Fuelbed Changes with Aging of Slash
From Ponderosa Pine Thinnings
Donald W. Carlton
and Stewart G. Pickford
ABSTRACT Fuel loading stratified by standard fuel size
transectat each samplepoint. Dead and downwoody
chtssesand .]itelbeddepth &lta fi•om ii preeonlmercially
thinnedstandsof ponderosa
pine (Pinusponderosa
Doagl. er
fuels were inventoried on the transect according to
Laws) indicatedthat changesin slashfitelbedsvaryingin age
J?om0 to 17 years can be describedmathematicallyas a
.[itnctionqf slashage a.d basal area o.ftreesrenwed. Least-
squaresequation of theform
I n(fitelbedcharacteristic)=
a + b'ln(basal area remtn'ed)+ c'(age)
gate R2 yahres
J?om0.74 to 0.96, and till equations
ssvre
signt.'ficant
at the 99-percent level. Prescribedburning producedt'ha•tgesin loadingand beddepththat werethe equiva-
lent of 2 to 20 or .lore yearsof naturalaging. Theseresults
shoaldbe of ase to hind managers,bat sincethey tire from
one drainage of the eastern side of the CascadeRange in
14•lshington
they may need adjastingbeforebeingtippliedto
other areas.
Throughout
much
ofthe
species'
wide
range
inthe
westernUnited States, ponderosapine developsdense,
stagnatedstandsof sapling- and pole-sizedreproduction. Timber and forageproductivityon bettersitescan
oftenbe increasedby precommercial
thinning.Thinning
is expensive,however,and it createsslashthat increases
fire hazardgreatly (Fahnestockand Dieterich 1962). A
manager'sdecisionto spendmoneyto reducethe hazard
dependsin part on the rate at which the hazard will
dissipateby naturaldecayof the slash.
Various studies have indicated that fire hazard dissi-
patesgradually(Long 1915, Fahnestockand Dieterich
1962, Fahnestock1968, Kill 1968, Wagenetand Oftbrd
1972, Albini and Brown 1978). A commonlyaccepted
rate for natural
abatement
of fire
hazard
in eastern
Oregonand Washington
is five yearsto returnto an MM
hazard level--moderate rate of spread,moderateresistance to control (USDA Forest Service 1968).
We studiedprecommercialthinningunits to estimate
the fuelbed characteristicsof thinning slashas a function of basal area removedand years since cutting.
Results indicated that changes in important fuelbed
characteristics
can be describedby mathematicalequations. These equations,or similar ones tbr specific
areas, should be useful in evaluatingfire hazard in
thinned areas, evaluating effectivenessof prescribed
burning, and perhapsother applicationsrequiringnumerical fuelbeddescriptions.
Methods
Our studydesignrequiredantteatedslashof various
ages. We selectedan area in the Entiatdrainageof the
eastsideCascadeRangeof Washington
becauseit containedpole-sizedstandsthat had beenprecommercially
thinnedat varioustimes over a 17-yearperiod.Sample
pointswere establishedin units thinnedin each of I1
years and in an unthinnedarea. The sampledstands
occupied various topographicsituationsat elevations
between 2,100 and 3,21)0 ti:et.
Twentysamplepointswere randomlylocatedwithin
each unit. Fuels data were collected along a 33-tbot
Brown's11974)technique,with depthsof fuelbedmeasuredas the high-particleinterceptat 2-footintervals
alongthefueltransect.
The intercept
wasdefinedasthe
distance from the surfaceof the forest floor to the tallest
deadwoodyfuel-particle
within6 inches
of thevertical
line throughthe measurement
point.
Pineneedlelitter and humusdepthweremeasured
at
twopointsalongeachfueltransect.
A 0.01-acrecircular
plot wasestablished
at eachsamplepoint.Eachstump
fromthinningwithinthisplotwasidentifiedby species,
and its diameter at stump top was recordedto the
nearest
inch.Percent
of•round
surface
covered
byslash
wasestimatedfrom l-ft- quadratsat the 20-tbotpoints
on fuel transects.
The percentof needlesremainingon
slashwasocularlyestimatedat eachplot center.
Data Analysis
Fuel loadings,litter and duff depths,and fuelbed
depthswere computed
for eachtransect.Total fuel
loadingvariedfrom 3.2 tonsper acrein an unthinned
stand(age= 0 years)to nearly20 tonsperacrein units
containing
the youngest
slash.Litter and duff depths
showed little consistentvariation with slash age, but
average
fuelbeddepthsdecreased
from 18.6 inchesin
the youngest
slashto 5.5 inchesin the oldestunits.
Fuelbeddepthsin unthinned
areasaveraged
only 1.9
inches (table 1).
Prethinningstockingvaried from 1,250 to 3,250
stemsperacre,butpercent
of originalstocking
removed
wasbetween80 and95 percenton all but the unit with
Table 1. Fuel loadings, litter and duff, and depth of
fuelbed.
Age of
Average fuel loading
slash
Yrs.
by diameterclass•
Depth
1-hr. 10-hr. 100-hr. 1,000-hr. Total Litter
Duff
- ......
Inches ......
Tonsper acre .............
17
15
13
12
11
10
0.46
.29
.35
.30
.24
.14
2.3
2.4
2.4
2.2
3.6
2.5
3.2
4.1
5.4
2.4
6.4
4.1
9.31
6.66
8.31
4.15
3.38
7.53
15.3
13.4
16.5
9.0
13.6
14.3
0.9
1.5
.7
1.1
.5
1.2
1.0
1.1
.7
1.2
.6
.5
7
6
.57
.54
2.7
2.4
4.5
7.0
.5
.4
.52
.74
.73
.05
4.2
2.7
4.1
.5
4.5
5.1
4.3
.4
14.4
13.8
19.8
17.7
16.7
3.2
.3
.3
3
2
1
0
6.60
3.90
10.53
9.12
7.58
2.30
1.0
.6
.7
.9
1.3
.6
1.0
1.0
Fuelbed
5.5
7.5
8.6
5.3
8.8
11.1
11.0
14.0
11.9
16.3
18.6
1.9
•Diameter classes are: 0-'/• inch = I hour; '/•-1 inch = 10 hour;
1-3 inches = 100 hour; 3-8 inches = 1,000 hour.
THE AUTHORS--Donald
W. Carlton
is a lbrcster with the Mount
Hood National Forest. Portland.Oregon. StewartG. PickIbrd is
associateprofessorof lbrestryat the Universityof Washington.
Seattle.
The work reported here was funded by. and performed in
ct•pcration with. the USDA ForestServicc'sPacific Northwest
ForestandRan[ze
Experiment
Station.
February 1982/JOURNAL
OF FORESTRY/91
17-year-old slash. Basal area removed (calculated at
stump top height) varied from 64 to 121 squarefeet;
averagestump diameter of the cut stemswas between
2.2 and 3.0 inchesfor all but one unit (table 2).
We hypothesized
thatchangesin physicalcharacteris-
Table 2. Original stocking per acre, percent of stocking
removed, basal area removed, and average diameter of
cut stems.
Ageof Prethinning Stocking Basalarea Averagediameter
tics of fuelbedsovertime couldbe approximated
by
some form of exponentialdecay.We thereforemade
slash
stocking
Years
17
15
13
12
11
10
Stems
1,445
2,550
3,250
2,100
2,745
1,250
Percent
75
86
85
85
89
90
7
1,460
91
6
3
2
1
0
2,595
2,295
1,385
1,785
1,375
95
92
81
87
.........
removed
cut stems
Square
least-squares
fits of the data in tables I and 2 to several
typesof exponentialequation.
s. The form of the equation that gavethe bestfit was:
In(property) =
a + b'ln(basalarearemoved)+ c-(ageof slash).
Table3 givesthe valuesof the constants
a, b, and c for
each physicalcharacteristicthat we estimatedin this
way.All of theseequations
gavehighlysignificant
(0.01) overall F-ratios and reasonable R• values. Resid-
ualsappeared
to be uniformlydistributed
overtheranges
of x-values.Figures I, 2, and 3 illustratetheserelationshipsgraphically.
We averagedthe estimatedpercentof needleretention
for each unit sampled. These data were then fitted by
leastsquaresto the equation:
(percentof needlesretained)=
1.028 exp( - 0.074-slashage).
We estimatedthe weightof needlesoriginallypresentin
the slashat the time of thinningfrom the numberof
treesremovedper squarefoot of cut basalarea by stem
diameterclass,multipliedby the estimatedneedleweight
for ponderosapine crowns from Brown et al. (1977).
removed
•et
80
108
94
70
99
69
•ches
3.28
2.63
2.17
2.35
2.36
2.83
64
2.49
121
109
71
96
2.42
2.68
2.96
2.80
56
1 Hour-- 052(cutb.a.)
e
054 (age)
t•
•-
lo
o)
.8
:•
5
.35
o
._1
• 1.c
2
.2
ii
This gavea meanvalueof 49.3 lb/fi2 of basalarea
removed. We used this value and the removed basal area
to computethe original slashneedle loadingfor each
plot, and then applied the estimatedpercentof needles
retainedto computecurrentneedleloading. Thesedata
werecombinedto yield the equation:
(needleloading,tons/acre)=
0.02535-(cut basal area) + 0.09482
•
/
/
5
[
10
•
15
co• -
20
Slash Age (yrs.)
Figure/. On•-ho.r tim•/a••/ /oa•in• v•r•.• s/asha•
th• ba•a/areac.t. Th• limit(• th• •/• •ata•)r slasha• i• /
to/7 •ar• an• fi)r ba•a/areac.t is h4 to/•2
exp(0.074-(slash
age)2).
52
SmallFuel•
The curvepredictssmallamountsof needlesremaining in the fuelbed for severalyearsafter most needles
woulddropfrom the slash.This residualneedleloading
was observedat the samplepointsand originatedfrom
needledrapefrom overstorycanopies.
e
.024(age)
148
Asexpected,
theaverage
high-log
height
• andaverage
high-particle
heightwerehighlycorrelated
(R2 = 0.95).
The averagehigh-logheightwas mosthighly correlated
withthepercent
of ground
surface
covered
byslash(R2
= 0.95). The equationsare:
(averagehigh-logheight) =
0.29'(averagehigh-particleheight) - 0.08
"
/
/
and
(percentslashgroundcover) =
6.9'(averagehigh-logheight) + 6.2
The equationswith which we describedour fuelbeds
give quantitativemeasuresof fuelbedchangesovertime;
we usedother reporteddatato helpjudgewhetheror not
the equationsare reasonable.The equationspredictthat
woody fuel loadings will decreaseby less than 25
percentduringthe first five yearsalter thinning.Significant decreases
in needleloading,however,will occurin
the secondthroughthe fourth year after thinning.The
needleretentionthat we predictfor years2 through4 is
tHigh-log
height•1s d•:fined
as thedistance
fromtheforest
floor to the tallestseveredbole within6 inchesof the vertical
line throughthe measurement
point.
92/JouRNALOFFoRESIR¾/February
1982
I/
/
5
/
10
/
15
20
Slash Age (yrs.)
Figure 2. Sm•/I-fi•el /ouding versassl•sh •ge and the
•re• cut. The /imit •f the fie/d d•t• •)r s/•sh •ge is I to 17
yearsandJbr bas•l •a cut is 64 to 132 squ•refeet.
within 50 •rcent of thatreposedby Albini and Brown
(1978). Our equationshowsneedleloadingswhich stabilize at about 5 percentof original valuesafter about
sevenyears•the resultof needlefallfrom overstorytrees
beingaddedto the fuelbed.We obse•ed bothneedlefall
and •ry old (7-13 years) retainedneedlesduring our
fieldwork, in approximatelytheseamounts.
Avg.High_ 1961cut
b.aI
49
Particle
-- eO66(age)
Table 3. Coefficients of best-fit equations describing
changes in fuelbed properties of slash of various ages.
Property
a
b
c
r2
Overall
F-ratio •
29
'•
•
21•1115
3C
2c
15
57
Fuelloading
•
1-hour
10-hour
100-hour
Total small
fuels
Total fuels
-2.955
- .682
- .927
-
Fuelbed depth
.543
1.184
.672
0,562
.434
.578
.516
.383
.494
-0.542
- .026
- .016
-
0.74
.91
.95
.96
.90
.92
.024
.020
.066
12.646 •
47.49 •
44,727 •
102.462
41.84 a
50.74 a
•See footnote1, table1 forexplanation
of fuelsizeclasses.
2All equationssignificantat the 99-percent/eve/ with2 and 9
degrees of freedom.
5
10
15
20
Slash Age (yrs.)
Figure'3. Averagehighparticle heighl versusslashage and the
basal area cut. The limit of the field data for slashage is I to
17 )'earsandfi)r basal area eul is 64 to 132 square.feet.
Woodyfuel loadingswill decreaseat nearlythe same
rates for years 5 to 10 as during the initial five years.
The rate of reductionis highestfor small fuel sizes, as
one would expect. Overall, our equationspredict that
after 20 years the total fuel loadingsare reducedby
about one-third, and fine fuel loadingsby about twothirds.
Fuel depthsdecreasedmorerapidly.The high-particle
interceptheightdecreasedby over25 percentin the first
5 years, declinednearly 50 percentafter l0 years,and
would probablyhave decreasedby 75 percentafter 20
years.The initial rate of fuelbeddepthchangeobserved
in this study is only half that reported by Fahnestock
and Dieterich (1962), but the differencemay be due to
the more xeric conditions (with slower funsal decay
rates) and lighter sno• loads in our studyarea than in
theirs. Also, Fahnestock and Dieterich used detached
branches,which shouldcompactat a differentrate than
unloppedtops.
Applications
Our estimatesof ratesof changein fuelbedproperties
shouldbe of interestin evaluatingpotentialfire hazard
in unitsproposedfor thinning. Much of the information
from theseestimatesmay be applicablein fire behavior
models already in existence(Rothermel 1972, Albini
1976). By making fire behaviorestimateson the basis
of fuelbed changes over time and combining these
estimateswith analysisof risk of ignition in thinnedor
adjacentunits, managersshould be in a better position
to evaluate the need for slash treatment.
The informationfrom this study shouldalso be of
interest in evaluatingthe effectivenessof slash treatments. As an example, we measuredfuelbed loadings
and depthsbeforeand after a seriesof prescribedbums
in thinningslashnear our studyarea. We then usedthe
equationsgiven in table 3 to estimatethe amountof
time by which burning acceleratedfuelbed changes.
Burning causeda decreasein needle loadingwhich was
the equivalentof two to three yearsof aging, on the
average.The reductionsin loadingsof total fuel and of
small woody fuel, expressedas an equivalentof aging,
were greaterthan the useful rangeof our equations(20
years). Certainly burning greatly lessensthe time to
reducewoody fuel loadingsin all size classes.
Burning produced reductionsin depth of fuelbed
equivalentto an aging period of 8 years in burns
conductedin the spring, and at least 20 yearsfor fall
burns. Since depth of fuelbed is an importantdeterminant of fire intensity,rate of spread,and resistanceto
control, the accelerationof fuelbed settling may be at
leastas importantas reductionsin fuel loading.
Herbaceousand shrubbyvegetationare also important
fuel componentsin thinnedstandsof ponderosapine.
We estimated these componentsbut did not include
them here.
Previous studies (McConnell
and Smith
1970, Sassamanet al. 1973, Richeson 1977) suggest
that effectsof untreatedslashon herbaceousproduction
are less significantthan the effects of thinning. Thus,
any increasein fire hazard due to understoryvegetation
associatedwith precommercialthinning shouldbe predictablelargelyon the basisof thinningintensityalone.
Finally, a word of caution. The studywas made in a
geographicallylimited area. Sampleunitswereclustered
within thinningsof different ages. Becauseclustering
tendsto reduce residualvariation,thesetwo facts may
make our regressionslook better than they really are.
Consequently.our work should be applied directly to
other areasonly with considerablediscretion.We have
shownthatthe approachis quiteworkable,however,and
will increasethe ability to evaluatefire hazardrealistically when ponderosapine in thinned precommercially. ß
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February 1982/JOURNAL
OF FOgESTg¾/93
frequencyand almost25 timesas productiveas medium
spreadareas; high spreadareas are almost 21/2times
more productivethan medium spread areas. Relative
differencesare even greater with a 10-yeartreatment
duration.
Second, most of the table 4 ratios are very high.
Extreme spreadratios are substantiallylower than the
rest, but extreme fuel types constituteonly a small
proportionof the Northern Region (accountedfor less
than 3 percentof total ignitions). For the balanceof
the region, between50 and 755 acreswould haveto be
protectedfor eachacre saved.Consequently,
a returnof
$50 to $755 per acresavedwouldbe requiredfor every
dollar spent per acre protected.A differenceof a few
dollars in average cost could, therefore, change the
requiredvalue yield by hundredsor even thousandsof
dollars. For example, with a low ratio of 50 to I and a
cost of $10 per acre protected,each acre savedwould
haveto yield benefitswith a presentvalue of $500. A
$20 per acre cost would requirean averageyield of
$1,000, and so forth. Given theserelationships,
it appearsthat the proportionof the NorthernRegionwhich
could be treatedeconomicallywould vary greatly with
even small differences
in treatment cost.
These results represent, in effect, the hypothetical
applicationof fuel managementto a set of conditions
which existed in the past. Differencesin theseconditions, particularlyfire weather,may decreasetheir reliability in evaluatingthe currentfeasibilityof fuel management.This problem, however,would be sharedby
even the most sophisticatedfuel-management
.model,
sincefuture fire weathercannotbe predicted.Results
are also predicatedon a set of assumptions
about fuel
management
effectiveness,
but refinementis impossible
without a great deal more data on fuel treatmentresponsethan currently exist. ß
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