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 <20 years),of low intensiry,and the ecosystemshave
dominantvegetationwell-adaptedto survivefire. At the other end of the spectrurnis the
high severityfue regime,wherefires are usuallyinfrequent(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,
Page52
17th Forcst Vegetathn Managcment Conl'erencc
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
Page53
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 .
Page 54
lTth Forest Veget:rtion Manauement Confercncc
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<ls
directly aheadof the forwarder,which then rolls over the lirnbs.reducingsoil conrpuetiorr
effects of the equipment and rnaking a rnore cornpacttuelbed. A third strategyis often the
resultof thinning operations:recruitnrcntof understoryvegetation,both shrubsanclherbs,
that rnaintainhigh moisturecontcntanclprovide l clarnpening
eff'ecton fire belravi,rr.
Such treatmentwill not always reduce potential fire behavior or fire severity. lf the
thinning is a selectiveor crown thinning that rernoveslarger trees,the averagefire
resistance
of the standwilldecreasebecausethe residualshave a sr-naller
uverageclianreter
(Jonversely.
height,
potential
fire
sevcrity
wrll
and
and
be highcr.
a low thinningrvill
rcclucepotentialfire scverityas lr\ienrgcresiclualtrce siz-ewill increasr-.PotentialI'ire
is likely to changeafter treatrnent(Trrtrlcl, Figure 2). An unthinltedstantlof
['rehavior
pine with fire behaviorchunrctcristrcs
potrderosa
of NFFI- Morlel 9 ([3urgarrlutrl
R o t h e n n e l1 9 t 1 4 ) w i l lh a v e f l a n t e l e n g t h s o f- l - - 5 t ' e e t u n t l e r n r o d e r a t e f i r e w e a l hAesr 't.a r r r l
entry for thinningcoulclincreasepotentialf ire behaviorby addingfuel loaclingancllircl
depth. The thinning treatmenfif fuels wcre rerroved (suchas rvholetree yarding)coulcl
reducefire behavior to Model 9 values,or c:oulclincreasefuel loading r:ven trevonrl1hc
exampleshown (a lop and scatter approach,for exarnple). If thc stanclis thinnetlhc:rvilv
enoughto allow herb and shrub understoryto establish,and thc hcrtrsare not curerl. this
atlditionof Iive fuels,even with the increasedclc:acl
fuel loacling.recluccsfire ht'havitrr'
['rclowthat of Model 9. Generally.shrubfoliar ntoisturewill relnainhigh and herlr
moisturewill declineover the surnrner,particularlyif the herbsare annuals.
Prescribed
burninghasbeertfoundto recluce
subsequent
wildfiredarnagcfhroughits
effectson suffacefuel loadingreductionand on rnortalityof understorytreesand shrubs
thatmightcarryfire into thecanopytrees(Hehns1979,Buukley1992).In n.roslof our
westernUnitedStatesforests,prescribecl
fire hasnot beenappliedwirlcly enou-{hto have
rnucheffecton wilclfirebehaviorat a landscane
scale.
MANA( ;IN(i CROWN F'tIF]I,S'I'0 PRI.]VI.]N'I'
C ROWN !'I R F]}tT.]H
A V I() R
T'hedevelopment
andmaintenarlce
of a forestrelativelyfreeof L:rownfire potcntialis
primarilydeperrdent
on managernent
of the structure
of crownfuels. Topography,
and
weather,
theother"legs"of the "fire behaviortriangle",areeitherfixeclor uncontrollahle.
The regulationof crown fire potentialcan be apprroached
frorn two cornplernrntarv
perspectives:
. Prevention
of conditions
that initiarccn;wnf ire
. Pteventionof conditionsthal rlli'rr.l,
spleaclof crown f ire
lTth Forest Vegctation Management ('ont'crcnce
l'agc 55
If eitheror both of thesecriteriacan be achievedwithin a stand,thenthe chanceof crown
tire, and/orblowup fire behavior,is very low. lf they can be achievedacrossthe entire
landscape,
thepossibilitiesof blowupsareessentially
eliminated.ln the following analysis,
theseconditionsaredescribedat the scaleof the stand,and thresholdconditionsare
definedand appliedto real forests.
Table l" Simulatedfuel changesrrfterthinningwith moderateslashdisposalancl
greenupthroughdeveloprnent
subsequent
of a shrub/herbunrlerstory.Valuesfor the
only andnot basedon realclata.
thinnedandgreenupconditionsarefor demonstration
(ireenup
Fuel Pararneter*
NFFL Model 9
ThinnedStanil
1-hr load
I0-hr load
100-hrload
Live herb load
Live woody load
Fuel depth
2.92
0.41
0.15
4.( X)
3.( X)
3.( X)
4.(X)
3.00
)
3.(X
0.(x)
0.(x)
0.(x)
0.(x)
l.(x)
2.(X)
0 . 2)(
0.-50
0.5()
* load in tons/acre,depth in f'eel
6
5
Modelg
*'ffi-Thin
--#Greenup
4
Flame
Length3
(FT)
2
1
0
10
46
Windspeed(MPH)
Figure2. Effectof changingfuel contlitions(seeTable l) on pote.ntial
fire trehavior'.
" with l-, l0-. and l(X)-hrfuel rnoistures
Environmental
conditionsare"r.noderate
at 6.7
ancl8% andlive fuel rnoisturcat l?tt(k.
Page 5(r
lTth Forest Vcgctation Managt:nrent ('onl'ercncc
ConditionsThat Initiate Crown Fire
A fire rnovingthrougha standof treesmay move as a surf'acefire, an independentcrown
fire, or as a specffumof intermediatetypesof fire. The initiation of crown fire behavioris
a function of surfacefireline intensityand critical parametersof the treecrown layer: its
heightabovegroundand moisturecontent(Van Wagner1977). The criticalsurface
intensitvneededto initiatecrowninsis:
lo = (-,76P12
( E q u a t i o nI )
where
lo = critical surfar:eintensity
C = empiricalconstant,derived as 0.010 (Van Wagner 1977)
z = crown base height
content)
h = heat of ignition (largely a function of crown r-noisture
can be calculatedfor a rilngerrt'crowrl
Theretore.valuesfor critical surfaceintensity,1,,,.
baseheightsand foliar moisturecontents(Table 2). Theserepresentminirnurnlevelso1'
f irelineintensitynecessaryto initiatecrr)wl'lfirc. [:quivalentflarne lengthsin Sl arrcl
'l'able3equivalents.Byranr's(195t)1
E n g l i s h u n i t s i u ' e p r e s e n t e d i n T ' a b lF
eo
3 r. t h e
relationbetweenf-lamelengthanc'lfireline intensitywas usedratherthan Thor"nas'(19(r3)
t'lrrrne
length model. Rotherrnel(1991) usesThornas'tnodel,which is basedctna2ll
p()werlaw. ratherthan Byrarn'sscluareroot model, in his technicalrep()rton predictinr
behavioraud size of crown fires. However,to dest:ribeinitiirtingcortditions.Bytanr's
ntoclellnay be more appropriate.anclis usedextensivelyfor estitnatingthe f-larnelengthof
spreadingsurfacefires (Albini l9'76.Rothermel19133).Levels of fireline intensityor flanre
length below thesecritical levelsrnay resultin fires that do not crown but are still of stand
replacementseverity. For the lirnited range of crown hase heights arrdfoliar rnoistures
shown in Tables 2 and 3, the critical levels of fireline intensity anclflame length appear
more sensitiveto heicht to crown basethan to foliar moisture.
If the structuraldimensions of a stand and inforrnation about late surnrnerfoliar nruisturc
are known. then critical levelsof fireline intensitythat will be associatedwith crown fire
for that stand can be calculated. Fireline intensity can be predicted fbr a rartgeof stand
fuel conditions,topographicconditions.and anticipatedweatherconditions.so it is
possibleto link on-the-groundconditionswith.the initiatingpotentiaIf-orcnlwn fires. The
potentialfor managementto avoid crown fire initiation involveskeeping | < lo. T'hiscan
be accomplishedby rnanagingsurfacefuels such that I is kept well below In (see previtrus
section),or by r-nanagingfor suffir.'ientcrown base heights sueh that Io is large (seeTaLrle
,)\
| 7th Forest Vcgetation Management Conl'erence
Pagtr 57
Table2. Critical levelsof fireline intensity(kW m- l ) associatedwith initiation of crown
fire activity in coniferousstands.
Height of Crown Basc (m)
Foliar Moisture
It
12
lfi
2t)
I6(X)
2461
4526
6968
t)731
1024
I l{t{I
2891
5 3 2r
t(l9l
| 1449
4t9
I l tt-s
2n8
3353
(r159
9482
13252
l(x)
419
l3s4
24nti
3tt30
7036
10833
15 1 4 0
120
606
t 7l 4
3l4tt
1841
t3904
13709
l9 l s9
Content ('/o)
2
70
30n
tt7I
tr0
362
9()
with criticallevelsof firelineirrtensity
fr'ornTable2.
Table3. Flarnelengthsassociated
equation.
usingByrarn's(19-59)
Foli:tr Moisturg
Hcightol'Crown Birsc
C o r r t c n(t% , )
lll()lors(noiroslliXrl)
l ( r( 5 3 ) 2 0 ( 6 6 )
2 (6)
4 (11)
10
l.l ('1)
l . t ' t ( 6 ) 2 .r ( r J ) 2 . 8( e )
ft0
t . 2( 4 \
1 . 9( 6 )
2.s(tt)
.r.0( 10) 4 . 0 ( l 3 ) 4 . 9 ( r 6 ) s . 7 ( l e )
90
1 . 3( 4 )
2 . 0( 1 )
2 . 1( L ) )
3 . 3( 1 0 ) 4 . 3( 1 4 ) 5 . 3( 1 7 ) ( r " l( 2 0 )
l(x)
1 . 3( 4 )
2.t (1)
2 . 8( e )
3 . 4( l r ) 4 . 6 ( 1 5 ) 5 . ( r ( l i t ) 6 . 5 ( 2 1 )
t20
l " 5( 5 )
2 . 1( 8 )
3 . 2( 1 0 ) 1 . 9( l . l ) 5 .I ( 1 7 ) 6 . 2Q l ' t 7. f ( 2 4 )
Page 5tl
6 (20)
8 (26)
1 2( 4 O )
1 . 1( 1 2 ) 4 . 6( r 5 ) s . l ( l 7 )
lTth Forcst Vcgetation M:rnagemcnt C'onl'crence
Conditions That Allow Crown Fire To Spread
The crown of a forest is similar to any otherporousfuel medium in its ability to carry fire
and the conditionsunderwhich fire will or will not spread.The net horizontalheatflux,
E, into the unburnedfuel aheadof the fire is describedby Van Wagner(1977):
E=Rdh
(Equation2)
where
E = net horizontalheatflux, kW m-2
R = rate of spread,tt'ts"c-1
d = bulk clensityof crown,kg m-3
h = heatof ignition.kJ kg-|
Crown fuelsaretypicallyconsidered
thosewhiehinteractin the crown fire process,
usuallyneedlesbut possiblyincludinglichens,fine branches,
etc. Crownfires will stopif
eitherR or d, the rateof spreador bulk densityof the crown,fall below somenrinirnunr
value.
lf surfacefirelineintensity(l) risesabovethecriticalsurfaceintensityneeclecl
to initiate
crownfire behavior(le), the crown will likely becomeinvolvedin combustion.However,
Van Wagner(1977, 1993)recognizesthreetypesof orownfire behaviorthat can be
describec'l
by criticallevelsof firelineintensity(l) andratesof spread(R): (1) a passivc
crown fire, in which I > Io, but R is not sutficientto maintainorownfire activity;(2) an
activecrown fire, whereI > [o and R is abovesomeminimum spreadrate;and (3) an
independent
of
crown fire, whereI > Io and R throughthe crown is largelyindependent
heatfrom the surfacefire intensityl. A "crown-fire-safe"
landscape
would have
suchthat,at most,a passivetype of crown fire would resultundersevere
characteristics
fire weather.
Criticalconditionscan be definedbelow which activeand independent
crowrlfire spreadis
To
fire
unlikely.
derivetheseconditions,visualizea crown
as a massof fuel passingby a
stationaryflaming front (Figure3). The massflow rate(S) is the productof rateof spread
(R) and crown bulk density(d), and the unitsof S are Kg rn-2 ,""-1, or rnassper unit
crosssectionareaof crown per unit time. It can be describedin the contextof Equation
2:
S=Rd=E/h
(Equation3)
The massflow ratehassomeminimumproduct,So, belowwhich crownfire spreadwill
not occur,and that is determinedby combinationsof therateof spread(R) andcrown
bulk density(d). Van Wagner(1971)empiricallyderiveda minimummassflow rateof So
= 0.05, which he favorablycomparedto other valuesin the literature. Therefore,even if
conditionsare favorablefor crown fire initiation,the potentialfor spreadof a crown fire
lTth Forcst Vegetation Management Conl'erence
Pagc59
B
Figure3. Critical conditionsfor massflow ratecan be visualizedby passinga forestalong
a "conveyorbelt" througha stationaryflaming front. A. Under severefire weatherand
high rateof spread,crown masspassesthroughtheflaming front rapidlyand achievesSo
> 0.05,andcrown fire occurs. B. Wherecrown bulk densityis lower underthe samerate
of spread,critical levelsof So cannotbe obtainedand the fue remainsa surfacefire.
Page60
lTth ForestVcgctationManagementConf'erencc
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 <0.05),
arbitrarythresholds
of R wereestablished
so that criticallevelsof d could be clefined.
The upperthresholdfor R for this exercise(R t ) wasdefinedas the maximumR of the
firesstudiedby Rothermel(1991),theeveningR of the Sundance
fire (1967),1.-J5
m secr. The rniddlerange(R2) wasdefinedby the maximumR of significantwind-drivenfires
sturliedby Rothermel,excludingthe eveningrun of the Sundancefire (PatteeCanyorr
1977,Sundance
afternoon1967,Sandpoint19t35,
Lily Lake l9tt0, Mink Creekl9l(lJ.Red
Bench l9lJl{,BlackTiger l9tt9),or 0"67m sec-l. The low end of therange(Rj) rvas
clefinecl
by theaveragernaximurnR of therunsof thesesarne7 fires.or 0.40 ,r, ,*.-l .
'['hcse
plovidea reasonable
rangeof threshold
valuesfor rateof spreacl.ln borealfbrests,
o rf n s e c - l , w i t h c r o w n b u l k c l e n s i t i e s g e n e r a l l y a b o v e 0 . 2
. f o h n s o(r1r( Y ) 2 ) e s t i r n a t e c l0R. 3
kg rn3. At the 1994TyeeFirein Washington,
R wasestirnated
at 0.3tt9nr sec-l irrnrixed
conif'erforestat MuclCreek(R. Sampson,
Tyeefire report),0.503
In sec-l in rnixed
c o n i f e r f o r e s t a t O k l a h o r n a G u l c h ( L . W i l l i a r n s , T y e e f i r e r e pl o
t )n. a
. 3r 4
r sn.d- c - l i n
lorlgepolepine/subalpine
fir forest(8. Walker.'Iyeefire report,frorn tatrleof ROS giverr
ternperatlrre/relative
hurnidityand HainesIndex),all within the rangedefinedby
Rotherrnel.
Ciiventhisrangeof R, and So = 0.05,alterrtative
levelsof d in equati()lt
3 are
(for Rj). bclowrvhichcrownfirr:
calculated
as0.037(for R1),0.074(for R2), and0.12-5
spreadwould be very unlikely.
A questiortable
in thisanalysisis thutR will rernainconstant
assunrption
asd is clruwn
dowrtto the criticallevel. [n fact, if R doesdeclineas d is reduced,then in theorya higher
levelof d rnightbe sustained
in theforestwithoutadditional
risk of crownfire spread.
This assun-rption
will be furtherdiscussed
later.
S'I'AND S'I'RUC'I'TJRT'S
AND CROWN I.'IRI.]
'l'he
preceding
analysis
hasestablished
criticallevelsof firelineintensitl'(l) anclseveral
possiblelevelsof crownbulk density(d) belowwhichcrownfire initiationandspreadare
unlikely. From theselevels,andwith additionalinformationon fire weather,surfar:efirel
"crown-fire-saf-e"
characters,
and standcharacters,
conditionsfor tbreststand:irnaylrc
clefined.It shouldbe ernphasized
forestis neitherfireproofnor
that a "crown-fire-safe"
likely to escapefree of wildfire damageat the standlevel. It is a standwhich is unlikelyto
generate
or allc'rw
the spreadof crown fire. Wildfire darnagewill clearlybe lessthanin
stands,but will not necessarily
unmanage(l
be low or absent.Management
of surfacefuels
lTth Forcst Vegetation Managcment Conl'crence
Pagc 6 |
is a corollary activity that can consffainsurfacefire intensityand reducechancesof crown
fire activity and damageto the residualstand.
Crown bulk densities(d) were calculatedfor ponderosapine (Pinusponderosa),Douglasftr (Pseudotsuga
menziesii),and grand fir (Abiesgrandis) for severaldiameter/density
cornbinations.Crown fuel biomassand crown volume were usedto constructa
rnass/volume
ratio (e.g.,crown bulk density)for eachspecies,sizeclass,and density.
Crown fuel as definedfor this report includesall foliar biomassplus 50 percentof other I hour tirnelagfuels (0 - 0.62 crn) in the crown. Otherfine fuels suchas lichenswere
ignoredfor this exercise,but would be sritical for somespeciesin other foresttypes (such
as subalpineftr[Abies lasiocarpa]). The "other" fuels hereare twigs and fine branches.
Half of thefine branchcategorywasconsidered
smallenoughto contributesignificant
process,
energyduringthe crown combustion
interpretedfrom datain Anderson(1969).
Foliar biomassof dominantsby tree speciesand sizeclasswas calculatedfrorn dataof
Browrr( 1971t)and Gholz et al. (1979),usingaverages
wheredatawereavailablefor a
speciesin both sources.Finebranchfuel additionswereaddedusingBrown's(l97tl) ratio
of foliar biornassto l-hr timelagsizeclassfor eachof the threespecies.The proportion
(at the 50% level)of 1-hrtimelagfuelsto foliar biomassrangedfrom I 1-lo/ofor sn.rallto
pine,26-20Vo
for smallto largeDouglas-fir,
largeponderosa
and2l-17% for srnalltcr
largegrandfir. Crown lichenbiomasswas not considered
in this analysis,but woulcl
increasecrown flammability for thosespeciescontainingsignificantamounts:granclfir and
sLrbalpine
fir.
At high treedensities,aggregatingthe crown fuel by sirnplernultiplicationof treebiornass
tir.nes
densitycan resultin overestimating
biomass.The total biomassper siteswascapped
at 50 tonnesha-1(about2(\ tac-l;. tris is a high crown fuel loacl(seeRothermel1991)
andprobablyresultsin someoverestimation
of crown bulk density,but only at very high
rreedensitiesof largeffees. Thesecombinations
arewell beyondthepossiblecritical
levelsof d definedearlier,so arerelativelyunimportantin evaluatingcriticalbulk densities.
Crown volurnesweredeterminedfrom Brown (1978)for eachof the threespeciesby size
class. Live crown lengthwas usedas the criterionfor crown volume,so thatdead
for any of the threespecies.
branchesbelowthe live crown were not considered
(ienerally,the bulk densityof deadbranchesbelowthe treecrown will be too low to
generate
crownfire activity,althoughlower branchesmay helpto sustainflameactivityup
into the crownfrom surfacefuel combustion.
Crown bulk densitiesfor the threespeciesby sizeclassanddensityshowedsome
consistent
trends(Table4). Crown bulk densitiesweregenerallyIowestfor ponderosa
pine, intennediatefor Douglas-fir,and highestfor grandfir. For any given densityof
trees,orown bulk densityincreasedwith tree size,suggestingcrown fuel biomass
increased
morethancrown volumeas ffee sizeincreased.For any giventreesize,crown
bulk densityincreasedwith treedensity.
Pagt 62
lTth Forest Vegetation Management Conl'crcncc
Table 4. Crown bulk density(kg m3) by diameterclassand densityfor ponderosapine
(regulartype, first row of eachtriplet), Douglas-fir(bold type, middle row), and grandfir
(italics,third row).
AverageDiameter
(dbh- cm)
Tree Densitv Per Hectare
50
100
200
400
800
1600
3200
7n
+o vl
t6L
3L4
ffiY (L15 TP/,\,
.001
.002
.002
.002
.003
.003
.00s
.007
.009
.0ll
.014
.018
.022
.027
.036
.043
.055
.013
.088
. r1 0
1.62
7-,0
.003
.004
.006
.007
.007
.0 1 2
.014
.014
.024
.028
.028
.047
.055
.056
.094
.l I I
.ll3
.189
.223
.226
.378
t9.0-5
.005
.009
.008
.010
.0 1 7
.0 t6
.02r
.034
.033
.041
.068
.066
.083
.136
.t32
.t66
.272
.263
.333
.545
.409
.010
.012
.0 t3
.020
.025
.026
.041
.049
.0s2
.082
.099
.t 03
.164
.198
.206
.328
.353
.287
.397
.353
.287
.01I
.019
.023
.023
.039
.047
.047
.078
.095
.095
.155
.t90
.190
.310
.247
.292
.313
.247
.03r
.023
.023
.062
.048
.047
.124
.096
.095
.248
.l9l
.247
.361
)<)
.247
.033
.042
.066
.083
.r33
.194
.210
l-vrcf,er
t.2l
0,5
1,5
3t.75
r-) q
I
L.-/
44.45
[.5
63.5
)cn
1 0 1. 6 +
4A+
.005
.167
Missingdatain cells denoteseithercrown bulk densitieswith unrealisticsizeldensity
combinations
or cellsfor which datawerenot available.
The sritical bulk densityfor independentcrown fire spreadcan be evaluatedin terrnsof
Table4. For example,if the middle value of maximum crown fire rate^ofspread(R) were
chosenas a planningcriterion,thenthe criticald would be0.074kg mr. It would be
lTth Forcst Vegetation Management Conl'erence
P:rge63
reachedfirst by grandfir, and last by ponderosa
pine.for a comparablesetof treesizes
and densities.The differencesbetweenspeciesare not as greatas the differencesbetween
densitiesand sizeclassesfor thesethreespecies,suggesting
controlof standstructureis
r-nore
inrportantthanspecies.However,in termsof fire tolerancein the presenceof a
surfacefire, choiceof specieswill be critical,as somespecieswill developfire tolerance
rnuchfasterthanothers,and somemay neveradaptbecauseof thin bark. The datain
Table4 arefor purestandsof one species.Crown bulk densitiesof cornbinations
of
pine,show little difference
species,suchas a 50-50mix of Douglas-firandponderosa
frorn the purespeciestables,but rnakearrimportantassumption:
that therewill not be
crown stratification.Stratificationwill be very irnportantwhereone specieseannraintain
itselfin subordinate
Lrrownpositions,which will affectboth biornassandcrown volume,
andthereforecrown bulk density.Stratificationcan alsoaffectheightto crown base.
Again.it rnustbe stressed
thatthesetablesarespecificto single-sized,
non-stratif
ied
stands.
()I''I'HI], CROWN }IIII,K I)I.]NSI'I'Y'I'HRI'SHOI,I)
AN I.]MPTRICAI,'I'I.]S'I'
Lirnitecl
ernpiricalinforrnation
existsto evaluate
thepredictability
of thecrownbulk
clensitythreshold.Much of the existingdatais from borealforesttypes(Van Wagner
1977.19c.)3).
Crownfire activityin severalstandsburnedin the 1994Wenatchee
fires
wereevaluatcdwherechangesin crown fire activityoccurred.Dataon standdianreter
andclensityweregatheredby ForestServicepersonnelin severalstanclsSorneof the
standshadalsobeentreatedwith prescribed
burningor pile burningto linritcriticallevels
of firelincintensity(lo)frorn occurring,
but all rnayserueasernpiricaltestsof crownbulk
clensity
thresholds.
Crownbulk densities
werecalculated
from thestaudcharacteristics
information,
andrelatedto presence
or absence
of crownfire activity(Table5).
(Jnthinncdsturtdstencledto be crowrrstratified,and the useof thesetaLrles
lnay have
for thosestands.
biasedtheestirnates
sornewhat
The lgg4 firesappearecl
to havea bulk clensity
thresholcl
of il =0.10 kg rn3.with crown
fire activitylikelyabovethethreshold
andno orownfire activitybelowit. Typicallyin this
a r e a . u n t h i n u e d s t a n d s h a v e d > 0 . 1 0 w h i l e t sh li an rnxel sc d
l o n o It r. r e a c h c a s e w h e r e a
thinned-unthinnecl
waspossible,crown fires in adjacentunthinnt:cl
cornparist'rt
stands
droppedto the groundas surfacefires whenthey reachecl
the thinneclstanrls.In cvery
case,the unthirrned
werelessthan l()0yearokl standswith utixesof
anclthinnedstanrls
Douglas-fir
pine. The criticallevelof d ernpirically
andponderosa
clerivecl
herefits
(
d
=
0
.
0
7
4
\
a
n
d
R
2
(
d
=
0
. 1 2 5 )T. h e
h a l f w a y b e t w e e n t h e l e v ecl sl doef r i v e d f r o r n R l
apparentincreasein criticalle'velsof d aboveR 1 rnaysuggestthatmaximulnratesof R
d e c l i na
e s c r o w n b u l k c l e n s i t y d e c r e aI nstehse. 1 9 9 4 f i r e s , i t a p p e a r s t h a t d : 0 . 1 0 i s
aboutthe highestcrownbulk densitya stancl
canrnaintain
andavoidthepotcrrtial
fbr
crownfire behavior.
Pagc (rJ
VegctationManagcmcntConfercncc
lTth F-orest
Table 5. Relationof estimatedcrown bulk densityto crown fire behavior. Standsare
rankedin descendingorder of crown bulk density.
Location
Stand
:urd
Trentrnent
Vicinity Mallen Rinch
Aver:tge
Stand
Diameter
(cmlinl)
Average
Stand
Density
( t h : t - ll a c l l
Estirnated
Crown Bulk
Density, d
(kgrn3)
Comments
tln
Fire Behavior
MC-unthinned 11 I'/l
r700[689]
= 0.1-5
Crown fire activity
Vicinity MarshRcsidencoMC-unthinnod l7 l7l
l7{x)[689]
= ().1-5
Crown firo activity
Navirrc Coulcc
MC-unthinned ll l7l
l(x)()
[4(x)l = ( f .l 0
Crtlwn firo aclivity
V i c i n i t y M a l l e nR a r r c h
MC-thinncd
30 [ l2l
5(x)[2U)l
= ( ).()c.)
Crttwn l-iresltrps
N a v i r r eC o u l c c
MC-thinncd
30 ll2l
2s()tr00l
= 0.05
Crown lirc stops
Mud Crcck
Mc-thinned
30 ll2l
250I l00l
= 0.05
Crown fire slops
MirrshRcsidcncc
MC-thinncd
24 p.5l
225 1t111
= 0.035
Crown l'iro stops
At levelsof d nearthe threshold,fuel treatmentrnayhelp to redusethe chancesof crown
fire activity. Two of the caseexarnples(Table5) werenearthe threshold.Navarre
Coulee,a standwith no fuel treatmont,was at the crown bulk densitythresholdand
exhibitedcrown fire behavior.At the Vicinity MallenRanchsite,pruningandpile burning
of thinning/pruning
debrisreducedthe fuel laddereffectandlimited I ( Io, so that a
runningcrown fire droppedto the groundthereeventhoughd was relativelyhigh.
Clrownhulk densitycan be usedasa criterionto lirnit crownfire behavior.By nraintaining
(cl)of <0.10kg rn3,activeor inclependent
stancls
crownfire
at crownbulk clensities
activitycanbe linritecl.Sucha criterionhasboth advantages
andlirnitations.The
include( I ) that empiricalevidenceseemsto supporttheexistenceof sucha
advantages
threshold;(2) thatd can be calculatedfrom typicalstandexamdata,so thatcrown fire risk
can be evaluatedat the standlevel;and (3) thatsilviculturalprescriptions
can be designed
"crown-fire-safe"
protection
Ievel
include
at the stand
to
from crown fire, to make
forests.
Limitationsof usingthis criterionare (l) that fire may still burn throughstandsanddo
significantdarnagedependingon the sizeand speciesof trees,(2) althoughstandscanbe
made"crown-fire-safe",
we do not cuffentlyknow how much of the landscape
needbe
(3) that othercornpetingand
landscape;
treatedthis way to makea "crown-fire-safe"
irnportantobjectivesntay limit the scaleof suchtreatmentson the landscape;and (4)
higherelevationforestshavenaturallyburnedthis way for millennia,so that attenlptstn
lTth Forest Vegetation Managcment Conl'ercnce
Page65
limit crown fire activity may be ecologicallyandeconornicallycost-effectiveonly in the
lower elevationforests.
Severalconstraints
to this analysisexist. The effectof slopewas not considered,
stands
with non-stratifiedcrowns were assumed,and crown bulk densitieswere calculatedfrom
crudedata. More validationof dimensionsof standswherecrown fires havebeen
observedto drop to the groundis needed.
DISCUSSI0N
It is clearthat foreststructurecan be manipulatedto reducethe severityof fire events.
The challengeis to rneldfuel strategies
with otherforestmanagement
objectives,and
beingableto selectproperfuel treatmentsandprioritiesfor treatment.In general,forests
with low severityfire regimesshouldhavehighestpdority for treatrnent,
and thatpriority
shoulddeclinein the high severityfire regirnes.The low severityfire regirnes(suchas
nrixedconifer)haveundergone
the rnostchangesincefire exclusionpolicieswereenacted.
high
levelsof bothrisk (chanceof a fire starting)andhazard(fuelsandtheir
anclhave
condition.suchas low fuel moisture).In the low severityfire regimes,strategies
that
potential
both surfaceand crown fire
are more likely to be adoptedthan in the
acldress
rnoclerate
to high severityfire regirnes,whereif fuel treatments
areappliedthey will havea
lcsssignificanteffectdue to generallylowerrisk"
sense,
either"tnoderate"
lrru conceptual
treatments
canbe considered
or "intensive".
Moderatetreatrnent
objectivemay focuson
areasarethosewherethe fuel manaBement
re<luction
of potential surfacefire intensity,so that | < Io. Firesmovingthroughthese
tirrestswill havelesschanceof crowningpotential.Low thinning,pruning,anclsurface
fuel treatrnent
with pile or broadcast
burningrnightbe amongthe fuel reducticln
includesthe techniques
techniqucs
applied.Intensivemanagement
aboveplus
<0.10 kg rn3,so thatevenunderseverefireweather
l'nanagelnentof
crownbulk clensity
fire is still likely to occur,but
thc fire is likely to renraina surfacefire" In bothtreatments,
will be higher. Fire severityin areasburnedrnay
thechancesof effectivefire suppression
species,the wildfire effect
bc lower,but if thetreesaresmallor areof lessfire-resistant
ruraystill be severe.
priority is high,thereis still rnuchuncertaintyabouthow
Wherefuel rnanagement
shouldbe applied. Ratherthanattempta "one-size-fits-all"
treatments
approach,there
nraybe insteada combinationof approaches
thatcanbe appliedto eachsituation
dependinguponthe foresttype andcompetingobjectives(Figure4). The gradientof
rangesfrom no treatmentat all to intensivetreatmentover the
treatmenton a landscape
possible)areshownin Figure4 and
steps(otherszLre
entirearea. Someintermediate
include"interior"no treatmentareassunoundedby intensivetreatmentareas,or no
by areaswith moderateto intensivetreatment.
treatmentenclavessurrounded
Pagc 66
lTth Forest Vegetation Management Conl'crcncc
LandscapeFuel ManagementAlternatives
No Treatment
Treatment
ffi vooerate
Treatment
ffi tnt"nsive
Irigure.1. Landscape
level approaches
to fuel rnanagement
crossa gradientfiom no
trcatnlentto intensivetreatmentacrossthe entirelandscape.Intermediate
approaches
(upperright, lower left) combinesomeareaof no treatrnentwith otherareaswhere
moderateto intensivetreatmentsare applied. [n theseexample,rnoderatetreatments
adclress
reductionof surfacefire intensity;intensivetreatments
addressboth surfacefire
anclcrownfire intensitv.
questionremains:how rnuchof a landscape
l.he $64,(X)0
needbe treatedto makeit
relltively fire-safe'/The answerto this questiondoesnot dependon fuelsmanagement
can pay for themselves;
alone. Someof thesetreatments
otherswill not, so will dependon
how rnuchmoneyis available,and how it canbe disributed. Fueltreatrnent
requires
access,so that "no treatrnent"
areasrnayby defaultincludeareaswhereroadsare not in
placeas well ls ireas wherespecificforeststructures(suchas late-successional
forest)are
desired.However,evenif the answersto theseperipheralquestionsareavailable,we
question.exceptto say
reallydort'tknow how to completelyanswerthefuel management
"firesafe".
that as more of the landscapeis treated,it becomesmore
In the 1994
Wenatc:hee
fires, areasthat had beenthinnedand prescribedburnedover the last 20 years
had si-unificantly
reducedfire behavior,but the topsof the standswerescorchedfrom heat
produceclat otherplaceson the landscape
that had not beentreated.The treatedareas
wereat a scaleof severalhundredacres,andclearlywerenot largeenoughto be effective
at reclucingfire severity,as most of the scorchedtreesdied.
| 7th Forest \/egetation Management Conl'erence
Pdge 67
We havemadesomegeat stridesin understandingthe effectsof foreststructureon fire
behavior,but somemajor challengesremain. Thesechallengesare largely landscape-level
issues,similar to many otherof our currentforestmanagementchallenges.
LITERATURE CITED
Agee,J.K. 1993.Fire ecologyof PacificNorthwestforests.
IslandPress.Covelo.CA.
Albini, F. 1976. Estirnatingwildfire behaviorandeffects.USDA For. Serv.Gen.Tech.
R e p .I N T - 3 0 .
Anderson,H.E. 1969. Heat transferand fire spread.USDA For. Serv.Res.Pap.INTBrown.J"K. 1978.Weightanddensityof crownsof RockyMountainconifers.USDA
For. Serv.Res.Pap.INT-197.
Buckley.A.J. 1992. Fire behaviourandfuel reductionburning:Bemm River wildfire,
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October,l9tl8. AustralianForestry55: 135-147
1984.BEHAVE: Firebehaviorpredictionandfuel
Burgart,R., andR.C.Rotherrnel.
rnocleling
system FUEL subsystem.USDA ForestServiceGen.Tech.Rep.INT161
Byranr.G.M. l9-59.Cornbustion
of forestfuels.[n: Brown,K.P.(ed) Forestfire:
Controland use. McGraw Hill. New York.
(lholz, H.A.,C.C.Grier,A.G. Carnpbell,
andA.T. Brown. 1979"Equations
for
estirlriltingbiomassand leaf areaof plantsin the PaoificNorthwest.ForestResearch
La[rRes.Pap.41. OregonStateUniversity.Corvallis,Oregon.39 pp.
Helrns,J.A. 1979. Positiveeffectsof prescribed
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Managemen
Nto t e s4 0 : l 0 - 1 3 .
.fohnson,E.A. I 992. Fire and vegetationdynamics:Studiesfrom the North American
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br,rreal
Keane.R., E.D. Reinhardt,and J.K. Brown. 1995. FOFEM: A first orderfire effects
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andpark
rnanagernent.
USDA For. Serv.Gen.Teoh.Rep.INT-GTR-320.
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TJSDAFor.Serv.Gen.Tech.Rep.INT-143.
R. 1991. Predictingbehaviorand sizeof crown fires in the northernRocky
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on combustion 1962August27 Septenrber
I ; Ithaca,
NY. New York: AsadernicPress.:844-859.
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V a n W a g n e r , C . E1. 9 9 3 .P r e d i c t i o n ocfr o w n f i r e b e h a v i o r i n t w o s t a n d s o f j a c k p i n e .
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Pagc 68
lTth Forest Vegetation Managcmcnt
Conl'erence