Leaf Area and Above- and Belowground
Growth Responses of Loblolly Pine to
Nutrient
and Water
Additions
TimothyJ. Albaugh, H. Lee Allen, Phillip M. Dougherty,Lance W. Kress, and
John S. King
ABSTRACT. A 2 x 2 nutrient and water factorial experiment with four replications was installed
in an 8-yr-old stand of Ioblolly pine (Pinus taeda L.) growing on an infertile, excessively drained
sandy site in Scotland County, North Carolina. After the fourth year of treatment, estimated stem
volume increment, total biomass production, and peak leaf area index (LAI) increased 152%,
99%, and 101%, respectively, with fertilization and 25%, 23%, 16%, respectively, with irrigation.
Stem volume growth efficiency (growth per unit LAI) increased 21% with fertilization, 9% with
irrigation, and 30% with both fertilization and irrigation. Total biomass production efficiency
increased 91% with fertilization, 29% with irrigation, and 120% with both fertilization and
irrigation. The observed increase in stem volume growth efficiency may have been due, in part,
to changes in biomass partitioning. However, altered partitioning patterns alone did not explain
the observed increase in total biomass production efficiency. We hypothesized that the change
in total biomass production efficiency may have been a result of greater allocation to foliage
(photosynthesizing tissue) and less allocation to fine roots (a high maintenance respiration
tissue) under fertilization and irrigation treatments. FOR.Sc•. 44(2):317-328.
Additional Key Words: Growth efficiency, biomass partitioning.
be requiredto manageforestplantationsin an economi-
NORDER
TO
MEET
INCREASING
DEMAND
for
forest
productson a declining land base,silvicultural treatments
like soil tillage,competitioncontrol,andfertilization
are commonly used to increasethe availability of site
resourcesand, in turn, productivityin forest plantations.
The conceptthat sitequality is a fixed or inherentproperty
of a site has been replaced with an understandingthat
productivityis largely dependenton resourceavailability
andthat siteresources,especiallynutrientsandwater,can
be manipulated.Clearly, a basic understandingof how
nutrient and water availability interact with prevailing
climatic conditionsto determineproductionpotentialwill
cally and environmentallysustainablemanner.
Biomassproduction
potentialof foreststandsiscorrelated
with the capacityof standsto interceptlight (Linder 1987,
Cannell 1989). Light interceptionis a functionof foliage
production,life span,and distribution.Of thesefactors,
foliage productionis believedto be the mostimportantin
determiningpotentialproductivity(Wang andJarvis1990).
A strongcorrelationbetweenstemwoodproductionandleaf
areahasbeendemonstrated
for loblollypine(PinustaedaL.)
(Teskey et al. 1987, Vose and Allen 1988). Maximum
abovegroundproductivityof temperateforestsshouldbe
T.J Albaugh(correspondingauthor), Box8008, North Carolina State University,Raleigh, NC 27695.8008--Phone: 919-515.3500; Fax: 919-5156193; E-mail:[email protected];
H.L. Allen, Box8008, Departmentof Forestry,NorthCarolinaState University,Raleigh,NC 27695-8008--E-mail: [email protected];P.M. Dougherty,Forest Science Laboratory,Westvaco Corp, POB 1950, Summerville, SC 29484--E-mail:
pmdoughCa•vestvaco.com;
L.W. Kress, USDAForestService,3041 CornwallisRoad, RTP,NC 27709; and J.S. King,Schoolof Forestryand Wood
Products,MTU, Houghton,MI 49931--E-mail: [email protected].
Acknowledgments:
This work contributesto the Global Change and Terrestrial Ecosystem(GCTE)core projectof the InternationalGeosphereB•osphereProgram(IGBP).We gratefullyacknowledgethe supportprovidedby the USDAForestServiceSouthem ForestExperimentStation, the
SouthemGlobalChangeProgram,the Departmentof Forestry,NorthCarolinaState University,and membersof the NorthCarolinaState Forest
NutritionCooperative.We wishto thank staff membersand graduatestudentsfor their assistancein the destructiveharvests.This paper has not
been subject to USDA Forest Service policyreview and should not be construedto representthe policies of that agency.The use of trade names
•n this paper does not implyendorsementby the associatedagenciesof the productsnamed, nor criticismof similarones not mentioned.
Manuscriptreceived July11, 1996. AcceptedSeptember 29, 1997.
Copyright¸ 1998 by the Societyof AmericanForesters
ForestSctence
44(2)1998 317
achievedat a leaf areaindex(LAI)(projected)
of approxi-
mately5 m2 m-2 based
onJarvisandLeverenz'
(1983)
theoretical
modelandtypicalstandlight extinctioncoefficients.Unfortunately,
manystandsproducelevelsof leaf
areawell belowthis optimum.In the southeastern
United
States,lownutrientandwateravailability,hightemperature,
andelevatedozonelevelshavebeenimplicatedas factors
responsible
fortheobserved
suboptimal
levelsof leafareain
pineplantations
(Teskeyet al. 1987,VoseandAllen 1988,
Colbertet al. 1990,Hennessey
et al. 1992,Stowet al. 1992).
Growthefficiency,the slopeof theproduction-leafarea
relationship,
depends
onthephotosynthetic
efficiencyof the
foliage,maintenance
respirationcost,and biomassallocation.Recentevidencesuggests
thatimprovednutrientavailabilitycanincrease
photosynthesis
(Zhang1993,Murthyet
al. 1996) and maintenance
respirationrates(Maier et al.
1998)of 1oblollypine.In addition,improvednutrientand
wateravailabilityhavebeenreportedtoreducebelowground
biomassallocationin stand-levelstudies(KeyesandGrier
1981,AxelssonandAxelsson1986,Vogt et al. 1986,Gower
et al. 1992,HaynesandGower 1995).
Previousstudies
haveexaminedtheimpactof standdevelopment(Kinersonetal. 1977),environmental
factors(Teskey
et al. 1987, Hennesseyet al. 1992, Stow et al. 1992), and
nutrients(Colbertet al. 1990, Vose andAllen 1988) on leaf
areaand growthefficiencyof 1oblollypine. However,no
studieshavequantifiedtheeffectsof bothnutrientandwater
availabilityonleafarea,production,
andgrowthefficiencyin
intermediate-aged
1oblollypinestands.Our objectiveswere
to determinetheeffectsof nutrientandwateravailabilityon
leaf area,above-andbelowground
productivity,andgrowth
efficiencyof 1oblollypine.
Methods
StudySite
The studywas establishedin the Sandhillsof Scotland
County,North Carolina(35øNlat., 79øWlong.) on a flat,
infertile, excessivelydrained, sandy, siliceous,thermic
PsammenticHapludultsoil (Wakulla series).Annual precipitationaverages1210mm(30 yr average),butextended
droughtsarepossibleduringthe growingseason.Average
annualtemperatureis 17øC(30 yr average).The site was
handplantedon a 2 x 3 m spacingwith 1oblollypine in
1985 afterfelling of thepreviousnaturallongleafpine (P.
palustris
Mill.) standandapplication
of Velpar
TMgrid
balls(17kgha-l).
Sixteen50 x 50 m (0.25 ha)treatmentplotswith30 x 30
m measurementplots centeredin the treatmentplot were
establishedin January1992 in the 8-yr-old stand.Through
plot selectionandthinninginitial meantree height,diameter, stand basal area, volume, LAI, and density (1260
stems
ha-l ) weresimilar(nostatistically
significant
differencesdetected)in all plotsprior to treatmentimposition.A 10m bufferseparated
thetreatmentplotsandin the
two cases where the buffer was less than 10 m, a 150 cm
deeptrenchbetweenthe treatmentplots was dug, lined
with plastic,and refilled. Completecontrolof nonpine
vegetationin the treatmentplots has been maintained
318
ForestSctence
44(2)1998
since 1992 through a combination of mechanical and
chemical(glyphosate)methods.Treatmentswere a 2 x 2
factorial
combination
of nutrition
and water additions
replicatedfour times.
The nutritiontreatments,which beganin March 1992
were (1) optimumnutritionthroughfertilizationor (2) no
addition.Optimumnutritionwasdefinedas(1) maintaining
a foliar nitrogen(N) concentration
of 1.3%, (2) maintaining
foliar macronutrient concentration:N
concentration ratios of
0.10 for phosphorus
(P), 0.35 for potassium
(K), 0.12 for
calcium(Ca), and0.06 for magnesium(Mg), and(3) maintainingfoliar boron(B) levelsgreaterthan 12 ppm.Foliar
nutrientstatuswasmonitoredmonthlyin eachplotusing35
fascicles(7 fasciclesfrom eachof 5 trees)collectedfrom a
branchterminalin theupperthirdof thecrownofcodominant
or dominanttrees.Fertilizerwasappliedasneededto meet
the statedtarget.
Water treatments,whichbeganin April 1993 were (1)
naturalprecipitation
and(2) naturalprecipitation
plusimgation appliedto meet the target soil water contentlevel
Irrigationeventsweretargetedto maintainsoilwatercontent
at greaterthan3.0 cm soilwatercontentin thesurface50 cm
of soil (40% available water content) as determinedfrom
volumetric soil water content measured with time domain
reflectometry
(TDR)(ToppandDavis 1985).The soilmoisturetargetwasbasedonevidencesuggesting
that40% of the
availablesoilwatercontentmaybea biologicallymeaningful
threshold
for photosynthesis
andgrowth(MyersandTalsma
1992). Water content was determined with a 50 cm TDR
probeattenlocationswithineachtreatmentplotonbiweekly
intervalsduringthe growingseason(March to November).
Percent available
soil water content in the surface 50 cm
(AWC) was calculatedas:
AWC = (MV- WP)/(FC- WP) x 100
(1)
where MV was the measuredvolumetric soil water content,
FC wasfield capacity(5.5 cm), WP waswiltingpoint(1 5
cm). For example,a measured
soil watercontentof 3 cm in
the surface50 cm of soil wouldresultin 38% A WC ((3 - 1.5)/
(5.5 - 1.5)x 100).The irrigationsystemconsistedof Rainbird
irrigationnozzlespositionedon 35 cm risersspaced10 m x
10 m aparton a headto headdesign.DuringeachirrigaUon
event2.5 cm of waterwasaddedto the plot. The system
operatedas neededto maintainthe targetsoil waterlevel
duringthe growingseason.
Estimationof LAl and StemVolume
StandLAI (projected)
wasmeasured
monthlyfromMarch
1992 to December1995 in eachof the 16 treatmentplots
usingthe Li-Cor LAI2000 plant canopyanalyzer(Li-Cor
1991).For eachmonthin eachplot, 20 LAI measurements
alonga transectonthe southsideof theplotweremadeat a
heightof 60 cm between0700 and 1000EST usinga 180ø
viewcap.Simultaneously,
abovecanopylightmeasurements
were collectedin an openfield adjacentto the studysite
wherethe light sensorhadan unobstructed
view of the sky.
The 20 measurements
were averagedto estimatemonthly
plot LAI, and the unadjusted
maximummonthlyLAI re-
cordedfor eachploteachyearwasusedasourestimateof
peakLAI.
In Decemberof eachyearfrom 1991(priorto treatment)
to 1995,measurements
of diameterat breast(1.4 m) height
(D), height(H), andlive crownlength(L) weremadeonall
treesin eachplot (-100 treesperplot).Stemwoodvolume
Wholetreestemwoodbiomass
regression
equations
(Ap-
pendix)
weredeveloped
using
D2xHandageasthepredictor
variables.
A totalof 48 trees(Table1)(24 control,8 irrigated,
8 fertilized,and8 fertilizedandirrigated)wereusedin the
wholetreestembiomass
regressions.
Wholetreefoliageand
branchwoodbiomass
regression
equations
(Appendix)were
was calculated as:
developed
using
D2x H, L, andageaspredictor
variables.
Wholetreefoliageandbranchwoodbiomassestimates
from
448trees(Table1)(124control,108irrigated,108fertilized,
and 108 fertilizedandirrigated)wereusedto developthese
where V was total outside bark volume in cubic meters
regression
equations.
Forty-eightof thetreeswerefromthe
(Sheltonet al. 1984). Stemwood volumewas calculatedfor
destructive
harvests
wherefoliageandbranchwoodbiomass
eachtreein a plot, summedto the plot level andscaledto
were directly measured.The additional 400 trees were
determine
volumeperhectare.
Stemwoodvolumegrowthfor
nondestructively
measuredand branchlevel foliage and
eachyear wasdeterminedasthedifferencebetweencurrent
branchwoodbiomass
regression
equations
wereappliedto
andpreviousyear stemwoodvolume.
eachbranchon eachtree andthen summedby tree for the
wholetreefoliageandbranchwoodbiomassestimates.
The
Estimationof BiomassProductionand Allocation
branch
level
regression
equations
were
developed
with
data
Standlevelbiomass
(dryweight)estimates
for 1992to 1995
fromthebranches
onthedestructively
harvested
trees.Branch
for foliage,branchwood,stemwood,andcoarseroot(>2 mm
diameter
and
distance
from
the
top
of
the
tree
were
usedas
diameter)
components
werecalculated
fromageandtreatment
predictor
variables
(Gillespie
et
al.
1994).
specific
wholetreeregression
equations
applied
toalltreesand
Regressionequationspredictingwhole tree coarseroot
thenscaledto an areabasisfor eachplot. The wholetree
andtaprootbiomass
weredeveloped
usingD asthepredictor
regression
equations
(Appendix)weregenerated
by destrucvariable(H, L, andagewerenot significantpredictorvaritivelyharvesting
treesfrom eachtreatment.
Standleveldry
ables).Coarserootestimates
presented
in ouranalyses
were
weightfine root (<2 mm diameter)biomassestimateswere
thesumof thecoarserootandtaprootestimates.
A totalof 23
calculated
byscaling
coredatato anareabasisforeachplot.
Destructive
sampling
of treecomponents
wascompleted trees(Table 1) (6 control,6 irrigated,5 fertilized,and 6
in February1992(priorto treatment
andonlyaboveground fertilizedandirrigated)wereusedto developthewholetree
components),
February1994andFebruary1996on 16trees
coarseandtaprootregression
equations.
Finerootbiomass
•n eachyear(Table1). Treeswereselected
to represent
the
was estimatedfrom coresamplescollectedeachmonthin
rangein H andD bytreatment
at thetimeof sampling.
H, D,
eachplotandscaledto anareabasis.In 1993and1994,all live
andL were measuredfor eachtree, and branchdiameterand
anddeadloblollypinerootswereremovedanddriedfrom20,
branchdistance
fromthetopof thetreeweremeasured
for
6.6 cm diameterx 10 cm deepcorescollectedin eachplot
eachlive branchon eachtree.All treeswerecutat ground
(Mignano1995).Samples
werescaledtoa 50 cmdepthusing
level,separated
intostemwood,branchwood,andfoliageby
Mignano's(1995) rootdistributionassessments.
In 1995,the
branch,anddriedat65øCtoaconstant
weight.In 1994(seven
sameprocedures
werefollowed,however,onlyfive, 15.2cm
trees)andin 1996(16trees),all coarse
rootsfoundin a square
diameterx 15 cm deepcoresweresampledin eachplot.
metercenteredon thetreestumpdownto 50 cm in the soil,
Productionestimatesfor stem wood, branch wood, and
andtheentiretaproot wereremovedanddriedat 65øCto a
coarseroot biomassfor a givenyearwere calculatedasthe
constant
weight.Treessampledin 1992wereselected
from
differencebetweenthebiomass
estimatefor thefollowing
treesweremovedpriorto treatmentimpositionto equilibrate
year and the given year for thesecomponents.
For exspacingandstanddensityamongplots.In 1994and 1996,
ample,the 1993 productionestimatefor stemwoodbiomass was the difference between the 1994 and 1993 biomass
treeswereselected
fromwithinthetreatment
plots,butthey
represented
lessthan1%of treesin theplots.Consequently, estimatefor stemwood. The foliage found on the trees
removalof harvested
treeshadminimalimpactonotherstand
whenourbiomassdeterminations
werecompletedin Febmeasurements
(e.g.,LAD.
ruary of 1992, 1994, and 1996 was largely produced
V = 0.00748+ (0.0000353
x D2x H)
Table1. Numberof treesin eachtreatmentusedto developwholetree biomassregressions
by yearandsampletype (destructiveor
nondestructive).
Numbersindicatethe numberof trees. Lettersindicatethe treatment where C is control,I is irrigated,Fis fertilized,
and R is fertilized and irrigated.
Year
Sample
type
1992
Destructive
16C
Foliage
16C
Branch
wood
16C
Stemwood
Coarse
root
1992
1993
1994
1994
1995
Nondestructive
Nondestructive
Destructive
Nondestructive
Nondestructive
20C, 20I, 20F, 20FI
20C, 20I, 20F, 20FI
4C, 4I, 4F, 4FI
20C, 20I, 20F, 20FI
20C, 20I, 20F, 20FI
20C, 20I, 20F, 20FI
20C, 20I, 20F, 20FI
4C, 4I, 4F, 4FI
20C, 20I, 20F, 20FI
20C, 20I, 20F, 20FI
4C, 4I, 4F, 4FI
2C, 2I, 1F,2FI
1996
1996
Destructive
4C, 4I, 4F, 4FI
Nondestructive 20C,20I, 20F,20FI
4C, 4I, 4F, 4FI
20C,20I, 20F,20FI
4C, 4I, 4F, 4FI
4C, 4I, 4F, 4FI
ForestSctence
44(2)1998 319
duringthe previousgrowingseasons
of 1991, 1993, and
1995, respectively.Consequently,
productionof foliage
for a givenyearwastheestimatedeterminedin February
of thefollowingyear.For example,1993foliageproduction was the estimate of the amount of 1993 foliage
determined
inFebruary,
1994.FairleyandAlexander'
s(1985)
decision
matrixwasusedto estimate
finerootproduction
of
eachplot from the monthlybiomassand necromass
esti-
tallymeasured
variablesthatquantifiednutrientand/orwater
availabilitywereusedto developa morebroadlyapplicable
mates. This method did not account for carbon losses to root
wherethedependent
variableandLAI werethesameasin the
basemodel,[50,[51,[52,[53,and•4 wereparameters
to be
estimated,
andN andW wereexperimentally
measured
esti-
linear model of the form:
Dependentvariable = •0 + ([•ILAD+ ([•2X LAIx N)
+ ([•3x LAI x W)
+ ([•4x LAIx Nx W)
exudationor mycorrhizalassociation.
StatisticalAnalyses
The statisticalsignificance
of thefertilization,irrigation,
andfertilizationby irrigationinteractioneffectsweredetermined usinganalysisof variance(SAS 1988). Treatment
(4)
matesof nutrientand wateravailability,respectively.
We selected
JanuaryfoliarN concentration
(%) (NCONC)
and a combinednutrientindex (sum of foliar N, P, and K
content
(mgfascicle-1))
(NCONT)to quantify
thenument
effectson annualincrementfor D, H, basalarea,volume,and
peak LAI were examinedfor 1992, 1993, 1994, and 1995.
Treatmenteffectson biomassproductionby component
(foliage,branchwood,stemwood,coarseroot,fineroot,and
total) for 1993, 1994, and 1995 were alsoevaluated.There
were no belowground
biomassproductionestimatesavailable for 1992.
Linearregression
wasusedto quantifythe relationship
betweenproductivity
andlightinterception.
Fortheseanalyses,datafor eachtreatmentwere pooledacrossall years
available(1992 to 1995for stemvolumeand1993to 1995for
totalbiomassproduction).
Our baserelationshipwasin the
form of a simplemodel:
treatment.
To quantifythewatertreatment,weexaminedthe
meangrowingseasonsoilwatercontent(cmof soilwaterin
thesurface50 cmof soil)(SMMEAN)andminimumgrowing
seasonsoil watercontent(cm of soil waterin the surface50
cm of soil) (SMMIN). The variablesNCONC andSMMEAN
wereselectedbecause
theywereusedto regulateournutrient
andwatertreatmentapplications,
respectively.
Thevariables
NCONT
and SMMIN
were used to determine if the more
balancednutrientavailabilityin our fertilizedplotsandif
extremesoilwaterconditions,
respectively,
affectedthebase
relationship.
Finally, initial standbasalareawastestedasa
covariatein themodel.All significance
levelswere{x< 0 05
RESULTS
Dependent
variable= [50+ ([•iLAI)
(2)
Nutrient and WaterAvailabilityAssessments
Averagedoverthe 4 yr of treatment,nonfertilizedplots
hadfoliar nutrientconcentrations
for N, P, K, Ca, andMg
where the dependentvariable was stem wood volume
increment
(m3ha-1 yr-1)ortotalbiomass
production
(Mg
ha-1 yr-1) andLAI waspeakleafareaindex(m2m-2).In
of 0.95%, 0.10%, 0.42%, 0.14%, and 0.07% and fertilized
orderto understand
howtreatmentaffectedtheslopeof the
volume-LAI or production-LAIrelationships,
we examinedan expanded
model:
plotshadconcentrations
of 1.29%,0.11%, 0.56%, 0.11%,
and 0.06%, respectively.From 1992 through1995, we
Dependent
variable= [•0+ ([•1LA/)+ ([•2X LA! X F)
+ ([•3X LA/X/) + ([•4X LAIx FX/)(3)
plots (Table 2). Nutrient levels have been maintainedas
closeas possibleto statedtargetsin light of difficulty in
achievingMg uptake.Foliar Mg levelsin the fertilized
plotsneverreachedthe targeteven thoughexchangeable
soil Mg levelsin the fertilizedplotswerehighcompared
added
475kgha-1N, 134kgha-1P,281kgha-1 K, 159kg
ha-1 Ca,146kgha-1 Mg,and3 kgha-1 B tothefertilized
wherethedependent
variableandLAI werethesameasin the
basemodel,[•0, [•1, [•2, [•3, and •4 wereparameters
to be
tountreated
plots(42kgha-1 in fertilized
plotsversus
17
kgha-1 innonfertilized
plotsin 1994).In 1993,additional
estimated,F (0 if nonfertilized, 1 if fertilized) and I (0 if
nonirrigated,
1 if irrigated)indicated
treatment.
Thissecond
modelprovidedfor directtestsof significanttreatmenteffectson the slopeparameter(growthor production
efficiency).Finally,if a treatment
termhada statistically
significanteffectontheslopeof thebasemodel,thenexperimen-
N requiredto maintaintheN targetwasnotaddedbecause
theMg:N ratioin thefertilizedplotshaddroppedto0.039
However,no visiblemacro-or microscopic
symptomsof
foliar Mg deficiencyweredetectedin the fertilized plots
Table2. Yearlyelementalapplication
rate(kgha-1)andprimarysourceforfertilizersappliedto fertilizedplotsin 1992,
1993, 1994, and 1995. No nutrient additions ware made to the nonfertilized plots during the study.
N
Year
1992
1993
1994
1995
(urea)
225
82
112
56
a Sulphate of potash magnesia.
320
ForestScience
44(2)1998
P
K
(TSP)
(KCI)
56
50
0
28
112
113
0
56
Ca
(gypsum)
135
0
0
24
Mg
(Sophomag')
56
56
0
34
B
(borate)
2
0
0
1
Table3. Averagesoilwater content(cmwater in upper50 cm of soil)and water inputs(mm of water) for irrigated
and nonirrigatedplotsfrom March 1 to November1 for 1992,1993,1994,and 1995.
Yc;•
Nonirrigated
Soilwatercontent
Waterinput
1992
1993
1994
1995
Irrigated
Soilwatercon•nt
Waterinput
2.68
866
2.60
866
2.55
2.85
2.85
735
885
990
2.86
3.03
3.91
1,403
1,052
1,591
Irrigatedplotsreceived91%, 19%, and61% morewater
during 1993, 1994, and 1995, respectively, than
nonirrigatedplots(Table 3). On a growingseasonbasis,
our soilwatertargetwasachievedin 1994 and 1995 (Table
3) In 1993,thetargetlevel couldnotbemaintainedduring
a 6 wk droughtin May andJuneand2 wk droughtsin July
andAugust.Soil watercontentin irrigatedplotsdropped
to aslow as2.4 cm(23% AWC) duringtheseperiodswhile
soil watercontentin nonirrigatedplotsdroppedto 1.7 cm
(5% AWC). Even thoughthe growingseasonsoil water
targetwas achievedfor irrigatedplotsin 1994, the target
wasnot maintainedthroughoutthe growingseason.Duringtwo periods,oneperiodin March to April anda second
periodin October,the soil water contentin the irrigated
plotsdroppedto 2.4 cm (23% AWC) while in nonirrigated
plotssoil watercontentwasaslow as 1.7 cm (5% AWC).
We wereunableto maintainthetargetedsoil watercontent
becauseour irrigation systemdid not supplywater at a
sufficientrate even when operatingat maximumcapacity
in 1993 and 1994. Theseproblemswere eliminatedfor the
1995 seasonwhenthe site experiencedtwo long drought
periods,a 60 day period in April and May and a 35 day
period in July and August.Throughouttheseperiods,the
target soil water contentwas maintainedin irrigatedplots.
In contrast,soil water contentdroppedto 1.6 cm (3%
AWC) in nonirrigatedplots.
ProductivityResponses
FertilizationsignificantlyincreasedD, H, basalarea,
volumeannualincrement,andpeakLAI in eachof the 4 yr
followingtreatment(Tables4 and5). In contrast,irrigation
Table4. Diameterat breastheight(D),haight,basalarea,and volumetreatmentmeansand root meansquareerror
for standingcrop(pre-and post-treatment)andannualincrement(1992,1993,1994,1995).Also,paakleafareaindex
(LAI)standingcroptreatmentmeansandrootmeansquareerrorby year.PeakLAIoccursin Augustor September
each year.
Variable
Control Irrigated Fertilized Fertilized
andirrigated Rootmeansquare
error
Pre-treatment
D (cm)
Height(m)
....................................................(beginningof 1992)..................................................
4.77
4.42
4.45
4.43
0.31
3.43
3.29
3.35
3.29
0.15
BasalArea(m2ha-q)
Volme (msha-q)
2.38
12.80
2.11
12.39
2.17
12.71
2.12
12.34
0.34
1.08
............................................................ (1992)..............................................................
D
1.72
1.77
2.64
2.64
Height
0.80
0.82
0.90
0.93
0.16
0.09
Basal Area
Volume
Peak LAI
1.83
4.63
0.63
1.81
4.38
0.62
2.94
6.98
1.01
2.83
6.76
0.92
0.34
1.02
0.20
............................................................ (1993)..............................................................
D
1.37
1.68
2.24
2.80
0.14
Height
0.57
0.77
0.86
1.25
0.06
Basal Area
Volume
Peak LAI
1.85
5.35
0.96
2.23
6.50
0.97
3.56
10.63
1.48
4.41
14.20
1.70
0.42
1.73
0.34
............................................................ (1994)..............................................................
D
1.37
1.50
2.29
2.33
0.10
Height
0.77
0.82
1.16
1.13
0.04
Basal Area
Volume
Peak LAI
2.19
7.92
1.24
2.44
8.78
1.24
4.56
17.81
2.29
4.67
19.04
2.23
0.36
1.84
0.33
............................................................ (1995)..............................................................
D
1.04
1.31
1.63
1.82
0.07
Height
0.59
0.88
1.33
1.47
0.08
Basal Area
Volume
Peak LAI
1.92
7.87
1.21
2.52
11.14
1.37
Post-treatment
D
Height
Basal Area
Volume
3.94
21.88
2.38
4.47
26.11
2.81
0.39
2.99
0.43
...................................................(end of 1995) ....................................................
10.3
10.7
13.2
14.0
0.59
6.2
6.6
7.6
8.1
0.33
10.2
38.6
11.1
43.2
17.2
70.0
18.5
78.4
1.69
7.95
ForestSctence
44(2)1998 321
Table5. Statisticalsummary(probability> F)oftreatmenteffects
on diameterat breastheight (D), height,basalarea, and volume
for pre-andpost-treatmentstandingcropandannualincrement
in 1992, 1993, 1994, and 1995.The same statisticsfor standing
cropof peak LAIfor 1992,1993,1994,and1995arealsopresented.
Fertilizer*
Variable
Pre-treatment
Irrigation Fertilizer
irrigation
....................(beginningof 1992)..........
D
0.271
0.336
0.322
Height
0.214
0.632
0.644
Basal Area
0.364
0.561
0.540
Volume
0.469
0.820
0.967
..............................(1992).......................
D
0.768
<0.001
0.036
0.784
Height
0.584
0.917
Basal Area
0.688
<0.001
Volume
0.654
0.001
0.980
Peak LAI
0.321
0.017
0.931
0.786
..............................(1993).......................
D
Height
Fertilization
significantly
increased
totalbiomass
productton
79%, 81%, and90% in 1993, 1994,and 1995,respectively
(Tables6 and7). Irrigationincreased
totalbiomass
production
19%, 10%,and23% in 1993, 1994,and 1995,respectively;
however,
onlythedifferences
in 1993and1995weresignificant
(Tables6 and 7). Over the 3 yr, fertilizationincreasedtotal
<0.001
0.099
0.014
biomass
production
19.6Mg ha-] (112%)whileirdgaOon
increased
totalbiomass
production
6.1Mgha-] (25%).
At the end of 1995, D, H, basal area, volume, and total
0.295
Volume
0.022
<0.001
0.188
Peak LAI
0.630
0.006
0.582
................................ (1994) .....................
0.160
<0.001
0.428
Height
0.716
<0.001
0.060
Basal Area
0.358
<0.001
0.706
Volume
0.306
<0.001
0.882
Peak LAI
0.999
<0.001
0.703
............................... (1995) ......................
<0.001
0.262
Height
<0.001
0.001
<0.001
0.096
Basal Area
0.019
<0.001
0.848
Volume
0.034
<0.001
0.761
Peak LAI
0.199
<0.001
0.539
......................(end of 1995)....................
D
0.106
<0.001
0.664
Height
0.034
<0.001
0.929
Basal Area
0.235
<0.001
0.869
Volume
0.146
<0.001
0.679
* Sulfate of potash magnesia.
didnotsignificantly
affectpeakLAI in anyyearandonlyhad
significantpositiveeffectsontheannualincrementfor D, H,
basalarea,andvolumeduring1993and1995,theyearswith
extendeddroughts.
The interactionbetweenfertilizationand
irrigationwas significantonly for heightgrowthin 1993
(Table 5). The height incrementin 1993 attributableto
irrigationwas greaterin the fertilized plots than in the
nonfertilizedplots.
PeakLAI, whichoccurredin Augustor September,increasedduringthefirst3 yr oncontrolplotsandin all years
ontreatedplots(Table4). Duringthe4 yr of treatment,
peak
LAI increased
from0.6to 1.3m2m-2 innonfertilized
plots
andfrom1.0to2.6m2m-2infertilized
plots(Table
4).Peak
LAI increased
54%, 65%, 82%, and100%,respectively
for
1992 through 1995, in fertilized plots compared to
nonfertilizedplots (Table 4). In control plots, peak LAI
322
stem
volume
growth
wasincreased
33.4m3ha-] (118%)with
fertilization,
and6.9m3ha-] (17%)withirrigation.
<0.001
<0.001
Post-treatment
Fertilizationsignificantlyincreased
stemvolumegrowth
52%, 109%,120%,and152%,respectively
for 1992through
1995 (Tables4 and 5). Irrigationincreasedstemvolume
growth30%,8%, and25%in 1993,1994,and1995,respectively, althoughonly theresponses
in 1993 and 1995 were
statistically
significant(Tables4 and5). Overthe4 yr period,
<0.001
0.018
D
tion (Tables4 and 5).
<0.001
Basal Area
D
remainedthesamein 1994and1995,whilepeakLAI in the
fertilizedplotsincreased
eachyearof thestudy.In 1995,peak
LAI wasslightly,but not significantly,increasedby irriga-
Forest
Sctence
44(2)1998
biomasswere significantlygreater(30%, 23%, 68%, 81%,
and 89%, respectively) in fertilized plots than in the
nonfertilizedplots (Tables4-7). Height was significantly
increasedby 7% in the irrigatedplots comparedto the
nonirrigatedplotsby the endof 1995. At thistime,D, basal
area,volumeandtotalbiomassweregreater(5%, 8%, 12%,
and 14%, respectively)in the irrigatedplotscomparedto
nonirrigated
plotsbutthesedifferences
werenotsignificant
No significantfertilizationby irrigationinteractionswere
foundfor any of thesevariablesat the endof 1995.
Allocation of BiomassProduction
Foliage,branch,stem,andcoarserootbiomass
production
were significantlyincreasedin all 3 yr, while fine root
productionwassignificantlyreducedin all 3 yr (Tables6 and
7) with fertilization.Irrigationsignificantlyincreasedfohage, branch,stem,and coarseroot biomassproduction•n
1993and 1995(Tables6 and7), theyearswith extendeddry
periods.Foliage biomassproductionwas significantly•ncreasedin 1994 with irrigation.Fine root productionwas
significantlyreducedwith irrigationin 1993.The interactive
effectsof fertilizationandirrigationwerenotsignificantfor
productionof any biomasscomponent
in anyyear.
Theallocationof thetotalbiomass
production
tofineroots
was lessin fertilizedplots (8%, 10%, and 6% in fertilized
plotsand24%, 24%, and18%in nonfertilizedplotsin 1993,
1994, and 1995, respectively)comparedwith nonfertilized
plotsandgenerallydecreased
overtimein all plots(Table6)
The 3 yr averageportionof total biomassproductionallocatedto belowground
components
was35%, 31%, 25%,and
23% for control,irrigated,fertilized,andfertilizedandirrigatedplots,respectively.
Forall treatments,
theproportion
of
total biomassproductionallocatedto belowground
componentswasreducedeachyear(32%, 31%, and24% in 1993,
1994, and 1995,respectively).
Of the biomassproductionallocatedaboveground,
proportionallymorebiomasswasallocatedto stemwoodproductionandthe stemwoodallocationincreased
eachyear;
Table6. Beginningof 1993andend of 1995biomasspoolsizeestimatesandyearlybiomassproductionestimates(Mg
ha-1yr-1)bytreatmentfrom1993through1995forfoliage,branchwood,stemwood,coarseroot,fineroot(<2mm)
biomasscomponentsandtotal biomass.Proportionalproduction(percantof total biomassin parentheses)data ara
presented.Root mean square errors are also shown.
Biomass
component
Foliage
Control
Branch wood
Stem wood
Coarse root
Fine root
Total
Foliage
Branchwood
Stemwood
Coarseroot
Fineroot
Irrigated
Fertilized
FertilizedandIrrigated
Rootmeansquare
error
...................................................(Pool size beginningof 1993).....................................................
1.8
1.7
3.6
3.4
0.3
2.2
4.7
2.2
0.9
11.8
2.0
4.4
2.1
0.9
11.1
3.1
6.6
3.5
1.3
18.1
3.0
6.4
3.4
0.8
17.1
0.4
0.7
0.4
0.2
1.8
......................................................................(1993 Production)
....................................................
2.0(24)
2.3(26)
4.3(32)
5.1(30)
0.4
0.7(9)
0.9(10)
1.8(13)
2.5(15)
0.3
2.2(27)
2.7(30)
3.7(27)
5.1(30)
0.6
1.0(12)
1.2(13)
2.4(18)
3.0(18)
0.3
2.3(28)
1.8(20)
1.4(10)
1.2(7)
0.3
Total
8.2
8.9
13.6
16.9
1.6
.........................................................................
(1994 production)
...................................................
Foliage
2.4(24)
2.8(24)
5.2(28)
6.2(30)
0.5
Branchwood
1.0(10)
1.1(9)
2.5(13)
2.6(13)
0.2
Stemwood
3.2(32)
3.5(30)
6.1(32)
6.4(31)
0.6
Coarseroot
1.1(11)
1.3(11)
3.1(16)
3.2(16)
0.2
Fineroot
2.4(24)
2.9(25)
2.0(11)
2.0(10)
0.4
Total
10.1
11.6
18.9
20.4
1.5
....................................................................(1995 production)
.......................................................
Foliage
2.7(28)
3.4(28)
5.6(30)
6.9(31)
0.6
Branchwood
1.0(10)
1.5(12)
2.4(13)
3.0(13)
0.4
Stemwood
2.9(30)
4.3(35)
7.0(37)
8.4(37)
0.9
Coarseroot
1.0(10)
1.3(11)
2.7(14)
3.1(14)
0.2
Fineroot
2.0(21)
1.7(14)
1.1(6)
1.2(5)
0.3
Total
Foliage
9.6
12.2
18.8
22.6
2.0
......................................................... (Pool size end of 1995).........................................................
3.0
3.5
5.7
6.9
0.6
Branch wood
Stem wood
Coarse root
Fine root
Total
4.9
13.0
5.3
0.8
27.0
5.5
14.9
5.8
1.2
30.9
9.8
23.4
11.7
0.7
51.3
however,thesechangeswereslight.Averagedoverall treatments,the proportionof aboveground
biomassproduction
representedby stemwoodwas42%, 45%, and46% in 1993,
1994,and 1995,respectively.
11.1
26.4
12.6
0.9
58.0
1.3
2.7
1.0
0.2
5.6
LAI x F x I interaction term was not significant). Total
biomassproduction efficiency increased 120% from 2.7
Mgha-1yr-1 perunitofLAI incontrol
plotsto6.0Mgha-1
35
GrowthEfficiency
Significant,positivelinear relationships
betweenstem
A_ir0gated
Fertilized
ßConlrol
Fe
rt
iIize
d+l
rrl
•t/•
30--,
volume
growth
andtotalbiomass
production,
andpeakLAI
wereobserved
[Equation
(2)].Ninety-three
percent
of the
variation
instemvolume
growth
and87%ofthevariation
in
• Ferllllzed+lrl'lgaled
•Fer
Iize
totalbiomassproduction
wereaccounted
for by peakLAI
(Table8). An additional
2% and8% of variationin stem
volumegrowthandtotalbiomassproduction,respectively,
were
accounted
forbythesignificant
positive
effect
that
.-=
fertilization
(LAI*F)andirrigation
(LAI*I)hadontheslope
of the expanded model (growth or production
efficiency)[Equation
(3)].
Stem volumegrowthefficiencyincreased28% from
7 1m3ha-1 yr-1perunitofLAI incontrol
plotsto9.2m3
ha-1 yr-1 perunitof LAI in fertilized
andirrigated
plots
(Table 8 and Figure 1). Stemvolumegrowthefficiency
wasincreased20% and8%, by fertilization andirrigation,
respectively(Figure 1) andtheir effects were additive (the
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Peak leaf area index
Figure1. Stemvolumagrowth(m3ha-1yr-1)perunitofpaakleaf
araa index for control, irrigated, fertilized, and fertilized and
irrigated plots during four years (1992-1995) of tha study.
Regression lines are shown for aach treatment.
ForestSctence
44(2)1998 323
3O
Table7. Statisticalsummary(probability> F)oftreatmenteffects
on biomasspool size (beginningof 1993 and end of 1995) and
production(1993-1995)for foliage, branchwood, stem wood,
coarse root, and fine root components and total biomass.
Fertilizer*
Biomass
component
Foliage
Branch wood
Stem wood
Coarse root
Fine root
Total
Foliage
Branch wood
Stem wood
Coarse root
Fine root
Total
Foliage
Irrigation Fertilizer
irrigation
.........(Pool sizebeginningof 1993)........
0.480
<0.001
0.887
0.535
0.441
0.511
0.080
0.357
0.001
<0.001
<0.001
0.260
<0.001
0.866
0.856
0.938
0.085
0.886
...................(1993 production).................
0.025
<0.001
0.241
0.012
0.017
0.021
0.020
0.034
<0.001
<0.001
<0.001
0.004
<0.001
0.146
0.213
0.194
0.387
0.154
0.493
0.252
0.317
0.204
<0.001
<0.001
<0.001
0.014
0.862
0.933
0.879
0.249
Total
0.062
<0.001
0.913
Branch wood
Stem wood
Coarse root
Fine root
Total
Foliage
Branch wood
Stem wood
Coarse root
Fine root
Total
<0.001
<0.001
<0.001
0.002
<0.001
<0.001
<0.001
<0.001
0.1 04
<0.001
1,5
2.0
2,5
3.0
3.5
4 0
laaf area index
Figure2. Totalbiomassproduction
(Mg ha-1yr-1}perunitof leaf
irrigated plots during three years (1993-1995} of the study.
Regressionlines are shown for each treatment.
volume growth than the basemodel. The othervariables
tested (NCONC, SMMIN, and initial basal area) were not
significant in the model. The significant measuredvariablesin thetotalbiomassproductionmodel[Equation(4)]
were NCONC and SMMIN. Again, the other variables
tested(NCONT, SMMEAN, andinitial basalarea) were not
significantin the model and the measuredvariable model
explainedonly 5% moreof the variationin total biomass
productionthanthe basemodel.
0.900
0.925
0.822
0.247
0.566
...............(Pool size ending 1995)...........
0.016
<0.001
0.258
0.152
0.113
0.217
0.018
0.092
1.0
area index for control, irrigated, fertilized, and fertilized and
.........................(1995 production)..............
0.010
<0.001
0.300
0.017
0.014
0.015
0.476
0.013
0.5
Peak
......................(1994 production)
..............
0.015
<0.001
0.263
Branch wood
Stem wood
Coarse root
Fine root
Foliage
0,0
Discussion
0.624
0.694
0.688
0.455
0.632
Fertilizationhadstrongpositiveeffectson leaf areaand
stemvolumegrowthefficiencyandbecauseof the multiphcativeeffectsof thesetwo components,
stemvolumegrowth
was dramaticallyincreased(Figure 1 and Table 4). Stem
volume
growth
efficiency
averaged
7.7m3ha-1yr-1perunit
ofLAIandcompared
favorably
withthe7.3m3ha-1yr-• per
yr-1perunitofLAIinfertilized
andirrigated
plots
(Table8
and Figure 2). Total biomassproductionefficiencywas
increased91% and 29%, by fertilizationand irrigation,respectively(Figure2), andtheir effectswere additive.
When replacing the nominal treatmentdesignations
with experimentallymeasuredvariablesin Equation(4),
NCONT and$MMEAN hadstatisticallysignificanteffects
on the slopeof the annualstem woodvolume growthto
LAI relationship (Table 8). However, these measured
variablesexplainedonly 1% moreof thevariationin stem
unit of LAI reportedfor loblollypine by Vose andAllen
(1988). The analysesof treatmenteffectson growth and
production
efficiencywerebasedonslopecomparisons
which
included all data. Becauseof the fertilizer effects on LAI, the
overlapping
rangein LAI acrossall treatments
wasfrom0.6
to 1.6.The trendsin growthandproductionefficienciesin
thisrangewerethesameasthoseobservedfor all data.
Similarto previousreports(Linder1987,Cannell1989),
we found that fertilization
increased leaf area more than it
Table 8. Models examined to understand the relationship be{ween stem wood volume or total biomass production and peak leaf area
index (LAI).Summarystatisticsand parameterestimatesare presented.
Dependentvariable
Stemvolume
Model
Regression
equation
Volme = -2.67+(9.52'LAI)
Basemodel[Equation(2)]
Expandedmodel[Equation(3)] Volume= -0.99+(7.12* LA1)
+(1.50*LAI*F)+(0.58* L,4I * W)
Continuous variable model
Volume=-1.44+(5.56' L,4/)
+(0.76* L,4I *NCONT)
[Equation(4)]
+(0.59* L,4I*SMME,4N)
See text for equation forms, variable definitions, and units.
324
Forest$ctence
44(2)1998
Totalbiomassproduction
n
s
R2 Regression
equation
n
64 6.7 0.93 Production
= 2.33+(7.26*LA1) 48
64 6.7 0.95 Production
= 6.40+(2.74*LA1) 48
+(2.49*L4/*F)+(0.80* LAI * W)
64 6.7 0.94 Production
= 3.63+(0.85*LA1) 48
+(3.01' LAI *NCONC)
+(0.96* LAI *SMMIN)
s
R2
5.1
5.1
0.87
0.95
5.1
0.92
•ncreasedstemvolumegrowthefficiency(61% and 28%,
respectively).
Clearly,thenativelevelof nutrientavailability
strongly
limitedleafareaproduction
andstemvolumegrowth
efficiencyat oursite.It wasnotpossible
to determine
which
of thenutrients
wereprincipallyresponsible
fortheincreased
leaf areaandstemvolumegrowthefficiencybecausemany
nutrientswere added. However,pretreatment
foliar N, K,
Our estimatesof productionand patternsof biomass
allocationcomparedwell with otherstudies.Stem wood
volume
increment
inourstudy
(4 to26m3ha-! yr-1)wasin
the samerangeas otherstudieshavinga similartreatment
design
[Snowdon
andBenson
1992(20to50m3ha-1yr-1),
LinderandBergh1996(5 to 30 m3 ha-1 yr-1)].Our
belowground
production
estimates
(3 to5 Mgha-1 yr-1)
and B concentrations were all below established critical
were lessthan, or in the rangeof thosefound in Vogt's
valuesfor loblolly pine (Allen 1987) and two of these
elements,N andK, havebeenlinkedto leaf areaproduction.
Quantifyingtheimportance
of additionalvariablesin the
expanded[Equation(3)] modeland the more mechanistic
model[Equation(4)] washindered
by thecolinearity
among
predictor
variables.
Thesmallamountof additional
variation
m stemwoodvolumegrowth(<2%) explainedby fertilization was due to the high correlationbetweenLAI and N
concentration
(r = 0.66). This result was not surprising
becauseincreasedloblolly pine foliage productionin responseto fertilizationhasbeenreported(Vose and Allen,
(1991)(5 to 11Mg ha-1 yr-1) survey
of Pinusspecies
in
1988).Themodel
R2values
presented
heremaybeinflated
due to the serial nature of the data.
Duringthe2 dryyears(1993and1995),irrigationsignificantlyincreasedannualstemwoodvolumegrowth(up to
30%),althoughtheseincreases
weremuchlessthangains(up
to 150%) observeddue to fertilizationduring thesesame
years.In contrastto the gain in stemvolumegrowthfrom
fertilization,over60% of thegainin volumegrowthdueto
irrigationresultedfrom increasedgrowthefficiencyrather
than increasedleaf area.The lack of a leaf arearesponseto
•rrigationcontrasts
with resultsfrom the Biologyof Forest
Growth (BFG) study in Australia, where substantialincreases
in leaf areaproduction
wereobservedwithirrigation
aloneduringdry years(Linder et al. 1987, Bensonet al.
1992).On our site,low soil watercontentandthe periodof
leafareaproduction
donot,generally,
occursimultaneously.
However, at the BFG site, low soil water content did occur
dunngtheperiodof foliageproduction.
Other factors may have influenced the weak growth
response to irrigation. Manogaran (1973), using
Thornwaite's(1948) water budgetapproach,concluded
thatirrigationwouldproducelittle additionalloblollypine
growth in our studyarea. Even thoughour site has low
water holdingcapacity,it usuallyhas frequentgrowing
seasonprecipitationand relatively low evaporativedemand(whencomparedto moresouthernandwesternareas
of the loblolly pinerange).In addition,leaf areamay still
below enough,evenonfertilizedplots,thatthedeepsandy
profile may provideenoughwater to meet transpiration
losses.When greaterleaf area is attainedon fertilized
plots,a positiveinteractionbetweenirrigationandfertilization may develop.Suchan effect hasbeenobservedfor
Eucalyptusplantationson sandysites(Tome and Peteira
1991).Finally, theweakgrowthresponse
in irrigatedplots
may haveresultedfrom waterstressin the irrigatedplots
during part of 1993 and 1994, when soil water content
droppedto 23% AWC. Myers andTalsma (1992) founda
rapiddecreasein predawnwaterpotentialoncesoil water
contentdroppedbelow 40% AWC.
temperateforestecosystems.
When comparedto foliage
productionestimatesfromthesamesurvey,ourestimates
(2to7Mgha-! yr-1)weregreater
than,
orintherange
ofthe
survey
data(2 to 6 Mg ha-1 yr-1).Patterns
of biomass
allocationfor plotsin thisstudyweresimilarto thosefound
in LinderandAxelsson's
studyonP. sylvestris
(1982)andin
Cannell's(1985) examinationof dry matterpartitioning.In
all threestudies,aboveground
production
associated
with
improvednutrition(fertilization)appeared
to be at the expenseof belowgroundbiomasscomponents.
At the same
time,shiftsin allocationaboveground
dueto improvednutrition were small relativeto the above-versusbelowground
trade-offs.
All regression
equations
developed
to predictcomponent
biomass were based on dimensional (D, H, and L) and
componentbiomassmeasuresfrom destructivelysampled
treesat thesite(Appendix).For all components
exceptthe
coarserootcomponent,
treatment
(F and/or/)hada statisticallysignificant
influenceontheparameter
estimates
for the
regression
equations.
Consequently,
thereportedsignificant
treatmentdifferencesin coarseroot biomassproduction
(Tables6 and 7) were the resultof statisticallysignificant
treatmentdifferencesin diametergrowth.Additionalevidence(Kress,pets.comm.)fromoursiteindicateda significant positiverelationshipbetweencoarseroot and stem
diametergrowththatwasunaffected
bytreatment
for 16trees
measured
in 1993and1994.Thelackof significant
treatment
effectsin the coarseroot biomassregression
equationmay
have been a result of our limited samplesize (23 trees);
however,oursamplesizefor coarserootbiomasswaslarger
thanmostreportedin theliterature(Santantonio
et al. 1977,
Kinersonet al. 1977,LinderandAxelsson1982,Vogt 1991,
andHaynesandGower 1995).
Therearetwohypotheses
regardingtheeffectsof nitrogen
availability on above- versusbelowgroundallocationof
biomassproduction.
The firsthypothesis
suggests
thatallocationto finerootproduction
decreases
andfinerootbiomass
decreases
or remainsthesamewith increased
nitrogenavailability(Petsson1983,Vogt et al. 1986,Goweret al. 1994,
HaynesandGower 1995).The secondhypothesis
suggests
thatallocationto fine rootproduction
remainsconstantbut
fine root biomassdecreases(Raich and Nadelhoffer 1989,
Hendrickset al. 1993).Ourresults,indicatinga proportional
andabsolute
decrease
in finerootproduction,
aresupportive
of thefirsthypothesis
andindicatethatthehypothesis
maybe
extended to include the effects of both nutrient and water
availability.
Comparisonof biomassallocationin our studyandthe
Kinersonet al. (1977) studywhichmeasured
productivityin
ForestSctence
44(2)1998 325
12- to 16-yr-old 1oblollypine standsin North Carolina
revealedseveralinteresting
differences.
While totalbiomass
production
in theirstandandourfertilizedplotsweresimilar,
ourfertilized
plotsproduced
upto 2 Mg ha-1 yr-1 more
foliage biomassin 1994 and 1995. Also, while foliage
biomassproduction
increased
substantially
eachyearin our
study,it remainedrelativelyconstantin theirs.At the same
time,theportionof totalbiomassproduction
allocatedto the
stemin theirstudy(60%) wasgreaterthanin ourstudy(37%
orless).Kinersonet al. (1977)indicatedthatstandagewasan
importantconsideration
when comparingproductivityin
forestecosystems,
and this may have been a factorin the
differences between the two studies. Yet these differences
wereevidentevenwhenthestands
werethesameage(ourlast
andtheir first year).
Our resultsindicatethatstanddevelopmental
stage,history of perturbation,
level of siteresourceavailability,and
standage shouldbe consideredwhen examiningtreatment
differencesin standproductivityand biomassallocation,
supportingthe conceptspresentedby Miller (1981) and
others(Ovington1957,Assman1970).While Miller (1981)
indicatedthatfertilizedstandswouldeventuallyreturnto the
samegrowthcurve as their nonfertilizedcounterparts,
this
probablywill not be the casein our studyas long as we
maintainthe treatments.
In light of thesedifferences,it may
be interestingin thefuture,whenmoredataareavailable,to
compareour plotsat the samedevelopmental
stagerather
than the samechronologicalage to understandtreatment
inducedchanges.However, given the low native nutrient
availabilityof thesiteandtheincreasein nutrientavailability
associated
with ourtreatments,
thecontrolplotsmaynever
reachthesamedevelopmental
stageasthefertilizedplots.In
thatregard,fine root biomassdecreased
slightlyandpeak
LATremained
at1.2m2m-2inthecontrol
plotsduring
1994
and 1995eventhoughcanopyclosurehadnot occurred.In
contrast,
asfertilizedplotsnearedcanopyclosurein 1995,we
observed a decrease in fine root biomass but an increase in
peakLAT.Apparently,thecontrolstandsmayhavealready
achieved"full" siteoccupancy
withrootclosurebutwithout
canopyclosure.
Stemvolumegrowthefficiencyandtotalbiomass
production efficiencyincreaseddue to fertilizationand irrigation
(Figures1 and2). The gainin stemvolumegrowthefficiency
resulted,in part, from proportionallygreaterallocationof
biomass production from belowground (fine roots) to
aboveground
components
(includingstem)(Table7). However,the changein allocationamongbiomasscomponents
didnotexplaintheobserved
increase
intotalbiomass
productionefficiency.Consequently,
at the standlevel,netcarbon
convertedintobiomassperunitof LATmusthaveincreased
with fertilizationandirrigationto accountfor the observed
increasein totalproduction
perunitLAT.Resultsfromother
researchat thesiteprovidedanexplanation
for theobserved
increase
in production
efficiency.Murthyet al. (1996)found
26% higherphotosynthesis
ratesin fertilizedplotsthanin
nonfertilized
plotsoverthelife of the1993foliagecohort.On
the other hand, Maier et al. (1998) found increasedstem
(130%) and branch(40%) wood maintenance
respiration
326
Forest
Sctence
44(2)1998
rateswithfertilization.Finerootrespiration
didnotincrease
duetofertilizationorirrigation,butfinerootrespiration
rates
wereupto 15timesgreaterthanstemorbranchwoodytissue
respiration
rates(Maier,pers.comm.).Basedontheobserved
changes
in allocation,
photosynthesis
rates,andrelativecomponentrespiration
rates,wehypothesized
thatincreased
total
biomassproductionefficiency,on a standbasis,resulted
whenmorebiomasswasallocatedto foliage(photosynthesizingtissue)andlessto fine roots(a highrespirationrate
tissue).Fertilizationand irrigation(high levels of soil resourceavailability)providedan environmentwherethese
changesoccurred.
Improvements
in growthefficiencyassociated
with fertilizationandirrigationin thisstudymaynotcontinuethrough
the life of the stands.Mencucciniand Grace(1996) suggest
that, in mature trees, increasedhydraulicresistanceand
maintenancerespirationresultsin reductionsin biomass
accumulationfor a givenlevel of LAT. Thesefactorswould
impactthe treatmentsin this studydifferently.Increased
hydraulicresistance
maybemoreimportantin fertilizedtrees
(wherethetreeswouldbetaller)whilemaintenance
respiration may be higherin the controlplots due to a larger
(absoluteandproportionally)
finerootbiomasscomponent.
While the net impactof thesefactorson growthefficiency
may be uncertain,the overallproductivityof the fertilized
and irrigatedplotsshouldremaingreaterthan the control
plotsdueto higherlevelsof LAT in theseplots.
Given the stageof standdevelopment
andexistingsite
conditions,
growthappearsto be limitedprimarilyby nutrientsandsecondarilyby water.This findingcontradicts
currentnutrientprescription
technology
thatwouldranksimilar
sitesas poor candidatesfor N+P fertilization.Due to the
overallpoorinherentsitenutrientavailability,theimposition
of anoptimumnutritiontreatment,
ratherthanN+P fertihzation alone,may be responsiblefor the strongresponseto
nutrientadditions.The potentialfor applyinga "complete"
nutritiontreatmentto enhancegrowthon droughtysitesis
intriguing.However,theremayberisksinvolvedin increasingleaf areaandreducingfinerootsonsuchsites.At thesame
time, while somemight argue that, operationally,better
potentialfertilizationsitesmay exist,the strategicimportanceof enhancinggrowthon sitesthat can be harvested
underalmostanyconditionwithoutsoildegradationcannot
be overlooked.
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ForestSctence
44(2) 1998 327
breast
height
(D),D2x height
(H), length
of thelivecrown
APPENDIX
Thisappendix
presents
thestand-andsite-specific
whole
treecomponent
biomassregression
equations
andsummary
statistics
usedfor ouranalyses.
The full modelformwas:
DEPVAR
= INDVAR
ß WINDVAR
F W F * W INDVAR
* F INDVAR
* F * W
whereDEPVAR was the dependentvariable(whole tree
component
dryweightfor stem,branch,foliage,coarseroot,
taproot),INDVARwastheindependent
variable[diameterat
(L), andAge],F was0 if nonfertilized
or 1 if fertilized,andW
was0 if nonirrigated
or 1 if irrigated.The wholetreebranch
wood,foliage,andstemcomponents
hadat leasttwo independentvariables,eachwith the full set of treatmentand
independent
variableinteractions
in thefull model.Nons•gnificanttermsweredroppedfromthe full modelto develop
a reducedmodelfor eachdependent
variablewhichwasused
toestimatewholetreecomponent
biomass.Whencalculating
the absolutedata from the log-log models,Baskerville's
(1972) adjustment
to theantilogarithm
[antilog(meansquare
error/2)] was used.
Model type, parameter estimates, and summary statisticsfor whole tree component biomass regressions.Parameter estimates for the
F(fertilizer), I (irrigation), Independent*F, and Independent*l are the values shown when F= 0 (nonfertilized)and I = 0 (nonirrigated).
When F= 1 (fertilized) and I= 1 (irrigated) then the parameter estimate for these terms is zero.
Parameter
Dependent
variable
Stem wood + bark
Type
Intercept INDVAR
transformed•
2.12 D2xH
Log-log
0.31 D2xH
D2xHxAge
Current-year
foliage
I
INDVAR*F
INDVAR*I
MSE a
Log-log
Coarseroot
Log-log
Taproot
Log-log
a Mean square error.
b Residualswere not biased,
ForestSctence
44(2)1998
-1.39
0
-4.52
-0.23
23.67
0
0.94
0
-1.44
-4).14
R2
rt
0.99
48
0.041 0.88
448
0.060 0.92
448
0
0
0
0
0
Age
0.58
0.11
0
0
0
0
0
0.17 . D2xH
3.09
3.55
170
1.42
0.12
-0.26
-0.18
0
L
1.51
0.64
0
Age
0.64
0.087
0
0
0
0
0
0
0
0
0
D• xHxAge
L xAge
328
F
L
D2xHxAge
L xAge
Branchwood
estimate
Not
D
D
-0.073 -2.45
0
0.13
-0.25
1.69
2.06
0
-0.35
0
0
0.166
0.097
0.78
0.92
23
23