Fuel Bed Characteristics
Sierra
Nevada
of
Conifers
Jan W. van Wagtendonk, U.S. Geological Survey, Yosemite Field Station, El Portal CA 95318; James M. Benedict, National Park Service, Santa Monica Mountains National
Recreation Area, Agoura Hills, CA 91301; and Walter M. Sydoriak, National Park Service, Bandelier National Monument, Los Alamos, NM 87544.
ABSTRACT. A studyoffuels in SierraNevadaconiferforestsshowedthat fuel beddepthandfuel bedweight significantlyvariedby tree speciesand developmental stageof the overstory.Specificvaluesfor depthand weightof woody,litter, anddufffuels are reported Therewasa significantpositiverelationshipbetween fuel beddepthand weight.Estimatesof woodyfuel weightusingtheplanar interceptmethodweresignificantly relatedto sampledvalues.Theserelationships canbe usedto estimate fuel weightsin thefield. West.J. Appl. For. 13(3):73-84.
Theaccumulation ofhazardous fuels throughout theSierra Nevadaover the past60 yr hasbecomean issueof recent •ncreased concern.In 1993,Congressdirectedthata studybe conducted to assess what information
is needed to make
decisionsfor the futuremanagement of the SierraNevada ecosystems. Oneof thecriticalfindingsof theSierraNevada Ecosystem Projectwasthatlive anddeadfuelsin theconifer foreststodayare moreabundantandcontinuousthanin the past(SNEP 1996). Combinedwith the growingdensityof homes,theseincreasesin fuels exacerbatethe threat of fire to the human and natural resources in the Sierra Nevada. Accurate methods are needed to assess fuel conditions in order to
developfire protectionand fuel mitigationplans.Lack of such accurate information reducesthe reliability of these
plans.
Prescribed fire hasbeenproposedas one of the most effectivetoolsfor reducingfuel hazards(van Wagtendonk 1996).The useof prescribed fire requiresreliableestimates
of fuelweightin orderto safelyaccomplish theburnsandto evaluatetheir effectivenessin meetingmanagement objectives(Reeberg1995).In addition,fuel bedweightanddepth areusedin theRothermel(1972) rateof spreadequationthat •scentralto theNationalFire DangerRatingSystem(Deemmg et al. 1977) andthefire behaviorpredictionsystemused NOTE:Jan W. van Wagtendonkis the corresponding authorand can be reached at (209) 379-1885; Fax: (209) 379-1886; E-mail:
[email protected]. The authorsthankall of the peoplewho helpedon thisproject.JoeCohooversawthe dataentry,andDianeEwell
spentmanyhoursin thefieldcollecting fuels.Shewasablyassisted by many volunteers includingCharisse Sydoriak,Kay Beeley,KathyTier, andKaren Kolbeck. Liam Bickford volunteered to enter all of the data.
by fire behavioranalysts(Rothermel1983).Fuel weightand deptharespecifiedfor standardized modelsthatareusedin bothof thesesystems(Anderson1982,Albini 1976). Fuelweightplaysadualrolein Rothermel's(1972)spread
rateequation.In thenumerator, it is multipliedby fuel heat contentto producea sourceof heat for the calculationof reactionintensity.Fuel weightis dividedby fuel depthin the denominatorof the equation to determine fuel bed bulk density,which actsas a heat sink. Increasesin fuel weight usuallycausereactionintensityto increasemorethanrateof spread.In fact, all elsebeingequal,asweightincreases, rate of spreadmay actuallydecreasebecausemorefuel mustbe raisedto ignitiontemperature (BurganandRothermel1984). Brown (1981) identifiesfuel bed bulk densityas one of the mostdifficultproperties to measurein thefield. Deeminget al. (1977) categorizedwoody fuels by size classesandduff fuelsby depthclassesthatbothcorrespond to fuel moisturetimelags(Table 1). Timelagis theamountof time necessary for a fuel componentto reach63% of its equilibriummoisturecontent(Lancaster1970). Deeminget al. (1977) stressthat the duff designations are very rough
approximations and shouldbe usedwith caution.For exTable 1. Timelag classes and corresponding woody fuel size classesand duff fuel depth classes(Deeming et al. 1977).
Timelagclass
Woodyfuelsizeclass Duff fuel depthclass ............................... (cm)..............................
1 hr 10 hr 100 hr 1000 hr
0.004).64 0.64-2.54 2.54-7.62 >7.62
0.00-0.64 0.64-1.91 1.91-10.16 >10.16
WJAF13(3)1998 73
ample, van Wagtendonkand Sydonak (1985) found good correlations betweenthemoisturecontents of duff andwoody 1hr and10hrtimelagfuels,butthecorrelations werepoorfor 100 hr andwere nonexistentfor 1000 hr timelagfuels. The forest floor also can be divided into litter and duff,
corresponding to the Oi layer andthecombinedOeandOa layers described in the literature on forest soils (Soil Survey Staff 1993). Litter is defined as freshly cast nonwoodyorganicmatterthat still retainsits morphological characteristics.
Duff
includes
both the fermentation
layer, where decompositionhas begun but the particles can still be recognized,and the humuslayer, where organicmaterialis compressedand in all statesof decay.As fuels,themostusefulcategoriesareprobablylitter andthe four duff depthlayers. Land managershavetraditionallyusedthe planarinterceptmethoddeveloped byBrown(1974a)toinventorydowned woodymaterial.Thismethodis alsothebasisfor photoseries for quantifyingnaturalforestresidues(BlonskiandSchramel 1981). Fuel weightis estimatedby countingthe numberof interceptsof woody fuel particlesin different size classes with a samplingplane.The intercepts arethenmultipliedby factorsderivedfrom physicalpropertiesof woodyfuels of RockyMountainconiferspecies. Theplanarinterceptmethod hasprovenusefulin areaswith plentifulwoodyfuelsresulting from managementactivitiesbut hasmet with lesssuccess in areas with natural fuels (Brown 1981). Although van Wagtendonket al. (1996) havedeterminedfactorsbasedon the physicalpropertiesof woody fuel particlesof Sierra Nevadaconifers,thereisa needfor moreaccuratewoodyfuel weightestimates. In the SierraNevada,the only publishedequationsfor estimatingforestfloor weightfrom depthwereprovidedby Agee(1973). Becausehesampledmixedstandsof ponderosa pine (Pinus ponderosa) and incense-cedar(Calocedrus decurrens)andstandsof white fir (Abiesconcolor)andgiant sequoia(Sequoiadendron giganteurn),individualspeciesre-
lationships couldnotbedetermined. Hisr2values, however, rangedup to 0.90 for total litter and duff for the white firgiant sequoiamix. Equationsfor estimatingweightsof duff fuelsandtotal fuels have alsobeendevelopedfor the Southwestand the Rocky Mountains.Unfortunately,conversionfactorsrecommendedby Brownet al. (1982) werebasedon a limited numberof studiesof bulk density including ponderosa pine and lodgepolepine (Pinus contorta) in the northern Rocky Mountains (Brown 1970, 1974b). Several studies of ponderosapine in the Southwesthave related fuel bed weight to fuel bed depth. Eakle and Wagle (1979) used
logarithmic regressions forthelitteranddufflayers, butr2 values were low (0.58 and 0.65, respectively). Linear regressionsthroughtheorigin werereportedby Ffolliott et
al.(1968,1976),butnor2values wereprovided. Harrington (1986)alsouseda linearregression andreported anr2of 0.78. Sackett (1979), however, was unable to establisha reliable relationshipfor predictingfuel weight from duff
depth. Woodard andMartin(1980)reported anr2of0.849 for lodgepolepine in Washington. 74
WJAF13(3)1998
The lackofinformauonon fuel weightanddepthof Sierra Nevadaconifersreducesthereliabilityof fuel hazardassessmentsandfirebehaviorpredictions. Thisstudywasdesigned to gatherthatinformation.Thispaperpresents datafor Sierra Nevadaconifersonwoodyfuelweightbysizeclass,litterand dufffuelweightby depthclass,litterandduffdepth,andfuel bedbulk density.We alsocomparewoodyfuel weightswith estimatesusingtheplanarinterceptmethod.In addition,we presentregression equations foreachspecies thatcanbeused to estimateduff fuel weight. Methods Fuels were collected
from stands of each of the 22
speciesof conifersoccurringin theSierraNevada.Species absentor not well-representedin YosemiteNational Park were sampledin adjacentnational forestsor in Sequoia and Kings Canyon National Parks (Figure 1). California torreya (Torreya californica), Pacific yew (Taxus brevifolia), andCaliforniajuniper (Juniperuscaliformca) had insufficientfuels for completesampling.Four stands of eachspecieswere sampledrepresentingdevelopmental stagesfromyoungto old. Treeswereapproximately2.5 to 10 cm in diam. in sapling stands, 10 to 60 cm in pole stands,60 to 120 cm in maturestands,andgreaterthan 120 cm in old stands. Each selected stand had an area of at least
300m2withanoverstory composed of 100%ofthedesired speciesandwasfree of recentdisturbancesuchasfire and tree mortality from insectsand diseases.Since adjacent trees could contributeto the fuels on a plot, only stands with at least 90% of fuels of the desired specieswere sampled. Four 11 m parallel transects3 m apartwere established beneatheach speciesand developmentalstagecombination. Fuel particle intersectionswith the samplingplanes were countedand categorizedby size class.Woody fuel depth was recorded at three points along each transect from the highestintersecteddeadparticleto the bottomof the litter layer (Brown 1974a). Five 20 x 50 cm subplots were systematicallyplacedalongeachtransectto collect woodyfuelsfor a total of 20 subplots/stand. Twenty 10 x 10 cm subplotsat the samelocationswere usedto collect litter and duff fuels. Duff samplesincludedincorporated conescales,bark flakes, and fine (0.0-0.64 cm) branches Litter and total duff depthwere measuredat the centerof the edge of each subplot adjacent to the transectline Freshly fallen litter was first separatedand sealed in plastic bags.Then fuels from the four timelag fuel diameter classesand four duff depth timelag classes(Table 1) were collectedand bagged.Soundand rotten samplesof the larger branchwoodwere baggedseparately.Samples were dried in a convectionoven at 65øCuntil weight loss stabilized.
Fuelweightsof theduff andwoodyfuelcomponents were analyzedusingtwo-way analysisof variancewith species and developmentalstageas the independentfactors.Twowayanalysisof variancewasalsousedfor woodyfueldepth, litter depth,andduff depthwith speciesanddevelopmental stageastheindependent variables.PosthocScheff6multiple
Location of
Yosemite National Park
SampleStands
.i
.
Yosemite ** *
Sequoiaand
Kings Canyon
.
Figure 1. Sampling locations for four stands each of 22 conifer species in the Sierra Nevada, California
and Nevada.
rangetestswereusedwitha one-wayanalysisof varianceto determinewhichspeciesmeanswere significantlydifferent from eachother (Scheff6 1959). Regressionanalysiswas usedto comparewoodyfuel weightsderivedin this study with the planar interceptmethod using Rocky Mountain (Brown 1974a) and SierraNevada (van Wagtendonket al. 1996) values.Estimatesof fuel weightas a functionof fuel depthweredetermined throughregression analysis.All significance tests were at the 0.05 level.
Rothermel(1983) and Albini (1976), who include litter and 1 hr duff fuels in their fuel models. Brown (1974a) also
includesthe litter layer in his fuel depthmeasurement. The equationfor fuel bedbulkdensityusedin thisstudyis:
BULKDENSITYBecl =
100 x WEIGHTwøøayWEIGHTœitterWE DEPTHwooay + DEPTHonehourduf f
Bulk densitiesfor litter, duff, and litter and duff combined
werecalculatedfrom theirrespectivedepthandweightvalues.Duff weightwasthesumof the four fuel depthclasses. Fuel bedbulk densitycalculationswere basedon the depth andweightof the woodyfuelsas well as the litter and the uppermostduff layer. This is consistentwith Burganand
where BULKDENSITY isinkgm-3, WEIGHT isinkgm-2, DEPTH is in cm.The depthof the 1 hr duff layeriseither0.64 cm or, in caseswherethetotalduff depthis lessthan0.64 cm, the total duff depth. Multiplying by 100 is necessaryto expressbulk densityon a per cubicmeterbasis. WJAF13(3)1998 75
Results 10 Woody
Fuel Bed Depth
Woodyfuel depthwassignificantlyaffectedby species, developmental stage,andtheirinteraction.Litter depthand duff depthweresimilarlyaffected.Woodyfueldepthvalues for the variousspeciesrangedfrom 1.24cm for foxtail pine (Pinusbalfouriana)to 9.27 cm for giantsequoia(Table2). Litterdepthwaslowestfor whitefir andredfir at 0.15 cm and highestfor sugarpine(P. lambertiana)at2.10cm.Therewas considerable variationin duffdepthsfroma highof 8.67cm for giantsequoiato a low of 1.28cm for westernjuniper. Multiplecomparisons dividedthewoodyfueldepthsinto six subsets with singleleafpinyon(P. monophylla)andfoxtail pine uniqueto the shallowestsubsetand Douglas-fir (Pseudotsuga me•ziesii),giantsequoia,and red fir (Abies mag•ifi'ca)uniqueto thedeepestsubset.Litter depthmeans
8
E
6
•
4
•
Litter
i
Duff
0
Saphng
Pole
Mature
Old
Developmental Stage Figure 2. Average fuel bed depth by developmental stage for 19
were groupedinto four subsets:the first subsetincludedall
Sierra
the specieswith meanlitterdepthslessthanthe 0.70 cm of limberpine (Pimps fieMlis); Jeffreypine (P. jeffreyi) was included in thesecond andthirdsubsets; andponderosa pine andDouglas-firwereuniqueto the fourth.There wereseven homogeneous subsets for duff depth,with foxtail pine and westernjuniperin theshallowest subsetandgiantsequoiathe
the 0.00-0.64 cm duff depth class while mountainhem-
Nevada
conifers.
(Pinussabiniana) hadthelowestweight(0.207kgm-2) in
lock(Tsugamertensiana) hadthehighest (0.927kgm-2). In the 0.64-1.91 depth class, foothill pine weighed the
least(0.706kgm-2) andwhitefir themost(2.675kgm-2).
deepest.
Theeffectof developmental stagecanbeseenin Figure2: growthof standsfromsaplingto pole,mature,andoldstages generallyresultedin increased depth.Therewasa slightdrop in litter depthbetweenthe pole andmaturestagesand in woodyfuel depthbetweenthe matureandold stages.
Limberpinehadparticularlydeepduff layerswith 9.193 kg
m-2 inthe1.191-10.16 cmdepthclassand4.582kgm-2 in the greater than 10.16 cm class. There was very little
western juniperduffwithonly0.733kgm-2 inthe1.91cm
Fuel Bed Weight
depthclassand no fuel in the greaterthan 10.16 cm class. Foxtail pine and western white pine (Pinus monticola) fuels in the deepestclass also were missing.
Litter and Duff Fuel Weight.--Species, developmental stage,and their interactionhad significanteffectson theweightof litteranddufffuelcomponents. Litterweights
Species means for litter weight were divided into five homogeneous subsets.Out of the 14 speciesin thegroupwith the lightestlitter, westernjuniper, white fir, westernwhite
variedfroma lowof 0.127kgm-2 forwhitefir toa high of 0.742kgm-2 for sugarpine(Table3). Foothillpine
pine,andredfir wereunique.Sugarpinewastheonlyunique memberof the heaviestgroup.The multiplecomparisons for
Table 2. Fuel bed depth for 19 Sierra Nevada conifers.
Species
Woodyfueldepth
Litterdepth
................................................................
Douglas-fir Foothillpine Foxtail pine Giant sequoia
76
Duffdepth
Litterandduffdepth
(cm) ..............................................................
6.51 4.50 1.24 9.27
0.33 1.35 0.19 0.37
4.40 3.23 1.60 8.67
Incense-cedar
5.84
0.20
4.87
5.07
Jeffreypine Knobconepine Limberpine Lodgepolepine
2.82 3.62 3.73 2.35
1.11 1.70 0.70 0.44
5.40 2.80 6.97 3.54
6.51 4.50 7.67 3.98
Mountain hemlock
3.52
0.36
6.05
6.4 I
Ponderosapine
4.34
1.87
7.88
9.75
Red fir
6.21
0.15
4.88
5.03
Singleleafpinyon Sugarpine Washoepine Westernjuniper Westernwhite pine
2.67 5.52 3.00 1.90 1.43
0.49 2.10 0.65 0.11 0.25
3.26 5.97 3.66 1.28 1.81
3.75 8.07 4.31 1.39 2.06
White fir
3.98
0.15
6.46
6.61
Whitebarkpine
2.19
0.49
4.93
5.42
All species
3.93
0.68
4.61
5.29
WJAF13(3)1998
4.73 4.58 1.79 9.04
Table 3. Average weight of litter and duff fuels of 19 Sierra Nevada conifers. Duff depthclass(cm) Species
L•tter
0.00-0.64
0.64- 1.91
I. 9 I- I 0.16
> I 0.16
Total
.......................................................................... (kgm-")........................................................................... Douglas-fir Foothill pine Foxtail vine G•ant sequoia Incense-cedar Jeffrey vine Knobconevine Limber vine Lodgepole vine Mountain hemlock Ponderosa vine Red fir Singleleaf pinyon Sugar vine Washoe vine Western juniper Western white vine White fir Whitebark vine
0.308 0.338 0.178 0.452 0.268 0.376 0.586 0.744 0.416 0.431 0.562 0.155 0.567 0.742 0.351 0.079 0.128 0.127 0.269
0.638 0.207 0.754 0.677 0.736 0.345 0.244 0.602 0.786 0.927 0_257 0.884 0.866 0.322 0.356 0.617 0.387 0.736 0.663
1.493 0.706 1.604 1.997 1.746 1.275 0.985 1.630 1.623 2.125 1.016 2.106 2.063 1.220 1.244 0.934 1.084 2.675 1.448
3.876 3.001 1.291 8.942 5.651 6.169 3.180 9.193 3.343 6.309 7.354 5.226 4.328 6.603 4.702 0.733 1.089 6.428 4.591
0.054 0.395 -2.477 0.232 1_177 0.428 4.582 0.042 1.522 2.682 0.583 0.624 0.574 0.286 --0.134 2.419
6.062 4.309 3.649 14.092 8.365 8.965 4.836 16.006 5.793 10.883 I 1.309 8.799 7.881 8.720 6.588 2.285 2.560 7.973 9.121
All species
0.370
0.579
1.525
4.843
1.022
7.999
duff weightsidentified sevensubsetsfor the 0.004).64 cm and0.64-1.91 cm depthclasses.Meansfor the greaterthan
I 0.16cmclassweregroupedintotwosubsets withall species butlimberpinein thefirstsetandthesevenspecieswith the leastduff in the secondclass.Figure3 showsthe effectof developmental stageon litter and duff weight.As stands developto olderstages,weightsin eachof thelitter andduff
2.54-7.62 cm and greaterthan7.62 cm soundclasses(Table 4). Ponderosapine hadthe leastamountof woodyfuel in the 0.004).64 cm classasdid knobconepine(Pittusattenuata)in the 0.64-2.54 cm class.Severalspecieshad no woodyfuels in the threelargestsize classes. Eight homogeneous subsetsof meanswere identifiedfor the small woodyfuels in the 0.00-0.64 cm class.The 0.64-
componentsincreased.
2.54 cm class was divided into five subsets with the heaviest
Woody Fuel Weight.---Woodyfuel weightin the three smallestsizeclassesandsoundfuelsgreaterthan7.62 cm in diam.wereall significantly affectedbyspecies, developmental stage,andtheirinteraction. Rottenfuelsgreaterthan7.62 cm in diameterdid not vary significantly.Red fir had the largestquantitiesof woodyfuel in the 0.004).64 cm and 0.64-2.54 cmsizeclasses,whilegiantsequoiadominatedthe
containingonly red fir andDouglas-fir.Speciesmeanswere groupedinto two overlappingsubsetsfor the2.54-7.62 size class, and giant sequoia was the only speciesthat did not occurin the lightestsubset.The meancomparisonsdid not divide thesoundandrottenclassesof fuelsgreaterthan7.62
14
12
10
•
Duff0.00-0.64 cm
•
Duff0.64-1.91 cm
ß ß
Duff1.91-10.16 cm Duff>7.62 cm
cm into subsets.
The increasein woody fuel weight with advancingdevelopmentalstagecan be seenin Figure 4. The threesmallest size classesincreasedbetweeneach of the developmental stages.Largesoundfuelsdecreasedslightlybetweenthepole andmaturestages,asdid largerottenfuelsbetweenthe pole, mature,and old stages. Bulk Density.--Average duff bulk densityrangedfrom
34.86kgm-3 forthe0.00-43.64 cmdepthlayerforfoothill pineto981.82kgm-3forthegreater than10.16cmlayerfor Douglas-fir (Table 5). Bulk density within the duff layer generallyincreasedwith depth. Red fir, singleleafpinyon, andsugarpine showeddecreasesin thegreaterthan 10.16cm depthclass,however,while westernjuniper and white fir decreased in the 1.9 I- I 0.16 cm class.
Litterbulkdensities variedfromalowof32.88kgm-3for foothill pinetoahighof 145.98 kgm-3forsingleleaf pinyon o
Sapling
Pole
Mature
Od
Developmental Stage Figure3. Average litter and duff weight by developmentalstage for 19 Sierra Nevada
conifers.
(Table 6). Foothill pine had the lowestvaluesfor the duff layer, the combinedlitter and duff layers,and the total fuel bed. Singleleaf pinyon had the highest bulk densitiesfor
duff(234.16kgm-3) andlitterandduffcombined (205.80 kgm-3).Limberpinehadthehighest valuefortotalfuel bedbulkdensity (40.21kgm-3). WJAF 13(3)1998 77
Table 4. Average weight of woody fuel components of 19 Sierra Nevada conifers
Species
0.0-0.64
Woody fuel component(cm) 2.54-7.62 >7.62 sound
0.64-2.54
>7.62 rotten
Total
.............................................................................. (kgm-2)........................................................................... Douglas-fir Foothillpine Foxtailpine Giant sequoia
0.370 0.064 0.185 0.130
0.473 0.093 0.140 0.403
0.153 0.067 -0.758
0.039 --1.045
Incense-cedar
0.245
Jeffreypine Knobconepine Limber pine Lodgepolepine
0.025 0.069 0.150 0.084
-----
0.282
0.139
0.318
--
0.984
0.196 0.075 0.215 0.142
0.073 0.091 0.456 0.175
--0.494 0.038
-----
0.294 0.235 1.315 0.439
Mountain hemlock
0.217
0.248
0.184
0.181
Ponderosa pine
0.013
0.215
0.260
0.428
0.079
0.139
0.140
Red fir
0.527
0.683
0.507
Singleleafpinyon
0.217
0.112
--
Sugarpine Washoepine Westernjuniper Westernwhitepine
0.143 0.018 0.037 0.089
0.178 0.107 0.110 0.162
0.389 0.028 0.079 0.087
--
--
--
0.086 0.023 ---
0.008 ----
1.036 0.224 0.325 2.336
0.830
0.995 1.996
0.329 0.804 0.176 0.226 0.338
White fir
0.290
0.370
0.195
0.175
0.003
1.033
Whitebarkpine
0.099
0.101
0.085
0.124
0.131
0.540
All species
0.156
0.227
0.196
0.161
0.019
0.759
Estimating Fuel Bed Weight
Fuelbedweightscanbeestimatedby predictinglitterand duff weightfromlitterandduffdepthor by determining the relationshipbetweencalculatedwoodyweightvaluesfrom the planarinterceptmethodwith woodyfuel weightsfrom this study. EstimatingLitter and Duff Weight.--Sinceregressions
through theoriginhadconsistently higherr2 valuesthan regressions withintercepts, theanalysiswasperformed with-
outintercepts. These r2values measure theproportion ofthe variabilityin weightabouttheoriginexplained bytheregression,however, andcannot bedirectlycompared to r2 values for modelsthatincludean intercept.
There was considerablevariation amongspeciesfor the litter layer regressions.Incense-cedar had the highest slope coefficient of 1.276, while foothill pine had the
lowestat0.1I I (Table7). Ther2values ranged from0.905 forlodgepole pineto0.355forfoothillpine.Ther2values for duff were higherand more consistentthan thoseof the litter, ranging from 0.978 for whitebark pine (Pinus albicaulis) to 0.643 for western white pine. The largest regressioncoefficient for duff was 2.592 for singleleaf pinyon, and the smallestwas 1.319 for Douglas-fir. When litter and duff were combined,the regressioncoefficients for individualspeciesrangedfrom 1.189 for sugarpineto 2.478 for singleleafpinyon,reflectingthe influenceof the
heavierduff layer.The r2 valuesrangedfrom0.797for westernwhite pine to 0.972 for whitebarkpine. When all
species werecombined, ther2valueforthelitterlayerwas 0.494 and the duff layer was 0.881. The equationfor the regression through the origin for total litter and duff weight as a functionof total litter and duff depth was:
!.4 0.00-0.64
I1
!.2
cm
0.64-2.54 cm
ß 2.54-7.62 cm >7.62 cm Sound
1
WEIGHT
>7.62 cmRotten
= 1.624 x DEPTH
r 2 = 0.937
whereWEIGHTisin kgm-2 andDEPTHisincm.Figure 5 is a plot of theregressionof all 1,520plots.As canbe seenin thefigure,the largestproportionof thedatapointsappeared near the origin.
Estimating Woody Fuel Weight.--For all species combined, the average total woody fuel weight in the
0.2
0.00-0.64 cm, 0.64-2.54 cm, and 2.54-7.62 cm size classes
was0.574kgm-2 (Table8). Calculated totalwoodyfuel
0
Sapling
Pole
Mature
Old
DevelopmentalStage
weight in the same classesusing the planar intercept method with fuel particle values from the Rocky Moun-
Figure4. Averagewoody fuel weight by developmentalstagefor
tains(Brown1974a)was0.630kgm-2.Calculations based
19 Sierra Nevada conifers.
on recently derived fuel particle values for the Sierra
78
WJAF13(3)1998
Table 5. Average bulk density of duff fuels of 19 Sierra Nevada conifers
Duff depthclass(cm)
Species
0.00-0.64
0.64-1.91
1.91-10.16 > 10.16 Totalduff .......................................................................... (kgm-2)...........................................................................
Douglas-fir Foothillpine Foxtailpine Giantsequoia
103.68 34.86 153.76 105.91
138.93 130.86 313.22 163.02
263.91 237.12 448.21 161.61
981.82 435.77 -240.90
152.53 130.61 210.67 162.00
Incense-cedar
117.20
153.58
203.68
378.65
180.93
Jeffreypine Knobcone pine Limberpine Lodgepole pine
54.48 44.71 112.03 124.40
119.20 134.14 164.86 188.60
239.89 370.76 337.91 279.51
607.25 392.53 532.85 --
168.48 219.71 223.80 162.59
Mountain hemlock
145.00
193.72
190.56
340.03
183.98
214.23
545.15
154.86
Ponderosa pine
40.83
83.85
Red fir
150.58
209.81
206.29
156.24
182.42
S•ngleleaf pinyon Sugarpine Washoepine Western juniper
136.09 50.35 60.29 150.03
296.81 108.06 151.52 254.60
464.39 368.30 266.73 239.21
362.07 184.69 936.51 --
234.16 160.96 172.79 178.06
66.07
Western whitepine
225.94
298.19
White fir
115.04
212.82
172.72
198.22
183.03
Whitebark pine
115.88
191.78
233.48
329.96
176.82
98.34
176.40
259.31
580.82
177.37
All species
Nevada (van Wagtendonk et al. 1996) yielded a total
--
138.71
WEIGHT= 0.935x CALCULATEDRock,Mountain s
woodyfuelweightof 0.563kgm-2 forthesameclasses.
r 2 = 0.790
Using each of the three methods,red fir had the heaviest combinedwoodyfuel weight,while Washoepine (Pinus washoensis)had the lightestwoody fuel weight. When the calculatedweightsfrom the planar intercept method are usedto predict the sampledweights, both the RockyMountainfuel particlevaluesandthe SierraNevada valuesproducedsignificantregressions throughthe origin (F•gure6). The equationsare:
WEIGHT = 0.858x CALCULATEDsierraNevada r 2 = 0.795
where WEIGHTis thepredicted weight inkgm-2based on thesamples, andCALCULATED istheweight inkgm-2from theplanar intercept method. Ther2values ofthetworegres-
Table6. Averagebulkdensityof fuelbedsof 19SierraNevadaconifers.Thefuelbedbulkdensityincludeswoody, litter, and I hr timelag duff fuels. Litter
Species
Duff
Litter and duff
Fuel bed
.............................................................................. (kgm-3)..........................................
Douglas-fir Foothillpine Foxtailpine Giantsequoia
101.333 32.876 89.909 140.511
152.533 130.605 210.665 162.000
143.464 85.890 187.976 160.416
20.629 12.407 33.395 31.823
Incense-cedar
144.775
180.928
185.041
20.744
168.482 219.711 223.803 162.586
139.352 113.822 189.771 151.174
21.107 20.325 40.209 32.948
Jeffreypine Knobcone pine Limberpine Lodgepole pine Mountain hemlock
Ponderosa pine
32.633 38.253 99.570 94.371 114.857
36.010
183.977
177.352
32.250
154.863
118.003
23.479
Red fir
120.049
182.423
172.139
35.581
Singleleafpinyon Sugarpine Washoepine Western juniper Western whitepine
145.981 40.763 56.404 70.532 49.357
234.164 160.960 172.785 178.064 138.705
205.797 117.153 145.048 147.378 122.948
30.846 28.243 16.571 20.934 27.295
White fir
78.424
183.030
179.786
24.831
Whitebark pine
59.440
176.817
154.970
27.233
All species
81.199
177.372
152.417
26.361 WJAF13(3)1998 79
Table 7. Regressionstatisticsfor litter, duff, and litter and duff weight (kg m-2) as e function of their respectivedepths (cm) for 19 Sierra Nevada conifers Each regression went through the origin, was based on 80 observatmns, and was s•gnificant at the 0.05 level, L•tter
Species
Duff
Coefficient
Douglas-fir Foothillpine Foxtailpine Giantsequoia
0.864 0. I 11 0.886 0.990
r2
Coefficient
Litter and duff
r2
Coefficient
r2
0.796 0.355 0.650 0.548
1.319 1.448 2.504 1.648
0.901 . 0.746 0.914 0.914
1.295 1.220 2.360 1.632
0.910 0.804 0.907 0.920
Incense-cedar
1.276
0.709
1.675
0.865
1.664
0.866
Jeffreypine Knobconepine Limberpine Lodgepolepine
0.358 0.339 0.889 0.951
0.823 0.636 0.789 0.905
1.707 1.646 2.337 1.671
0.864 0.896 0.946 0.904
1.496 1.274 2.255 1.612
0.874 0.902 0.955 0.912
Mountain hemlock
1.102
0.883
1.876
0.913
1.848
0.917
Ponderosa pine
0.276
0.669
1.402
0.912
1.233
0.921
Red fir
0.530
0.446
1.727
0.932
1.722
0.937
Singleleafpinyon Sugarpine Washoepine Westernjuniper
0.906 0.304 0.600 0.832
0.845 0.623 0.570 0.780
2.592 1.396 1.870 1.798
0.883 0.880 0.863 0.932
2.478 I. 189 1.719 1.763
0.900 0.897 0.862 0.924
Westernwhitepine
0.542
0.319
1.422
0.643
1.485
0.797
White fir
1.050
0.888
1.518
0.918
1.572
0.922
Whitebarkpine
0.540
0.878
1.895
0.978
1.802
0.972
All species
0.363
0.494
1.750
0.881
1.624
0.937
sionsshowthat thereis a slight improvementwhen using SierraNevadavaluesovervaluesfromtheRockyMountains (Figure 6). Discussion
Fuel Bed Depth Most woodyfuel depthswereslightlyshallowerthanthe 6.10 cm specifiedin standardfuel modelsfor short-needled conifersand long-needledconifersas describedby Albini (1976). Giantsequoia,Douglas-fir,andredfir exceededthis value;while incense-cedar andsugarpine were within 0.30
cm of thestandard value.Thesespecies,alongwith foothill pine, ponderosapine, and white fir, grow at low- to midelevationsand typicallydrop more branchesand twigs than thehigherelevationspecies.An exceptionis knobconepine, whichgrowson thelowerridgetopsin a stand-replacing fire
60
30
20
5
10
15
2O
25
30
35
Duff Depth(cm)
Figure 5. Regressionlines for duff weight as a function of duff depth for all 19 Sierra Nevada conifer species combined.
80
WJAF13(3)1998
regimewherewoodyfuelsseldomgeta chanceto accumulate. Differencesbetween Sierra Nevada fuel bed depth values and those in the standardized fuel models could result
in significantoverpredictions of fire spreadandintensity. Litter fuel depth appearedto be a function of needle morphology. Coniferswithmediumto longneedleshadthe deepestlitterlayers,whilethosewith singleshortneedlesor scaleshad the shallowestlitter layers.Long needlestendto form more porousfuel bedsthan shortand flat needlesas evidencedby themeanlitter andduff weights(Table3). The thinlitterlayersfortheshort-needled high-elevation whitebark pine, lodgepolepine,andwesternwhite pinecouldindicate thatlow growthpotentialhasan effecton needleproduction and litter depth. Studiesin otherregionsshowsimilarrelationshipsw•th growing conditions.In Arizona, Eakle and Wagle (1979) reporteda litter depthfor ponderosa pine of 1.46 cm, whfie Brown (1970) found litter depthsfor ponderosapine •n wetter, more mesic Montana to averagejust over 2 cm. Lodgepolepine duff depthsin Wyoming were 2.34 cm (Brown 1974b),while in theCascadeMountainsof Wash•ngton they averaged5.02 cm (Woodardand Martin 1980) Differencesin duff depth also appearto be related to growing conditions.Ponderosapine, white fir, and g•ant sequoia,the three specieswith the deepestduff layers, all grow in productivelow- to mid-elevations;in the absenceof fire, they tend to accumulatelarge quantitiesof orgamc material.Deepduff layerswerealsorecordedfor limberpine, a speciesthat growsin cold conditionsat high elevations Thesetreesgrowto considerable age,andfire is a rareevent, givingduff ampletime to accumulate.Althoughthisis also thecasefor whitebarkpineandfoxtailpine,thesespeciesd•d not showa similarresponse.Somespecies,suchas western juniperandfoxtailpine,growonrockyexpanses of exposed granitewherewindsandotherfactorsprecludethe buildup of fuels.
Table 8. Woody fuel weight for the combined 0.00-0 64 cm, 0.64-2.54 cm, and 2 54-7 62 cm size classesfrom this study and calculatedfrom the planar intersectmethod using valuesfrom the RockyMountains (Brown 1974a) and from the Sierra Nevada (van Wagtendonk et al. 1996)
Woody fuel weight
Species
Thisstudy
RockyMountains
SierraNevada
......................................................... (kgm-2)....................................................... Douglas-fir Foothillpine Foxtailpine Giantsequoia
0.997 0.224 0.325 1.292
1.145 0.292 0.300 1.186
0.955 0.212 0.259 1.202
Incense-cedar
0.666
0.847
0.771
Jeffreypine Knobconepine Limberpine Lodgepole pine
0.294 0.235 0.821 0.401
0.347 0.240 0.698 0.579
0.306 0.171 0.786 0.484
Mountain hemlock
0.649
0.863
0.761
Ponderosa pine
0.488
0.479
0.505
Red fir
1.717
1.718
1.521
Singleleafpinyon Sugarpine Washoepine Westernjuniper Westernwhitepine
0.329 0.710 0.153 0.227 0.339
0.381 0.704 0.233 0.239 0.354
0.375 0.669 0.179 0.259 0.209
White fir
0.855
1.037
0.809
Whitebarkpine
0.286
0.334
0.274
All Species
0.579
0.630
0.563
Previously,moststudiesthatreporteddepthsfor species that occur in the Sierra Nevada combined the litter and duff
layers.In general,thesevalueswerelessthantheonesfound m this study.For instance,combineddepthsfor ponderosa pinein theSouthwest wereone-thirdof thosefromthisstudy (Ffolliottetal. 1968,1979,EakleandWagle 1979).Kittredge (1955) measuredlitter and duff depthin the SierraNevada and found values for white fir, red fir, sugarpine, and 80
ponderosa pineto betwo-thirdsto one-halfthosefoundhere. It is possiblethat 40 yr of additionalaccumulationwithout fire has allowed fuels throughoutthe SierraNevadato increase.Kittredge(1955) alsoconductedthe only previous studyof knobconepinelitter andduff depth.In thesouthern Californiacoastranges,he foundvaluesthatweresimilarto thosereportedhere.Sinceknobcone pineisa short-lived firedependentspecies,theremightnotbe enoughtime for fuels to accumulate tohighlevels.If fire isnotallowedin knobcone pine stands,considerable deadanddownwoodyfuelsaccumulateasthe standsdie and fall apart.
,,,,'"' /? Line of Exacl Agreement • '"'"/ •
R2=0.795
•
Litter and duff weight differencesalso appearedto be relatedto needlemorphologyandgrowingconditions.Conifers with single short needlesgrowing at mid- to high-
60 Y=0.939 X • Sierra Nevada
,..,"
.,.'
ß
elevations, such as white fir and red fir, tended to have
40
o
.'"/
,'" /
• Rocky Mounlain! Y=0,858 X
.,,'•/
R2=0,790
0
20
40
60
Fuel Bed Weight
80
Calculated Woody FuelWeight(kgm'2) Figure 6. Regressionlines for woody fuel weight derived by direct sampling in this study as a function of weights calculated using the planar intercept method with values from the Rocky Mountains (Brown 1974a)andthe SierraNevada (vanWagtendonk et al. 1996).
heavier litter weights.Mid- to low-elevationconiferswith fasciclesof multiplemediumto longneedles,suchas sugar pine, knobconepine, andponderosa pine, hadheavierlitter loads. Growing conditionsand tree size could be affecting litter weightsof specieswith scalelikeleaves;thelowestlitter weightwasrecordedby the high-elevationwesternjuniper, while themid-elevationgiantsequoiahada moderatelyhigh litter weight.Other species,however,defy simpleexplanations.For instance,whitebarkpine andlimberpinegrow in similarconditionsandhavesimilarneedlemorphologies; yet their litter weights are at the two extremes. The litter weight values determinedin this study were similarto thoseof previousstudiesof SierraNevadaconifer species.In the SierraNevada,van Wagtendonk(1974) re-
ported litterweights of0.442kgm-2forponderosa pineand 0.292kg m-2 forincense-cedar. Ageeet al. (1978)found slightly lower valuesfor ponderosapine, sugarpine, and
WJAI • 13(3)1998 81
g•ant sequoia•n the southernS•erraNevada Their htter weight for white fir was twice that reportedhere, but they attributedthat to a heavy crop of deciduouscone scales, whichwerenotincludedaslitterin thisstudy.Ponderosa pine
threesmallests•zeclassesthandid Washoepine Branching habitandtreesizeappearto •nfluencetheamountof woody fuels.Species,suchasredfir, whitefir, andDouglas-fir,have
litterinthedrierSouthwest averaged 0.224kgm-2(Sackett
Giantsequoia,by its sheersize,age,andslowdecayrate,can accumulatelarge amountsof woodyfuelsover time. Other species,suchas Washoepine, foothill pine, and western juniper,haveparticularlyopencrownswithsmallamounts of branchwood. Knobcone pineretainsmuchof itsbranchwood in the crownbeforebeingburnedby a stand-replacing fire
1979), while in the northernRocky Mountainsits average
was0.324kgm-2 (Brown1970). In the absenceof fire, duff accumulation is the result of the
differencebetweenduff productionand decomposition. Modelsof forestfloor buildupin the SierraNevadahave shown maximum total accumulation to be reached after about
100yr withoutfire (Agee 1973,vanWagtendonk1985).In climatesmore conduciveto decomposition, the accumulation peakssoonerat a lower total amount.Conversely,in climatesthatarefavorablefor growthbut notfor decomposition, accumulation rates are fast and total accumulation
high. The variationin totalduff weightamongspecieswas so greatthatnosinglefactorcanexplainthedifferences.It does appear,however,that dueto productivitylevels,speciesin hot, dry and in cold, dry environmentshave lighter duff layers.For instance,westernjuniper,westernwhitepine,and foxtail pinehadthe lowestvaluesfor duff weight,followed by foothillpineandknobcone pine.Mid-elevationconifers, suchasredfir andDouglas-fir,in areaslittle affectedby fire suppression activitiesgenerallyhad moderateamountsof duff. On theotherhand,specieswith theheaviestduff layers were either mid-elevationspecies,suchas ponderosapirie andgiantsequoia,wheretheeffectsof fire suppression have beenmostpronounced, or high-elevation long-livedspecies, suchaslimberpine,whitebarkpine,andmountainhemlock, wherefiresseldomoccurandproductivityoutpacesdecomposition. The duff amountsin this studywere greaterthan those found in previous studiesin the Sierra Nevada by van Wagtendonk(1974) for incense-cedar and ponderosapine andby Kittredge(1955) for ponderosa pine,knobconepine, giantsequoia,whitefir, andred fir. As with duff depth,the additional20 and 40 yr of fuel accumulationwithoutfire couldexplainthesedifferences.Drier growingconditionsin the Southwestcould explainthoselower duff weightsfor ponderosa pinethanthosefoundin theSierraNevada(Ffolliott et al. 1968, 1979,Eakle andWagle 1979, Sackett1979). Woodyfuel weightsin thesmallestthreesizeclasseswere heavierin this studythan thosespecifiedin standardized models(Anderson1982, Albini 1976) generallyused for SierraNevadaconifers.The short-needledfuel model(Model
8)specifies awoody weight of1.121 kgm-2compared tothe 0.855kgm-2found hereforwhitefir.Thelong-needled fuel model(Model9) usesa valueof 0.796kgm-2;ponderosa pineinthisstudy hadaweight of0.488kgm-2.If, however, theweightof thelitter anduppermost duff layerareaddedto
thewoody weights, thevalues are1.748kgm-2and1.307kg m-2, respectively. Theselattervaluesareprobably more realisticsincelitterandtheuppermost duff layerusuallyare burnedby the passingfire front. There was considerable variationin woody fuel weight amongspecies;red fir had 10 timesmorewo6dyfuelsin the 82
WJAF13(3)1998
numerous small branches that contribute to the fuel load.
Previousstudies of woodyfuelweightsof SierraNevada species havebeenbasedprimarilyonBrown's(1974a)planar interceptmethod.ParsonsandDeBenedetti(1979) reported weightsfor woody fuels less than 7.62 cm in diam for ponderosapine and giant sequoiathat were slightlylower than thosefound in this study;while weightsfor white fir werenearlytwiceashigh.Highervalueswerealsofoundfor ponderosapine, white fir, and incense-cedarby van Wagtendonkand Sydoriak(1987). In studiesof ponderosa pinein theSouthwest, Sackett(1979, 1980)usedplotsfor the 0.00-0.64 cm sizeclassandthe planarinterceptmethodfor thelargerclassesof woodyfuelsandfoundweightscomparableto thoseof this study. Fuel Bed Bulk Density
Bulk density is calculatedfrom weight and depth of woody,litter,andduffcomponents. Species withshallowbut heavybeds,suchas singleleafpinyon,hadhigh bulk dens•ties;specieswith deepbut light beds,suchas foothillp•ne, hadlow bulkdensities.A combination of needlemorphology andgrowingconditions contributes to thesedifferences. Th•s variationin bulk densitywill havea profoundeffecton fire behavior.Up to a point,moreporousfuelbedswill burnw•th greaterintensitythan denserbeds.The bulk densitiesreportedhere can be usedto constructcustomfuel models (Burganand Rothermel1984). In the SierraNevada,Stephens(1995) foundbulk dens•tiesfor ponderosa pineandwhitefir to increasewith depth
Ponderosa pinevaried from22.40kgm-3forthelitterlayer to264.00kgm-3 forthedeepest dufflayer.Forwhitefir,he recorded a minimum of 52.00kg m-3 anda maximum of 338.20kgm-3 forthesame layers. These values aresimdar to thosefoundin thisstudy. Forestfloor bulk densitiesfrom thisstudyalsocompared well with studiesin otherregions.The only SierraNevada speciesto havebulkdensitiespreviouslyreportedareponderosapine,Douglas-fir,andlodgepole pine.In theSouthwest,
ponderosa pinebulkdensities varied from61.13kgm-3 for the combinedlitter and duff layers(Ffolliott et al. 1976) to
211.07kgm-3 withtheaddition ofwoody fuels(Harrington 1986). The most comparablevaluesare for the combined litter andduff layers,however,andvaluesthererangedupto
151.01kg m-3 (EakleandWagle1979).Thesecompare favorably withthe118.00kgm-3 foundinthisstudy forthe combinedponderosa pine litter andduff. In thenorthernRockyMountains,Brown(1970) recorded
a bulkdensity of 76.89kgm-3 forponderosa pinecombined litterandduff.In ponderosa pinestandswithgrassandshrub understories,however, Brown (1981) found median bulk
densittes forlitterlayers rangtng from2190kgm-3 to29.00 kgm-3 Inthesame regton, Douglas-fir stands withs•mdar understones hadhtterbulkdensities from25.30kgm-3 to 58 10kgm-3 (Brown1981).
addtttonof wetght and depth data from thts study, these esttmates canbeimprovedevenmore Ftnaladjustments can then be madeto the calculatedesttmatesso they reflect the differencesbetweencalculatedandsampledfuel weight.
Two studiesof lodgepole pinein Washington andWyomingfoundbulkdensities slightlylessthanthosereported
Conclusion
here(WoodardandMartin 1980, Brown 1970). Calculated
forestfloorbulkdensities fromtheweightanddepthfigures tn Kittredge(1955)for ponderosa pine,whitefir, andredfir were similar to the ones found here. The calculated bulk
densityfor knobcone pine growingin southernCalifornia was half that of the Sierra Nevada stands.
EstimatedLitter and Duff Weight Theequations presented herewill allowreliablepredicttonsof litter andduff weightto be madebasedon depth. By using this easily measurablefuel characteristic,the timeconsumingcollectionandweighingof largeamounts of duff fuels are avoided.Theseequationsshouldreplace the ones by Agee (1973) that were for mixed stands of ponderosapine and incense-cedarand for white fir and giant sequoia.They are also more appropriateto usefor ponderosapine in the Sierra Nevada than the equations from theSouthwest(Eakle andWagle 1979,Ffolliott et al. 1968, 1976, Harrington 1986). Althoughdirectcomparisons with otherregression methods,suchaslinearandlogarithmic,cannotbemade,thehigh
r2 values indicate thevalidityofusing regressions through the origin.In addition,the appropriateness of thisapproach wasindicatedby thelargenumberof observations thatwere nearzerofor bothweightanddepth(Figure5). The regressionequationscan be usedby managersand researchersinterestedin any singlelayer or combination of layers. For instance,the methodusedby the National ParkServiceto determinetheeffectiveness of prescribed fire treatmentsusesseparatelitter andduff measurements (Reeberg1995). Brownet al. (1982) includethelitter layer wtth the woody fuels but do not provide equationsto convertlitter depthtOlitter weight.The Albini (1976) fuel modelsincludelitter andthe uppermostduff layer in the 1 hr ttmelagfuel class.User-definedcustomfuel modelscan useany combinationof litter andduff fuels to reflect local conditions(BurganandRothermel(1984). Althoughit is possiblethat interactionsamongspeciescouldalter productionand decompositionrates, the equationscould be used to approximateestimatesof duff weight in mixed speciesstands.For instance,if some measureof abundancesuchas basalareaor canopycoveris available,the equationscan be appliedin proportionto thatmeasure. EstimatedWoody Fuel Weight Thepopularityof theplanarinterceptmethodof inventorytngwoodyfuelswarrantstheinvestigation intoits applicabthtyin regionsotherthanthenorthernRockyMountains.In a studyof Sierra Nevadaconifers,van Wagtendonket al.
(1996)showedthatregionalvariationin thephysicalproperttes of woody fuel particlescould result in fuel weight estimates that differ from as much as 40.8% less to 8.3% more
thanthosecalculatedfromRockyMountainvalues.With the
This studyhasshownthatregionaldifferencesin fuel bed propertiesare real and shouldbe taken into consideration when planningand conductinghazardousfuel treatments. Since fuel bed depthplays suchan importantrole in fire behavior,thesedifferencescouldresultin overpredictions of fire intensityand rate of spreadif the standardizedfuel modelsare used.The refinementssuggested herewill provide the most accurateestimatesof fuel bed properties possibleconsistentwith time and fundingconstraints. The importanceof havingaccurateestimatesof fuel weightand depthin predictingfire behaviorand measuringfire effects ß cannotbe overstated.As land managersstrive to meet the conflictingresourceobjectivesof maintainingnaturalprocessesandreducinghazardousfuels,they will needthe best available information.
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