The Importance of Biotic vs. Abiotic Drivers of Local Plant Community Composition Along Regional ...

14 Pages·2015·0.91 MB·English

RESEARCHARTICLE
The Importance of Biotic vs. Abiotic Drivers
of Local Plant Community Composition Along
Regional Bioclimatic Gradients
KariKlanderud1,2*,VigdisVandvik2,DeborahGoldberg3
1 DepartmentofEcologyandNaturalResourceManagement,NorwegianUniversityofLifeSciences,Ås,
Norway,2 BiologyDepartment,UniversityofBergen,Bergen,Norway,3 DepartmentofEcologyand
EvolutionaryBiology,UniversityofMichigan,AnnArbor,Michigan,UnitedStatesofAmerica
* [email protected]
Abstract
Weassessediftherelativeimportanceofbioticandabioticfactorsforplantcommunitycom-
positiondiffersalongenvironmentalgradientsandbetweenfunctionalgroups,andasked
whichimplicationsthismayhaveinawarmerandwetterfuture.Thestudylocationisa
OPENACCESS uniquegridofsitesspanningregional-scaletemperatureandprecipitationgradientsinbore-
Citation:KlanderudK,VandvikV,GoldbergD(2015) alandalpinegrasslandsinsouthernNorway.Withineachsitewesampledvegetationand
TheImportanceofBioticvs.AbioticDriversofLocal associatedbioticandabioticfactors,andcombinedbroad-andfine-scaleordinationanaly-
PlantCommunityCompositionAlongRegional
sestoassesstherelativeexplanatorypowerofthesefactorsforspeciescomposition.Al-
BioclimaticGradients.PLoSONE10(6):e0130205.
thoughthecommunityresponsestobioticandabioticfactorsdidnotconsistentlychangeas
doi:10.1371/journal.pone.0130205
predictedalongthebioclimaticgradients,abioticvariablestendedtoexplainalargerpropor-
AcademicEditor:CristinaArmas,Instituteof
tionofthevariationinspeciescompositiontowardscoldersites,whereasbioticvariables
EcologyandBiodiversity,CHILE
explainedmoretowardswarmersites,supportingthestressgradienthypothesis.Significant
Received:February16,2015
interactionswithprecipitationsuggestthatbioticvariablesexplainedmoretowardswetter
Accepted:May17,2015
climatesinthesubalpineandborealsites,butmoretowardsdrierclimatesinthecolderal-
Published:June19,2015 pine.Thus,wepredictthatbioticinteractionsmaybecomemoreimportantinalpineandbo-
Copyright:©2015Klanderudetal.Thisisanopen realgrasslandsinawarmerfuture,althoughmorewinterprecipitationmaycounteractthis
accessarticledistributedunderthetermsofthe trendinoceanicalpineclimates.Ourresultsshowthatbothlocalandregionalscalesanaly-
CreativeCommonsAttributionLicense,whichpermits
sesareneededtodisentanglethelocalvegetation-environmentrelationshipsandtheirre-
unrestricteduse,distribution,andreproductioninany
gional-scaledrivers,andbioticinteractionsandprecipitationmustbeincludedwhen
medium,providedtheoriginalauthorandsourceare
credited. predictingfuturespeciesassemblages.
DataAvailabilityStatement:Allvegetationdataare
availablefromtheNordicvegetationdatabase(NVD,
http://www.givd.info/ID/EU-00-018).Allotherrelevant
dataarewithinthepaper.
Introduction
Funding:ThisstudywasfundedbytheNorwegian
ResearchCouncil(http://www.forskningsradet.no/) Consequencesofclimatechangeforspeciesassemblagesandbiodiversitymaydependonhow
throughtheprogrammeNORKLIMAandthe
therelativeimportanceofbioticandabioticinteractionsshiftswiththeenvironment.Thus,
SeedClimproject,grantnumber184912/S30toVV.
disentanglingtherelativeimpactofbioticandabioticfactorsonspeciescomposition,and
Thefundershadnoroleinstudydesign,data
howthesevarywithenvironmentalconditions,areurgentlyneededtounderstandhow
collectionandanalysis,decisiontopublish,or
preparationofthemanuscript. climatechangeaffectsecologicalprocessesandbiodiversityacrossregions[1].Thisisarecent
PLOSONE|DOI:10.1371/journal.pone.0130205 June19,2015 1/14
Bioticvs.AbioticDriversofPlantCommunityComposition
CompetingInterests:Theauthorshavedeclared restatementoftheclassicaldebatewithinecologicaltheoryaboutthedeterminantsofplant
thatnocompetinginterestsexist. communityspeciescomposition[2,3]:Abioticfactors,suchassoilmoisture,nutrientsandpH
aretraditionallypredictedtomoredirectlyaffectspeciesestablishmentandsurvivalunder
stressfulenvironmentalconditions,suchasinthealpine[4].Bioticinteractions,ontheother
hand,suchascompetitionfromdenserandhighervegetationandalargeramountoflitter,are
oftenpredictedtoplayamoreimportantroleforspeciescoexistenceinmoreproductivehabi-
tats[5,3,6,7,8],butsee[9,10,11].Negativebioticinteractionsarealsofoundinstressfulenvi-
ronments(e.g.[12]),whereasmorerecently,anumberofstudieshavedocumentedsignificant
impactsofpositivebioticinteractionsforplantsincold,dry,orinfertileunproductivehabitats
[13,14,15,16,17,18].Thus,bioticinteractionsmaybeequallyimportantinstressfuland
moreproductivehabitats,althoughtherelativeimportanceofdifferenttypesofbioticinterac-
tions(positivevs.negative)mayshift[19].
Empiricalassessmentoftrendsintheimpactofspeciesinteractions,whethernegativeor
positive,alongbroad-scaleproductivityorstressgradientsaretypicallybasedonmeta-analyses
ofexperimentalmanipulationofneighboursandquantificationofresponsesintermsofplant
growth,orlesscommonly,survival[20,16].However,suchindividual-leveldatacannotneces-
sarilybescaleduptoconsequencesoftheseinteractionsatcommunity-level.Totakeanex-
treme(andunlikely)example:ifallspecieswerestronglyreducedingrowthandsurvivalby
competition,butsufferedsimilarreductions,theirrelativeabundancewouldnotbeaffected
[21].Thus,approachestoassesseffectsofbioticinteractionsonentirecommunities
areneeded.
Inthispaperwetakeanon-experimentalapproachdisentanglingtherelativeimportanceof
interactionsamongplantsvs.theabioticenvironmentforstructureofentirecommunities.We
firstassessthevariationinfine-scalevegetationaccountedforbylocalbioticvs.abioticvari-
ables,wherethebioticvariablesareproxiesfortheintensityofinteractions,suchasthecover
ortheheightofvegetation.Wethencomparetherelativevariationaccountedforbylocalbiotic
vs.abioticvariablesacrosssitesthatdifferssubstantiallyinenvironmentalstressandproductiv-
ity.Whilethisobservationalapproachcannotunambiguouslyisolatecauseandeffect,itdoes
avoidexperimentalartefactsandcanmoreeasilyfocusontheentirecommunityratherthan
simplemeasuresofindividualgrowthforoneorafewcomponentspecies.Theuseofobserva-
tional,ratherthanexperimentaldataalsofacilitatescomparisonacrossdifferentsites
andscales.
Hierarchicalmultivariatevariationpartitioningapproaches[22],seealso[23]areoftenused
toassesstherelativeimportanceofdifferentlocal-andregional-scaleenvironmentalfactorsfor
communitycomposition(e.g.[24,25,26,27]).However,theseapproachesestimatevariation
withouttakingintoaccountdifferencesinresponsesacrossenvironmentalgradients,i.e.all
localcommunitiesmaynotrespondtothesameunderlyingenvironmentalfactors,andthey
maynotrespondinthesamewaytothosefactors.Wehypothesizethattherelativeimportance
ofbioticandabioticfactors,andpossiblythedirectionofbioticeffects(positivevs.negative),
differsbetweenlocalcommunities.Thus,wesupplementthevariationpartitioningapproach
withreplicatedlocal-scaleordinationanalysestoallowassessmentoftherelativeimportanceof
differentenvironmentalfactorswithineachlocalcommunityalongbroader-scaleclimategra-
dients.Inourstudy,wethuscombinebroad-andfine-scaleordinationanalysesofvegetation
datafromlocalsiteswithinaregional-scalegridofsitestocomparetheeffectofbioticvs.abiot-
icfactorsalongenvironmentalgradients.
Theimpactsofbioticandabioticfactorsonplantcommunitiesalongelevationgradients
haveprimarilybeenstudied,orinterpreted,astemperaturegradients[16],butsee[28].Tem-
peratureandprecipitationare,however,oftencorrelatedalongelevationgradients,andrecent
studieshavepointedattheimportanceofprecipitation,soilmoisture,andtheirinteractions
PLOSONE|DOI:10.1371/journal.pone.0130205 June19,2015 2/14
Bioticvs.AbioticDriversofPlantCommunityComposition
withtemperatureandlocalenvironmentalfactorsasdriversofcommunitydynamics[29,30,
31].Bothtemperatureandprecipitationregimeshavechangedworldwideduringthelastcen-
tury,andarepredictedtoincreaseby2.3–4.6°Cand5–30%,respectively,inNorthernEurope
towards2100[32].Towardshigherelevations,thereisalsoanincreasingharshnessandde-
creasingproductivity.Highertemperaturesandprecipitationratesmayincreaseproductivity
intheseareasandtherebyaltertherelativeroleofbioticandabioticinteractions.Inaccordance
withthis,vegetationcanopyheightandlittercoverhaveincreasedinalpineandarcticplant
communitiesduringthelast20–30years,bothinclimatewarmingexperiments[33,29]andin
unmanipulatedmonitoringplots[30].Thesestudiesandothers(e.g.[34]),alsoshowthatgra-
minoidshaveincreasedmoreinabsoluteabundancethanforbs,especiallyinsiteswithhigh
ambientsitetemperatureandsoilmoisture[30],andforbsdecreaseinabsoluteabundancein
thewarmestsites[30].Graminoidsarebettercompetitorsfornutrientsandlightthanmost
forbs[35],andconsequently,thedeclineofforbsmaybeduetobioticinteractionsratherthan
adirectclimateeffect.Thissuggeststhatclimatewarmingand/orincreasedprecipitationmay
directlybenefitgraminoids,whereasbioticinteractionscausedbygraminoiddominance,may
decreaseforbabundance.
Toseparatetheeffectsoftemperatureandmoistureontherelativeimportanceofbioticand
abioticfactors,weestablishedtwelvestudysitesalongnaturaltemperatureandprecipitation
gradientsinsouthernNorway,suchthatthesetwomainclimatevariablesvariedindependently
(Table1)andallfactorsotherthanclimatewereasidenticalaspossible;includingvegetation
type,bedrock,aspect,slope,andlanduse[36].Withineachsiteinthisregional‘climategrid’,
wesampledvegetationandassociatedbioticandabioticenvironmentalconditionsonafine-
scale.
Weusedthesedatatotestthefollowinghypotheses:1)Thecommunityresponsetolocal-
scalevariationinbioticandabioticvariablesisnotconsistentacrossbroad-scalebioclimatic
gradients.2)Thelocalabioticenvironmentaccountsformorevariationinspeciescomposition
instressfulandunproductivealpineanddrysitesthaninthemorebenignandproductivebo-
realandmesicsites.3)Bioticvariablesaccountformorevariationinspeciescompositionin
warmerandwettersites.4)Forbsaremorestronglyaffectedbybioticinteractionsthangrami-
noids.Weconcludebydiscussingtheimplicationsoftheseresultsforgrasslandspeciescompo-
sitionunderwarmerandwetterfutureclimates.Ourresultsshowthattheimportanceof
abioticvsbioticvariablesforspeciescompositionshiftalongthebroadscaletemperatureand
precipitationgradientsandweproposethatbothlocalandregionalscalesanalysesiscrucialto
beabletodetectanddisentanglethedifferentdriversoflocalvegetation-
environmentrelationships.
MaterialsandMethods
Thestudywasconductedattwelvesitessituatedfromthecontinentaleasttotheoceanicwest
andfromthealpinetotheborealclimaticzonesinthefjordlandscapeofsouthernNorway.
Thesitesaredistributedacrossauniqueclimategridwiththreelevelsofsummertemperature
(meanoffourwarmestmonths)replicatedatfourlevelsofmeanannualprecipitation(Fig1,
Table1).Theclimatedataareinterpolatedwith100mresolution[37]fromthenormalperiod
1961–1990[38].Thesiteswereselectedtobeassimilaraspossiblewithrespecttoallfactors
otherthanclimatetofacilitatecomparisonsbetweensites.Thesitesareallgrasslandsassociated
withcalcareousbedrock.Mostofthesitesaresouth-westexposedslopesofcatwentydegrees
inclination,exceptone(Boreal3)thatiseastexposed.Allsitesaremoderatelygrazed.Geo-
graphicaldistancebetweensitesisonaverage15kmandrangesfrom175km(Boreal1andBo-
real4)to650m(Boreal2andSubalpine2;thesearealso400ma.s.l.apart).Withinsites,all
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Bioticvs.AbioticDriversofPlantCommunityComposition
Table1. Altitude,annualprecipitation,summertemperature,numberofplotssampledandmean±standarddeviationsofpredictorvariablesatal-
pine,subalpine,andborealgrasslandsitesalongprecipitationgradients(low[1]tohigh[4])insouthernNorway.
Site Sitename Altitude Precip Temp No pH LOI Moisture Vegetation Litter Bryophyte
(ma.s.l.) (mm) (°C) plots (%) (%) height(cm) cover cover(%)
(%)
ALPINE1 Ulvhaugen 1208 596 6.17 20 5.7 7.9 37.5±5.7 3.6±1.1 17.6±7.1 13.0±13.7
±0.2 ±2.4
ALPINE2 Låvisdalen 1097 1321 6.45 20 5.9 8.2 40.0±5.4 3.5±1.3 12.4±3.4 17.1±8.5
±0.2 ±2.7
ALPINE3 Gudmesdalen 1213 1925 5.87 19 6.1 15.3 50.8±6.9 4.6±2.2 15.0±5.5 20.4±25.4
±0.2 ±4.3
ALPINE4 Skjellingahaugen 1088 2725 6.58 10 6.1 7.1 47.6±6.6 4.1±0.9 12.4±6.9 28.0±16.9
±0.6 ±3.8
SUB-ALPINE Ålrust 815 789 9.14 20 5.4 9.1 33.1±5.2 3.3±1.2 11.3±4.1 17.1±17.1
1 ±0.2 ±3.1
SUB-ALPINE Høgsete 700 1356 9.17 20 5.2 12.9 36.5±1.5 8.9±2.8 13.2±6.2 34.9±14.5
2 ±0.2 ±0.8
SUB-ALPINE Rambæra 769 1848 8.77 18 5.4 10.0 37.5±7.4 5.3±2.5 11.4±4.0 29.9±17.8
3 ±0.3 ±3.2
SUB-ALPINE Veskre 797 3029 8.67 10 5.8 13.4 50.2±3.8 4.1±1.2 8.2±3.2 25.4±17.2
4 ±0.2 ±2.5
BOREAL1 Fauske 589 600 10.3 10 5.3 10.7 34.8±3.0 9.4±2.6 15.3±5.5 9.7±6.9
±0.1 ±1.4
BOREAL2 Vikesland 474 1161 10.55 10 5.2 14.7 35.0±2.8 8.6±4.1 16.1±7.5 50.1±20.1
±0.2 ±2.1
BOREAL3 Arhelleren 431 2044 10.60 10 5.2 10.1 43.8±2.0 15.9±5.2 10.2±2.8 41.9±22.9
±0.1 ±1.3
BOREAL4 Øvstedal 346 2923 10.78 5 5.3 10.1 31.5±5.7 11.9±2.8 9.1±4.1 40.7±22.8
±0.2 ±3.7
LOIreferstoLoss-On-Ignition,ameasureofsoilorganicmatter.
doi:10.1371/journal.pone.0130205.t001
Fig1.Mapandstudydesign.Locationofthetwelvestudysitesalongtemperatureandprecipitation
gradientsinthefjordlandscapeofsouthernNorway.
doi:10.1371/journal.pone.0130205.g001
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Bioticvs.AbioticDriversofPlantCommunityComposition
plotsweresituatedintheselectedgrasslandwithin5blockswithatotalareaofca.75–200m2.
AlltheplantcommunitiesarewithintheplantsociologicalassociationPotentillo-Festucetum
ovinae[39].Inthealpine,thistypetendstowardsPotentillo-Poligoniumvivipari,andinsome
ofthelowlandsites,theytendtowardsNardo-Agrostiontenuis[39].
Vegetationsamplingwasconductedin25x25cmplotsrandomlypositionedwithinfive
blocksateachsite.Thenumberofplotsdiffersbetweensitesduetothedesignofatransplant
experimentconductedbetweenthesites(Table1).Weestimatedpercentagecoverofallvascu-
larspecies,andthetotalcoverofbryophytes,litter,andbaresoilineachplot.Weusedaruler
atfourfixedpointsineachplottomeasurethemeanheightofthevegetation.Allofthesevari-
ablesareimportantproxiesfortheintensityofpositiveaswellasnegativebioticinteractions
amongplantsinborealandalpinegrasslands.Heightofthevascularvegetationmayindicate
theintensityofcompetitionforlight.Increasingvascularcovermayalsoindicateincreasing
competition(forlightorbelowground),butunderharshenvironmentalconditionsincreasing
coverhasalsobeenshowntofacilitaterecruitment[40,41]andplantgrowth[35].Bryophytes
maylimitseedlingemergenceandgrowthbylimitingaccesstosoil,shadingorallelopathicin-
teractions[15,42],limitgrowthofthevascularvegetation[43],orfacilitateotherspeciesdue
totheirwaterholdingcapacities[44].Similarly,littercovermaylimitplantrecruitmentby
shading,actasaphysicalbarriertoseedlingsandshoots(e.g.[45,46],orfacilitateplantgrowth
byprovidingnutrientsthroughdecompositionorbyamelioratingdroughtortemperatureex-
tremes[47].Allthesitesareonnon-protectedprivatelyownedland,andpermitsfordoing
fieldsamplinghasbeengivenfromthetwelvelandowners.Noprotectedspeciesweresampled.
ThesampledvegetationdataaredepositedintheNordicvegetationdatabase(NVD,http://
www.givd.info/ID/EU-00-018)[48].
Toquantifythelocalabioticenvironment,wemeasuredsoilpH,moisture,andorganiccon-
tent(proxyfornutrientavailability)ineachplot.Theseareimportantdeterminingfactorsfor
plantspeciescompositionanddiversityinthissystem[24,40].Wewereabletotakethesoil
samplesfromdirectlyunderneaththesampledvegetationineachoftheplots,because,after
thevegetationwassampled,allplotsweredugupandtransplantedtoanewsiteforanotherex-
periment.Thesoilsampleswerestoredcoldandbroughttothefreezerdirectlyaftersampling.
Beforeanalysesthesoilwasthawedandputthroughatwommsieve.SoilpHwasmeasured
afteradding50mldistilledwaterto25gsoilandmixingfortwohours.Tomeasurewatercon-
tent,thesoilwasweighed,andthendriedat105°Cfor24hoursandleftonehourinadesicca-
torbeforeweighingagain.Loss-on-ignition(LOI)wasusedtoestimatesoilorganicmatter;the
soilwasburntforanothersixhoursat550°Candthenleftonehourinthedesiccatorbefore
weighing.WealsomeasuredsoilmoistureinthefieldbyusingaSM200Soilmoisturesensor
(Delta-TDevicesLtd,UK)atfourfixedpointsperplotandcalculatedtheaverage.Soilmois-
turewaspositivelycorrelatedbetweenthesoilsampledfromthefieldandthedirectfieldmea-
surements,andtoobtainmoresamplingpointsonatimescale,weusedtheaverageofthesoil
sampleandfieldmeasurementsintheanalyses.
Statisticalanalyses
Todeterminewhetherthedesignofthegridwithorthogonaltemperatureandprecipitation
gradientscorrespondedtoclearvegetationpatternswithrespecttothesetwofactors,wefirst
examinedtheregional-scalepatterninvegetation-environmentrelationshipsbymeansofaca-
nonicalcorrespondenceanalysis(CCA;[49])withallsites,climatevariables,andlocalenviron-
mentalvariablesincluded.Unimodal-basedmethodswerechosenbecausethegradientsinthe
overallcompositionaldatawererelativelystrong(gradientlengthaxis1=3.69,axis2=2.5,
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Bioticvs.AbioticDriversofPlantCommunityComposition
axis3=3.32,axis4=2.30SD,assessedbydetrendedcorrespondenceanalysiswithdefault
options).
Wethenpartitionedthecommunitydataintograminoidandforbspeciescompositionand
conductedaseriesofanalysesonthesetwodatasetstodeterminetherelativeimportanceofthe
bioticandabioticvariablesforlocalcommunitycompositionwithineachdatasetalongthe
bioclimaticgradients.
First,astandardhierarchicalvariationpartitioning[22],seealso[23]wasusedtoassessthe
overallimportanceofthedifferentgroupsofexplanatoryvariablesonregional(among-site)
andlocal(within-site)scales.Thiswasdoneinthreesteps:(1)Weassessedthetotalvariation
accountedforbyeachlocalabiotic(soilmoisture,pH,LOI)orbiotic(vegetationheightand
cover,bryophytecover,littercover,baresoil)variable.Forthevariable‘vegetationcover’,the
sumofallforbspecies’coverswasusedasapredictorforthegraminoiddataset,andthesumof
allgraminoidspecies’coverswasusedasapredictorfortheforbdataset.Similarly,wequanti-
fiedthevariationexplainedbyeachofthethetwogroups(abiotic,biotic)whereonlysignifi-
cant(p<0.05)variables,asassessedbyforwardselection,wereretained.(2)Foreachofthe
analysesabove,variationatthewithin-siteorlocalscalewasquantifiedbyrunningtheanalyses
describedabovewithdummyvariablesrepresentingallthetwelvesitesasco-variablesinpartial
CCAs.(3)Variationattheamong-siteorregionalscalewascalculatedbysubtractingthis
local-scalevariation(calculatedinstep2)fromthetotalvariationexplainedbythatvariableor
group(calculatedinstep1).
Second,totestwhethervariationexplainedbybioticandabioticvariablesdifferedamongthe
12localsites(i.e.,hypothesis1)weconductedaseriesofsite-wiseordinationanalyses.Asitcould
beeasiertopickupbioticorabioticdrivenvariabilityinmoreheterogeneousdatasets,weused
multivariatedispersiontostatisticallycomparethemagnitudeofwithin-siteheterogeneityin
speciescompositionamongsitesformultivariatespeciescompositionaldata[50].Thisanalysis
showsthatthewithin-sitecompositionalvariabilityiscomparableinmagnitudeamongsites
(multivariatedispersion:0.37±0.04),andanydifferenceswereunrelatedtothebroad-scale
temperatureandprecipitationgradients(temperature;r2=0.05;precipitation:r2=0.02).The
speciescompositionwithineachsitewasrelativelyhomogenous(gradients<2.5SD,assessedby
detrendedcorrespondenceanalyses),andwethereforeusedredundancyanalysis(RDA;[51])to
assesstheproportionofvariationingraminoidandforbspeciescompositionexplainedbyabiotic
andbioticfactorsforeachofthesites.SiteBoreal4wasremovedfromthedatabeforeanalyses
duetolowsamplesize.Foreachofthe11remainingsites,wethenestimatedthevariationinthe
graminoidandforbspeciescompositionexplainedbyeachexplanatoryvariablebothseparately
(8variables,graminoidsandforbcommunity,11sites;totalof176ordinationanalyses)and
jointlyforthetwogroupsofexplanatoryvariables(bioticvsabiotic;twocommunities,11sites
foratotalof44ordinationanalyses).Inthegroup-wiseordinationanalysesweusedforwardse-
lectionwithineachdataset,andincludedonlysignificantvariables(p<0.1)ineachofthetwo
groups,or,ifnoneweresignificant,thebestpredictorineachgroup,inthefinalanalyses.We
usedpresence/absencedataanddownscalingofrarespeciesinallordinations,which,alongwith
theforwardselections,wereconductedinCanoco5[52].
Weanalysedtheoutputfromthesesite-wiseordinationanalysestotestwhethertherelative
importanceofbioticvs.abioticvariablesindeterminingspeciescompositionchangesinconsis-
tentwaysalongtemperatureandprecipitationgradients(hypothesis2,3),orvariesbetween
thefunctionalgroups(graminoids,forbs;hypothesis4).Wefirstfittedsimplelinearregressions
toassessifthevariationinforborgraminoidspeciescompositionexplainedbyeachofthe
localenvironmentalvariables(individualbioticandabioticvariables,quantifiedinthesite-
wiseanalysesdescribedabove)variedsystematicallyalongtheregional-scaletemperatureand
precipitationgradients.Wethentestedtherelativeexplanatorypowerofbioticandabiotic
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Bioticvs.AbioticDriversofPlantCommunityComposition
factorsforlocalcommunitypatternsmoreformallyinananalysiswheretheproportionofthe
totalexplainedvariationaccountedforbythebioticvariablesintheRDAanalyseswereusedas
responsevariableinanANOVAwithtemperature(alpine,subalpine,boreal),precipitation
(levels1-4),orfunctionalgroup(forborgraminoid),andtheirinteractionsasexplanatoryvari-
ables.Proportionaldatawereusedintheseoverallmodelstoavoidbiasesduetomethodologi-
calartefactscausingdifferentfractionsofthetotalvariationexplainedamongdatasets[53].
TheseanalyseswereperformedinRversion2.15.3[54]usingRStudioversion0.96.331(RStu-
dio,Inc.,Boston,Massachusetts,USA)andthecarpackage[55].
Results
Thecanonicalcorrespondenceanalysis(CCA)showsthatthemainpatternsinplantcommu-
nitycompositioninthefulldatasetreflectthetemperature(axis1)andprecipitation(axis2)of
thesitesdistributedwithintheclimategridinsouthernNorway(Fig2).Thesitemeanbiotic
andabioticenvironmentalvariablesarealsorelatedtothesemajorgradients.Asexpected,veg-
etationheightincreasestowardswarmerclimates,soilmoistureincreaseswithprecipitation,
whereaspHishigherinalpinesites.Interestingly,graminoidcoverandrichnessincreasewith
precipitationandsoilmoisture,whereasforbcoverisnegativelycorrelatedwiththesevariables
andwiththegraminoids(Fig2).Baresoil,indicatingdisturbanceorbadconditionsforplant
growth,ismostcommonindryalpinesites,andhighlycorrelatedwithforbspeciesrichness.
Bryophytecoverontheotherhand,isnegativelycorrelatedwithforbspeciesrichness,and,to-
getherwithsoilorganiccontent(LOI),highesttowardswarmerandwettersites.
Fig2.Overallordinationofthegrid.Canonicalcorrespondenceanalysis(CCA)ofmeasuredbiotic(green)
andabiotic(black)environmentalvariablesandalpine(▲),sub-alpine(●)andboreal(▼)sitesalonga
precipitationgradient(1–4fromlighttodarkblue,seeFig1)insouthernNorway.LOIreferstoLoss-On-
Ignition,ameasureofsoilorganicmatter.Eigenvaluesaxis1=0.472,axis2=0.248,axis3=0.182,axis
4=0.137.Onlythetwofirstaxesareshown.
doi:10.1371/journal.pone.0130205.g002
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Bioticvs.AbioticDriversofPlantCommunityComposition
Theseregional-scalepatternsarereflectedintheamong-sitevariationcomponentsinboth
thegraminoidandtheforbspeciescomposition(Fig3A).Vegetationheight,soilmoistureand
pHarethemostimportantvariablesforbothfunctionalgroups,withsoilmoisturesomewhat
moreimportantforgraminoids,andpHmoreimportantforforbs.Thelowexplanatorypower
ofallenvironmentalvariablesatthelocalscale(Fig3B;abioticvariablesaverage3.17%±1.38
forgraminoids,2.82±0.66forforbs;bioticaverage1.20%±0.38forgraminoids,1.69±0.42
forforbs)suggeststhateitherthemeasuredvariablesarenotimportantforspeciescomposition
withinsites,orthatthecommunityresponsestothesevariablesdifferbetweensites.
Todistinguishbetweenthesepossibilities,vegetation-environmentrelationshipswereana-
lysedseparatelyforeachsite.Theexplanatorypowerofbothbioticandabioticvariablesfor
localcompositionalpatternsincreasessubstantiallyinthesite-wiseanalysesrelativetothe
analysesontheentiredataset(Fig3C;abioticvariablesaverage13.83%±0.93forgraminoids,
11.69±1.00forforbs;bioticaverage8.85%±2.58forgraminoids,9.78±1.96forforbs,across
sites).Theregressionslopesofthelinearmodelsindicatethatthislocalexplanatorypowervar-
iessystematicallybetweenbioticandabioticvariables,alongtheclimategradients,andbetween
forbsandgraminoids(Table2).Theexplanatorypowerofindividuallocalbioticvariables
changesmoreconsistentlyinresponsetotemperaturethantheabioticvariables:Allbioticvari-
ableshaveconsistentlypositive(ifrarelystatisticallysignificant)regressionslopesandthus
likelyexplainmoreofthevariationinbothforbandgraminoidspeciescompositiontowards
warmersites(Table2).Asexpected,forbshavesteeperregressionslopesforthebioticvariables
thangraminoids.Thepatternsalongtheprecipitationgradientarelessclear,butsoilpHex-
plainsmoreofgraminoidspeciescompositiontowardswettersites(Table2).TheANOVAre-
sultsconfirmthesepatterns–theproportionofthetotalexplainedvariationinthesite-wise
RDAsexplainedbythebioticvariables(relativetotheabiotic)increasessystematicallywithin-
creasingtemperature,andasignificantinteractionwithprecipitationsuggeststhattherelative
roleoflocalbioticandabioticenvironmentalvariablesshiftsalongthebroadscaleclimategra-
dients(Table3,Fig4).Moreover,bioticvariablesseemtoexplainmoretowardswettersubal-
pineandborealsites,butmoretowardsdryersitesinthealpine(Table3,Fig4).
Fig3.Variationexplainedatbroadandfinegeographicalscales.Compositionalvariationingraminoidandforbspecies(%oftotaldeviance[CCA]or
variation[RDA])explainedbyabioticandbioticenvironmentalvariables,groupedandseparately,atthea)regional/amongsitescaleandb)local/withinsite
scalewhenallsitesarepartofthesamecanonicalcorrespondenceanalysis(CCA).Partc)showsparallelresultsforseparatesite-wiseredundancy
analyses(RDA)analysesforeachofelevengrasslandsitesinsouthernNorway.LOIreferstoLoss-On-Ignition,ameasureofsoilorganicmatter.
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Bioticvs.AbioticDriversofPlantCommunityComposition
Table2. Regressionslopesandassociatedp-values(*p<0.05,·p<0.1,nosymbol=notsignificant)ofrelationshipsbetweenvariationinforb
andgraminoidspeciescompositionexplainedbylocalabioticandbioticenvironmentalvariablesinredundancyanalyses(RDA)oflocalgrass-
landspeciescompositionalongbroadscaletemperatureandprecipitationgradientsinsouthernNorway.
Temperature Precipitation
Forbs Graminoids Forbs Graminoids
Abioticvariables -3.153* -1.852 +0.339 +0.596
pH -0.067 -0.859 +0.323 +0.521*
LOI +0.072 +0.857 +0.078 +0.095
Moisture +0.411 -0.898 -0.136 +0.347
Bioticvariables +0.254 +1.191 +0.215 -0.056
Vegetationheight +1.318 +1.227 +0.196 +0.109
Littercover +1.222· +0.978 +0.164 -0.107
Bryophytecover +1.040· +1.025 +0.099 +0.029
LOIreferstoLoss-On-Ignition,ameasureofsoilorganicmatter.n=11.
doi:10.1371/journal.pone.0130205.t002
Discussion
VegetationandenvironmentalanalysesofalpineandborealgrasslandsofsouthernNorway
showthattheabioticandbioticdriversoflocal-scalevariationinplantcommunitycomposi-
tionshiftalongtheregional-scaletemperatureandprecipitationgradients.Althoughthepat-
ternsarenotstrong,ourresultstosomeextentsupportthepredictionsofadecreasingroleof
abioticandanincreasingroleofbioticenvironmentalvariablesforspeciescompositionto-
wardswarmerclimates(e.g.[4,5,3]).Interactionsbetweentemperatureandprecipitationindi-
cate,however,thattheimportance(asmeasuredbyvariationexplained)ofbioticvs.abiotic
variablesvariesincomplexways,insomecasesdrivenbythespecificbioticorabioticvariable.
Theshiftbetweenlocalabioticandbioticdeterminantsofspeciescompositiontowards
warmersitesreflectstheproductivitygradientintheregion,withplantcommunitybiomass
Table3. ANOVAestimates,F-andP-valuesoftherelativeproportionoftheexplainedvariationaccountedforbybiotic(vsabiotic)variablesinre-
dundancyanalyses(RDA)ofgraminoidandforbspeciescompositioningrasslandsitesalongtemperatureandprecipitationgradientsinsouth-
ernNorway.
Predictors df Estimates F P
Intercept 43.45
Temperature 2 6.91 0.011
Sub-alpine -4.99
Boreal 17.54
Precipitation 3 2.71 0.096
2 4.05
3 -5.44
4 -19.55
Temperature×Precipitation 5 3.57 0.037
Sub-alpine×2 -7.75
Boreal×2 -10.73
Sub-alpine×3 43.12
Boreal×3 5.56
Sub-alpine×4 29.94
Temperatureandprecipitationareexpressedfactorialvariables(threetemperaturelevels:alpine,sub-alpine,boreal,andfourprecipitationlevels1–4).
Functionaltype(graminoidvsforb)anditsinteractionswerenotsignificantandthereforenotincludedinthefinalmodel.n=22.
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Bioticvs.AbioticDriversofPlantCommunityComposition
increasingfromalpinetoborealsites[56].Thus,ourresultsareinlinewiththepredictions
thatinthealpine,wherevegetationcoverissparser,theimpactsoftheabioticenvironmentis
mostimportantforspeciescomposition,whereasinthelowlandswhereplantbiomassis
higher,effectsofthebioticenvironmentaremoreprevalent.Nevertheless,althoughtheroleof
localbioticenvironmentalvariablesslightlyincreasedtowardswarmerandmoreproductive
sitesinourstudyarea,precipitationgradientsshowednoconsistenttrendandeventhetem-
peraturepatternwasrelativelyweakandnoisy.Wesuggestthatthisisbecausebioticinterac-
tionsrepresenttheneteffectsofbothfacilitativeandcompetitiveprocesses,withtheroleof
competitionpredictedtodecrease,andfacilitationtoincrease,towardsstressfulenvironments
(thestressgradienthypothesis;[57,13,16]).Althoughourstudycannotdistinguishbetween
competitiveandfacilitativeprocesses,ourresultssuggestthatbioticinteractionsareimportant
forplantcommunitystructurealsoincold,low-productivitysystems.Competitiveinteractions
havebeenshowntoaffectrecruitmentalsoinalpine[58,59,41]andarctic[15]plantcommu-
nities.Inlinewiththesefindings,otherstudiesinourclimategridhavealsoshownthatthe
vegetationcanopylimitsseedlingrecruitmentbothinalpineandborealsites,althoughlesspro-
nouncedinthealpine[60].Ontheotherhand,anintactvegetationcanopy,andadenselitter
andbryophytecover,mayalsofacilitateplantrecruitmentandgrowthintheclimaticallysevere
alpinesitesbyproviding,shelter,nutrientsormoisture[13,16,18].
Thebioticvariablestendedtoexplainahigherproportionofthevariationinforbthanof
graminoidspeciescompositiontowardswarmersitesinthestudyarea.Forbsmaybemoresus-
ceptibletobioticinteractionsthangraminoids,andthenegativecorrelationintheoverallordi-
nationinFig1betweenforbcoverandgraminoidsalsoindicatesacompetitiverelationship
betweenthetwofunctionalgroups,whichisinlinewithresultsfromagraminoidremovalex-
perimentperformedinthesamegridofsites.Thismaysuggestthatthepresenceofgraminoids
inthecommunitycandecreaseforbcover,inparticularwhenprecipitationandsoilmoisture
increase[56].Increasedabundanceofgraminoidsattheexpenseofforbsisfoundinexperi-
mentswherewarmingandnutrientadditiongenerallybenefitgraminoidsmorethanforbs[61,
62,63,36].Thisislikelybecausegraminoidsrespondfasterthanmostoftheforbstothenutri-
entsadded[62],andtherebyout-competethelow-statureforbsforspaceandlightbytheir
higherstandingbiomass.Inadditiontotheheightofthevegetation,litterandbryophytecover
Fig4.Variationexplainedbyabioticvs.bioticvariables.Relativevariationinspeciescompositionexplainedbyabiotic(black)andbiotic(grey)
environmentalvariablesinsite-wiseredundancyanalyses(RDA)performedacrosselevengrasslandsitesinalpine,subalpineandborealsitesalonga
precipitationgradientfromlow(1)tohigh(4)insouthernNorway.LOIreferstoLoss-On-Ignition,ameasureofsoilorganicmatter.
doi:10.1371/journal.pone.0130205.g004
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