Annual changes in MODISvegetation indices of Swedish coniferous forests in relation

RemoteSensingofEnvironment114(2010)2719–2730

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RemoteSensingofEnvironment

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AnnualchangesinMODISvegetationindicesofSwedishconiferousforestsinrelationtosnowdynamicsandtreephenology

A.M.Jönssona, ,L.Eklundha,M.Hellströma,L.Bärringa,b,P.Jönssonc

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DepartmentofEarthandEcosystemSciences,DivisionofPhysicalGeographyandEcosystemAnalysis,LundUniversity,Sölvegatan12,SE-22362Lund,SwedenRossbyCentre,SwedishMeteorologicalandHydrologicalInstitute,SE-60176Norrköping,SwedenCentreforTechnologyStudies,EnvironmentandSociety,MalmöUniversity,20506Malmö,Sweden

articleinfoabstract

Remotesensingprovidesspatiallyandtemporallycontinuousmeasuresofforestre ectance,andvegetationindicescalculatedfromsatellitedatacanbeusefulformonitoringclimatechangeimpactsonforesttreephenology.Monitoringofevergreenconiferousforestismoredif cultthanmonitoringofdeciduousforest,asthenewbudsonlyaccountforasmallproportionofthegreenbiomass,andtheshootelongationprocessisrelativelyslow.Inthisstudy,wehaveanalyzeddatafrom186coniferousmonitoringsitesinSwedencoveringboreal,southern-boreal,andboreo-nemoralconditions.Ourobjectivewastoexaminethepossibilitytotrackseasonalchangesinconiferousforestsbytime-seriesofMODISeight-dayvegetationindices,testingthecoherencebetweensatellitemonitoredvegetationindices(VI)andtemperaturedependentphenology.Therelationshipsbetweentwovegetationindices(NDVIandWDRVI)andfourphenologicalindicators(lengthofsnowseason,modeledonsetofvegetationperiod,treecoldhardinesslevelandtimingofbudburst)wereanalyzed.

Theannualcurvesproducedbytwocurve ttingmethodsforsmootheningofseasonalchangesinNDVIandWDRVIweretoalargeextentcharacterizedbytheoccurrenceofsnow,producingstableseasonaloscillationsinthenorthernpartandirregularcurveswithlesspronouncedannualamplitudeinthesouthernpartofthecountry.MeasuresbasedonthresholdvaluesoftheVI-curves,commonlyusedfordeterminingthetimingofdifferentphenologicalphases,werenotapplicableforSwedishconiferousforests.Evergreenvegetationdoesnothaveasharpincreaseingreennessduringspring,andthemeltingofsnowcanin uencethevegetationindicesatthetimingofbudburstinborealforests.However,theinterannualvariationinVI-valuesforspeci ceight-dayperiodswascorrelatedwiththephenologicalindicators.Thisrelationcanbeusedforsatellitemonitoringofpotentialclimatechangeimpactsonnorthernconiferousspringphenology.

©2010ElsevierInc.Allrightsreserved.

Articlehistory:

Received12February2010

Receivedinrevisedform18June2010Accepted18June2010Keywords:

ConiferousforestMODISNDVI

NorwaysprucePhenologyScotspineTIMESATWDRVI

1.Introduction

Therecentglobalwarminghasinducedearliertimingofspringevents(IPCC,2007;Parmesan,2006).Inameta-analysiswithdatafromEurope,theaverageadvanceofspring/summerwasestimatedtobe2.5daysperdecadeduringtheperiodof1971–2000(Menzeletal.,2006).Changesintreephenologymayhaveimplicationsforforestmanagement,inparticularduringthephaseofforestregenerationwithselectionofsuitabletreespeciesandprovenances(Larsen,1995;Saxeetal.,2001).Lea ngand oweringarevisiblesignsoftreeresponsetoseasonalchanges,precededbyphysiologicalprocessesassociatedwithrecoveryfromwinterdormancy.Boththeseasonalcoldhardinesslevel(i.e.toleranceoffreezingtemperatures)andtimingofbudburstareaffectedbythecumulativeimpactofambienttemperature(Chuine,2000;Kalbereretal.,2006).Inawarmingclimate,thetreesmaynotdevelopthesamecoldhardinessastoday,

Correspondingauthor.Tel.:+46462229410.

E-mailaddress:Anna_Maria.Jonsson@nateko.lu.se(A.M.

Jönsson).0034-4257/$–seefrontmatter©2010ElsevierInc.Allrightsreserved.doi:

10.1016/j.rse.2010.06.005

andtheymaybehardenedforashorterperiodoftime.De-hardeningandbudburstmayoccuralreadyinthebeginningoftheyearwhentheseasonaltemperatureprogressionisslowandtheriskoftemperaturebacklasheswithsuddenfrostepisodesishigh,whichhasraisedconcernsofanincreasedriskforfrostdamage(Cannell&Smith,1986;Jönssonetal.,2004;Morinetal.,2009).

Monitoringofforestphenologyisrequiredtoassesstheimpactofongoingclimatechange,whichmayvaryamongtreespeciesduetodifferentsensitivitytoseasonalchangesintemperature,lightandprecipitation.Satelliteremotesensinghastheadvantageofprovidingspatiallyandtemporallycontinuousmeasuresofforestre ectance,andremotelysenseddataareusedincalculationsofdifferentvegetationindices.TheNormalizedDifferenceVegetationIndex(NDVI)(Tucker,1979)isoneofthemostwidelyusedindices,andithasbeenappliedtomonitoringofspringphenologicaleventsoverlargeareas(Beaubien&Hall-Beyer,2003;Clelandetal.,2007;Schwartzetal.,2002;Stöckli&Vidale,2004;Zhangetal.,2006).NDVIisbasicallyameasureof‘greenness’,andithasbeenrelatedtotheabsorptionofphotosynthet-icallyactiveradiation(Graceetal.,2007)andGPP(Olofssonetal.,2008).AratherhighcorrespondencebetweenNDVIandleafareaindexhas