AcaciakoainHawai‘i:FacingtheFutureProceedingsofthe2016Symposium,Hilo,HI:www.TropHTIRC.org,www.ctahr.hawaii.edu/forestry

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SILVICULTURE

STANDMANAGEMENTOFKOA:THINNINGANDFERTILIZATION

TravisIdol,Ph.D. (UniversityofHawai‘iatMānoaDepartmentofNaturalResources

andEnvironmentalManagement),PaulScowcroft(USDAForestService,Instituteof

PacificIslandsForestry),J .B.Friday(UniversityofHawai‘iatMānoaDepartmentof

NaturalResourcesandEnvironmentalManagement)

InmanyareasofHawai‘i,Acaciakoa(koa)hasbeennaturallyregeneratedthroughsoilscarification

orotherdisturbancessuchasfire,resultinginsingle-agecohortsofdensebutpatchystands.

Managingthesestandsfortimberproductionusuallyincludesthinningtomaintaingrowth

potentialandpreventsuppressionthroughintraspecificcompetion.Othertreatments,suchas

fertilizationorgrasscontrol,mayhaveadditionalpositiveeffectsontheselectedtrees("crop

trees").

Agrowingbodyofresearchoffersinsightsintoresponsestothinningandothertreatmentsfor

dense,naturally-regeneratedkoastands.ScowcroftandStein(1986)studieda10-year-oldkoa

standonMauisituatedat1100mabovesealevel(asl)onnutrient-poorsoilthatreceivedon

average2500mmmeanannualprecipitation(MAP).Standdensityat10yearswas2100stems

ha1andamajorityoftreeswereconsidereddyingorweak.Experimentaltreatmentsincluded

thinningto50%residualdensityandfertilizationwithN-P-K(10-30-10)at460kgha-1andMgSO4

at170kgha-1.Thinningsignificantlyincreasedstandrelativegrowthrateforthefirst2-3years,

untilanoutbreakofkoaloopermoth(Scotorhythrapaludicola)defoliatedthestand.Fertilization

hadnosignificantindependentoradditionaleffecttothinning.

PearsonandVitousek(2001)studieda9-year-oldkoastandonHawai‘iIslandsituatedat1500m

aslona2000-3000year-oldlavaflowsoil(MaunaLoa)thatreceivedonaverage2500mmMAP.

Standdensitywas16,000stemsha-1withabasalareaof21m2ha-1.Experimentaltreatments

includedthinningto50%residualbasalareaandfertilizationwith70kgha-1ofnitrogen(N)as

ammoniumnitrate.Thinningincreasedmeanannualstemdiameter(DBH)incrementfrom0.4to

1.1cm.Fertilizationhadnosignificantindependentoradditionaleffecttothinning.

AcaciakoainHawai‘i:FacingtheFutureProceedingsofthe2016Symposium,Hilo,HI:www.TropHTIRC.org,www.ctahr.hawaii.edu/forestry

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Inthesamelocation,Scowcroftetal.(2007)studieda23-year-oldkoastand.Standdensitywas

approximately900stemsha-1.Experimentaltreatmentsincludedthinningaroundselectedcrop

trees(alltreeswithoverlappingortouchingcrowns),herbicidegrasscontrol,andfertilizationwith

750kgha-1ofphosphorus(P)astriplesuperphosphate.Thefullcombinationoftreatments

resultedinasignificantincreaseinannualDBHincrementfrom0.5to1.09cm.Thisdifferencewas

duetoasignificantdeclineinDBHincrementovertimeinthecontrolplotsvsnodifferenceover

timeinthefullytreatedplots.

Bakeretal.(2008)studied30-year-oldkoastandsonHawai‘iIslandsituatedat1500maslona

1500-300year-oldlavaflowsoil(MaunaLoa)thatreceivedonaverage1200mmMAP.

Experimentaltreatmentsincludedthinningaroundselectedcroptreestodifferentsetdistances,

representingresidualstanddensitiesfrom900to200stemsha-1,andherbicidegrasscontrol.After

3years,stemDBHincrementwastwiceasgreatwiththinningto900stemsha-1andalmost4times

asgreatat200stemsha-1.Grasscontrolhadnosignificantindependentoradditionaleffect.

Idoletal.(2017)studieda9year-oldkoastandsituatedonHawai‘iIslandat1500maslonadeep

ashsoil(MaunaKea)thatreceivedonaverage2000mmMAP.Experimentaltreatmentsincluded

thinningaroundselectedcroptreestoaresidualstanddensityof500stemsha-1(4.5-mradius),

fertilizationwith600kgha-1ofPastriplesuperphosphate,andherbicidegrasscontrol.Thinning

increasedannualstemDBHincrementfrom1.0to2.0cm.Fertilizationandgrasscontrolhadno

significantindependentoradditionaleffectonDBHincrement.Forthinnedtrees,Pfertilization

significantlyincreasedheightgrowthfrom0.5to1.0mover2years.Gapclosureafterthinning

progressedataratethatwasprojectedtoresultinfullcanopyclosureafterapproximately5years.

Conclusionsandrecommendationsfromthisstudyincludethefollowing:

1.Thetimeoffirstthinningshouldtakeplaceat6-8yearsofage,dependinguponinitialdensityand

averagecroptreesize.

2.Croptreeselectionthinningisrecommendedtofocuseffortsontreeswiththebesttimber

potential.Thethinningintensity(radius)shouldbeaswideaspracticable,uptotheexpected

crownarearequirementoftreesatthefirstcommercialthinningorharvest.

3.ThinningatayoungageshouldapproximatelydoublestemDBHincrement,maintaininggrowth

potentialoftreesinotherwiseoverstockedstands.

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4.FertilizationwithPisrecommendedatarateofatleast500kgha-1.Thisappearstobemore

importantforolderstands,buttheremaybeimprovementsinheightgrowthevenforyounger

stands.

5.Giventhegreaterexpenseofthinningvsfertilizationandthepotentialincreaseinheightgrowth,

repeatedPfertilizationmayimprovethecanopydominanceofcroptreesandreduceoreliminate

theneedforadditionalpre-commercialthinningasthinnedgapsclosein.

Figure1:A12-year-oldpotentialcroptreeonMaunaKearespondswithhealthycrowndevelopment

fouryearsafterrelease

AcaciakoainHawai‘i:FacingtheFutureProceedingsofthe2016Symposium,Hilo,HI:www.TropHTIRC.org,www.ctahr.hawaii.edu/forestry

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Figure2:Alllessvigorousorforkedtreeswerefelledwithina15-footradiusofeachpotentialcroptree

inthisstudyina9-year-oldstandonMaunaKea.

Figure3:Theyoungkoastandontheleftwasregeneratedbyscarificationbybulldozerandfencingto

excludecattle.Ahealthyseedbankhadbeenbuiltupinthesoilbytheremainingoverstorykoatrees

inthesepasturesonMaunaKea.

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References

Baker,P.J.,Robinson,A.P.,Ewel,J.J.,2008.SuddenandsustainedresponseofAcaciakoacroptrees

tocrownreleaseinstagnantstands.CanadianJournalofForestResearch38,656-666.

Idol,T.W.,MartinezMorales,R.,Friday,J.B.,Scowcroft,P.G.2017.Precommercialreleasethinningof

potentialAcaciakoacroptreesincreasesstemandcrowngrowthindense,8-year-oldstandsin

Hawai'i.ForestEcologyandManagement392:105-114.

Pearson,H.L.,Vitousek,P.M.,2001.Standdynamics,nitrogenaccumulation,andsymbioticnitrogen

fixationinregeneratingstandsofAcaciakoa.EcologicalApplications11,1381-1394.

Scowcroft,P.G.,Friday,J.B.,Idol,T.W.,Dudley,N.,Haraguchi,J.,Meason,D.,2007.Growthresponse

ofAcaciakoatreestothinning,grasscontrol,andphosphorusfertilizationinasecondaryforestin

Hawai‘i.ForestEcolManage239,69-80.

Scowcroft,P.G.,Stein,J.D.,1986.StimulatinggrowthofstagnatedAcaciakoabythinningand

fertilizing.ResearchNote-PacificSouthwestForestandRangeExperimentStation,USDAForest

Service,8pp.