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Fuel Bed Characteristics of
Sierra Nevada 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 study of fuels in Sierra Nevada conifer forests showedthat fuel bed depth and fuel bed weight
significantly varied by tree species and developmental stage of the overstory. Specific values for depth and
weight of woody, litter, and duff fuels are reported There was a significant positive relationship between fuel
bed depth and weight. Estimates of woody fuel weight using the planar intercept method were significantly
related to sampled values. These relationships can be used to estimate fuel weights in the field. West. J. Appl.
For. 13(3):73-84.
The accumulation of hazardous fuels throughout the Sierra
Nevada over the past 60 yr has become an issue of recent
•ncreased concern. In 1993, Congress directed that a study be
conducted to assess what information is needed to make
decisions for the future management of the Sierra Nevada
ecosystems. One of the critical findings of the Sierra Nevada
Ecosystem Project was that live and dead fuels in the conifer
forests today are more abundant and continuous than in the
past (SNEP 1996). Combined with the growing density of
homes, these increases in fuels exacerbate the threat of fire to
the human and natural resources in the Sierra Nevada. Accu-
rate methods are needed to assess fuel conditions in order to
develop fire protection and fuel mitigation plans. Lack of
such accurate information reduces the reliability of these
plans.
Prescribed fire has been proposed as one of the most
effective tools for reducing fuel hazards (van Wagtendonk
1996). The use of prescribed fire requires reliable estimates
of fuel weight in order to safely accomplish the burns and to
evaluate their effectiveness in meeting management objec-
tives (Reeberg 1995). In addition, fuel bed weight and depth
are used in the Rothermel (1972) rate of spread equation that
•s central to the National Fire Danger Rating System (Deem-
mg et al. 1977) and the fire behavior prediction system used
NOTE: Jan W. van Wagtendonk is the corresponding author and can be
reached at (209) 379-1885; Fax: (209) 379-1886; E-mail:
jan_van_wagtendonk@usgs.gov. The authors thank all of the people who
helped on this project. Joe Coho oversaw the data entry, and Diane Ewell
spent many hours in the field collecting fuels. She was ably assisted by many
volunteers including Charisse Sydoriak, Kay Beeley, Kathy Tier, and Karen
Kolbeck. Liam Bickford volunteered to enter all of the data.
by fire behavior analysts (Rothermel 1983). Fuel weight and
depth are specified for standardized models that are used in
both of these systems (Anderson 1982, Albini 1976).
Fuel weight plays a dual role in Rothermel' s (1972) spread
rate equation. In the numerator, it is multiplied by fuel heat
content to produce a source of heat for the calculation of
reaction intensity. Fuel weight is divided by fuel depth in the
denominator of the equation to determine fuel bed bulk
density, which acts as a heat sink. Increases in fuel weight
usually cause reaction intensity to increase more than rate of
spread. In fact, all else being equal, as weight increases, rate
of spread may actually decrease because more fuel must be
raised to ignition temperature (Burgan and Rothermel 1984).
Brown (1981) identifies fuel bed bulk density as one of the
most difficult properties to measure in the field.
Deeming et al. (1977) categorized woody fuels by size
classes and duff fuels by depth classes that both correspond
to fuel moisture timelags (Table 1). Timelag is the amount of
time necessary for a fuel component to reach 63% of its
equilibrium moisture content (Lancaster 1970). Deeming et
al. (1977) stress that the duff designations are very rough
approximations and should be used with caution. For ex-
Table 1. Timelag classes and corresponding woody fuel size
classes and duff fuel depth classes (Deeming et al. 1977).
Timelag class Woody fuel size class Duff fuel depth class
............................... (cm) ..............................
1 hr 0.004).64 0.00-0.64
10 hr 0.64-2.54 0.64-1.91
100 hr 2.54-7.62 1.91-10.16
1000 hr >7.62 >10.16
WJAF 13(3) 1998 73
ample, van Wagtendonk and Sydonak (1985) found good
correlations between the moisture contents of duff and woody
1 hr and 10 hr timelag fuels, but the correlations were poor for
100 hr and were nonexistent for 1000 hr timelag fuels.
The forest floor also can be divided into litter and duff,
corresponding to the Oi layer and the combined Oe and Oa
layers described in the literature on forest soils (Soil
Survey Staff 1993). Litter is defined as freshly cast
nonwoody organic matter that still retains its morphologi-
cal characteristics. Duff includes both the fermentation
layer, where decomposition has begun but the particles
can still be recognized, and the humus layer, where or-
ganic material is compressed and in all states of decay. As
fuels, the most useful categories are probably litter and the
four duff depth layers.
Land managers have traditionally used the planar inter-
cept method developed by Brown (1974a) to inventory downed
woody material. This method is also the basis for photo series
for quantifying natural forest residues (Blonski and Schramel
1981). Fuel weight is estimated by counting the number of
intercepts of woody fuel particles in different size classes
with a sampling plane. The intercepts are then multiplied by
factors derived from physical properties of woody fuels of
Rocky Mountain conifer species. The planar intercept method
has proven useful in areas with plentiful woody fuels result-
ing from management activities but has met with less success
in areas with natural fuels (Brown 1981). Although van
Wagtendonk et al. (1996) have determined factors based on
the physical properties of woody fuel particles of Sierra
Nevada conifers, there is a need for more accurate woody fuel
weight estimates.
In the Sierra Nevada, the only published equations for
estimating forest floor weight from depth were provided by
Agee (1973). Because he sampled mixed stands of ponderosa
pine (Pinus ponderosa) and incense-cedar (Calocedrus
decurrens) and stands of white fir (Abies concolor) and giant
sequoia (Sequoiadendron giganteurn), individual species re-
lationships could not be determined. His r 2 values, however,
ranged up to 0.90 for total litter and duff for the white fir-
giant sequoia mix.
Equations for estimating weights of duff fuels and total
fuels have also been developed for the Southwest and the
Rocky Mountains. Unfortunately, conversion factors rec-
ommended by Brown et al. (1982) were based on a limited
number of studies of bulk density including ponderosa
pine and lodgepole pine (Pinus contorta) in the northern
Rocky Mountains (Brown 1970, 1974b). Several studies
of ponderosa pine in the Southwest have related fuel bed
weight to fuel bed depth. Eakle and Wagle (1979) used
logarithmic regressions for the litter and duff layers, but r 2
values were low (0.58 and 0.65, respectively). Linear
regressions through the origin were reported by Ffolliott et
al. (1968, 1976), butno r 2 values were provided. Harrington
(1986) also used a linear regression and reported an r 2 of
0.78. Sackett (1979), however, was unable to establish a
reliable relationship for predicting fuel weight from duff
depth. Woodard and Martin (1980) reported an r 2 of 0.849
for lodgepole pine in Washington.
74 WJAF 13(3) 1998
The lack ofinformauon on fuel weight and depth of Sierra
Nevada conifers reduces the reliability of fuel hazard assess-
ments and fire behavior predictions. This study was designed
to gather that information. This paper presents data for Sierra
Nevada conifers on woody fuel weight by size class, litter and
duff fuel weight by depth class, litter and duff depth, and fuel
bed bulk density. We also compare woody fuel weights with
estimates using the planar intercept method. In addition, we
present regression equations for each species that can be used
to estimate duff fuel weight.
Methods
Fuels were collected from stands of each of the 22
species of conifers occurring in the Sierra Nevada. Species
absent or not well-represented in Yosemite National Park
were sampled in adjacent national forests or in Sequoia
and Kings Canyon National Parks (Figure 1). California
torreya (Torreya californica), Pacific yew (Taxus
brevifolia), and California juniper ( Juniperus califormca)
had insufficient fuels for complete sampling. Four stands
of each species were sampled representing developmental
stages from young to old. Trees were approximately 2.5 to
10 cm in diam. in sapling stands, 10 to 60 cm in pole
stands, 60 to 120 cm in mature stands, and greater than 120
cm in old stands. Each selected stand had an area of at least
300 m 2 with an overstory composed of 100% of the desired
species and was free of recent disturbance such as fire and
tree mortality from insects and diseases. Since adjacent
trees could contribute to the fuels on a plot, only stands
with at least 90% of fuels of the desired species were
sampled.
Four 11 m parallel transects 3 m apart were established
beneath each species and developmental stage combina-
tion. Fuel particle intersections with the sampling planes
were counted and categorized by size class. Woody fuel
depth was recorded at three points along each transect
from the highest intersected dead particle to the bottom of
the litter layer (Brown 1974a). Five 20 x 50 cm subplots
were systematically placed along each transect to collect
woody fuels for a total of 20 subplots/stand. Twenty 10 x
10 cm subplots at the same locations were used to collect
litter and duff fuels. Duff samples included incorporated
cone scales, bark flakes, and fine (0.0-0.64 cm) branches
Litter and total duff depth were measured at the center of
the edge of each subplot adjacent to the transect line
Freshly fallen litter was first separated and sealed in
plastic bags. Then fuels from the four timelag fuel diam-
eter classes and four duff depth timelag classes (Table 1)
were collected and bagged. Sound and rotten samples of
the larger branchwood were bagged separately. Samples
were dried in a convection oven at 65øC until weight loss
stabilized.
Fuel weights of the duff and woody fuel components were
analyzed using two-way analysis of variance with species
and developmental stage as the independent factors. Two-
way analysis of variance was also used for woody fuel depth,
litter depth, and duff depth with species and developmental
stage as the independent variables. Post hoc Scheff6 multiple
Location of
Sample Stands Yosemite
National Park
.i .
Yosemite
**
*
Sequoia and
Kings Canyon .
Figure 1. Sampling locations for four stands each of 22 conifer species in the Sierra Nevada,
California and Nevada.
range tests were used with a one-way analysis of variance to
determine which species means were significantly different
from each other (Scheff6 1959). Regression analysis was
used to compare woody fuel weights derived in this study
with the planar intercept method using Rocky Mountain
(Brown 1974a) and Sierra Nevada (van Wagtendonk et al.
1996) values. Estimates of fuel weight as a function of fuel
depth were determined through regression analysis. All sig-
nificance tests were at the 0.05 level.
Bulk densities for litter, duff, and litter and duff combined
were calculated from their respective depth and weight val-
ues. Duff weight was the sum of the four fuel depth classes.
Fuel bed bulk density calculations were based on the depth
and weight of the woody fuels as well as the litter and the
uppermost duff layer. This is consistent with Burgan and
Rothermel (1983) and Albini (1976), who include litter and
1 hr duff fuels in their fuel models. Brown (1974a) also
includes the litter layer in his fuel depth measurement. The
equation for fuel bed bulk density used in this study is:
B ULKDENSITYBecl =
100 x WEIGHTwøøayWEIGHTœitterWEIGHTønehøura"f[
DEPTHwooay + DEPTHonehourduf f
where BULKDENSITY is in kg m -3, WEIGHT is in kg m -2,
DEPTH is in cm. The depth of the 1 hr duff layer is either 0.64
cm or, in cases where the total duff depth is less than 0.64 cm,
the total duff depth. Multiplying by 100 is necessary to
express bulk density on a per cubic meter basis.
WJAF 13(3) 1998 75
Results
Fuel Bed Depth
Woody fuel depth was significantly affected by species,
developmental stage, and their interaction. Litter depth and
duff depth were similarly affected. Woody fuel depth values
for the various species ranged from 1.24 cm for foxtail pine
(Pinus balfouriana) to 9.27 cm for giant sequoia (Table 2).
Litter depth was lowest for white fir and red fir at 0.15 cm and
highest for sugar pine (P. lambertiana) at 2.10 cm. There was
considerable variation in duff depths from a high of 8.67 cm
for giant sequoia to a low of 1.28 cm for western juniper.
Multiple comparisons divided the woody fuel depths into
six subsets with singleleaf pinyon (P. monophylla) and fox-
tail pine unique to the shallowest subset and Douglas-fir
(Pseudotsuga me•ziesii), giant sequoia, and red fir (Abies
mag•ifi'ca) unique to the deepest subset. Litter depth means
were grouped into four subsets: the first subset included all
the species with mean litter depths less than the 0.70 cm of
limber pine (Pimps fieMlis); Jeffrey pine (P. jeffreyi) was
included in the second and third subsets; and ponderosa pine
and Douglas-fir were unique to the fourth. There were seven
homogeneous subsets for duff depth, with foxtail pine and
western juniper in the shallowest subset and giant sequoia the
deepest.
The effect of developmental stage can be seen in Figure 2:
growth of stands from sapling to pole, mature, and old stages
generally resulted in increased depth. There was a slight drop
in litter depth between the pole and mature stages and in
woody fuel depth between the mature and old stages.
Fuel Bed Weight
Litter and Duff Fuel Weight.--Species, developmen-
tal stage, and their interaction had significant effects on
the weight of litter and duff fuel components. Litter weights
varied from a low of 0.127 kg m -2 for white fir to a high
of 0.742 kg m -2 for sugar pine (Table 3). Foothill pine
Table 2. Fuel bed depth for 19 Sierra Nevada conifers.
10 Woody
• Litter
i Duff
8
E 6
• 4
0 Saphng Pole Mature Old
Developmental Stage
Figure 2. Average fuel bed depth by developmental stage for 19
Sierra Nevada conifers.
(Pinus sabiniana) had the lowest weight (0.207 kg m -2) in
the 0.00-0.64 cm duff depth class while mountain hem-
lock (Tsuga mertensiana) had the highest (0.927 kg m-2).
In the 0.64-1.91 depth class, foothill pine weighed the
least (0.706 kg m -2) and white fir the most (2.675 kg m-2).
Limber pine had particularly deep duff layers with 9.193 kg
m -2 in the 1.191-10.16 cm depth class and 4.582 kg m -2 in
the greater than 10.16 cm class. There was very little
western juniper duff with only 0.733 kg m -2 in the 1.91 cm
depth class and no fuel in the greater than 10.16 cm class.
Foxtail pine and western white pine (Pinus monticola)
fuels in the deepest class also were missing.
Species means for litter weight were divided into five
homogeneous subsets. Out of the 14 species in the group with
the lightest litter, western juniper, white fir, western white
pine, and red fir were unique. Sugar pine was the only unique
member of the heaviest group. The multiple comparisons for
Species Woody fuel depth Litter depth Duff depth Litter and duff depth
................................................................ (cm) ..............................................................
Douglas-fir 6.51 0.33 4.40 4.73
Foothill pine 4.50 1.35 3.23 4.58
Foxtail pine 1.24 0.19 1.60 1.79
Giant sequoia 9.27 0.37 8.67 9.04
Incense-cedar 5.84 0.20 4.87 5.07
Jeffrey pine 2.82 1.11 5.40 6.51
Knobcone pine 3.62 1.70 2.80 4.50
Limber pine 3.73 0.70 6.97 7.67
Lodgepole pine 2.35 0.44 3.54 3.98
Mountain hemlock 3.52 0.36 6.05 6.4 I
Ponderosa pine 4.34 1.87 7.88 9.75
Red fir 6.21 0.15 4.88 5.03
Singleleaf pinyon 2.67 0.49 3.26 3.75
Sugar pine 5.52 2.10 5.97 8.07
Washoe pine 3.00 0.65 3.66 4.31
Western juniper 1.90 0.11 1.28 1.39
Western white pine 1.43 0.25 1.81 2.06
White fir 3.98 0.15 6.46 6.61
Whitebark pine 2.19 0.49 4.93 5.42
All species 3.93 0.68 4.61 5.29
76 WJAF 13(3) 1998
Table 3. Average weight of litter and duff fuels of 19 Sierra Nevada conifers.
Duff depth class (cm)
Species L• tter 0.00-0.64 0.64- 1.91 I. 9 I- I 0.16 > I 0.16 Total
.......................................................................... (kg m-") ...........................................................................
Douglas-fir 0.308 0.638 1.493 3.876 0.054 6.062
Foothill pine 0.338 0.207 0.706 3.001 0.395 4.309
Foxtail vine 0.178 0.754 1.604 1.291 -- 3.649
G•ant sequoia 0.452 0.677 1.997 8.942 2.477 14.092
Incense-cedar 0.268 0.736 1.746 5.651 0.232 8.365
Jeffrey vine 0.376 0.345 1.275 6.169 1_ 177 8.965
Knobcone vine 0.586 0.244 0.985 3.180 0.428 4.836
Limber vine 0.744 0.602 1.630 9.193 4.582 16.006
Lodgepole vine 0.416 0.786 1.623 3.343 0.042 5.793
Mountain hemlock 0.431 0.927 2.125 6.309 1.522 10.883
Ponderosa vine 0.562 0_257 1.016 7.354 2.682 I 1.309
Red fir 0.155 0.884 2.106 5.226 0.583 8.799
Singleleaf pinyon 0.567 0.866 2.063 4.328 0.624 7.881
Sugar vine 0.742 0.322 1.220 6.603 0.574 8.720
Washoe vine 0.351 0.356 1.244 4.702 0.286 6.588
Western juniper 0.079 0.617 0.934 0.733 -- 2.285
Western white vine 0.128 0.387 1.084 1.089 -- 2.560
White fir 0.127 0.736 2.675 6.428 0.134 7.973
Whitebark vine 0.269 0.663 1.448 4.591 2.419 9.121
All species 0.370 0.579 1.525 4.843 1.022 7.999
duff weights identified seven subsets for the 0.004).64 cm
and 0.64-1.91 cm depth classes. Means for the greater than
I 0.16 cm class were grouped into two subsets with all species
but limber pine in the first set and the seven species with the
least duff in the second class. Figure 3 shows the effect of
developmental stage on litter and duff weight. As stands
develop to older stages, weights in each of the litter and duff
components increased.
Woody Fuel Weight.---Woody fuel weight in the three
smallest size classes and sound fuels greater than 7.62 cm in
diam. were all significantly affected by species, developmen-
tal stage, and their interaction. Rotten fuels greater than 7.62
cm in diameter did not vary significantly. Red fir had the
largest quantities of woody fuel in the 0.004).64 cm and
0.64-2.54 cm size classes, while giant sequoia dominated the
14
12
10
• Duff0.00-0.64 cm
• Duff0.64-1.91 cm
ß Duff 1.91-10.16 cm
ß Duff>7.62 cm
o Sapling Pole Mature Od
Developmental Stage
Figure 3. Average litter and duff weight by developmental stage
for 19 Sierra Nevada conifers.
2.54-7.62 cm and greater than 7.62 cm sound classes (Table
4). Ponderosa pine had the least amount of woody fuel in the
0.004).64 cm class as did knobcone pine (Pittus attenuata) in
the 0.64-2.54 cm class. Several species had no woody fuels
in the three largest size classes.
Eight homogeneous subsets of means were identified for
the small woody fuels in the 0.00-0.64 cm class. The 0.64-
2.54 cm class was divided into five subsets with the heaviest
containing only red fir and Douglas-fir. Species means were
grouped into two overlapping subsets for the 2.54-7.62 size
class, and giant sequoia was the only species that did not
occur in the lightest subset. The mean comparisons did not
divide the sound and rotten classes of fuels greater than 7.62
cm into subsets.
The increase in woody fuel weight with advancing devel-
opmental stage can be seen in Figure 4. The three smallest
size classes increased between each of the developmental
stages. Large sound fuels decreased slightly between the pole
and mature stages, as did large rotten fuels between the pole,
mature, and old stages.
Bulk Density.--Average duff bulk density ranged from
34.86 kg m -3 for the 0.00-43.64 cm depth layer for foothill
pine to 981.82 kg m -3 for the greater than 10.16 cm layer for
Douglas-fir (Table 5). Bulk density within the duff layer
generally increased with depth. Red fir, singleleaf pinyon,
and sugar pine showed decreases in the greater than 10.16 cm
depth class, however, while western juniper and white fir
decreased in the 1.9 I- I 0.16 cm class.
Litter bulk densities varied from a low of 32.88 kg m -3 for
foothill pine to a high of 145.98 kg m -3 for singleleaf pinyon
(Table 6). Foothill pine had the lowest values for the duff
layer, the combined litter and duff layers, and the total fuel
bed. Singleleaf pinyon had the highest bulk densities for
duff (234.16 kg m -3) and litter and duff combined (205.80
kg m-3). Limber pine had the highest value for total fuel
bed bulk density (40.21 kg m-3).
WJAF 13(3) 1998 77
Table 4. Average weight of woody fuel components of 19 Sierra Nevada conifers
Woody fuel component (cm)
Species 0.0-0.64 0.64-2.54 2.54-7.62 >7.62 sound >7.62 rotten Total
.............................................................................. (kg m-2) ...........................................................................
Douglas-fir 0.370 0.473 0.153 0.039 -- 1.036
Foothill pine 0.064 0.093 0.067 -- -- 0.224
Foxtail pine 0.185 0.140 -- -- -- 0.325
Giant sequoia 0.130 0.403 0.758 1.045 -- 2.336
Incense-cedar 0.245 0.282 0.139 0.318 -- 0.984
Jeffrey pine 0.025 0.196 0.073 -- -- 0.294
Knobcone pine 0.069 0.075 0.091 -- -- 0.235
Limber pine 0.150 0.215 0.456 0.494 -- 1.315
Lodgepole pine 0.084 0.142 0.175 0.038 -- 0.439
Mountain hemlock 0.217 0.248 0.184 0.181 -- 0.830
Ponderosa pine 0.013 0.215 0.260 0.428 0.079 0.995
Red fir 0.527 0.683 0.507 0.139 0.140 1.996
Singleleaf pinyon 0.217 0.112 -- -- -- 0.329
Sugar pine 0.143 0.178 0.389 0.086 0.008 0.804
Washoe pine 0.018 0.107 0.028 0.023 -- 0.176
Western juniper 0.037 0.110 0.079 -- -- 0.226
Western white pine 0.089 0.162 0.087 -- -- 0.338
White fir 0.290 0.370 0.195 0.175 0.003 1.033
Whitebark pine 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
Fuel bed weights can be estimated by predicting litter and
duff weight from litter and duff depth or by determining the
relationship between calculated woody weight values from
the planar intercept method with woody fuel weights from
this study.
Estimating Litter and Duff Weight.--Since regressions
through the origin had consistently higher r 2 values than
regressions with intercepts, the analysis was performed with-
out intercepts. These r 2 values measure the proportion of the
variability in weight about the origin explained by the regres-
sion, however, and cannot be directly compared to r 2 values
for models that include an intercept.
1
!.4
0.00-0.64 cm
I 1 0.64-2.54 cm
!.2 ß 2.54-7.62 cm
>7.62 cm Sound
>7.62 cm Rotten
0.2
0 Sapling Pole Mature Old
Developmental Stage
Figure 4. Average woody fuel weight by developmental stage for
19 Sierra Nevada conifers.
There was considerable variation among species for the
litter layer regressions. Incense-cedar had the highest
slope coefficient of 1.276, while foothill pine had the
lowest at 0.1 I I (Table 7). The r 2 values ranged from 0.905
for lodgepole pine to 0.355 for foothill pine. The r 2 values
for duff were higher and more consistent than those of the
litter, ranging from 0.978 for whitebark pine (Pinus
albicaulis) to 0.643 for western white pine. The largest
regression coefficient for duff was 2.592 for singleleaf
pinyon, and the smallest was 1.319 for Douglas-fir. When
litter and duff were combined, the regression coefficients
for individual species ranged from 1.189 for sugar pine to
2.478 for singleleaf pinyon, reflecting the influence of the
heavier duff layer. The r 2 values ranged from 0.797 for
western white pine to 0.972 for whitebark pine. When all
species were combined, the r 2 value for the litter layer was
0.494 and the duff layer was 0.881. The equation for the
regression through the origin for total litter and duff
weight as a function of total litter and duff depth was:
WEIGHT = 1.624 x DEPTH
r 2 = 0.937
where WEIGHT is in kg m -2 and DEPTH is in cm. Figure 5
is a plot of the regression of all 1,520 plots. As can be seen in
the figure, the largest proportion of the data points appeared
near the origin.
Estimating Woody Fuel Weight.--For all species
combined, the average total woody fuel weight in the
0.00-0.64 cm, 0.64-2.54 cm, and 2.54-7.62 cm size classes
was 0.574 kg m -2 (Table 8). Calculated total woody fuel
weight in the same classes using the planar intercept
method with fuel particle values from the Rocky Moun-
tains (Brown 1974a) was 0.630 kg m -2. Calculations based
on recently derived fuel particle values for the Sierra
78 WJAF 13(3) 1998
Table 5. Average bulk density of duff fuels of 19 Sierra Nevada conifers
Duff depth class (cm)
Species 0.00-0.64 0.64-1.91 1.91-10.16 > 10.16 Total duff
.......................................................................... (kg m-2) ...........................................................................
Douglas-fir 103.68 138.93 263.91 981.82 152.53
Foothill pine 34.86 130.86 237.12 435.77 130.61
Foxtail pine 153.76 313.22 448.21 -- 210.67
Giant sequoia 105.91 163.02 161.61 240.90 162.00
Incense-cedar 117.20 153.58 203.68 378.65 180.93
Jeffrey pine 54.48 119.20 239.89 607.25 168.48
Knobcone pine 44.71 134.14 370.76 392.53 219.71
Limber pine 112.03 164.86 337.91 532.85 223.80
Lodgepole pine 124.40 188.60 279.51 -- 162.59
Mountain hemlock 145.00 193.72 190.56 340.03 183.98
Ponderosa pine 40.83 83.85 214.23 545.15 154.86
Red fir 150.58 209.81 206.29 156.24 182.42
S•ngleleaf pinyon 136.09 296.81 464.39 362.07 234.16
Sugar pine 50.35 108.06 368.30 184.69 160.96
Washoe pine 60.29 151.52 266.73 936.51 172.79
Western juniper 150.03 254.60 239.21 -- 178.06
Western white pine 66.07 225.94 298.19 -- 138.71
White fir 115.04 212.82 172.72 198.22 183.03
Whitebark pine 115.88 191.78 233.48 329.96 176.82
All species 98.34 176.40 259.31 580.82 177.37
Nevada (van Wagtendonk et al. 1996) yielded a total
woody fuel weight of 0.563 kg m -2 for the same classes.
Using each of the three methods, red fir had the heaviest
combined woody fuel weight, while Washoe pine (Pinus
washoensis) had the lightest woody fuel weight.
When the calculated weights from the planar intercept
method are used to predict the sampled weights, both the
Rocky Mountain fuel particle values and the Sierra Nevada
values produced significant regressions through the origin
(F•gure 6). The equations are:
WEIGHT = 0.935 x CALCULATEDRock,Mountain s
r 2 = 0.790
WEIGHT = 0.858 x CALCULATEDsierraNevada
r 2 = 0.795
where WEIGHTis the predicted weight in kg m -2 based on
the samples, and CALCULATED is the weight in kg m -2 from
the planar intercept method. The r 2 values of the two regres-
Table 6. Average bulk density of fuel beds of 19 Sierra Nevada conifers. The fuel bed bulk density includes woody,
litter, and I hr timelag duff fuels.
Litter Duff Litter and duff Fuel bed
Species .............................................................................. (kg m-3) ..........................................
Douglas-fir 101.333 152.533 143.464 20.629
Foothill pine 32.876 130.605 85.890 12.407
Foxtail pine 89.909 210.665 187.976 33.395
Giant sequoia 140.511 162.000 160.416 31.823
Incense-cedar 144.775 180.928 185.041 20.744
Jeffrey pine 32.633 168.482 139.352 21.107
Knobcone pine 38.253 219.711 113.822 20.325
Limber pine 99.570 223.803 189.771 40.209
Lodgepole pine 94.371 162.586 151.174 32.948
Mountain hemlock 114.857 183.977 177.352 32.250
Ponderosa pine 36.010 154.863 118.003 23.479
Red fir 120.049 182.423 172.139 35.581
Singl eleaf pinyon 145.981 234.164 205.797 30.846
Sugar pine 40.763 160.960 117.153 28.243
Washoe pine 56.404 172.785 145.048 16.571
Western juniper 70.532 178.064 147.378 20.934
Western white pine 49.357 138.705 122.948 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
WJAF 13(3) 1998 79
Table 7. Regression statistics for litter, duff, and litter and duff weight (kg m-2) as e function of their respective depths (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 Duff Litter and duff
Species Coefficient r 2 Coefficient r 2 Coefficient r 2
Douglas-fir 0.864 0.796 1.319 0.901 1.295 0.910
Foothill pine 0. I 11 0.355 1.448 . 0.746 1.220 0.804
Foxtail pine 0.886 0.650 2.504 0.914 2.360 0.907
Giant sequoia 0.990 0.548 1.648 0.914 1.632 0.920
Incense-cedar 1.276 0.709 1.675 0.865 1.664 0.866
Jeffrey pine 0.358 0.823 1.707 0.864 1.496 0.874
Knobcone pine 0.339 0.636 1.646 0.896 1.274 0.902
Limber pine 0.889 0.789 2.337 0.946 2.255 0.955
Lodgepole pine 0.951 0.905 1.671 0.904 1.612 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
Singleleaf pinyon 0.906 0.845 2.592 0.883 2.478 0.900
Sugar pine 0.304 0.623 1.396 0.880 I. 189 0.897
Washoe pine 0.600 0.570 1.870 0.863 1.719 0.862
Western juniper 0.832 0.780 1.798 0.932 1.763 0.924
Western white pine 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
Whitebark pine 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
sions show that there is a slight improvement when using
Sierra Nevada values over values from the Rocky Mountains
(Figure 6).
Discussion
Fuel Bed Depth
Most woody fuel depths were slightly shallower than the
6.10 cm specified in standard fuel models for short-needled
conifers and long-needled conifers as described by Albini
(1976). Giant sequoia, Douglas-fir, and red fir exceeded this
value; while incense-cedar and sugar pine were within 0.30
cm of the standard value. These species, along with foothill
pine, ponderosa pine, and white fir, grow at low- to mid-
elevations and typically drop more branches and twigs than
the higher elevation species. An exception is knobcone pine,
which grows on the lower ridgetops in a stand-replacing fire
60
30
20
5 10 15 2O 25 30 35
Duff Depth (cm)
Figure 5. Regression lines for duff weight as a function of duff
depth for all 19 Sierra Nevada conifer species combined.
regime where woody fuels seldom get a chance to accumu-
late. Differences between Sierra Nevada fuel bed depth
values and those in the standardized fuel models could result
in significant overpredictions of fire spread and intensity.
Litter fuel depth appeared to be a function of needle
morphology. Conifers with medium to long needles had the
deepest litter layers, while those with single short needles or
scales had the shallowest litter layers. Long needles tend to
form more porous fuel beds than short and flat needles as
evidenced by the mean litter and duff weights (Table 3). The
thin litter layers for the short-needled high-elevation whitebark
pine, lodgepole pine, and western white pine could indicate
that low growth potential has an effect on needle production
and litter depth.
Studies in other regions show similar relationships w•th
growing conditions. In Arizona, Eakle and Wagle (1979)
reported a litter depth for ponderosa pine of 1.46 cm, whfie
Brown (1970) found litter depths for ponderosa pine •n
wetter, more mesic Montana to average just over 2 cm.
Lodgepole pine duff depths in Wyoming were 2.34 cm
(Brown 1974b), while in the Cascade Mountains of Wash•ng-
ton they averaged 5.02 cm (Woodard and Martin 1980)
Differences in duff depth also appear to be related to
growing conditions. Ponderosa pine, white fir, and g•ant
sequoia, the three species with the deepest duff layers, all
grow in productive low- to mid-elevations; in the absence of
fire, they tend to accumulate large quantities of orgamc
material. Deep duff layers were also recorded for limber pine,
a species that grows in cold conditions at high elevations
These trees grow to considerable age, and fire is a rare event,
giving duff ample time to accumulate. Although this is also
the case for whitebark pine and foxtail pine, these species d•d
not show a similar response. Some species, such as western
juniper and foxtail pine, grow on rocky expanses of exposed
granite where winds and other factors preclude the build up
of fuels.
80 WJAF 13(3) 1998
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 classes from this
study and calculated from the planar intersect method using values from the Rocky Mountains (Brown 1974a) and
from the Sierra Nevada (van Wagtendonk et al. 1996)
Species
Woody fuel weight
This study Rocky Mountains Sierra Nevada
......................................................... (kg m-2) .......................................................
Douglas-fir 0.997 1.145 0.955
Foothill pine 0.224 0.292 0.212
Foxtail pine 0.325 0.300 0.259
Giant sequoia 1.292 1.186 1.202
Incense-cedar 0.666 0.847 0.771
Jeffrey pine 0.294 0.347 0.306
Knobcone pine 0.235 0.240 0.171
Limber pine 0.821 0.698 0.786
Lodgepole pine 0.401 0.579 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
Singleleaf pinyon 0.329 0.381 0.375
Sugar pine 0.710 0.704 0.669
Washoe pine 0.153 0.233 0.179
Western juniper 0.227 0.239 0.259
Western white pine 0.339 0.354 0.209
White fir 0.855 1.037 0.809
Whitebark pine 0.286 0.334 0.274
All Species 0.579 0.630 0.563
Previously, most studies that reported depths for species
that occur in the Sierra Nevada combined the litter and duff
layers. In general, these values were less than the ones found
m this study. For instance, combined depths for ponderosa
pine in the Southwest were one-third of those from this study
(Ffolliott et al. 1968, 1979, Eakle and Wagle 1979). Kittredge
(1955) measured litter and duff depth in the Sierra Nevada
and found values for white fir, red fir, sugar pine, and
80
• 60
• 40
o
Line of Exacl Agreement • '"'"/
,,,,'"' /?
Sierra Nevada •
Y=0.939 X ,..,"
R2=0.795 .,.' ß
.'" / • Rocky Mounlain!
,'" / Y=0,858 X
.,,'•/ R2=0,790
0
20 40 60 80
Calculated Woody Fuel Weight (kg m '2)
Figure 6. Regression lines 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 Sierra Nevada (van Wagtendonk
et al. 1996).
ponderosa pine to be two-thirds to one-half those found here.
It is possible that 40 yr of additional accumulation without
fire has allowed fuels throughout the Sierra Nevada to in-
crease. Kittredge (1955) also conducted the only previous
study of knobcone pine litter and duff depth. In the southern
California coast ranges, he found values that were similar to
those reported here. Since knobcone pine is a short-lived fire-
dependent species, there might not be enough time for fuels
to accumulate to high levels. If fire is not allowed in knobcone
pine stands, considerable dead and down woody fuels accu-
mulate as the stands die and fall apart.
Fuel Bed Weight
Litter and duff weight differences also appeared to be
related to needle morphology and growing conditions. Coni-
fers with single short needles growing at mid- to high-
elevations, such as white fir and red fir, tended to have
heavier litter weights. Mid- to low-elevation conifers with
fascicles of multiple medium to long needles, such as sugar
pine, knobcone pine, and ponderosa pine, had heavier litter
loads. Growing conditions and tree size could be affecting
litter weights of species with scalelike leaves; the lowest litter
weight was recorded by the high-elevation western juniper,
while the mid-elevation giant sequoia had a moderately high
litter weight. Other species, however, defy simple explana-
tions. For instance, whitebark pine and limber pine grow in
similar conditions and have similar needle morphologies; yet
their litter weights are at the two extremes.
The litter weight values determined in this study were
similar to those of previous studies of Sierra Nevada conifer
species. In the Sierra Nevada, van Wagtendonk (1974) re-
ported litter weights of 0.442 kg m -2 for ponderosa pine and
0.292 kg m -2 for incense-cedar. Agee et al. (1978) found
slightly lower values for ponderosa pine, sugar pine, and
WJAI • 13(3) 1998 81
g•ant sequoia •n the southern S•erra Nevada Their htter
weight for white fir was twice that reported here, but they
attributed that to a heavy crop of deciduous cone scales,
which were not included as litter in this study. Ponderosa pine
litter in the drier Southwest averaged 0.224 kg m -2 (Sackett
1979), while in the northern Rocky Mountains its average
was 0.324 kg m -2 (Brown 1970).
In the absence of fire, duff accumulation is the result of the
difference between duff production and decomposition.
Models of forest floor buildup in the Sierra Nevada have
shown maximum total accumulation to be reached after about
100 yr without fire (Agee 1973, van Wagtendonk 1985). In
climates more conducive to decomposition, the accumula-
tion peaks sooner at a lower total amount. Conversely, in
climates that are favorable for growth but not for decompo-
sition, accumulation rates are fast and total accumulation
high.
The variation in total duff weight among species was so
great that no single factor can explain the differences. It does
appear, however, that due to productivity levels, species in
hot, dry and in cold, dry environments have lighter duff
layers. For instance, western juniper, western white pine, and
foxtail pine had the lowest values for duff weight, followed
by foothill pine and knobcone pine. Mid-elevation conifers,
such as red fir and Douglas-fir, in areas little affected by fire
suppression activities generally had moderate amounts of
duff. On the other hand, species with the heaviest duff layers
were either mid-elevation species, such as ponderosa pirie
and giant sequoia, where the effects of fire suppression have
been most pronounced, or high-elevation long-lived species,
such as limber pine, whitebark pine, and mountain hemlock,
where fires seldom occur and productivity outpaces decom-
position.
The duff amounts in this study were greater than those
found in previous studies in the Sierra Nevada by van
Wagtendonk (1974) for incense-cedar and ponderosa pine
and by Kittredge (1955) for ponderosa pine, knobcone pine,
giant sequoia, white fir, and red fir. As with duff depth, the
additional 20 and 40 yr of fuel accumulation without fire
could explain these differences. Drier growing conditions in
the Southwest could explain those lower duff weights for
ponderosa pine than those found in the Sierra Nevada (Ffolliott
et al. 1968, 1979, Eakle and Wagle 1979, Sackett 1979).
Woody fuel weights in the smallest three size classes were
heavier in this study than those specified in standardized
models (Anderson 1982, Albini 1976) generally used for
Sierra Nevada conifers. The short-needled fuel model (Model
8) specifies a woody weight of 1.121 kg m -2 compared to the
0.855 kg m -2 found here for white fir. The long-needled fuel
model (Model 9) uses a value of 0.796 kg m-2; ponderosa
pine in this study had a weight of 0.488 kg m -2. If, however,
the weight of the litter and uppermost duff layer are added to
the woody weights, the values are 1.748 kg m -2 and 1.307 kg
m -2, respectively. These latter values are probably more
realistic since litter and the uppermost duff layer usually are
burned by the passing fire front.
There was considerable variation in woody fuel weight
among species; red fir had 10 times more wo6dy fuels in the
82 WJAF 13(3) 1998
three smallest s•ze classes than did Washoe pine Branching
habit and tree size appear to •nfluence the amount of woody
fuels. Species, such as red fir, white fir, and Douglas-fir, have
numerous small branches that contribute to the fuel load.
Giant sequoia, by its sheer size, age, and slow decay rate, can
accumulate large amounts of woody fuels over time. Other
species, such as Washoe pine, foothill pine, and western
juniper, have particularly open crowns with small amounts of
branchwood. Knobcone pine retains much of its branchwood
in the crown before being burned by a stand-replacing fire
Previous studies of woody fuel weights of Sierra Nevada
species have been based primarily on Brown' s (1974a) planar
intercept method. Parsons and DeBenedetti (1979) reported
weights for woody fuels less than 7.62 cm in diam for
ponderosa pine and giant sequoia that were slightly lower
than those found in this study; while weights for white fir
were nearly twice as high. Higher values were also found for
ponderosa pine, white fir, and incense-cedar by van
Wagtendonk and Sydoriak (1987). In studies of ponderosa
pine in the Southwest, Sackett (1979, 1980) used plots for the
0.00-0.64 cm size class and the planar intercept method for
the larger classes of woody fuels and found weights compa-
rable to those of this study.
Fuel Bed Bulk Density
Bulk density is calculated from weight and depth of
woody, litter, and duff components. Species with shallow but
heavy beds, such as singleleaf pinyon, had high bulk dens•-
ties; species with deep but light beds, such as foothill p•ne,
had low bulk densities. A combination of needle morphology
and growing conditions contributes to these differences. Th•s
variation in bulk density will have a profound effect on fire
behavior. Up to a point, more porous fuel beds will burn w•th
greater intensity than denser beds. The bulk densities re-
ported here can be used to construct custom fuel models
(Burgan and Rothermel 1984).
In the Sierra Nevada, Stephens (1995) found bulk dens•-
ties for ponderosa pine and white fir to increase with depth
Ponderosa pine varied from 22.40 kg m -3 for the litter layer
to 264.00 kg m -3 for the deepest duff layer. For white fir, he
recorded a minimum of 52.00 kg m -3 and a maximum of
338.20 kg m -3 for the same layers. These values are simdar
to those found in this study.
Forest floor bulk densities from this study also compared
well with studies in other regions. The only Sierra Nevada
species to have bulk densities previously reported are ponde-
rosa pine, Douglas-fir, and lodgepole pine. In the Southwest,
ponderosa pine bulk densities varied from 61.13 kg m -3 for
the combined litter and duff layers (Ffolliott et al. 1976) to
211.07 kg m -3 with the addition of woody fuels (Harrington
1986). The most comparable values are for the combined
litter and duff layers, however, and values there ranged up to
151.01 kg m -3 (Eakle and Wagle 1979). These compare
favorably with the 118.00 kg m -3 found in this study for the
combined ponderosa pine litter and duff.
In the northern Rocky Mountains, Brown (1970) recorded
a bulk density of 76.89 kg m -3 for ponderosa pine combined
litter and duff. In ponderosa pine stands with grass and shrub
understories, however, Brown (1981) found median bulk
densittes for litter layers rangtng from 21 90 kg m -3 to 29.00
kg m -3 In the same regton, Douglas-fir stands with s•mdar
understones had htter bulk densities from 25.30 kg m -3 to
58 10 kg m -3 (Brown 1981).
Two studies of lodgepole pine in Washington and Wyo-
ming found bulk densities slightly less than those reported
here (Woodard and Martin 1980, Brown 1970). Calculated
forest floor bulk densities from the weight and depth figures
tn Kittredge (1955) for ponderosa pine, white fir, and red fir
were similar to the ones found here. The calculated bulk
density for knobcone pine growing in southern California
was half that of the Sierra Nevada stands.
Estimated Litter and Duff Weight
The equations presented here will allow reliable predic-
ttons of litter and duff weight to be made based on depth.
By using this easily measurable fuel characteristic, the
time consuming collection and weighing of large amounts
of duff fuels are avoided. These equations should replace
the ones by Agee (1973) that were for mixed stands of
ponderosa pine and incense-cedar and for white fir and
giant sequoia. They are also more appropriate to use for
ponderosa pine in the Sierra Nevada than the equations
from the Southwest (Eakle and Wagle 1979, Ffolliott et al.
1968, 1976, Harrington 1986).
Although direct comparisons with other regression meth-
ods, such as linear and logarithmic, cannot be made, the high
r 2 values indicate the validity of using regressions through
the origin. In addition, the appropriateness of this approach
was indicated by the large number of observations that were
near zero for both weight and depth (Figure 5).
The regression equations can be used by managers and
researchers interested in any single layer or combination
of layers. For instance, the method used by the National
Park Service to determine the effectiveness of prescribed
fire treatments uses separate litter and duff measurements
(Reeberg 1995). Brown et al. (1982) include the litter layer
wtth the woody fuels but do not provide equations to
convert litter depth tO litter weight. The Albini (1976) fuel
models include litter and the uppermost duff layer in the 1
hr ttmelag fuel class. User-defined custom fuel models can
use any combination of litter and duff fuels to reflect local
conditions (Burgan and Rothermel (1984). Although it is
possible that interactions among species could alter pro-
duction and decomposition rates, the equations could be
used to approximate estimates of duff weight in mixed
species stands. For instance, if some measure of abun-
dance such as basal area or canopy cover is available, the
equations can be applied in proportion to that measure.
Estimated Woody Fuel Weight
The popularity of the planar intercept method of invento-
rytng woody fuels warrants the investigation into its applica-
bthty in regions other than the northern Rocky Mountains. In
a study of Sierra Nevada conifers, van Wagtendonk et al.
(1996) showed that regional variation in the physical proper-
ttes of woody fuel particles could result in fuel weight
estimates that differ from as much as 40.8% less to 8.3% more
than those calculated from Rocky Mountain values. With the
addttton of wetght and depth data from thts study, these
esttmates can be improved even more Ftnal adjustments can
then be made to the calculated esttmates so they reflect the
differences between calculated and sampled fuel weight.
Conclusion
This study has shown that regional differences in fuel bed
properties are real and should be taken into consideration
when planning and conducting hazardous fuel treatments.
Since fuel bed depth plays such an important role in fire
behavior, these differences could result in overpredictions of
fire intensity and rate of spread if the standardized fuel
models are used. The refinements suggested here will pro-
vide the most accurate estimates of fuel bed properties
possible consistent with time and funding constraints. The
importance of having accurate estimates of fuel weight and
depth in predicting fire behavior and measuring fire effects
ß cannot be overstated. As land managers strive to meet the
conflicting resource objectives of maintaining natural pro-
cesses and reducing hazardous fuels, they will need the best
available information.
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