. FinedeadI'uelswill oftenbe at moisturecontentsof l0o/oor less,while foliar moistureof live understoryvegetationwill in the wetterthannormalsunrmerof or higher. In the dry easternCascades be at 100o/o was 125(k,, 1995,averageshrubmoisturecontentin Septernber and grasseswereat 93% (J.K.Agee,unpublished data).Thesefuelswill havea darnpening effecton fire hchavior. is annual,orperennialgrasses andfbrbscure,thefine However,if thegrasscornponent cleadfuelcan increasefirelineintensity.Post-fireanalysesof fire rlanrageto plantr.rtitrrr NationalForest treesatierthe 19f17fires in the FlayfbrkDistrictof the Shasta-Trinity (Weatherspoon betweengrasscoverarrcl andSkinner1995)showeda positiverelationship clamage and a negativerelationshipbetweenforb coveranddarnage(Figurel) , rnost likely becausegrasseswerecureclandforbswerenot. Management of foreststructure to reducefuelscanbe donemanually.rnechanically, or operatio115. ollepotentialrernovultechlriquc throughprescribed burning.In harr,resting is
Percent of Plantations
with Moderateto Extreme Damage
Figurel. Fire damageto mixedconif'erplantationin the l9tJ7fires was worsewhengrass coverwas higherbut lesswhenforb coverwas higher(Weatherspoorr anrlSkinner1995). Moderateto extremedarnageincludesmore than 10%of treeswith >50a/(scorchtcr = >50o/o Coverclasses are0 = not presenUI = I -2()(/o:2 treeswith crownsconsumeci. 2 1 - 5 V nu : n d3 = 5 l - l ( X ) 7 o .
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whole-treeyarding, with trees cut by feller-bunchershauled to landings with grapple skidders, and delimbing occurring at landings. Debris is chipped or burned there. Other fuel removal strategiesmay include manual tree cutting and pile burning with sites scatteredthrough the woods, or prescribedburning under moist conditions. These strategieswill generally result in less fuel and more moderate fire behavior after treatment. Adjusting fuel errangementmay be another strategyto produce a less flarnnrablefuelbed. ln thinning operationswith harvesterforwarderequiprnent,limbing is clonein the woo 0.05,andcrown fire occurs. B. Wherecrown bulk densityis lower underthe samerate of spread,critical levelsof So cannotbe obtainedand the fue remainsa surfacefire.
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canbe lirnitedby So < 0.05. The problemis thusreducedto one wherecriticalconditions for R (rateof spread)and d (crown bulk density)may be defined,below which crown fires will not spread.Individualcrown torching,andcrown scorchof varyingdegrees,rnaystill occur. Defininga setof criticalconditionsof R andd thatrnaybe definedby rnanagernent activitiesis difficult. For givenlevelsof d abovesomeminirnunl R couldpotentially increasein the presence of strongwindsand allow an independent crown fire (sensuVan WagnerlL)77)to move througha stand. For this exercise.to defineconditionssuchthat crownfire spreadwould be unlikely(thatis, produotsof R andd suchthatSn
