J Veg Sci. 2018;1–11. wileyonlinelibrary.com/journal/jvs  | 1

Journal of Vegetation Science

© 2018 International Association for Vegetation Science

Received:19June2017  |  Accepted:2December2017DOI:10.1111/jvs.12601

S P E C I A L F E A T U R E : V E G E T A T I O N H I S T O R Y

The role of climate, forest fires and human population size in Holocene vegetation dynamics in Fennoscandia

Niina Kuosmanen1,2  | Laurent Marquer3,4 | Miikka Tallavaara2 | Chiara Molinari3 |  Yurui Zhang2,5 | Teija Alenius6 | Kevan Edinborough7 | Petro Pesonen8 |  Triin Reitalu9 | Hans Renssen5, 10 | Anna-Kari Trondman11 | Heikki Seppä2

SpecialFeature“Millennialtocentennialvegetationchange”(Eds.ThomasGiesecke,PetrKuneš&TriinReitalu).

1DepartmentofForestEcology,FacultyofForestryandWoodSciences,CzechUniversityofLifeSciencesPrague,Praha6,CzechRepublic2DepartmentofGeosciencesandGeography, UniversityofHelsinki,Helsinki,Finland3DepartmentofPhysicalGeographyandEcosystemScience,LundUniversity,Lund,Sweden4GEODE,UMR-CNRS5602,UniversitéToulouseJeanJaurès,Toulouse,France5DepartmentofEarthSciences,VUUniversityAmsterdam,Amsterdam,TheNetherlands6DepartmentofPhilosophy,History,CultureandArtStudiesArchaeology,UniversityofHelsinki,Helsinki,Finland7InstituteofArchaeology,UniversityCollegeLondon,London,UK8ArchaeologicalFieldServices,NationalBoardofAntiquities,Helsinki,Finland9InstituteofGeology,TallinnUniversityofTechnology,Tallinn,Estonia10DepartmentofNaturalSciencesandEnvironmentalStudies,UniversityCollegeofSoutheastNorway,Telemark,Norway11DepartmentofBiologyandEnvironmentalScience,LinnaeusUniversity,Kalmar,Sweden

CorrespondenceNiinaKuosmanen,DepartmentofForestEcology,FacultyofForestryandWoodSciences,CzechUniversityofLifeSciencesPrague,Praha6,CzechRepublic.Email:kuosmanen.niina@gmail.com

Funding informationPEDECO;CzechScienceFoundation(GAČR);EBOR;AcademyofFinland

Co-ordinatingEditor:ThomasGiesecke

AbstractQuestions:Weinvestigatedthechangingroleofclimate,forestfiresandhumanpopu-lationsizeinthebroad-scalecompositionalchangesinHolocenevegetationdynamicsbeforeandaftertheonsetoffarminginSweden(at6,000calyrBP)andinFinland(at4,000calyrBP).Location:SouthernandcentralSweden,SWandSEFinland.Methods:HoloceneregionalplantabundanceswerereconstructedusingtheREVEALSmodelonselectedfossilpollenrecordsfromlakes.Therelativeimportanceofclimate,firesandhumanpopulationsizeonchangesinvegetationcompositionwasassessedusing variationpartitioning. Past climate variablewasderived from the LOVECLIMclimatemodel. Fire variablewas reconstructed from sedimentary charcoal records.Estimatedtrendinhumanpopulationsizewasbasedonthetemporaldistributionofarchaeologicalradiocarbondates.Results:Climateexplainsthehighestproportionofvariationinvegetationcomposi-tionduringthewholestudyperiodinSweden(10,000–4,000calyrBP)andinFinland(10,000–1,000calyrBP),andduringthepre-agriculturalperiod.Ingeneral,firesex-plainarelatively lowproportionofvariation.Humanpopulationsizehassignificanteffectonvegetationdynamicsaftertheonsetoffarmingandexplainsthehighestvari-ationinvegetationinSSwedenandSWFinland.Conclusions:Mesolithichunter-gathererpopulationsdidnotsignificantlyaffectveg-etationcompositioninFennoscandia,andclimatewasthemaindriverofchangesatthat time.Agricultural communities,however,hadgreatereffectonvegetationdy-namics,andtheroleofhumanpopulationsizebecameamoreimportantfactorduringthelateHolocene.Ourresultsdemonstratethatclimatecanbeconsideredthemaindriveroflong-termvegetationdynamicsinFennoscandia.However,insomeregionsthe influenceofhumanpopulation sizeonHolocenevegetationchangesexceededthatofclimateandhasalongevitydatingtotheearlyNeolithic.

K E Y W O R D S

climate,fire,humanpopulationsize,pollen,REVEALSplantabundance,variationpartitioning

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1  | INTRODUCTION

Pollen-based reconstructions of Holocene vegetation dynamicshavedemonstratedtheinfluenceofanthropogenicactivityonfor-estcompositionandlandscapeopennesssincethemid-HoloceneinCentralandNorthernEurope(Fyfe,Woodbridge,&Roberts,2015;Marquer etal., 2014, 2017). Fennoscandian pollen data demon-stratetheregionaldifferencesinforestcompositionthroughouttheHolocene, and thisvariation has been generally connected to thechangesinclimate(Birks,1986;Heikkilä&Seppä,2003;Milleretal.,2008).Inaddition,fireisanimportantdisturbancefactorinborealforests that profoundly affects forest age, structure, compositionandsuccessiondynamics (Bradshaw,Lindbladh,&Hannon,2010).AlthoughMesolithichunter-gatherersmayhavealteredlocal-scalevegetation composition through fires and favouring food plants(Hörnbergetal.,2005;Regnell,Gaillard,Bartholin,&Karsten,1995),the regional scalevegetation change in Fennoscandia presumablyremained under natural drivers longer than in other European re-gions.Thecurrentintensiveanthropogenicinfluenceandpredictedacceleratedwarming of the boreal biome (Christensen, Kumar, &Aldrian,2013)maycausecompositionalandstructuralchangestothe presentvegetation.Therefore, understanding the complex in-teractionsbetweennatural-andhuman-inducedchangesinthepastregionalvegetationdynamics can shed light on the future effectsofachangingclimateonecosystemsthatareheavilyinfluencedbyhumanactivity.

The strongest effect of human impact on vegetation inFennoscandia is connected to the onset of agriculture, which wasoftenaccompaniedby increasedhuman-induced fires (Granström&Niklasson 2008; Huttunen, 1980). The earliest signs of agriculturearefoundatca.6,000calibratedyearsbeforepresent(calyrBP)inSSweden(Sørensen&Karg,2012;Welinder,2011).InSWFinlandthefirstunambiguousevidenceofcultivationisdated4,000–3,500calyrBP (Lahtinen,Oinonen,Tallavaara,Walker, & Rowley-Conwy, 2017;Taavitsainen,Simola,&Grönlund,1998).InEFinland,earliestsignsofcereal cultivation inpalynological recordsare reportedalready fromthe early Neolithic ca. 6,400–5,200cal yr BP (Alenius, Mökkönen,Holmqvist, & Ojala, 2017; Alenius, Mönkkönen, & Lahelma, 2013),whereasfirstarchaeologicalevidenceofcultivation,alongsideincreas-ingpalynologicalevidence,isdetectedafter3,200calyrBP(Lavento,2001;Taavitsainenetal.,1998).

Studies concerning the role of human impact and climate onvegetation changes during themid- and lateHolocene inNEuropehavebeenmainlyqualitative,basedon interpretationofpollendataorarchaeologicalfindings.Recently,Reitaluetal. (2013),Kuosmanenetal. (2016)andMarqueretal. (2017)haveaddressed thisquestionbymeansofvariationpartitioning,usingdifferentproxiestoassesstheroleplayedby the anthropogenic influence.Reitalu etal. (2013) de-rived thehuman impactvariable frompollen records,Marqueretal.(2017) used the anthropogenic land-cover change scenario (ALCC;Kaplanetal.,2010),whileKuosmanenetal. (2016)utilizedahumanpopulation sizeproxy,basedon the temporal frequencydistribution

ofradiocarbon-datedarchaeologicalfindings(Lechterbecketal.,2014;Woodbridgeetal.,2014).Thisapproachassumesacorrelationbetweentheamountofarchaeologicalmaterialatanyparticulartimepointandthehumanpopulationsizeatthattime(Shennan&Edinborough,2007;Tallavaara,Pesonen,&Oinonen,2010).Anadvantageofthisproxyisitsindependencefromthereconstructedvegetationdynamics.

Here we employ this independent proxy of human populationsize together with climate and regional fires to statistically assesstheir relative importance on the variation in the Holocene vegeta-tioncomposition inFennoscandiaduring three timeperiods: (1) thewholestudyperiodinSweden(10,000–4,000calyrBP)andFinland(10,000–1,000calyrBP); (2)beforetheonsetoffarminginSweden(at10,000–6,000calyrBP)andinFinland(at10,000–4,000calyrBP);and (3)after theonsetof farming inSweden (at6,000–4,000calyrBP)andinFinland(at4,000–1,000calyrBP).ThelengthofthestudyperiodsbetweenSwedenandFinlanddiffersduetothelimitationsofdataonhumanpopulationsize.

2  | METHODS

2.1 | Study area

ThestudyareaislocatedinFennoscandiaandisdividedintofourre-gions;(1)southern(S)Sweden,(2)central(C)Sweden,(3)southwest(SW)Finlandand (4)southeast (SE)Finland (Figure1).Thenorthernlimitof thestudy region inSweden is locatedat61.5°N,while theborderbetweenSandCSwedenisat58.3°N.InFinland,thenorth-ern limitof thestudy region isat62.5°N,and theborderbetweenSWandSEFinland isat25.5°E (alongLakePäijänne).Thesestudyregionswerechosenbecausetheavailabilityofahighamountofdataforpollen-basedREVEALSestimates,humanpopulationsizedataandcharcoaldata.

2.2 | Regional plant abundances

Weusedpollen records from33 lakes selected from theEuropeanPollen Database (Fyfe etal., 2009; Giesecke etal., 2014), theLANDCLIMpollendataarchive(Marqueretal.,2017;Trondmanetal.,2015,2016)orprovideddirectlybydatacontributors(AppendixS1).An important criterion for the selected pollen records was robustchronological control and adequate Holocene time resolution. TheREVEALSmodel(Sugita,2007)wasrunseparatelyforeachstudyre-gionusingfivepollenrecordsforCSweden,17forSSweden,fiveforSEFinlandandsixforSWFinland(Figure1).

Withineachregion,theREVEALSmodelwasusedtoconvertpol-lenpercentagesofthe23mostimportantpollentaxaintoproportionalvegetationcover(AppendixS4)for200-yeartimewindowsfollowingMazieretal.(2012),Trondmanetal.(2015)andMarqueretal.(2017).Descriptions ofHolocene changes invegetation cover are depictedinFigure2andAppendixS4.Thestudyperiodcovers10,000–1,000calyrBPcorrespondingwiththetimeofavailablehumanpopulationsizedata.Additionallytotaxa-specificREVEALSestimates,nineplant

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functionaltypes(PFTs;AppendixS5),groupedintermsofbioclimaticlimits afterTrondman etal. (2015),were defined in order to assessbroad-scaletrendsindifferentplantecosystemsduringtheHolocene.

2.3 | Explanatory variables of Holocene vegetation changes

2.3.1 | Climate

Theclimatevariables for thestudy regionswereobtained fromtheLOVECLIMclimatemodelaspresentedbyZhang,Renssen,andSeppä(2016).Theversionappliedinthisstudyexplicitlyrepresentscompo-nents of the atmosphere, oceans (including sea ice) and vegetationin the climate system with intermediate complexity (Goosse etal.,2010).Thespatialresolutionofthemodelis5.6°×5.6°andthreegridcellswereused toprovide regional temperaturedata for the studyregions.SandCSwedenfellintothesamelatitudinalband,andthusthe climatedata are the same for these regions.The climate varia-bles includewinterDecember–January–February (DJF)andsummer

June–July–August (JJA)temperaturesexpressedas30-yearaverageand as anomalies from the preindustrialmean (550–250cal yrBP).Climateparameterswereaveragedover200-yearbinsforthestatisti-calanalyses(Figure2;AppendixS4).Precipitationdatawereexcludeddue to the largeuncertainty in the simulations.Theappliedclimatesimulationprovidesrealistictrendsonlargesub-continentalscale,es-peciallyforFennoscandia,asdemonstratedbyinter-modelandmodeldatacomparisons(Zhang,Renssen,Seppä,&Valdes,2017a,b).

2.3.2 | Forest fires

The regional fire variable was reconstructed based on sedimentarycharcoalrecordsfrom19lakes.Charcoaldatacoveringpartorallofthelast10,000yearswereacquiredfromthelatestversionoftheGlobalCharcoal Database (GCD v3; Marlon etal., 2016), from previouslypublishedsyntheses(Clear,Molinari,&Bradshaw,2014)orprovideddirectlybytheauthors(AppendixS2).ToretrieveregionalcompositesofchangesinfireactivityovertheHolocene,charcoalaccumulationinsedimentswastransformedandstandardizedaccordingtoaprotocol

F IGURE  1 Locationofthefourstudyregions.Theradiocarbon-datedarchaeologicalsitesforhumanpopulationsizeproxyareshownasgreycircles,siteswithpollenrecordaremarkedasgreencirclesandsiteswithcharcoalrecordaremarkedwithblackpolygons.Thecorrespondencebetweenthelabelofeachsite,thesitenameandthecharacteristicsofeachsitearegiveninAppendicesS1andS2

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(b) Sweden C

% % % % % %% %% %% % % % % %% % % % % % %SPD

4,0004,5005,0005,5006,0006,5007,0007,5008,0008,5009,0009,500

10,000–1 0 1 2 –2 –1 0 1 –2–1 0 1 0 0.0006 10 20 40 10 20 40 10 20 40 10 10 10 20 20 10 10 10 10 10 10 20 10 10 10 10 10

SumT WinT

(a) Sweden S

(c) Finland SW

(d) Finland SE

°C difference frompreindustrial mean

z-score oftransformed

charcoal influx

Age

(Cal

yea

r BP

)Pice

aPinu

sAlnu

sBetu

la

Carpinu

s

Corylus

Fagus

Fraxinu

s

Quercu

s

Tilia

Ulmus

Salix

Junip

erus

Callun

a v.

Artemisi

a

Cypera

ceae

Filipen

dula

Gramine

ae

Plantag

o l.

Plantag

o m.

Rumex

a.

Cereali

a-typ

e

Secale

cRegional

fires

Humanpopulation

size

4,0004,5005,0005,5006,0006,5007,0007,5008,0008,5009,0009,500

10,000–1 0 1 2 –2 –1 0 1 –2 0 2 0 0.0006 10 20 40 10 20 40 60 20 10 10 10 20 10 10 10 10 10 20 10 20 10 10 10 10 10

SumT WinT

10% % % % % %% %% %% %% % % %% % % % % % %SPD°C difference from

preindustrial meanz-score of

transformedcharcoal influx

Age

(Cal

yea

r BP

)

PiceaPinu

sAlnu

sBetu

la

Carpinu

s

Corylus

Fagus

Fraxinu

s

Quercu

s

Tilia

Ulmus

Salix

Junip

erus

Callun

a v.

Artemisi

a

Cypera

ceae

Filipen

dula

Gramine

ae

Plantag

o l.

Plantag

o m.

Rumex

a.

Cereali

a-typ

e

Secale

cRegional

fires

Humanpopulation

size

1,0001,5002,0002,5003,0003,5004,0004,5005,0005,5006,0006,5007,0007,5008,0008,5009,0009,500

10,000–1 0 1 2 –2–1 0 1 –2–1 0 1 0 0.0006 20 40 60 20 10 20 40 60 10 10 10 10 10 20 20 10 10 10 10 10 10 20 10 10 10 10 10

SumT WinT

% %% %% % % % %% % % % %% %% % % % % % %SPD°C difference frompreindustrial mean

z-score oftransformed

charcoal influx

Age

(Cal

yea

r BP

)

Picea

Pinus

Alnus

Betula

Carpinu

s

Corylus

Fagus

Fraxinu

s

Quercu

s

Tilia

Ulmus

Salix

Junip

erus

Callun

a v.

Artemisi

a

Cypera

ceae

Filipen

dula

Gramine

ae

Plantag

o l.

Plantag

o m.

Rumex

a.

Cereali

a-typ

e

Secale

cRegional

fires

Humanpopulation

size

1,0001,5002,0002,5003,0003,5004,0004,5005,0005,5006,0006,5007,0007,5008,0008,5009,0009,500

10,000–1 0 1 2 –2–1 0 1 –2–10 1 0 0.0006 20 40 60 20 40 10 20 40 60 10 10 10 10 10 10 10 10 10 10 10 10 10 20 40 10 10 10 10 10

SumT WinT

% % % % % %% %% %% %% % % %% % % % % % %SPD°C difference frompreindustrial mean

z-score oftransformed

charcoal influx

Age

(Cal

yea

r BP

)

Picea

Pinus

Alnus

Betula

Carpinu

s

Corylus

Fagus

Fraxinu

s

Quercu

s

Tilia

Ulmus

SalixJu

niperu

s

Callun

a v.

Artemisi

a

Cypera

ceae

Filipen

dula

Gramine

ae

Plantag

o l.

Plantag

o m.

Rumex

a.

Cereali

a-typ

e

Secale

cRegional

fires

Humanpopulation

size

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describedbyPower,Marlon,Bartlein,andHarrison(2010)andwidelyused within the paleofire community (Daniau etal., 2012; Molinarietal.,2013).Thesmoothcurveswereconstructedbydeterminingfit-tedvaluesat200-year intervals (Figure2;AppendixS4).Compositecurveswereproducedwith theRpackagepaleofire (Blarquezetal.,2014;RFoundationforStatisticalComputing,Vienna,Austria).

2.3.3 | Human population size

Changes in human population size were reconstructed using thetemporalfrequencydistributionofarchaeologicalradiocarbondates(Shennan&Edinborough,2007;Shennanetal.,2013).TheSwedishdata were extracted from the EUROEVOL data set that containsover 14,000 archaeological radiocarbon dates from the NeolithicinNWEurope,spanningtheLateMesolithicandEarlyBronzeAge(Shennan etal., 2013; Timpson etal. 2014; Manning, Colledge,Crema, Shennan, & Timpson, 2016). The Swedish samples weresupplementedbyMesolithicdates(Edinborough,2005)anddividedintosouthern(414dates)andcentral(352dates)subsetsaccordingtothestudyregions.DuetotheNeolithicfocusoftheEUROEVOLdata, the Swedish sampleswere restricted to cover the period of10,000–4,000cal yr BP. The Finnish data set (Oinonen, Pesonen,& Tallavaara, 2010; Tallavaara etal., 2010) covers the period of10,000–1,000calyrBPbothinSW(692dates)andSE(315dates)regions.

Summedprobabilitydistributionsofcalibratedradiocarbondates(SPDs)werecreatedforeachregion,andevaluatedforrandomsam-pling bias and calibration effects using a Monte-Carlo simulationapproach (Shennanetal., 2013;Timpsonetal. 2014) andby imple-menting the methodology used in Crema, Habu, Kobayashi, andMadella(2016;AppendixS6).Toreducethepotentialbiasofoversam-plingwithinsites,multipledates from individual sitesweregroupedintonon-overlappingbins,suchthatafterorderingthedatesatonesite,thenextdatewasonlyassignedtoanewbinifthereweremorethan200radiocarbonyearsbetweenitandthepreviousdate(Shennanetal., 2013;Tallavaara etal., 2010). Finally, SPDswere binned intoconservative200-yearbinsby averaging summedprobabilityvalueswithineachbin(Figure2,AppendixS4).

2.4 | Statistical analysis

Variationpartitioning (Borcard,Legendre,&Drapeu,1992)wasper-formed to assess the relative importance of climate, regional firesandhumanpopulationsizeonvariationinHolocenevegetationcom-position. In the analysis, vegetation composition derived from theREVEALSestimateswereusedasresponsematrix,whileclimate,re-gionalfiresandhumanpopulationsizewereusedasexplanatoryvari-ables. REVEALS estimates were Hellinger-transformed (Legendre &

Gallagher,2001)toassessthesignificanceofeachconstrainingvaria-bleandanANOVApermutationtestwasrunwith999randomizations.

Variationpartitioningwas first performed for thewhole studyperiod inallregions,namelyat10,000–4,000calyrBPinSandCSweden, and at 10,000–1,000calyrBP in SWandSEFinland. Inorder toevaluate theeffectof theexplanatoryvariablesonvege-tationbefore theonsetof farming,variationpartitioningwasper-formedfortheperiodof10,000–6,000calyrBPforCandSSweden,andfortheperiodof10,000–4,000calyrBPforSWandSEFinland.To assess the relative importance of the explanatoryvariables onvegetationchangesafterthebeginningofagriculture,variationpar-titioningwasperformedfortheperiodof6,000–4,000calyrBPforS andCSweden, and for theperiodof4,000–1,000calyrBP forSWandSEFinland.Toseparatelyevaluatetherelativeimportanceofsummerandwintertemperaturesonvegetation,variationparti-tioningwithsummerandwintertemperatureasindividualexplana-toryvariableswasperformedforallthreestudyperiods,thewholeHolocene,thepre-agricultureandtheagriculturalperiods,forallthestudyregions.

Inordertoassessthechangeintheroleofclimate,regionalfiresandhumanpopulation size during theHolocene, amovingwindowapproach(Reitaluetal.,2013)wasemployed.Thisapproachenablesperformingvariationpartitioning for subsetsofdata in shorter timeperiods, hence providing information on how the relative roles ofclimate and human population size change over time.Movingwin-dowanalysiswasperformedforallstudyregionsin2,000-yeartime windows,withten200-yearbinsineachtimewindow.

3  | RESULTS

3.1 | Variation partitioning results

3.1.1 | The Holocene

Climatealoneexplainedthehighest fractionofvariation invegetationduringthewholestudyperiodforallregions(Figure3).Therelativeim-portanceofwintertemperaturehadhighexplanatorypowerforthevari-ationinvegetationcoverinSandCSweden,whilesummertemperatureexplainedahigherproportionofthevariationinvegetationinSWandSEFinland(AppendixS7).Thevariationexplainedbyhumanpopulationsizeandforestfireswasrelativelylow.Humanpopulationsizehadasignifi-cantimpactonvegetationchangesinCSwedenandSWFinland,whileforestfireshadasignificanteffectinSSwedenandSEFinland.Climate,forestfiresandhumanpopulationsizetogetherexplainedatleast70%ofthevariationinlong-termvegetationinallregions.VariationpartitioningperformedwithPFTsshowedsimilarresultsasthetaxa-basedanalysis,withoverallhigherexplanatorypoweroftheusedexplanatoryvariablesineachstudyperiod(AppendixS8).

F IGURE  2 Holocenechangesintheexplanatoryvariables:simulatedsummer(SumT)andwinter(WinT)temperatures(fromtheLOVECLIMmodel),fires(charcoalinfluxz-scores)andhumanpopulationsizedataexpressedassummedprobabilitydistributions(SPD)ateachstudyregion(a)SSweden,(b)CSweden,(c)SWFinlandand(d)SEFinland(seeFigure1)andthemeanofREVEALSestimatesof23taxa

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3.1.2 | Pre- agriculture

Beforetheonsetoffarmingclimatealoneexplainedmostofthevari-ationinvegetation,withwintertemperaturesexplainingahigherpro-portionofvariation than summer temperatures (Figure3;AppendixS7). Fires had a significant effect on vegetation composition inFinland,explaining3%of thevariation invegetation inSWand4%inSEFinland.Humanpopulationsizeexplained5%of thevariationinvegetationinSWFinland,butwasnotsignificantinotherregions.Climate,forestfiresandhumanpopulationsizeexplainedatleast65%ofthevariationinvegetationcompositionbeforetheonsetofagricul-tureinallregions.

3.1.3 | Agricultural time

HumanpopulationsizeexplainedthehighestamountofthevariationinvegetationcompositioninSSwedenandSWFinland(6%and9%,

respectively;Figure3).Individually,climateexplainedarelativelylowfractionofvariationinvegetationinallregions,withsummertemper-aturedrivingahigherproportionofvariationthanwintertemperature(AppendixS7).Forest firesdidnotexplain individuallyanyvariationin vegetation.However, togetherwithhumanpopulation size, theyexplained 6% of variation in vegetation dynamics in SE Finland. Ingeneral,arelativelyhighproportionofvariationinvegetationwasleftunexplainedinallregions.

3.1.4 | Moving window approach

Results of the moving window approach (Appendix S9: Figure S1)demonstrate the strong impact of climate on vegetation dynamicsfrom the early to themid-Holocene in S andC Sweden, especiallybetween8,600–6,500cal yrBP,when climate explained40%–60%ofthevariationinvegetation.Inbothregionstherelativeimportanceofclimatewasnotably lowerat6,500–5,000calyrBP.Therelative

F IGURE  3 Percentageofvariationinvegetationcompositionexplainedbyclimate(wintertemperature:WinT,summertemperature:SumT),forestfires(Charc)andhumanpopulationsize(Pop)inSSweden,CSweden,SWFinlandandSEFinlandduringa)thewholestudyperiod(10,000–4,000calyrBPinSwedenand10,000–1,000calyrBPinFinland),b)beforetheonsetoffarmingat10,000–6,000calyrBPinSwedenandat10,000–4,000calyrBPinFinland,and(c)aftertheonsetoffarmingat6,000–4,000calyrBPinSwedenandat4,000–1,000calyrBPinFinland.Values<0arenotshowninthefigure.Statisticalsignificance(***p < .001;**p < .01,*p < .05andp < .1)ofeachparameteronvariationinvegetationisshownnexttothecorrespondingVenndiagram

Sweden S

Sweden S

Sweden S

Sweden C

Sweden C

Sweden C

etamilCetamilC etamilCetamilCForestfires

Forestfires

Forestfires

Forestfires

Populationsize

Populationsize

Populationsize

Populationsize

14

13 21435 3

Unexplained variation = 19

39

28

4

Unexplained variation = 30

Finland SW

Finland SW

Finland SW

Finland SE

Finland SE

Finland SE

27

23

18

3

1

Unexplained variation = 27

27

12

37 2

Unexplained variation = 22

WinT***SumT*Charc**Pop

WinT***SumT***CharcPop **

WinT ***SumT *CharcPop *

WinT***SumT***Charc**Pop

(a)

(b)

(c)

1

2

4

143

13 1949

1

Unexplained variation = 19

WinT**SumT*CharcPop

Climate Forestfires

Populationsize

42

27 0

1

Unexplained variation = 35

WinT***SumTCharcPop

Climate Forestfires

Populationsize

22 21

5

3

Unexplained variation = 33

WinT***SumT***Charc .Pop*

Climate Forestfires

Populationsize

30 17

1310

4

Unexplained variation = 27

WinT***SumT**Charc *Pop

Climate Forestfires

Populationsize

6

11

Unexplained variation = 110

WinTSumTCharcPop

Climate Forestfires

Populationsize

22

1

10

Unexplained variation = 94

WinTSumT *CharcPop

Climate Forestfires

Populationsize

9

31

Unexplained variation = 59

WinTSumTCharcPop*

Climate Forestfires

Populationsize

5

01 6

Unexplained variation = 97

WinTSumTCharcPop

Climate Foresfires

Populationsize

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importanceoffireswasgenerallylow,explainingca10%ofthevaria-tionbetween9,000–8,000calyrBPinbothregions,andalsobetween6,000–5,500calyrBPinSSweden. InSSwedentheimportanceofhumanpopulationsizeincreasedat7,000–6,500calyrBP,explainingupto60%ofthevariationinvegetation(Figure4).InCSwedentheexplanatoryvalueofhumanpopulationsizeremainedrelatively lowthroughouttheHolocene.

InFinland,theresultsofthemovingwindowapproach(AppendixS9:FigureS2)demonstratenotablefluctuations intheimportanceofclimateinbothregions.InSWandSEFinlandclimateexplained20%–60% of the variation during ca. 1,000-year periods around8,000,5,700and3,800calyrBPwithlowerimportanceinbetween.In SW Finland the explanatory value of fires was higher (20%)around6,000calyrBP,withlowervaluesduringthelateHolocene.In SE Finland fires explained the highest amount of variation invegetation during the early Holocene. In general, the importanceofhumanpopulationsizewas low,exceptaround5,000calyrBP,when it explainedover20%of thevariation invegetation. In SWFinlandtherelativeimportanceofhumanpopulationsizeincreased

from 3,500cal yr BP exceeding the explanatory value of climateby explaining 20% of the variation in vegetation (Figure4). In SEFinlandtherelativeimportanceofhumanpopulationsizeremainedlowduringtheHolocene.

4  | DISCUSSION

4.1 | Role of climate in Holocene vegetation dynamics

Theimportanceofclimateasadriverofvegetationchangewashigh-estduringthebeginningoftheHoloceneThermalMaximum(HTM)ca.8,000–7,000calyrBP,andagainaround6,000–5,000calyrBP,corresponding to themain temperature shiftsduring theHolocene.Thiscanbeseenespecially inSSwedenandSWFinland (Figure4).Asimilar trendofhigher relative importanceofclimate in theearlyHolocene is also suggested byMarquer etal. (2017; Figure4). ThestrongroleofclimateinvegetationdynamicsatthebeginningoftheHTMisinagreementwiththewell-establishednorthwardrangeshift

F IGURE  4 Relativeimportanceofclimate,firesandanthropogenicinfluenceonvariationinvegetationdynamicsthroughtheHoloceneinNEurope.Thefractionofvariationinvegetationexplainedbyhumanpopulationsize,regionalfiresandclimatein(a)SSwedenand(b)SWFinlandforten200-yearsubsetsofdatain2,000-yearmovingtimewindows.(c)Thefractionofvariationexplainedbyclimateandhumanimpactfor14subsetsofdatain1,200-yearmovingtimewindowinEstonia(Reitaluetal.,2013).(d)ThefractionofvariationexplainedbyclimateandlanduseaverageacrosssouthernScandinaviaandBalticcountries(Marqueretal.,2017).ForSSweden,SWFinlandandEstoniatheresultsofeachtimewindowareshownfromthemidpointofthetimeperiod.TheusedtimewindowsareshowninAppendixS9:TablesS1,S2

20 40 60 80 1000

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population sizeHumanClimate Fires% of RV variation explained

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oftemperatetreetaxasuchasCorylus,Tilia,Ulmus and Quercusduringthisperiod (Milleretal.,2008;Seppäetal.,2015), andcanbeseenasanincreaseoftemperatebroad-leafforestcoverinFennoscandia.Thiscorrespondswiththepresentecologicalunderstandingthattem-peratureisoneofthemainlimitingfactorsforthenorthernlimitsofthe temperate tree species (Jackson, Betancourt, Booth, & Grayd,2009;Woodward,Lomas,&Kelly,2004).

Althoughtheimportanceofhumanpopulationsizeonvegetationdynamics increases from the early Neolithic in Sweden and duringthe late-Holocene (ca 3,500cal yr BP) in Finland, climate remainsanimportantdriverbehindvegetationcoverchanges,especiallyinCSwedenandSEFinland(AppendixS9:FiguresS1,S2).Similarly,there-sultsofReitaluetal.(2013)andMarqueretal.(2017)demonstratethestrongeffectofclimateonvegetationdynamicsinEstoniaandsouth-ernScandinaviaduringthelast1,000years.Ifthiscontrolofclimateonboreal forest compositionwill prevail, theprojected acceleratingfuturewarminginnorthernregions(Christensenetal.,2013)willactasthemaindriveroffuturechangesintheborealecosystems,atleastatregionalscale.

4.2 | Role of fires in Holocene vegetation dynamics

The change from a natural to amore human-induced fire impactonvegetationdynamicsisnotclearlydefinedinourdata.Ahigherinfluenceofforestfiresonvegetationcompositionduringthepre-agriculture periodwas detected for both Sweden and Finland. InSSweden,theslightincreaseintheroleoffiresduringthebegin-ningoftheagriculturalperiodcoincideswiththeincreasingeffectof human population size (Figure4.),while in C Sweden fires didnotshowanyeffectonvegetationcomposition.Itisplausiblethatduring this time human-induced fires were local fires, which didnothavenotableimpactonregionalvegetationcomposition.Sinceslash-and-burnpracticeswereoftenusedinconnectionwithearlyfarming (Alenius, Haggrén, Koivisto, Sugita, & Vanhanen, 2017;Huttunen,1980),thenotablylowimportanceofforestfiresonveg-etationduringtheagriculturalperiodinSWFinlandwassurprising.Although the charcoal dataused in this study reconstruct the re-gionalfirehistory,theunevendistributionofthesedimentarychar-coalrecords inthestudyregionsmightbiastheresults.However,changes in vegetation dynamics are partly explained by the jointeffectoffiresandhumanpopulationsize,whereaspre-agriculturefires and climate have a notable joint effect on variation in veg-etation.Therefore, it isplausiblethatthe increased jointeffectofhumanpopulationsizeandfiresindicatethechangefromnatural-tomorehuman-inducedfiresaffectingvegetationdynamicsduringthelateHolocene.

4.3 | Increasing anthropogenic influence from early Neolithic

The comparison between our results and those of Marquer etal.(2017) for S Scandinavia and the Baltic region demonstrate an in-creasingroleofhumanpopulationsizeonvegetationdynamicsfrom

themid-HoloceneinSSwedensuggestingincreasinglandusebythegrowingpopulation(Figure4).ItisplausiblethattherapidincreaseinhumanpopulationsizeinSSwedenwasduetosuitableclimatecon-ditions during theHTM (Tallavaara& Seppä, 2012) and the spreadofmore intensive land-usepracticesfromCentralEurope.Theshifttoagrariansocietygraduallychangedthelandscape,whenNeolithiccommunitiesstartedtomigrate inland.Broad-leaf forestswere firstusedaswoodlandpasturesandthenclearedforcultivation(Lagerås,1996;Sköld,Lagerås,&Berglund,2010).

The low human impact on vegetation dynamics in SW Finlanduntil3,500calyrBP is in linewith thesmallhumanpopulationsizeandwith the lateranthropogenic influenceonvegetation inFinlandcompared to S Scandinavia and theBaltic region (Fyfe etal., 2015;Marquer etal., 2017). The increasing importance of human impactin SWFinland correspondswith theonset of agriculture ca4,000–3,500cal yr BP (Taavitsainen etal., 1998), suggesting that humanpopulation sizewas themajordriverofvegetationdynamicsduringthelateHolocene,especiallyduringtheperiodof3,000–1,000calyrBP.Growingagrariancommunitiesclearedtheforestforpasturesandarableland.Cultivatedfieldswerecommonlyleftasfallowforafewyearsinordertomaintainsoilproductivity.ThismayhavecausedanincreaseintheproportionofBetula and Alnustogetherwithshrubsinthevegetationcover(Reitaluetal.,2013).

The lower importance of human population size on regionalvegetationchangeinCSwedenandSEFinlandcouldbeexplainedby the fact that these regionsweremore forested and less pop-ulatedduring the studyperiod.Alenius etal. (2013) showed that,forexample, inEFinlandforestclearancesandcultivation intensi-fiedonlyfromAD600onwards,coincidingwith increasinghumanpopulationsize.Duetothe landuplift,substantially largeareasofSEFinlandwereraisedrapidlyabovetheBalticsea levelafter thedeglaciation,incontrasttoSWFinland(Tikkanen&Oksanen,2002)andthereforeSEFinlandlacksthick,fine-grainedmineralsoillayers,such as clay and silts,whichprovide fertile soils for cultivation inSW Finland. It is plausible that, due to the lack of suitable areasformoreintensivefieldcultivation,humanpopulationsizeremainedrelativelylowinSEFinlandandanimalhusbandrytogetherwithspo-radicslash-and-burncultivationwerepracticed longer than inSWFinland(Taavitsainenetal.,1998).

4.4 | Issue of causality in the variation partitioning analysis

It is also important to note that variation partitioning as amethoddoesnot consider the causality, but only the co-variationbetweenvariablesthroughtime.Thecausalitybetweenclimateandvegetationisrelativelyclear,whilethecausalitybetweenchangesinvegetationand inhunter-gathererpopulation sizeand forest fires ismoredif-ficult todeduce.Hunter-gathererpopulationswere small and scat-tered,affectingmainlythelocal-scalevegetationdynamics(Hörnbergetal., 2005;Regnell etal., 1995). In Finland, thedecline in hunter-gatherersalongwiththedeclineoftemperateforestscoincideswiththe expansion of spruce and consequently the boreal ecosystem

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(Seppä etal., 2009). Boreal forests provide lower food abundancecompared to temperatemixed forests, and ithasbeenargued thatthisecosystemchangecausedthedeclineinhunter-gathererpopula-tionsize(Tallavaara&Seppä,2012).Itisalsoprobablethatinsteadofacausalrelationshipbetweenhumanpopulationsizeandvegetation,thesevariablesco-varybecauseofthechangesinclimate.Similarly,thecausalitymaybedifficulttodistinguishbetweenfiresandvegeta-tion.Ohlsonetal.(2011)showedthatfirefrequencyinFennoscandiadecreasedafterthespruceexpansion.However,itisstillspeculativeifspruceexpandedbecauseoffirereductionorthefiresdecreasedduetothespruceexpansion.Sincedisentanglingtheseissuesofcau-salitywithvariationpartitioningischallenging,futurestudieswouldbenefitfromtheuseofothermethodsthatprovidemoreinsightintothedirectionof the interactionbetween response andexplanatoryvariables.

5  | CONCLUSION

Hunter-gathererpopulationsdidnotsignificantlyaffectvegetationdynamics inFennoscandia.ClimatewasthemaindriverofchangeduringtheHoloceneandlarge-scaleshiftsinvegetationwerelikelydriven by climate.However,with relatively stable climatic condi-tions, theroleofclimatemaydecreaseandother factors, suchasdisturbances, site characteristics, competition and human impactmayhaveagreatereffectonvegetationcomposition.Agriculturalpopulations,however,highlyaffectedvegetationcomposition,andtheroleplayedbyhumanpopulationsizebecamemoreimportantduringthelateHolocene.Thereisaclearregionaldependencere-latedtohumanpopulationsize,i.e.theeffectofhumanimpactonvegetationchangewasnotablyhigherinSSwedenandSWFinland,where land use was more intensive, than in C Sweden and SEFinland.

Thevariationpartitioningapproachprovidesimportantinsightsintothedriversofpastvegetationdynamicsandprovidesasteppingstoneforfindingmoreprecisemethodstoassesstheprocessesbe-hindpastchangesinlong-termvegetationdynamics.TherelativelyhighimportanceofclimatethroughouttheHolocenesuggeststhat–evenafter theonsetof farming–climate, togetherwithhumanimpact, remained an important driver of changes in broad-scalevegetationdynamics.Althoughtheanthropogenicimpactmayhaveperiodically and regionally overruled the effect of climate alreadyfromtheearlyNeolithic,climatecanbeconsideredthemaindriverof long-termregionalvegetationdynamics inFennoscandiaduringtheHolocene.

ACKNOWLEDGEMENTS

Financial support was provided by the PEDECO project (number16-23183Y) funded by the Czech Science Foundation (GAČR) andEBOR project, funded by the Academy of Finland. We thank theLANDCLIMprojectcoordinatedbyM.J.Gaillardfordatacontributions.

ThisstudyisalsoacontributiontothePAGESLandCover6kworkinggroup.

ORCID

Niina Kuosmanen http://orcid.org/0000-0002-9688-3797

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SUPPORTING INFORMATION

Additional Supporting Information may be found online in the supportinginformationtabforthisarticle.Appendix S1MetadataforpollenrecordsAppendix S2MetadataforcharcoalrecordsAppendix S3TheREVEALScorrectionsforthe23planttaxaAppendix S4VegetationcompositionandexplanatoryvariablesAppendix S5Plantfunctionaltypes(PFTs)Appendix S6Summedprobabilitydistributions(SPD)ofarchaeologicalradiocarbondatesAppendix S7 Variation partitioning results for winter and summertemperatureAppendix S8Variationpartitioningresultswithplantfunctionaltypes(PFTs)Appendix S9Resultsfrommovingwindowanalysis

How to cite this article:KuosmanenN,MarquerL,TallavaaraM,etal.Theroleofclimate,forestfiresandhumanpopulationsizeinHolocenevegetationdynamicsinFennoscandia.J Veg Sci. 2018;00:1–11. https://doi.org/10.1111/jvs.12601