Effects of Fire Intensity on
Competitive Dynamics Between
Red and Black Oaks and
Mountain
Laurel
W. Keith Moser, Mark J. Ducey, and P. Mark S. Ashton,
Yale Schoolof Forestry and EnvironmentalStudies,360
ProspectStreet,New Haven, CT 06511.
ABSTRACT. We investigated
thecompetitive
response
of oakandmountainlaurel toprescribed
fire ontwo
sitesin northeastern
Connecticut.
Afternearlya decade,thecompetitive
positionof oakonmoderatelyburned
portionsof both siteswas little better,and in somerespectsworse,than on adjacentunburnedcontrols.
However,onportionsof bothsiteswherefire hadkilledmuchof theoverstory,oakregrowthwasvigorousand
had escapedthedensemountainlaurel layer. The resultssuggestthat light understory
fires aloneare not
sufficient
for oakregeneration,
andthatprescribed
fire shouldbe usedonlyaspart of an integratedstrategy
involvingharvestingof theoverstory.Ecologically,theresultspoint to theimportanceof severedisturbances
in maintainingthestructureandfunctionof oak ecosystems.
North.J. Appl.For. 13(3):119-123.
Although
oaks
form
asignificant
portion
ofthe
overstorywith shrubcompetitionand the accumulationof
acrossmuchof theirrangein North America, successful
oak
regenerationhas remainedan elusive silviculturalgoal in
manyecosystems
(Hannah1987,Crow 1988,Abrams1992).
Fire,whichisnota majorfeatureof modemforestlandscapes
in the northeastern
United States,appearsto havebeenan
importantfactorpromotingoakdominancein bothpre- and
postsettlement
forests(Abrams1992). Despitesomedisagreement,a broadrangeof ecologicalandhistoricalstudies
suggests
early uplandforestsin the Northeastexperienced
understory
fireswith a returnperiodbetween1 yr and15 yr
(Day 1953, Buell et al. 1954, Little 1974, Russell 1983),
althoughfire regimesmay shift with both culturalchanges
andchangesin the physicalclimate(Thompsonand Smith
1970,Overpecket al. 1990, 1991).Numerousauthorshave
suggestedthat an absenceof fire limits oak regeneration
(Lorimer1984,Hostetal. 1987,AbramsandNowacki1992),
whileotherauthors
havepointedtoabroadrangeof problems
NOTE:
Theauthors
thankYale UniversityandtheYaleForestfor support
during this research.W. Keith Moser's currentaddressis Tall
Timbers Research,Inc., Rte. 1, Box 678, Tallahassee,FL 32312.
This researchwas sponsored
by the U.S. Departmentof Energy,
Office of EnergyResearch,EnvironmentalSciencesDivision,Office of HealthandEnvironmental
Research,
underappointment
to
theGraduateFellowships
for GlobalChangeadministered
by Oak
Ridge Institutefor ScienceandEducation.Dr. David M. Smithand
two anonymous
reviewersprovidedvaluablecomments.
shadetolerants
in theunderstory
(Hannah1987,Crow1988,Lorimer
et al. 1994). However,experienceswith prescribedfire in
northeasternoak forestshave been mixed (Niering et al.
1970, Nyland et al. 1982, Wendel and Smith 1986, Boemer
et al. 1988).
An understory
shrubof particularmanagement
concernin
southern
New Englandis mountainlaurel.Mountainlaurel's
highshade-tolerance
andaggressive
vegetativegrowthhabit
make it a stem competitorfor oaks on a variety of sites
(Chapman1950). In this study,we examinedthe resultsof
two controlledbumson the competitivedynamicsbetween
red and black oaks and mountainlaurel. The hypothesis
drivingthebumswasthatforcingthelaurelto resproutfrom
thegroundsurfacewould"leveltheplayingfield," allowing
oakseedlings
to gainaheightadvantage
andeventuallyreach
sufficientsizeto allow successful
standregeneration.
Study Area and Methods
Two siteswerestudied,bothontheYale-MyersForestin
northeastern
Connecticut.Both siteswere approximately8
ac in size. Before the initial treatment, each site had been a
single stand,with a denseunderstoryof large mountain
laurel, includingnumerousstems> 2 in. and over 6 ft in
height.Prescribed
bumswerecarriedoutduringthespringon
NJAF13(3)1996 119
half of each site the Red Front site was burned in 1985, and
the Buell Brook site in 1984. On both sites,the fire burned
more intenselythan plannedon half of the burnedarea,
resultingin neareliminationof the overstory.
The
Red Front
site is located
on well-drained
and
adjusted (SAS, SAS Inst, 1990) The secondanalysis
involved calculating the mean annual height and basal
diameterincrementfor thepairedoakandmountainlaurel,
to assess
the relativeratesof growthof the two species
Differences within treatments were assessed using
somewhatexcessivelywell-drained Charlton-Hollis fine
sandy loams (USDA 1981). At the time of the initial
treatmentin 1985, the standwas approximately70 yr old.
Wilcoxon's signed rank test, and treatments were com-
Theprefirecanopy
hada basalareaof 140ft2/ac,witha
Inst. 1990).
densityof 225 stems/ac> 2 in., excludingmountainlaurel.
Speciescompositionwas dominatedby black oak, white
pine, white oak, and black birch. On the lightly burned
portion of the stand,40% of the basalarea and 30% of the
density were eliminated, chiefly due to girdling of the
smallerstems.On the severelyburnedarea,basalarea,and
densitywere reducedby 60% and 70%, respectively.
The Buell Brook site is located on moderately well
drained Woodbridge extremely stony fine sandy loam
(USDA 1981). The stand was 80 yr old at the time of
treatmentin 1984. The prefire canopyhad a basal area of
165ft2/acand425stems/ac
> 2 in, andwasdominated
by
northernred oak, white oak, black birch, and red maple.
Little overstorymortality occurredon the lightly burned
area, although scarringof sometrees was evident. However, on the severelyburnedportion, basalarea and density were reducedby over 95%.
We measured oak and mountain laurel regrowth on
both sitesduring the 1993 growing season,as part of a
broadstudyon fire andunderstorydynamics(Ducey et al.
1996). The three areas of each site (unburned control,
moderateburn with little overstorydamage,and severe
burn with heavy overstorydamage)were delineated,and
fourpointswerelocatedsystematically
in eacharea.Around
each point, four milacre plots aligned on the cardinal
directionswere usedto estimatethe heightdistributionof
oak and mountain laurel. We measuredthe height of all
individualsto the nearest0.5 in., defining an individual as
a group of stems either connectedabovegroundor by
belowgroundparts discerniblethroughvigorous shaking.
We then divided a 1/50th ac circular plot centeredon the
plot point into quarters.On each 1/200th ac quarter, we
measuredthe total height and basaldiameter of the tallest
red or black oak seedlingand its tallest direct mountain
laurel competitor,and removedbasalsectionswhich were
agedin the laboratoryundera microscope.Sectionswere
not taken for first-year seedlings.If no mountainlaurel
wasjudged sufficientlycloseto shadethe seedlingor be
shadedby it, or if no oakswere presenton the 1/200th ac
quarter,thenno mountainlaurel samplewastaken.
Data from the plot quarterswere analyzedin two ways.
First, each quarterwas classedin a table by two factors:
presenceor absenceof oak, and dominanceby oak or
mountainlaurel, basedon height. If oak was absent,then
the quarterwas classedas laurel-dominated;the classof
oak absent but dominant
formed
a structural
zero in the
table. The table was analyzedwith site and treatmentas
dependentvariables using maximum-likelihoodlogistic
analysisfor stratifiedsamples;contrastswereBonferonni120
NJAF13(3)1996
pared using the Kruskal-Wallis test. Both tests were
Bonferonni-adjusted
for multiplecomparisons(SAS, SAS
Results
Densityandstockingof oakonthemilacreplotsareshown
in Table 1. The height distributionof oaks and mountain
laurel basedon the milacre plots is shownin Figure 1
Although the number of oaks observedwas too small to
permitrigorousstatisticalanalysis,somequalitativefeatures
are commonto both sites.First is the wide variability in size
of mountainlaurel, with large numbersof tall stemson the
controlplots.While theoverallheightof themountainlaurel
wasreducedon the moderatelyburnedplots,so wasthatof
the oaks.However,on the plotswith heavyoverstorydamage, both oak and mountainlaurel experiencedmore rapid
regrowth,and the tallest oaks are bigger than the tallest
mountainlaurel. This effect is greateston the Buell Brook
site, which is more mesic and had the highestoverstory
mortality.Only severelyburnedplotshadoaksover4.5 ft tall
Presenceand dominanceof oak on the quarterplots is
shown in Table 2. Stand effects and stand-treatment
interac-
tionswere not statisticallysignificant(likelihoodratio chlsquare8.78, 6 df, P > 0.05). However, overall treatment
effectswere highly significant(likelihoodratio chi-square
81.04, 6 df, P < 0.0001). Contrastsindicatedthat the control
and moderateburns were not significantlydifferent (chlsquare5.29, 2 df, P > 0.05). However, both control and
moderatebumswere significantlydifferentfrom the severe
bum (chi-square20.21 and 11.01, 2 df, P < 0.0001 andP <
0.05, respectively).
Averagesfor mean annual height incrementand mean
annualdiameterincrementare shownin Figure2 andFigure
3. Oak ageson the controlplot quartersrangedfrom 1 to 18
yr old; mountainlaurel agesrangedfrom 8 to 57. On the
severelyburnedquarters,both oak and laurel showedrapid
sproutre-establishment,
with all laurel and all but one oak
datingwithin 3 yr followingdisturbance.
No oakspredated
Table 1. Density and stocking of red and bleck oaks by site and
treatment.
Site andtreatment
Red Front
Control
Moderate bum
Severebum
Buell Brook
Control
Moderatebum
Severebum
Density,#/ac
Stocking,
all oak
% milacres
oak >4.5'
440
310
31
25
0
0
1,500
63
6
810
31
0
75
100
0
69
2,600
10,100
Red Front
lO
Buell Brook
16
9
14
12-
8-
I
i
Oak
Laurel
108
6-
4
2
0-
14
12
10
8
6
4
2
0
14
Control
Moderate Burn
Severe Burn
12-
Figure 2. Mean annual height increment of oak and mountain
laurel. Differences between oak and mountain laurel are significant
only for the severe burn (Wilcoxon's signed rank test: severe
10
8
6
burn, P< 0.001; control and moderate burn, P< 0.05). Differences
between treatments are significant for both oak and mountain
laurel (Kruskal-Wallis test: oak, P< 0.001; laurel, P< 0.01).
Cumulative
Propotion
Figure 1.
mountain
Height distribution of red and black oak ("...) and
laurel (
) by site and treatment.
the fire. Age distributionswere similar on the moderately
burnedquarters,except that 30% of oaks were 2 yr old or
younger.Most largeoaksontheburnedplotswereresprouted
advancegrowth.
This is true despitethe fact that mountainlaurel itself grew
beston the openplots(Figure2 and3); thereis no evidence
eitherthat mountainlaurel was stronglyset back by severe
fire, or thatit is hamperedin any way by the highlight levels
now on these sites.In this context, laurel can be seenas shade-
tolerantrather than shade-demanding.
Table 2 indicatestheineffectiveness
of burningaloneasa
strategyfor controllingmountainlaurel and enhancingoak
0.12
I
Discussionand Applications
Thecombination
of stocking,species
heightdistributions,
o 0.1
and the agesof the oak and mountainlaurel indicatethat the
initial hypothesisbehind the burns was incorrect. While
burningdid "level the playingfield," on the plotswherethe
•-
burnwentasplanned,mountainlaurelstill won.It is only on
theplotswheresignificantamountsof sunlightwerereleased
to the forestfloor that oak was able to competeefi'ectively.
E 0.06
TabIs 2. Presence and dominance of oak on 1/200th ac quarter
-• 0.04
Oak
Laurel
0.08
plots, by site and treatment.
Oak present,
Site and treatment
Oak present, Oak absent,
laurel
laurel
•- 0.02
oak dominant
dominant
dominant
Control
Moderate Burn
Severe Burn
Buell Brook
6
13
94
38
44
0
56
44
6
and
Control
Moderate Burn
Severe Burn
6
31
100
44
6
laurel are significant for both burned treatments (Wilcoxoh's
signed rank test: control, P< 0.05; moderate burn, P< 0,05; severe
burn, P < 0.001 ). Differences between treatments are significant
0
Red Front
50
63
0
0
r
Control
•
Moderate Burn
Severe Burn
Figure 3. Mean annual diameter increment at root collar of oak
mountain
laurel.
Differences
for both oak and mountain
0.001; laurel, P< 0.05).
between
oak
laurel (Kruskal-Wallis
and
mountain
test: oak, P <
NJAF 13(3)1996 121
reproduction.While stockingand dominanceof oak on the
quarterplotsdid showa small,nonsignificant
improvement
fromthecontrolto themoderately
burnedplots,boththesize
of theoaksandtheproportion
of thegroundsurfaceonwhich
oakisfreeto growareinadequate
for successful
regeneration
of the stand.No oaksobservedon the moderatelyburned
plotsexceededSander's(1972) 4.5 ft sizecriterion,indicatinglikely regeneration
failureif theoverstorywereharvested
now. Furthermore,the large fraction of plot quarterson
whichthelargestoaksareonlyfirst or secondyearseedlings
indicatesoak may be having difficulty persistingon the
moderatelyburnedsites;many of theseoakswere deador
absent
onsubsequent
sitevisits.Chapman
(1950)andKittredge
andAshton(1992) observed
thatoakregeneration
persisted
well in the gapsbetweenclumpsof largemountainlaurel.
However,onthemoderately
burnedsites,thelarge,clumped
structure
typicalof thecontrols
wasreplacedbyanextremely
dense,monolayerstructurewith few gaps.Thus, burning
may havecreatedan environmentmorehostileto oak establishmentthanthat previouslyon the site.Theseresultsare
consistent
withthoseof Nylandet al. (1982),whoconsidered
that singlebumsare not likely to enhanceoak recruitment
significantly.Giventhelackof apparentharmto themountainlaurel,we hypothesize
thatevenrepeatedbumsmaynot
besufficientto controlunderstory
densities
andimprovethe
competitivepositionof oak.
By contrast,ontheopenplots,thesizeandstockingof oak
indicatethatit will probablydominatethenextstand,despite
the fact that stockingwasinadequatebeforeoverstorymortality. Thisresultis surprisinggiventheimportanceof large
advancedregenerationto oak successunder most circumstances(Sander 1972, Crow 1988, Smith and Ashton 1993).
While much of the growthincreasecan be explainedby
increased
lightavailability,it ispossiblethatincreased
nutrientavailabilityfollowingfire isacontributing
factor(Boemer
et al. 1988,Gilliam 1988).Certainly,in a highlight environment, oak shouldbe ableto allocatea greaterfractionof its
biomassto roots, and thus better take advantageof any
nutrientpulsewhich occurs(Tilman 1988).
From a managementstandpoint,prescribedfire is not a
sufficientconditionfor obtaininggoodadvancedregeneration of oak.While fire cantemporarilyreducethe quantity
andcoverof competingspecies,if the conditionsfavoring
development
of a dense,shade-tolerant
understory
remainin
placeafterthefire,thenoakwill findnoadvantage.
However,
fire may play an importantsupportingrole in the contextof
harvestsdesignedto reduceoverstorydensityandallow oak
establishment.
Here,thesurprisingsuccess
of oakfollowing
overstorymortalityin our studysuggests
that fire may enhanceoak vigor aboveandbeyondthat expectedin normal
harvesting.However,it may be thatlessrisky andexpensive
measures,
suchasdeliberatecrushingof mountainlaurelwith
skidders,may provide a sufficient advantageto oak. A
controlledcomparison
of fire withmechanicaltreatments
for
laurel controlwould help fill this knowledgegap.
Lorimeret al. (1994) assertthatthemechanism
by which
fire enhances
oaksurvivalandgrowthis unclear,butpointto
the importanceof competitionin the understory.Basedon
122
NJAF13(3)1996
theseresults,we suggestthattherole of fire asa disturbance
is to releasetotal growingspace(Oliver andLarson1990)
Wherethe overstoryis still sufficientlydenseto inhibitoak
growthrelativeto thatof itscompetitors,
fire will notconfer
anadvantage.
If theeffectof fireisprimarilyindirect(Lorimer
et al. 1994), i.e., competitionreductionand increasedresourceavailabilityratherthan somephysiologicalbonus,
thenothertypesof disturbance
may suffice.This view may
helpexplainthemixedrecordof fire asa management
toolin
oakforests(Nieringet al. 1970,Nyland et al. 1982,Wendel
andSmith 1986,Boemeret al. 1988).It alsoemphasizes
the
importantrole of severedisturbances,
both naturaland human-induced,in determiningthe structureand diversityof
oak ecosystems
(Abrams 1992, Reice 1994).
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