Height-diameter allometry and above ground biomass in tropical montane forests

20 Pages·2017·2.87 MB·English

RESEARCHARTICLE
Height-diameter allometry and above ground
biomass in tropical montane forests: Insights
from the Albertine Rift in Africa
Ge´rardImani1,2*,FaustinBoyemba2,SimonLewis3,4,NsharwasiLe´onNabahungu5,
KimCalders3,6,LouisZapfack7,BernardRiera8,ClarisseBalegamire9,AidaCuni-Sanchez3
1 BiologyDepartment,Universite´OfficielledeBukavu,Bukavu,DRCongo,2 PlantDepartment,Universite´
deKisangani,Kisangani,DRCongo,3 DepartmentofGeography,UniversityCollegeLondon,London,
a1111111111 UnitedKingdom,4 SchoolofGeography,UniversityofLeeds,Leeds,UnitedKingdom,5 SoilLaboratory,
a1111111111 InternationalInstituteofTropicalAgriculture/Kalambosite,Kalambo,DRCongo,6 EarthObservation,
a1111111111 ClimateandOpticalGroup,NationalPhysicalLaboratory,Teddington,Middlesex,UnitedKingdom,7 Plant
BiologyandphysiologyDepartment,UniversityofYaounde´1,Yaounde´,Cameroun,8 Laboratoired’Ecologie
a1111111111
ge´ne´rale,MuseumNationald’HistoireNaturelle,Brunoy,France,9 CentredeRechercheenSciences
a1111111111
NaturellesdeLwiro,Lwiro,DRCongo
*[email protected]
OPENACCESS Abstract
Citation:ImaniG,BoyembaF,LewisS,
NabahunguNL,CaldersK,ZapfackL,etal.(2017) Tropicalmontaneforestsprovideanimportantnaturallaboratorytotestecologicaltheory.
Height-diameterallometryandaboveground Whileitiswell-knownthatsomeaspectsofforeststructurechangewithaltitude,littleis
biomassintropicalmontaneforests:Insightsfrom
knownontheeffectsofaltitudeonabovegroundbiomass(AGB),particularlywithregardto
theAlbertineRiftinAfrica.PLoSONE12(6):
changingheight-diameterallometry.Toaddressthisweinvestigate(1)theeffectsofaltitude
e0179653.https://doi.org/10.1371/journal.
pone.0179653 onheight-diameterallometry,(2)howdifferentheight-diameterallometricmodelsaffect
abovegroundbiomassestimates;and(3)howotherforeststructural,taxonomicandenvi-
Editor:DafengHui,TennesseeStateUniversity,
UNITEDSTATES ronmentalattributesaffectabovegroundbiomassusing30permanentsampleplots(1-ha;
alltrees(cid:21)10cmdiametermeasured)establishedbetween1250and2600maslinKahuzi
Received:January19,2017
BiegaNationalParkineasternDemocraticRepublicofCongo.Foreststructureandspecies
Accepted:June1,2017
compositiondifferedwithincreasingaltitude,withfourforesttypesidentified.Different
Published:June15,2017
height-diameterallometricmodelsperformedbetterwiththedifferentforesttypes,astrees
Copyright:©2017Imanietal.Thisisanopen gotsmallerwithincreasingaltitude.Abovegroundbiomassrangedfrom168to290Mgha-1,
accessarticledistributedunderthetermsofthe
buttherewerenosignificantdifferencesinAGBbetweenforeststypes,astreesize
CreativeCommonsAttributionLicense,which
decreasedbutstemdensityincreasedwithincreasingaltitude.Foreststructurehadgreater
permitsunrestricteduse,distribution,and
reproductioninanymedium,providedtheoriginal effectsonabovegroundbiomassthanforestdiversity.Soilattributes(Kandacidity,pH)
authorandsourcearecredited. alsosignificantlyaffectedabovegroundbiomass.Resultsshowhowforeststructural,taxo-
DataAvailabilityStatement:Allrelevantdataare nomicandenvironmentalattributesaffectabovegroundbiomassinAfricantropicalmon-
withinthepaperanditsSupportingInformation taneforests.Theyparticularlyhighlightthattheuseofregionalheight-diametermodels
files:S1Table.Plotspecificheight-diameter
introducessignificantbiasesinabovegroundbiomassestimates,andthatdifferentheight-
allometricmodelsrelatingheight(inm)todiameter
(incm),theAkaikeInformationCriteria(AIC), diametermodelsmightbepreferredfordifferentforesttypes,andtheseshouldbeconsid-
variationexplainedbythemodel(R)andtheRoot eredinfuturestudies.
MeanSquaredError(RSME).S1Fig.Correlation
betweenabovegroundbiomass(AGBinMgha-1)
andothersfloristics,soilandstructuralattributes.
S2Table.Correlationbetweensoilsattributes.S3
Table.Soilmaincharacteristicsperforesttype.
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Height-diameterallometryandabovegroundbiomassintropicalmontaneforests
Funding:Thepresentworkhasbeenfinancially Introduction
supportedbytheFCCCproject(Appuiàl’UNIKIS:
Tropicalforestsplayamajorroleintheglobalcarbonbalance[1–4].Withincreasinginterest
"ForêtsetChangementClimatiqueauCongo")
fundedbytheEuropeanCommission,implemented inREDD+,animportantchallengefacingecologistsandforestersistoquantifyaspreciselyas
bytheCenterforInternationalForestryResearchin possiblethecarbonstocksandtheirfluxesatdifferentspatialscales[5].Significantmilestones
partnershipwithUniversityofKisangani(UNIKIS) havebeenreachedinthelastdecadethankstothedevelopmentofbroad-scaleremotesensing
andSynergiesDevelopment(RSD)forlogistical
approaches[2,6,7].However,localforestbiomassestimationsbasedonfieldmeasurements
support.SimonLewisissupportedbythe
commonlyrepresentthefoundationforthecalibrationandvalidationofremotesensingmod-
EuropeanResearchCouncil.Thisfunderhadarole
els[8–11].Asaconsequence,uncertaintiesanderrorsinlocalbiomassestimationsmaypropa-
onlyinstudydesign,datacollectionandsoil
analysis. gatedramaticallytobroad-scaleforestcarbonstockassessment[12–14].
Abovegroundbiomass(AGB)isthemajorpoolofbiomassintropicalforests[15].Inthe
Competinginterests:Theauthorsdeclarethatthey
field,theAGBofatree(orTAGB)isgenerallyestimatedeitherdirectlyorindirectly.Inthe
havenoconflictsofinterest.
firstcasetreesareharvested,weightedanddriedandweightedagain;whileinthesecondcase
allometricequationsareusedtoestimatebiomassfromanumberofvariables,mostlytree
diameter,wooddensity,andsometimes,treeheight[16].Itiswellknownthatbiomassesti-
matesaremoreaccurateifheightisincludedintheseallometricequations[16–19].However,
samplingtreeheightisdifficultintropicalforests[20],especiallyinmontaneareas,becauseof
thesteepslopesandthedifficultyofseeingtreecrowns[21].Therefore,inmostcases,tree
heightsaremeasuredforanumberofindividuals,andaheight-diametermodelisappliedto
estimatetheheightfortheremainingtrees[17,22].Severalauthorshaveshownthesignificant
biasesinbiomassestimatesassociatedwithusingregionalheight-diametermodels[19,23–26]
andhighlightedtheneedforlocalsitespecificheight-diametermodels.
However,thereisadebateonwhichtypeofmodelshouldbeselectedtobuildalocalsite-
specificheight-diametermodel.Whilesomeauthorssupporttheuseofapowerlawmodel
[18,27](theonepredictedbythemetabolictheoryofecology[28,29]),someotherssupporta
secondorderpolynomialofthelog-logdata[30](theonewhichconsidersthesaturationof
treeheightwithtreediameter),andotherspreferatrulyasymptoticmodel[17,23,31,32](for
furtherdetailssee[25]).Ithasbeenhighlightedthatthepowerlawmodelisunrealisticbiologi-
callybecauseofthebasicassumptionoffactorslimitingtreegrowthinheightbutnotindiam-
eters[33],andmostrecentstudieshavechosenatrulyasymptoticmodel(e.g.Brienenetal.
[34]).Amongtheasymptoticmodels,Feldpauschetal.[17]foundthattheWeibullmodelwas
themostappropriateforbiomassprediction,asitreduceserrorinsmall-diameterclasses.
Baninetal.[35]andKearsleyetal.[19]preferredanonlinear3-parameterexponentialmodel.
Tworecentstudies,whichincludedanotherasymptoticmodel,theMichaelis-Mentenmodel
[25,33],preferredthislaterone.Allthesestudiesfocusedontropicallowlandforesttypes,and
toourknowledge,theshapeoftheheight-diameterallometryhasnotbeenstudiedindetailin
tropicalmontaneforests.OnlyLedoetal.[26]assessedonemontaneforestinPeruand
showedthatathree-parameterWeibullfunctionwasthemostaccurateheight-diameterallo-
metricmodel.
Itiswellknownthattreeheightusuallydecreaseswithincreasingelevation[36].The
rationshipbetweenheightanddiameterisalsorelatedtospecies,climatic,soilcharacteristics,
regionandeventreediversity[18,35,37,38].Thepresenceofbamboo,commonindisturbed
highmontaneareas,andwindmayalsoalterforeststructure[39–41].Toourknowledge,
exceptMugashaetal.[21]inTanzania,nootherstudyhasassessedheight-diameterrelation-
shipsinmontaneforestsinAfrica.InTanzania,Mugashaetal.[21]distinguishedfourvegeta-
tiontypes:lowlandforest,montaneforest,miombowoodlandandAcaciasavanna.The
height-diameterallometricmodeldifferedaccordingtovegetationtype[21].Inmontanefor-
ests,forexample,theseauthorsshowedthattheexponentialmodelofWykoffwasthepre-
ferredmodel.
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Height-diameterallometryandabovegroundbiomassintropicalmontaneforests
Apartfromtheeffectsofheight-diameterrealtionshipsonAGB,itiswellrecogniziedthat
severalotherforeststructuralandtaxonomicattributesaffectAGB.Withregardtoforest
structure,stemdensityandstandbasalarea,whichtendtoincreaseanddecreaserespectively
withincreasingaltitude[36,42],affectAGB.Withregardtotaxonomicattributes,speciesrich-
nesshasbeenlinkedwithAGB.Generally,thereisadeclineintreespeciesrichnesswith
increasingaltitude[36,43,44],becauseofagreaterroleofenvironmentalfilteringathigherele-
vations(e.g.coolertemperatures,fog,reducedlightincidenceandhigherrelativehumidity).
Environmentalparameters,suchasclimateandsoilsalsoaffectAGB[32,45,46].Thus,quanti-
fyingtherelationshipsofaltitudeonheight-diameterrelationships,stemdensityandstand
basalareawillassistinunderstandinghowthesecontributetoAGBchangeswithtemperature,
anessentialpracticalquestion,asalmostallforestsarecurrentlywarming,andwillwarmcon-
siderablyinthefuture[47].
TheAlbertineriftisabiodiversityhotspot[48]withgreatforeststructuralandfloristic
diversity,and,therefore,isaconservationpriorityzone[49,50].Acarbonprojectcouldpro-
videfundsforenhacingconservationprogrammesinthisregion[51].However,littleisknown
aboutAGBestimatesinthemontaneareasoftheregion.Toourknowledge,nostudyhas
assessedAGBintropicalmontaneforests(TMF)intheAlbertineRift.Infact,fewstudieshave
assessedAGBinTMFinAfricaandmosthavefocusedinTanzania[52–56].
ToimproveourunderstandingofAGBestimatesthefactorsaffectingit,especiallywith
regardtotheAlbertineRiftregion,theobjectivesofthisstudywereto:(1)determinehowalti-
tudeandotherfactorsaffectheight-diameterallometryindifferentTMFs,(2)assesshowdif-
ferentheight-diameterallometricmodelsaffectAGBestimates;and(3)determinehowforest
structural,taxonomicandenvironmentalattributesaffectAGB.
Materialsandmethods
Studyarea
ThisstudyfocusedonthemontaneforestsofKahuziBiegaNationalPark(NP)anditssur-
roundings(Fig1).AresearchpermitwasobtainedfromICCN(InstitutCongolaisdeConser-
vationdelaNature)andbytheDirectorofKahuzi-BiegaNationalPark.Toworkinthe
communityforest,permissionwasobtainedfromMrBulonvu,memberofvillagecommittee.
Weconfirmthatthefieldstudiesdidnotinvolveendangeredorprotectedspecies.
TheKahuziBiegaNP,locatedineasternDemocraticRepublicofCongo(DRC),ispartof
theAlbertineRiftregion.Establishedasaforestreservein1937,itbecameaNationalParkof
6000km2in1975,withtheaimofprotectingthegorillas(Gorillaberingeigraueri)foundinthe
area[57].IntheNPaltituderangesfrom~650to~3320masl.
Kahuzi-BiengaNPhasabimodalrainfallregime,withmostrainsfallinginSeptember-
DecemberandMarch-April.Annualrainfallrangesbetween1500and2000mm,meanannual
temperatureis20˚C,andhumidityiscloseto76%[59].However,importantclimaticdiffer-
encescanbeobservedwithincreasingaltitude(colderandwetter),withfogbeingacommon
featureabove2900masl.Sometimesfrostcanbeobservedabove3300masl,especiallyduring
themonthofJuly.AccordingtoDewitteetal.[60],Kahuzi-BiengaNPanditssuroundings
haveaAcrisolsandFerrasolsumbricsoiltypes.Thegeologicalsubtrateisvolcanicrichin
basalt[61].
MostofKahuzi-BiegaNPiscoveredbyclosed-canopyforest.Whilebelow1250maslthere
issemi-decidiousandevergreenlowlandrainforest,between1250and2600maslthereare
montaneforests[59,62].Montaneforestsareclassifiedintofourdistincttypesbecauseoftheir
floristicandstructuralcharacteristics:Submontane(1250to1500masl),lowermontane(1500
to1800masl),middlemontane(1800to2400masl)anduppermontane(2400to2600masl)
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Height-diameterallometryandabovegroundbiomassintropicalmontaneforests
Fig1.StudysiteandlocationofplotsinandaroundKahuziBiegaNationalPark.BackgrounddataadaptedfromPlumptre
etal.[58].
https://doi.org/10.1371/journal.pone.0179653.g001
(seeImanietal.[44,63]).SubmontaneforestsaredominatedbyAnonidiummannii,Strombo-
siascheffleriandTrichilia welwichii;lowermontaneforestsbyTrilepisiummadagascariensis,
CarapagrandifloraandDrypetesdinklagei;middlemontaneforestsbyMacaranga neomil-
braediana;anduppermontaneforestsbyHageniaabyssinicaandRapaneamelanophloeos.Sub
montaneandlowermontaneforestshavehigherfloristicdiversityandlargertrees(diameter
andheight)thantheothertwoforesttypes.Uppermontaneforestshaveshortertreeswith
twistedstemsandmanyepiphytesontheirbranches.Naturalbamboo(Sinarundinariaalpine)
formationscanbefoundintheuppermontaneforest.
TheNPstudiedispartoftheAlbertineAfromontaneBiodiversityHotspot[48].Several
endangeredanimalandplantsspeciesarefoundinthisNP,suchasgorillas(Gorillaberingei
graueri),elephant(Loxodontaafricanavar.cyclotis),forestbuffalo(Synceruscaffernanus),
lion(Panthera leo)andgiantforesthog(Hylochoerusmeinertzhageni)[64,65].Commercial
loggingneveroccurredonthisNP.Surroundingcommunities,whicharemainlyfarmersof
Pygmy,BashiandLegatribes,arenotallowedtohunt,fish,collectfirewoodorothernon-
timberforestproductsinsidetheNP,butsomedo.Becauseofcivilunrestandlackoflaw
reinforcement,somepartsoftheNP,mostlybetween1400-1700maslhavebeenconsider-
ablydegraded,duetofirewoodcollection,mineralactivitiesandcharcoalproduction(Pers.
Obs.).
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Height-diameterallometryandabovegroundbiomassintropicalmontaneforests
Fig2.Montaneforeststratificationandsamplingdesign.
https://doi.org/10.1371/journal.pone.0179653.g002
Studydesignandfieldmeasurements
Thirtyplotsof1hawereestablishedatdifferentaltitudeswithinthemontaneforestbelt.After
preliminaryobservations,plotsweresetuprandomlyatleast300mfromeachotherinparts
oftheforestthatwerenotdegraded.Becauseoftheabovementioneddegradationbetween
1400-1700maslinsidetheNP,twelveplotswereestablishedwithinthisaltitudinalzonein
non-disturbedcommunityforestoutsidetheNP.Afteraclusteranalysis,plotsweregrouped
intofourforesttypesinrelationtotheirfloristicandforeststructure(seeImanietal.[44]for
details),sothatsixplotswereidentifiedassubmontaneforest,sixaslowermontaneforest,14
asmiddlemontaneforestandfourasuppermontaneforest(Fig2).Althoughamorebalanced
designwouldhavebeenpreferredwewereunavailabletogobacktotheforesttosetupmore
plotsine.g.uppermontaneforests.Whilethevariationinforeststructureinmiddlemontane
forestsisgreaterthaninotherforesttypes(seeResults),probablyrelatedtotheplotbeing
locatedinthewindward/leewardsideofthemountain,localvariationinslopeandexposureto
wind,weareconfidentthattallertreesorgreaterplotAGBwouldnotbefoundevenifmore
plotsweresetupinuppermontaneforests.
Weassumethattheforestplotsstudiedhavenotbeendisturbedbydirecthumanimpacts,
butmayhavebeendisturbedbynaturaleventsinthepast.
Inallplotsalllivefree-standingwoodystems(cid:21)10cmdiameterat1.3malongthestem
fromtheground(orabovebuttresses/deformitiesifpresent)weremeasuredandstemswere
identifiedtospecieswhenpossible.SamplesofunidentifiedtreesweretakentotheHerbarium
ofCentreforResearchinNaturalSciencesLwiro(LWI),INERAMulungu(MLGU)and
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Height-diameterallometryandabovegroundbiomassintropicalmontaneforests
depositedinLwiroHerbarium.Unidentifiedtreesrepresented0.15%ofthetreessampled(26
individualsof16797sampled),andtheywerefoundinSubmontaneandlowermontanefor-
ests.Taxonomyfollowedtheplantlist(www.theplantlist.org).Treeheightwasalsomeasured
inanumberoftreesperspeciesanddiameterclassineachplot,followingArcangelietal.[22].
Intotal,3834ofthe16797trees(cid:21)10cmdiameter(23%)weresampledforheight,usingahyp-
someterlaserAce1000Rangefinder.Theseincludedanaverageof25%oftreesineachplot
coveringarangeofdiameterclasses.PlotaltitudewasrecordedusingaGARMINGPSMAP62
stchandheldGlobalPositioningSystemdevice.
Ineachplotfoursoilsamplesofthelayer0-30cm,whichistherichestinorganicmatter
[66],werecollected.Eachsoilsampleconsistedofthecombinationofthreecorescollected
withinasubplotof50x50m[67].SoilsamplesforanalysisweretakentoIITA(International
InstituteofTropicalAgriculture)laboratoryinKalamboresearchstation(DRC)wherethey
wereair-dried.
Analysisofsoilsamples
BulkdensitywasdeterminedusingaKopeckycylinder,fromwhichsoilwasweightedand
driedtoconstantweight[68].Sand/silt/claypercentagewasobtainedusingthehydrometer
method[69]whilesoilpH,acidity,exchangeableAluminium,Carbon,Nitrogen,Phosphorous
andPotassiumweredeterminedusingstandardFAOprotocols[69].
Thebestheight-diameterallometricmodel
Sevenheight-diameterallometricmodelscommonlyusedtoestimateheightfromdiameter
[21,25]wereconsideredinthisstudy:GompertzorWinsorS-Shaped(Functionsm1)[70],
logistic(Functionm2)[71],WeibullS-shaped(Functionm3),Richards(Functionm4),
Michaelis–Menten(Modelm5),Power(Modelm6)andasecondorderpolynomial(Model
m7):
• Height ¼ a(cid:3)expð(cid:0) b (cid:3) expð(cid:0) c(cid:3)diameterÞ ðm1Þ
• Height ¼ a=ð1 þ exp½ðb (cid:0) diameterÞ=c(cid:138) ðm2Þ
• Height¼aþðb(cid:0) aÞexp½(cid:0) expðcÞdiameter^d(cid:138) ðm3Þ
• Height ¼ aþðb(cid:0) aÞexp½(cid:0) expðcÞ(cid:3)diameter(cid:138) ðm4Þ
• Height ¼ a(cid:3) diameter=ðbþdiameterÞ ðm5Þ
• Height ¼ a(cid:3) diameterb ðm6Þ
• Height ¼ aþb(cid:3)logðdiameterÞþc(cid:3)ðlogðdiameterÞÞ2 ðm7Þ
Wherea,b,canddaremodelparametersestimatedusingthenlsfunctioninnlmeandmin-
pack.lmpackagesofR[72].Thebestmodelfor(i)eachplotand(ii)eachforesttype(sub,
lower,middleanduppermontane)wereassessed.Thebestmodelwasselectedconsidering:
AkaikeInformationCriterionorAIC[73],RootMeanSquaredError(RMSE)andvariation
explainedbythemodel(R2).Overall,thebestmodelisonewhichhaslowAICandRMSEbut
highR2[21,73,74].
Factorsaffectingtheslendernesscoefficient
Theslendernesscoefficient(themeanoftheratiobetweenheightanddiameterofallindividu-
alsmeasuredonfield)wascalculatedforeachplot[75].Fifteenvariables,includingaltitude,
slope,stemdensity,treespeciesrichness,andelevensoilvariables(pH,H+,exchangeable
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Height-diameterallometryandabovegroundbiomassintropicalmontaneforests
Aluminium(Al3+),Carbon,Nitrogen,Phosphorous,bulkdensity,sand,silt,andclaypercent-
age)wereassessedaspotentialfactorsaffectingtheslendernesscoefficient.Meanplotslope
wasextractedfromASTERglobaldigitalelevationmodelmaps(https://wist.echo.nasa.gov/~
wist/api/imswelcome/)at30mpixelresolutionusingtheslopefunctioninArcGIS.Stemden-
sity(numbertreesha-1)includedalltrees(cid:21)10cmdiameter.Treespeciesrichnesswasdeter-
minedastotalnumberoftreespecies(identifiedspecieswithavalidLatinnameplusunique
morphospecies)(cid:21)10cmdiameterobservedinagivenplot.
AGBestimates
TheChaveetal.[30]equationincludingdiameter,wooddensityandtreeheightwasusedto
estimatetheAGBofeachtreeintheplot.Thebesttaxonomicmatchofwooddensitytoeach
stemwasextractedfromaglobaldatabase[76,77]followingLewisetal.[32].Forthetrees
whoseheightwasnotsampledinthefield,theirheightwasestimatedusing(1)thebestheight-
diameterallometricmodelforeachplotasdeterminedinprevioussection,(2)thebestheight-
diameterallometricmodelforeachforesttypeasdeterminedinprevioussection,(3)theFeld-
pauschetal.[17]height-diameterallometricmodelforEastAfricaand(4)theFeldpausch
etal.[17]height-diameterallometricmodelforCentralAfrica.
Statisticalanalysis
RstatisticalsoftwareR3.02wasusedforallstatisticalanalyses[78].AnovaandTukey-HSD
testsorKruskal-Wallis(ifnonhomogeneityofvariance,forH andD )andmultiple
mean mean
comparisonofKruskal-Wallis,using‘kruskalmc’inR,wereusedtoevaluatesignificantdiffer-
encesin(i)soilcharacteristics,(ii)structuralparameters(meanheight,meandiameter,basal
area,stemdensity,stemdensityoflargetree(cid:21)50cm,woodymassdensity,speciesrichness,
AGB)and(iii)slendernesscoefficientsbetweenforesttypes,afterdatatransformation
(orthogonality).
Alinearregressionandapearsoncorrelationtestwereusedto(i)assesshowaltitude
affectedheight-diameterrelationshipsand(ii)whichenvironmentalattributesarecorrelated
toslendernesscoefficient.Amultipleregressionmodelwasusedtoinvestigatehowenviron-
mentalattributesaffectedheight-diameterrelationships.Inthislatercaseonlyvariableswith
p<0.05wereretainedinthefinalmodelusing‘updatefunction’.Beforerunningthisregres-
sion,acorrelationbetweensoilvariableswasassessedusingPearsoncorrelation.Onlynon-
autocorrelatedvariables(significantatp<0.01)wereconsideredinthemultipleregression.
Thereby,thevariablesretainedinthisanalysiswereAl3+,K+,P,C/Nratio,silt,sand,bulkden-
sity,slopeandstemdensity.Variableswererandomlypermutedinthemultipleregression,so
thatvariableorderdidnotaffectthepvalue.
AIC,RMSEandR2wereusedtodeterminethebestheight-diametermodelforeachforest
type.WhenAICwassimilarbetweentwomodels,astudenttestwasappliedtoconfirmifthe
treeheightsestimatedusingoneoranothermodelweresignificantlydifferentfromeach
other.Pearsoncorrelationwasusedtoassesstherelationshipbetweenstructuralparameters,
soilcharacteristicsandAGB.T-testswereusedtodeterminedifferencesbetweenAGBcalcu-
latedusingdifferentapproachespresentedintheprevioussection.
Results
Height-diameterallometricmodels
AccordingtoAIC,andRSMEvalues,themodelthatperformedbetterwasnotthesameacross
foresttypesandmountains(Table1).TheGompertz(Modelm1)performedbetterforSub
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Height-diameterallometryandabovegroundbiomassintropicalmontaneforests
Table1. Localsitespecificheight-diameterallometricmodelsrelatingheight(inm)todiameter(incm).
Foresttype Models Modelsparameters Selectioncriteria
a b c d RSME AIC
Submontane(1250–1500) Model1 30.61 2.7 0.95 3.66 4698.36
Model2 29.2 26.8 0.71 3.68 4704.6
Model3 30.3 27.8 -5.5 1.5 3.66 4699.18
Model4 34 -3.1 -3.7 3.69 4709.55
Model5 60.5 81.98 3.8 4757.88
Model6 1.50 0.69 4;03 4862.17
Model7 -12.31 5.27 0.96 3.76 4741.57
Lowermontane(1500–1800) Model1 30.0 3.2 0.94 3.07 4983.82
Model2 28.04 24 10.3 3.09 4993.32
Model3 29.04 27.2 -5.8 1.7 3.08 4988.48
Model4 33.4 -5.2 -3.5 3.12 5014.14
Model5 70.25 97.11 3.34 5146.39
Model6 1.24 0.75 3.58 5281.67
Model7 -15.02 6.45 0.91 3.2 5065.38
Middlemontane(1800–2400) Model1 21.54 2.3 0.94 2.98 8201.36
Model2 20.88 19 11.5 3.01 8228.07
Model3 23.29 27 -2.8 0.9 2.97 8186.18
Model4 22.7 -1.71 -3.3 2.97 8184.93
Model5 33.89 40.85 3.02 8234.56
Model6 2.09 0.55 3.22 8453.27
Model7 -18.68 12.08 -0.65 2.98 8199.96
Uppermontane(2400–2600) Model1 12 2.3 0.9 2.42 1672.03
Model2 11.8 11.3 7.4 2.43 1673.65
Model3 na na na na na na
Model4 12.3 -2.8 -2.6 2.42 1670.35
Model5 17.21 19.65 2.43 1672.65
Model6 2.34 0.43 2.49 1688.49
Model7 -15.26 11.57 -1.17 2.41 1667.91
TheperformedmodelwasselectedusingAkaikeInformationCriteria(AIC)andRootMeanSquaredError(RSME).Thebestmodelforeachforesttypeis
showninbold.
https://doi.org/10.1371/journal.pone.0179653.t001
montaneandlowermontaneforestswhiletheRichardsAsymptotic(Modelm4)performed
betterforthemiddlemontaneforestsandthesecondorderpolynomial(Modelm7)performed
betterfortheuppermontaneforests(Table1).
Ifeachplotisconsideredseparately,thereislimitedagreementbetweenplotsofthesame
foresttype.TheLogis(Modelm2)performedbetterfor50%oftheplotsinboththeSubmon-
taneandlowermontaneforests,andfor35%ofthemiddlemontaneforests.ThePower
(Modelm6)performedbetterfor50%oftheuppermontaneforests(seeS1Table).Forthe
middlemontaneforests,forwhich14plotsweresampled,fourdifferentmodelswerepre-
ferreddependingontheplot,highlightingthevariabilityinheight-diameterallometryeven
withinagivenforesttype.TheMichaelis-Menten(Modelm5)andtheWeibull(Modelm3)
onlyoutperformedothermodelsinfourandoneplotsrespectivelyofthe30plotssampled
(S1Table).
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Height-diameterallometryandabovegroundbiomassintropicalmontaneforests
Theeffectsofheight-diameterallometricmodelchoiceonAGB
estimates
Theuseofdifferentheight-diameterallometricmodelshadimportanteffectsonAGBesti-
mates(Fig3,Table2).Comparedwithusingthebestheight-diameterallometricmodelper
plot,theuseofthebestmodelperforesttypehadlittleeffectonAGBestimatesinSubmontane
andmiddlemontaneforests(-4.3and3.7%changeinAGBrespectively),butconsiderable
effectsinloweranduppermontaneforests(-24.9and34.67%changeinAGBrespectively,see
Table2).However,thesedifferenceswerenotsignificantifforesttypeisnottakenintoaccount
(Table2).
TheuseofFeldspauchEastAfricaheight-diameterallometricmodelproducedevengreater
overestimates:25.5%,37.6%and78.4%changeinAGBinSubmontane,middleandupper
montaneforestsrespectively(Table2).
TheuseofFeldspauchCentralAfricaheight-diameterallometricmodelalsoproduced
greatoverestimates:38.8%,52.1%and105%inSubmontane,middleanduppermontanefor-
estsrespectively(Table2).Forlowermontaneforests,FeldspauchEastAfricaandFeldspauch
CentralAfricamodelsproducedsmallerchangesonAGB(-1.4and10%,seeTable2).These
differenceswereallsignificantevenifforesttypewasnottakenintoaccount(Table2).
AGBestimatesanditsrelationshipwithforestandenvironmental
attributes
AGBrangedfrom168Mgha-1intheuppermontaneforeststo290Mgha-1inmiddlemon-
taneforests(Table3).However,nosignificantdifferencesinAGBwereobservedamongthe
foresttypesstudied(Table3).Thisisrelatedtothefactthatuppermontaneforestshadsignifi-
cantlylowermeanheight,meandiameter,andnumberofspeciesbutgreaterstemdensity
thanotherforesttypes(Table3).Ifindividualplotsareconsidered,AGBrangedfrom90Mg
ha-1inoneplotintheuppermontaneforestto588Mgha-1inaplotinmiddlemontaneforests
(seeFig3).IfAGBisplottedagainstaltitude,AGBhasacertainbell-shaperelationshipwith
altitude(seeFig3).
AGBwasfoundtobesignificantlypositivelycorrelatedwithBA,SD ,D andH
50 mean mean
butnotwithstemdensity,WMDorspeciesrichness(Table4,S1Fig).Withregardtoenviron-
mentalattributes,AGBwassignificantlynegativelycorrelatedwithsoilpH(atp<0.05)and
significantlypositivelycorrelatedwithpotassium(Table4,S1Fig).
Slendernesscoefficientsandsoilsamples
Slendernesscoefficientssignificantlychangedwithincreasingaltitude(R2=0.39;r–0.474,
p<0.01,seeFig4a).Uppermontaneforesthadasignificantlydifferentslendernesscoefficient
comparedwiththeotherforesttypes(Fig4b).
Amongtheforestandenvironmentalattributesconsidered,potassiumcontent,sandand
siltcontentsignificantlyaffectedtheslendernesscoefficient(Table5).Manysoilvariableswere
autocorrelated(S2Table).
Withregardtosoilcharacteristics,importantdifferenceswereobservedbetweenforest
types(S3Table).Ingeneral,Submontaneforestsanduppermontaneforestswerethemost
different,withlowerandmiddlemontaneforestshavingvaluesrangingbetweenboth
extremes.Soilcharacteristicsarediscussedindetailinanotherpublication(Imanietal.in
preparation[79]).
PLOSONE|https://doi.org/10.1371/journal.pone.0179653 June15,2017 9/20
Height-diameterallometryandabovegroundbiomassintropicalmontaneforests
Fig3.Abovegroundbiomass(AGBinMgha-1)withregardtoaltitude.AGBcalculatedusing(a)thebestheight-diametermodelperforesttype,(b)
thebestheight-diameterallometricmodelperplot,(c)Feldpauschetal.(2012)height-diameterallometricmodelforEastAfricaand(d)Feldpauschetal.
(2012)height-diameterallometricmodelforCentralAfrica.pvalueand“r”ofPearsoncorrelationbetweenAGBandaltitudeareindicatedineachplot.
Significantcorrelationsatp<0.01.
https://doi.org/10.1371/journal.pone.0179653.g003
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