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Implications of increased deciduous cover on stand structure and aboveground carbon pools of Alaskan boreal forests
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Title: Implications of increased deciduous cover on stand structure and aboveground carbon pools of Alaskan boreal forests
Series Title: Alexander, H. D., M. C. Mack, S. Goetz, P. S. A. Beck and E. E. Belshe. 2012. Implications of increased deciduous cover on stand structure and aboveground carbon pools of Alaskan boreal forests. Ecosphere 3(5):45 http://dx.doi.org/10.1890/ES11-00364.1
Physical Description: Journal Article
Creator: Mack, Michelle
Alexander, Heather D.
Goetz, Scott
Beck, Pieter S. A.
Belshe, E. Fay
Publisher: Ecological Society of America
Place of Publication: Washington. D. C.
Publication Date: May 24, 2012
 Subjects
Subjects / Keywords: aboveground net primary production (ANPP)
Alaska, USA
biomass
boreal forest
carbon
climate change
deciduous
fire
Picea mariana
snag
succession
woody debris
 Notes
Abstract: Fire activity in boreal forests has increased recently with climate warming, altering stand structure and composition in many areas. Changes in stand dynamics have the potential to alter C cycling and biophysical processes, with feedbacks to global and regional climate. Here, we assess the interactions between fire, stand structure, and aboveground C accumulation and storage within boreal forests of interior Alaska, where increased fire severity is predicted to shift forest composition from predominantly black spruce (Picea mariana) to greater deciduous cover. We measured aboveground biomass and net primary productivity (ANPP) of trees and large shrubs, snags, and downed woody debris across 44 midsuccessional (20–59 years since fire) stands of varying deciduous importance value (IV), determined by relative density, basal area, and frequency of deciduous trees and large shrubs within each stand. Aboveground biomass, ANPP, and deciduous snag biomass increased significantly with increased deciduous IV and years since fire. Deciduous IV had little influence on evergreen snag biomass and downed woody debris, but both C pools decreased with years since fire. Forest type also affected stand structure and C pools. Black spruce stands had shorter trees with less basal area and aboveground biomass and slower rates of biomass accumulation and ANPP compared to those dominated by trembling aspen (Populus tremuloides) or Alaska birch (Betula neoalaskana). These parameters in black spruce stands were similar to mixed stands of black spruce and aspen but were often lower than mixed stands of black spruce and Alaska birch. Much of the biomass accumulation in deciduous stands was attributed to higher treelevel ANPP, allowing individual stems of deciduous species to accumulate more stemwood/bark faster than black spruce trees. If increased fire activity shifts stand composition from black spruce to increased deciduous cover, ANPP, aboveground tree/large shrub biomass, and deciduous snag biomass will increase, leading to increased aboveground C pools in mid-successional forest stands of interior Alaska. While species dominance shifts like these will impact aboveground patterns of landscape-level C cycling in boreal forests, variations in soil C pools and forest properties like albedo must also be assessed to accurately determine implications for global and regional climate.
Acquisition: Collected for University of Florida's Institutional Repository by the UFIR Self-Submittal tool. Submitted by Michelle Mack.
Publication Status: Published
Funding: Funding for this research was provided by NASA Ecosystems and Carbon Cycle Grant NNX08AG13G, NOAA Global Carbon Cycle grant NA080AR4310526 and the Bonanza Creek Long Term Ecological Research Site program funded by NSF DEB-0620579 and USDA Forest Service, Pacific Northwest Research Station, grant PNW01-JV11261952-231.
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Source Institution: University of Florida Institutional Repository
Holding Location: University of Florida
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This item is licensed with the Creative Commons Attribution License. This license lets others distribute, remix, tweak, and build upon this work, even commercially, as long as they credit the author for the original creation.
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Implicationsofincreaseddeciduouscoveronstandstructure andabovegroundcarbonpoolsofAlaskanborealforestsHEATHERD.ALEXANDER,1, MICHELLEC.MACK,1SCOTTGOETZ,2PIETERS.A.BECK,2ANDE.FAYBELSHE11DepartmentofBiology,UniversityofFlorida,Gainesville,Florida32611USA2WoodsHoleResearchCenter,Falmouth,Massachusetts02540USA Citation: Alexander,H.D.,M.C.Mack,S.Goetz,P.S.A.Beck,andE.F.Belshe.2012.Implicationsofincreased deciduouscoveronstandstructureandabovegroundcarbonpoolsofAlaskanborealforests.Ecosphere3(5):45.http:// dx.doi.org/10.1890/ES11-00364.1Abstract.Fireactivityinborealforestshasincreasedrecentlywithclimatewarming,alteringstand structureandcompositioninmanyareas.ChangesinstanddynamicshavethepotentialtoalterCcycling andbiophysicalprocesses,withfeedbackstoglobalandregionalclimate.Here,weassesstheinteractions betweenfire,standstructure,andabovegroundCaccumulationandstoragewithinborealforestsof interiorAlaska,whereincreasedfireseverityispredictedtoshiftforestcompositionfrompredominantly blackspruce( Piceamariana )togreaterdeciduouscover.Wemeasuredabovegroundbiomassandnet primaryproductivity(ANPP)oftreesandlargeshrubs,snags,anddownedwoodydebrisacross44midsuccessional(20–59yearssincefire)standsofvaryingdeciduousimportancevalue(IV),determinedby relativedensity,basalarea,andfrequencyofdeciduoustreesandlargeshrubswithineachstand. Abovegroundbiomass,ANPP,anddeciduoussnagbiomassincreasedsignificantlywithincreased deciduousIVandyearssincefire.DeciduousIVhadlittleinfluenceonevergreensnagbiomassand downedwoodydebris,butbothCpoolsdecreasedwithyearssincefire.Foresttypealsoaffectedstand structureandCpools.Blacksprucestandshadshortertreeswithlessbasalareaandabovegroundbiomass andslowerratesofbiomassaccumulationandANPPcomparedtothosedominatedbytremblingaspen ( Populustremuloides )orAlaskabirch( Betulaneoalaskana ).Theseparametersinblacksprucestandswere similartomixedstandsofblackspruceandaspenbutwereoftenlowerthanmixedstandsofblackspruce andAlaskabirch.MuchofthebiomassaccumulationindeciduousstandswasattributedtohighertreelevelANPP,allowingindividualstemsofdeciduousspeciestoaccumulatemorestemwood/barkfaster thanblacksprucetrees.Ifincreasedfireactivityshiftsstandcompositionfromblacksprucetoincreased deciduouscover,ANPP,abovegroundtree/largeshrubbiomass,anddeciduoussnagbiomasswillincrease, leadingtoincreasedabovegroundCpoolsinmid-successionalforeststandsofinteriorAlaska.While speciesdominanceshiftslikethesewillimpactabovegroundpatternsoflandscape-levelCcyclinginboreal forests,variationsinsoilCpoolsandforestpropertieslikealbedomustalsobeassessedtoaccurately determineimplicationsforglobalandregionalclimate.Keywords: abovegroundnetprimaryproduction(ANPP);Alaska,USA;biomass;borealforest;carbon;climate change;deciduous;fire; Piceamariana ;snag;succession;woodydebris. Received 19December2011;revised2April2012;accepted24April2012; published 24May2012.Corresponding Editor:Y.Pan. Copyright: 2012Alexanderetal.Thisisanopen-accessarticledistributedunderthetermsoftheCreativeCommons AttributionLicense,whichpermitsrestricteduse,distribution,andreproductioninanymedium,providedtheoriginal authorandsourcesarecredited. E-mail: hdalexander@ufl.edu v www.esajournals.org1May2012vVolume3(5)vArticle45

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INTRODUCTIONWildfireistheprimarydisturbanceinthe borealforestbiome(Johnson1992,Payette1992, GoldammerandFuryaev1996)andisafundamentaldriverofecosystemprocesses,including foreststanddevelopment(VanCleveetal. 1983 a ),biogeochemicalcycles(Amiroetal. 2009),andenergyandwaterbalance(McGuire etal.2006,Randersonetal.2006).Overthelast threedecades,firefrequency,extent,andseverity haveincreasedinmanyareasoftheborealforest, probablyduetoclimatewarminganddrying (KasischkeandTuretsky2006,Sojaetal.2007), andglobalclimatechangemodelspredicta furtherintensifyingfireregimeasatmospheric temperaturesincrease(Stocksetal.1998,Tchebakovaetal.2009).Becauseborealforestscontain alargeproportionofglobalterrestrialcarbon(C) stocks(KasischkeandStocks2000),therehas beengreatinterestinunderstandingtheeffectsof analteredfireregimeontheseecosystemsand potentialfeedbackstoclimatewarming(e.g., Randersonetal.2006,Bond-Lambertyetal.2007, Johnstoneetal.2010 a ,Becketal.2011).Directly, increasedfireactivitycanreduceCstoredin vegetationandorganicsoilsandincreaseatmosphericCO2concentrations(Balshietal.2009), creatingapositivefeedbacktoclimatewarming (Kasischkeetal.1995,Kurzetal.2008).Firescan alsoinitiateanarrayofindirecteffectsonforest regrowthandstanddynamicsthatinfluenceC accumulationpatterns(Kasischkeetal.2000)and surfaceenergyfluxes(Randersonetal.2006)and ultimatelydeterminewhetheranalteredfire regimewillleadtoapositiveornegative feedbacktoclimatewarming(Goetzetal.2007). Oneofthemostimportantwaysanintensifyingfireregimemayindirectlyimpactboreal foreststanddynamicsandCpoolsisbyaltering demographicprocessesandcreatingapatternof forestregrowththatdiffersfromthepre-fire stand.Acrosstheborealforest,post-firerecruitmentoftreesandlargeshrubsislargely controlledbythedepthofthesoilorganiclayer (SOL)becausethisdeterminesthedistancea seedling  srootmustgrowbeforereachingthe mineralsoil,whichprovidesamorestable moistureenvironmentthantheSOL(Johnstone andChapin2006,Greeneetal.2007).With increasingfireseverity,SOLconsumptionincreases,decreasingSOLdepthandincreasing mineralsoilexposure(KasischkeandJohnstone 2005,Turetskyetal.2010).Thesepost-fire changesinSOLdepthandsoilcharacteristics canstronglyinfluencesuccessionaltrajectoriesof dominantplantspecies(Kasischkeetal.2000) becauseofvariableplantrecruitmentstrategies exhibitedbypotentialcolonizers(Johnstoneand Chapin2006)andthetendencyforpost-fire recruitmentpatternstopredictfuturestand dynamics(Johnstoneetal.2004,Johnstoneetal. 2010 b ,Johnstoneetal.2010 c ).Differencesin dominantvegetationamongsuccessionaltrajectoriescan,inturn,alterstandattributesandC storagebecauseofdifferencesinvegetation longevity,productivity,growthhabit,litterquality,andflammability.Thus,understandinghow post-firestanddynamicsandCpoolsvarywith shiftsindominantvegetationisessentialfor predictinghowborealforestswillinfluence futurepatternsofland-atmosphereCexchange andscalingupstand-levelCbudgetstothe landscape.Yet,despitetheimportanceofpostfirestanddynamicsandCaccumulationpatterns withinborealforeststoclimatefeedbacks,our currentknowledgeofhowtheseparametersvary withanintensifyingfireregimeremainslimited. Toaddressthislimitation,wequantified abovegroundtreeandlargeshrubbiomass,net primaryproductivity(ANPP),snagbiomass,and downedwoodydebriswithinborealforestsof interiorAlaska,wheremanyareaspreviously dominatedbytheevergreenconiferblackspruce ( Piceamariana (Mill.)B.S.P.)arepredictedtoshift tostandswithgreatercoverofdeciduousspecies suchasAlaskabirch( Betulaneoalaskana Sarg.) andtremblingaspen( Populustremuloides Michx.) (Ruppetal.2002,Chapinetal.2004,Johnstoneet al.2010 b ).Thisshiftisexpectedbecauserecent increasesinburnareaandseverityhavefavored theestablishmentandgrowthoflight-seeded deciduousspecies,whichreadilygerminateon highmoisturemineralseedbeds(Johnstoneand Kasischke2005,JohnstoneandChapin2006, Johnstoneetal.2010 b ).Incontrast,post-fire recruitmentofblackspruceoccursviaalarge aerialseedbankcontainedwithinsemi-serotinous cones(Viereck1973).Largeinputsofrelatively heavyseedsoccurringsoonafterthefiredisturbancemaximizesblackspruceestablishment followinglow-severityfires(JohnstoneandChav www.esajournals.org2May2012vVolume3(5)vArticle45ALEXANDERETAL.

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pin2006).Thishasfacilitatedahistoricalpattern ofsuccessionalself-replacementfollowingfire (Kurkowskietal.2008)andcontinueddominanceofthesestandsacrossthelandscape (Chapinetal.2004). Weexpectedafire-inducedshiftindominant vegetationtohaveimportantimplicationsfor standdynamicsandabovegroundCbalance duringpost-firesuccessionbecauseoffundamentaldifferencesbetweenevergreenanddeciduousfunctionaltypes.Blackspruce  severgreen growthformanddense,darkfoliagecreatecold, moistunderstoryconditions.Theseconditions, combinedwithrecalcitrantleaflitter,limit decompositionandnutrientavailabilityand promoteathickorganichorizonandpermafrost development(FlanaganandVanCleve1983), whichcanlowertreeproductivityandstanding biomass(VanCleveetal.1983 b ).Thus,the proportionofCstoredinblacksprucestands maybehigherinSOLcomparedtoaboveground treecomponents.Incontrast,deciduoustrees andlargeshrubslosetheirfoliageannually, creatingaregularsupplyoforganicmaterialto theforestfloor.Thislitterismoreeasily decomposedbecauseofitshigherqualityand themorefavorable(warmer/drier)understory conditions,leadingtofasterratesofsoilnutrient turnoverandincreasednutrientavailability(Van Cleveetal.1983 a ).Leaflitterinputsalso minimizemossgrowth,restrictingpermafrost development,increasingnutrientuptakeability (VanCleveetal.1983 a ).Alloftheseconditions tendtopromotelowhighratesofproductivity andlargeamountsofstandingbiomasswithin deciduousstandsandathinSOL(VanCleveet al.1983 b ). Toevaluatetheeffectsofincreasingdeciduous dominanceonstanddynamicsandaboveground CpoolswithinborealforestsofinteriorAlaska, wesampled44mid-successional(20–59years sincefire)foreststandsvaryingincomposition frompredominantlyblacksprucetopredominantlydeciduoustreesandlargeshrubs.Our primaryobjectivewastounderstandtheinfluenceofincreasingdeciduousdominanceon abovegroundCpoolsacrossadiverselandscape, independentofdeciduousspeciescomposition. Wewereinterestedin  deciduousness  froma broadperspectiveforseveralreasons:(1)most deciduoustreesandlargeshrubs(e.g.,willows) arelight-seededandwind-dispersed(Walkeret al.1986),characteristicsusuallyassociatedwith increasedabundancefollowinghigh-severity fires(e.g.,JohnstoneandChapin2006),(2)all deciduoustreesandlargeshrubsrepresenta sourceofcompetitionwithblackspruce(Imo andTimmer1999),(3)attemptstoscaleCpools tothelandscapeareoftenbasedonsatellite imageryandtendtousethelighter-colored foliageofdeciduousspeciesasawholebutdo notdistinguishbetweendifferentdeciduous species(e.g.,Becketal.2011),and(4)theamount ofbiomasscontainedindeciduousspeciesis importantforundersta ndingsurfaceenergy fluxesandwaterbalance(Amiroetal.2006,Liu andRanderson2008).WefocusedonmidsuccessionalCdynamicsbecausethisstagemay becomemoreprevalentwithanincreaseinfire frequency,burnarea,andfireseason,andmost previousCbudgetsandmodelsutilizedatafrom maturestands(e.g.,Goweretal.2001),whilethe pervasivenessoffireacrossthelandscapeclearly leadstoamosaicofdifferent-agedstands.We alsoprovideestimatesofdownedwoodydebris andsnagbiomassbecausetheseareimportant yetunderstudiedCpoolsinborealforests (Hardenetal.2000,Bond-Lambertyetal. 2002 b )andbecauseweexpectedfireseverityto influencenotonlydemographicprocessesbutto alsodeterminewhetherfire-killedtreesentered thesnagorwoodydebrisCpool.Whetherdead treesareincontactwiththeforestfloorcould affectCdynamicsthroughoutthesuccessional cyclebyinfluencingthesusceptibilityoftheseC poolstodecomposition(Bond-Lambertyetal. 2002 b )and/orsoilburial(Hagemannetal.2010). Inaddition,becauseoursamplingencompassed standsofvaryingspeciescomposition,ourdata providedauniqueopportunitytoassessthe effectsofforesttypeonmid-successionalCpools.MATERIALSANDMETHODSStudyareaThisstudytookplaceacrossa ; 800km2area ofuplandborealforestsofinteriorAlaska(Fig. 1).Climateiscontinental,withlong,coldwinters andwarm,drysummers.Averagemonthly surfaceairtemperatureinFairbanks,AKfrom 1971–2001rangedfrom 23 8 CinJanuaryto17 8 C inJuly(AlaskaClimateResearchCenter2012). v www.esajournals.org3May2012vVolume3(5)vArticle45ALEXANDERETAL.

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Nearlyhalf(47 % )ofannualprecipitation(260 mm)occursduringthegrowingseason(June– August).ClimateishighlyinfluencedbygeographicboundingbytheBrooksRangetothe north,theAlaskanRangetothesouth,andthe MackenzieRangetotheeast,whichlimitsthe influenceofpolarairmassesfromthenorthand oceanicairmassesfromthesouth.Permafrostis discontinuousinthisarea(Pe we 1975),andsoils rangefrompoorly-drainedGelisolstowellFig.1.(A)StudyareainAlaskawith(B)sampledfirescarslabeledandsamplingsitesrepresentedbysolid triangles.Allfirescarsofintermediateage(20–59yearssincefire)atthetimeofsamplingareshownindarkgrey. Youngerfirescarsareshowninlightgrey. v www.esajournals.org4May2012vVolume3(5)vArticle45ALEXANDERETAL.

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drained,permafrost-freeInceptisols(Dyrness andViereck1982).Thecontinentalclimatealso provesconducivetofrequentwildfires.Fire returnintervalsinborealforestsofwestern NorthAmericarangefrom50–150years(Payette 1992).SiteselectionToassesstheeffectsofincreasingdeciduous dominanceonabovegroundCpools,wesurveyedroad-accessible,mid-successionalfirescars (20–59yearsafterfire)withininteriorAlaska duringsummers2008–2010(Fig.1).Firescars weremappedbytheAlaskaFireServiceand theirlocationsacquiredfromtheAlaskaGeospatialDataClearinghouse(U.S.Departmentof theInteriorBureauofLandManagement2012). Withineachfirescar,welocatedforeststandsof varyingdeciduousdominanceusingacombinationofsatelliteimagerydepictingdeciduous versusevergreencover(Becketal.2011)and groundobservations.Ourgoalwastofindat leastthreestandsperfirescarrangingfrom predominantlyconiferoustopredominantlydeciduouscanopycompositionthatcouldhave originatedfollowingfirewithinastandpreviouslydominatedbyblackspruce.However,this wasnotalwayspossible,andsomefirescars haveonlyonestandortwoormorestandswith similarcanopycomposition.Weassessedthe possibilityofblackspruceoriginationbylooking forfire-killedstems,stumps,andlogsofblack spruce,regeneratingblackspruceintheunderstory,andbyavoidingsoutheastfacingslopes, whereblacksprucetypicallydoesnotgrow.Firekilledblackspruceweredifferentiatedfrom whitespruce( P.glauca )basedonsize,location ofcones,andtwigmorphology.Wealsoavoided samplingpoorly-drainedblacksprucestands, whichhavealongerburnreturnintervaland rarelytransitiontoothertreespecies. Intotal,wesampled53standswithin16fire scars.Fourstandswereremovedafterdeterminingthatactualtreeagesweresubstantiallyolder thanthefireage,suggestingthesestandsdidnot burn,orhadlow-severityfires,despitetheir locationwithinaknownfirescar.Fouradditional standswereremovedbecauseofknownpresamplingcharacteristicsthatrenderedthem differentfromallothersampledstands:two wereriparianwithsubstantialwhitespruce dominanceandtwowerealow-lying,moist blacksprucestands.Anadditionalstandthat consistentlyproducedoutlyingdata,likelydue toitslocationalongahillsidewithconsiderable drainage,wasalsoremoved.Thisleft44stands forouranalyses.Abovegroundbiomassandnetprimary productivity(ANPP)oftreesandlargeshrubsStandinventoriesandallometricequations wereusedtoestimateabovegroundbiomassof treesandlargeshrubs( Salix and Alnus spp.). Withineachstand,weestablisheda100-mlong transectsubdividedintofive20-mlongsubsections.Alonga10-mportionofeach20-m subsection(note:afewstandsin2008were measuredalonga5-mlongsection),wemeasureddiameteratbreastheight(DBH;1.4mtall) orbasaldiameter(BD;forstems 1.4mtall)of livinganddeadtreesandlargeshrubsfalling within1mtoeithersideofthetransect.This generatedfivereplicatesub-plotswithineach stand. Standinventorieswereconvertedtoliveand deadbiomassusingallometricequations.For mostspecies,wederivedallometricequations (AppendixA)fromrawharvestdata(Bonanza CreekLongTermEcologicalResearchSite2009; seeYarieetal.2007fordetailsoftreessampled). Thebest-fitpowerequations( y axb; x BDor DBH; y drymass)relatingBDandDBHtoeach treecomponentandabovegroundtotalbiomass weregeneratedbylogtransformingthedataand fittingabest-fitlinetodeterminethevalueof a andtotestsignificanceofthefit(SigmaPlot version11).They-interceptofthelinearequation generatedfromthelog-transformeddatawas thenback-transformedtodeterminethevalueof b .Powerequationswereconfirmedusingthe powerregressionfunctioninMicrosoftExcel.For Larixlaricina (DuRoi)K.Koch(only5individualssampled)and Salix spp.,publishedequationswereused(Bond-Lambertyetal.2002 a ).For alder( Alnus spp.),equationswereproduced fromrawharvestdatapreviouslyacquiredin interiorAlaska(F.S.ChapinIII, unpublisheddata ). Standingdeadbiomassforalderwasestimated directlyfromharvestdata;forallotherspecies,it wasestimatedasthedifferencebetweentotallive biomassandlivecrownbiomass.Leafareaindex (LAI)wasmeasuredat20-mintervalsalongeach v www.esajournals.org5May2012vVolume3(5)vArticle45ALEXANDERETAL.

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transectusingaLAI-2000PlantCanopyAnalyzer (LI-CORBiosciences,Lincoln,Nebraska,USA). AbovegroundNPP(ANPP)foreachstandwas estimatedasthesumofannualcrowngrowth andsecondarygrowth.Annualcrowngrowthfor treeswasestimatedusingallometricequations (AppendixA)computedfromrawharvestdata asdescribedaboveforbiomass.Crowngrowth forlargeshrubs( Salix and Alnus spp.)was computedasthesumoffoliagebiomass(acquiredfromallometricequations)andnew branchbiomass,whichwasestimatedasthe averageproportionoflivebranchbiomass attributabletonewgrowth.Thisproportion wasbasedonpreviousharvestsandwas10 % for Alnus spp.(F.S.ChapinIII, unpublisheddata ) and17 % for Salix spp.(Macketal.2008).Standspecificestimatesofsecondarytreegrowthwere computedusingannualdiameterincrement increasesofcanopytreesdeterminedfrom growthringmeasurements.At20-mintervals alongeach100-mtransect,werandomlyselected atreeofeachcanopydominantandobtaineda woodslaborcorefromthebaseofthetree( ; 20 cmabovetheorganicsoil).Woodslabswere driedat60 8 C,sequentiallysandedusingfiner andfinergritstoobtainasmooth,clearsurface, andscannedat1200dpi.Ringnumberandwidth weredeterminedusingWinDendro(Regent Instruments,Ontario).Secondarygrowthoftree speciesnotsampledbutpresentinthetransect (typically 10 % oftotalbasalarea)wasestimatedusingtheaveragegrowthincrementofall sampledtreeswithinastand.Themeanaverage annualringwidthforthelast5yearswasused withstemwood/barkallometryequationsto calculatesecondarygrowth.Secondarygrowth for Alnus and Salix spp.wereestimatedas29.8 % growthperyear,basedonestimatesfor S.pulchra Cham.(Shaver1986).WepresentANPPatthe standlevel,whichisANPPsummedforalltrees dividedbythesamplingarea,andANPPtreeat thetreelevel,whichisANPPofthestand dividedbystanddensity.DownedwoodydebrisDownedwoodydebriswasmeasuredalong each20-msubsectionofthe100-mtransectline usingthelineinterceptmethod(Brown1974). Downedfinewoodydebris(FWD)wastallied withinfivesizeclasses,anddownedcoarse woodydebris(CWD; 7cm)wasrecordedby species,diameter(cm),anddecayclassaccording toManiesetal.(2005).Treeswereconsidered downedwoodydebrisandnotsnagsiftheywere atangle 45degreestotheforestfloor.Field dataforFWDwereconvertedtowoodmassper unitareausingmultipliervaluesfromthe NorthwestTerritories,whichtakeintoaccount specificgravity,tilt,andmeansquareddiameter (Nalderetal.1997).AllFWDcalculationswere basedonmultipliervaluesforblackspruce becausewecouldnotidentifyFWDbyspecies. CWDfielddatawereconvertedtomassperunit areausingdecayclassesanddensityvalues derivedfromwoodofthesameorsimilartree species(e.g.,whitebirchsubstitutedforAlaska birch,andblacksprucesubstitutedforwhite spruce)withinOntario,Canada(TerMikaelianet al.2008).Tiltwasassumedtobe1forallCWD estimates.StatisticalanalysisAsuiteofuni-andmultivariateregression modelswerefittedtothedatatodeterminehow muchofthevariationinCpoolscouldbe explainedbydeciduousimportancevalue(IV) andtwootherpotentiallyimportantexplanatory variables,yearssincefireandtotalstemdensity, whichhavebeenshowninotherstudiesto influenceCpools(e.g.,Macketal.2008).To determinethedeciduousIVofeachstand,we groupedtreeandlargeshrub( Salix spp.and Alnus spp.)speciesbytheirleafphenology (evergreenvs.deciduous)andthencalculated anIVofthedeciduouscomponentofeachstand basedonstandinventorydata,whereIV relativedensity relativebasalarea relative frequency.Allresponsevariablesweretestedfor normalityandhomogeneityofvariancepriorto analysesandweretransformedusinglogarithmic (biomassandANPP)orsquare-root(snagsand woodydebris)transformationswhentheydid notmeettheseunderlyingassumptions.Wethen generatedlinearmodelscontainingeachofthe threeexplanatoryvariablesindependentlyand everycombinationofthesevariablesandtheir interactionsusingthebasepackageinR(RCore DevelopmentTeam2010).Sinceouroriginal studydesignwastosampleseveralforeststands ofvaryingdeciduousdominancewithinthesame firescar,wealsomodeledourdatausing  fire v www.esajournals.org6May2012vVolume3(5)vArticle45ALEXANDERETAL.

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scar  asarandomeffect.However,thisdidnot improveourmodels,sowasleftoutoffuture analyses.Weselectedthe  best-fit  multivariate modelforeachdependentvariablebasedon AkaikeInformationCriteria(AIC)valuescorrectedforsmallsamplesizes(AICc)andcomparedthismodeltothemoreparsimonious, univariatemodels.TheAICestimatesgoodness offitforastatisticalmodelandiscomputedas2 k 2(ln) L ,where k isthenumberofparametersin thestatisticalmodel,and L isthemaximized valueofthelikelihoodfunctionfortheestimated model.WespecificallyusedAICc,whichtakes intoaccountsamplesizeandpreventsoverfitting ofthemodel(BurnhamandAnderson2004).We present  goodness-of-fit  resultsforeachsignificantunivariatemodelandthe  best-fit  multivariatemodelbasedontransformeddata,butstandlevelmeansandstandarderrorsarepresented foruntransformeddata. TobetterunderstandhowcompositionalvariationsamongforeststandsinfluenceCpools,we subdividedstandsintoforesttypesbasedonthe relativecontributionofeachspeciestototalstand biomass.Speciesoccurringinfrequently( L.laricina P.balsamifera L.,and P.glauca (Moench) Voss)weregroupedintoasingle  other  category, andwillowsandaldersweregroupedintoa  tall shrub  category.Anyspeciesconstitutingatleast 20 % ofstandbiomasswasconsideredtobea dominantstandcomponent.For27of44stands, blackspruce( n 14),aspen( n 12),Alaskabirch ( n 3),orshrubs( n 1)clearlydominated, havinggreaterthan66 % ofstandbiomass.The otherstandsgroupedintoblackspruce/shrub( n 4),blackspruce/aspen( n 3),blackspruce/ aspen/shrub( n 3),blackspruce/Alaskabirch( n 6),andblackspruce/Alaskabirch/shrub( n 2). Thesingleshrubstandwasremovedfromour analyses,andthestandscontainingamixof shrubsandtreesweregroupedwithstands containingthesamedominanttreecomponent. Thisapproachgeneratedfiveforesttypes(AppendixB). WecomparedstandcharacteristicsandCpools amongforesttypesofsimilaragesusingan ANOVA(SASversion9.2).Todothis,we groupedforesttypesintotwoagecategories: (1)20–39yearssincefireand(2)40–59years sincefire.ThisgroupingyieldedasingleAlaska birchstandintheyoungeragecategoryand singleaspenstandintheolderagecategory. Thus,wepresentdatafromthesestandsfor comparativepurposes,butremovedthemfrom statisticalanalyses.Toimproveourunderstandingofhowabovegroundbiomass,ANPP,and ANPPpertreevarywithyearssincefireamong foresttypes,wetestedforaninteractioneffectof foresttypeandyearssincefire,andwhen significant,performedpost-hocmultiplecomparisonteststodeterminedifferencesamongslopes (GraphPadPrismversion5.0).Becauseour samplesizewasreducedbydividingstandsinto foresttypes,wereportsignificanceforall analysesofforesttypeat a 0.10. F -valueswere computedbasedonTypeIIIsumsofsquare.RESULTSAbovegroundNPPandCpoolsasafunctionof deciduousIV,yearssincefire,anddensityModelcomparisonsindicatedthatbothdeciduousIVandyearssincefiresignificantlyinfluencedtotalabovegroundbiomassoftreesand largeshrubsandtheirabovegroundproductivity (ANPP)(Table1),whilestanddensityhadno detectableeffect.Abovegroundbiomassand ANPPexhibitedasignificantlinearincreasewith increaseddeciduousIVandyearssincefire(Fig. 2;Table1).Biomassrangedfromalowof58.0 6 30.0g/m2ina26-year-oldblacksprucestandto 11,614.8 6 1475.6g/m2ina40-year-oldAlaska birchstand.ANPPvariedfrom8.4 6 3.9 g m 2 yr 1inthesame26-year-oldblackspruce standto833.3 6 128.3g m 2 yr 1ina51-year-old Alaskabirchstand.Whenconsideredindependently,deciduousIVandyearssincefireexplained17 % and33 % ofthevariationinbiomass and26 % and12 % ofthevariationinANPP, respectively.Includingbothindependentvariablesinamultivariatemodelsubstantially improvedourabilitytoexplainvariationsin biomassandANPP,capturing64 % and48 % of theirvariability,respectively.Thelackofstronger correlationsamongdependentandindependent variablesmayreflectlandscapeheterogeneity,as standsweresampledacrossavastareaencompassingarangeofaspects,slopes,andsoil conditions.Notably,however,thetrendof increasingabovegroundbiomasswithincreasing deciduousIVwasdetectedwithinindividualfire scars(datanotshown),suggestingtheimporv www.esajournals.org7May2012vVolume3(5)vArticle45ALEXANDERETAL.

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tanceofthisexplanatoryvariableregardlessof otherfactorssuchasstandageorlandscape variability. Evergreensnagbiomassdecreasedlinearly withyearssincefire,whiledeciduoussnag biomassincreased(Table1,Fig.2).Deciduous snagbiomassalsoexhibitedasignificantlinear increasewithincreaseddeciduousIV.Density hadnoeffectoneitherdependentvariable. Evergreensnagbiomassdeclinedfrom2472.4 6 912.0g/m2ina20-year-oldstandtonobiomass innumerousstandsofvariousages.Although yearssincefirewasasignificantexplanatory variableofevergreensnagbiomass,itexplained only13 % ofthevariability.Modelcomparisons indicatedthatnomultivariatemodelpredicted evergreensnagbiomassbetterthanaunivariate modelbasedonyearssincefire.Whenconsideredindependently,yearssincefireanddeciduousIVexplained12 % and24 % ofthevariability indeciduoussnagbiomass,respectively,which increasedfromnobiomassinseveralblack sprucestandsto1600.2 6 557.6g/m2ina51year-oldAlaskabirchstand.Includingboth explanatoryvariablesandtheirinteractionina multivariatemodelexplained49 % ofthevariabilityindeciduoussnagbiomass. Downedwoodydebrisbiomassexhibiteda morecomplicatedtrend,withallthreemodeled independentvariablessignificantlyexplaining variationinthisparameter(Table1,Fig.2). Downedwoodydebrisexhibitedonlyamodest increasewithincreasingdeciduousIVandstand density,butdeclinedwithincreasingyearssince fire,rangingfrom157.0 6 43.4g/m2ina50-yearoldblacksprucestandwith6.2treesm 2to 4162.8 6 1558.9g/m2ina39-year-oldblack sprucestandwith5.7tree/m2.DeciduousIVand densityexplainedonly7 % and11 % ofthe variationindownedwoodydebris,whileyears sincefireexplained23 % .The  best-fit  multivariatemodelincludedallthreeexplanatoryvariables,capturing29 % ofthevariabilityindowned woodydebris.However,modelcomparisons indicatethatthismodelwasonly0.7AICcunits differentfromtheunivariatemodelwithonly yearssincefireasanexplanatoryvariable, suggestingthatthemorecomplexmodelisno betteratexplainingvariationsindownedwoody debristhanthemoreparsimoniousunivariate model. Table1.Summaryofsignificantunivariateandthe  best-fit  multivariateregressionmodelstopredict abovegroundCpoolswithinborealforestsofinteriorAlaska( n 44).DependentvariableModeltypeIndependentvariable(s) P adj. R2AICcD AICcAboveground biomass(g/m2) SignificantunivariateDeciduousIV( )0.0020.1760.235.6 Yearssincefire( ) 0.0010.3351.026.4  Best-fit  multivariate DeciduousIV ( ) ,Yearssincefire ( ) 0.0010.6424.60.0 ANPP(g m 2 yr 1)SignificantunivariateDeciduousIV( )0.0010.26276.513.8 Yearssincefire( )0.0150.12284.521.8  Best-fit  multivariate DeciduousIV ( ) ,Yearssincefire ( ) 0.0010.48262.70.0 Evergreensnag biomass(g/m2) Significantunivariate Yearssincefire ( ) 0.0110.13343.70.0 Deciduoussnag biomass(g/m2) SignificantunivariateDeciduousIV( )0.0130.12326.921.1 Yearssincefire( )0.0010.24320.414.6  Best-fit  multivariate DeciduousIV ( ) ,Yearssincefire ( ) DeciduousIV 3 Yearssincefire ( ) 0.0010.49305.80.0 Downedwoody debris(g/m2) SignificantunivariateDeciduousIV( )0.0460.07257.59.2 Yearssincefire( )0.0010.23249.00.7 Density( )0.0150.11255.67.3  Best-fit  multivariate DeciduousIV ( ) ,Yearssincefire ( ) Density ( ) 0.0010.29248.30.0 Notes: Candidateunivariatemodelsincludeddeciduousimportancevalue(IV),yearssincefire,andtotalstemdensity. Multivariatemodelsincludedallcombinationsofthesevariables.OnlythebestfitmultivariatemodelwiththelowestAICc(AkaikeInformationCriteriacorrectedforsmallsamplesizes)ispresented,whileallunivariatemodelsexhibitingasignificant effectareshown. D AICcindicatesthedifferencebetweenAICcvaluesforthebestfitmodelamonguni-andmultivariate models.Aplusornegativesignindicatesthedirectionoftheeffectoftheindependentvariableonthedependentvariable. v www.esajournals.org8May2012vVolume3(5)vArticle45ALEXANDERETAL.

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Fig.2.VariationsinabovegroundCpoolsasafunctionofdeciduousimportancevalue(IV)andyearssincefire across44mid-successionalborealforeststandswithininteriorAlaska. v www.esajournals.org9May2012vVolume3(5)vArticle45ALEXANDERETAL.

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Standcharacteristics,abovegroundNPP,and CpoolsacrossforesttypesAmongyoungerstands(20–39yearssince fire),density,height,diameter,basalarea,LAI, abovegroundbiomass,andANPPofblack sprucestandsconsistentlydifferedfromaspen stands(Table2).Blacksprucestandscontained treesthatwerethreetimesshorterthanaspen stands(2.2 6 1.3mvs.6.4 6 2.0m)withhalfthe basalarea(6.2 6 4.7vs.12.2 6 5.2cm/m2)and LAI(0.5 6 0.5vs.1.1 6 0.6m2/m2).Biomass containedwithinlivebranchesandstemwood/ barkwas2.7and5.4timeslowerinblackspruce standscomparedtoaspenstands,andtotal biomasswasfourtimeslessinblackspruce (675.0 6 493.1g/m2)comparedtoaspen(2705.8 Table2.MeanstandcharacteristicsandabovegroundCpoolsfor20–39-year-oldstandsdistributedacrossboreal foresttypeswithininteriorAlaska.Parameter Foresttype df FP Blackspruce ( n 10) Blackspruce Alaskabirch ( n 4) Blackspruce Aspen ( n 4) Aspen ( n 11) Alaskabirch ( n 1) Standcharacteristic Density(trees/m2)4.93.07.13.72.63,250.590.63 (4.4)(0.9)(8.0)(5.3) Treediameter(cm)1.1A1.4A1.4A2.0B2.83,254.94 0.01 (0.3)(0.4)(0.5)(0.8) Basalarea(cm2/m2)6.2A7.1A9.4AB12.2B20.23,252.660.07 (4.7)(4.9)(5.7)(5.2) Height(m)2.2A3.3A3.3A6.4B7.13,2510.70 0.001 (1.3)(1.3)(2.4)(2.0) Leafareaindex(m2/m2)0.5A0.5A0.5A1.1B0.23,242.930.06 (0.5)(0.2)(0.3)(0.6) Abovegroundbiomass(g/m2)Foliage92.098.5162.3145.1179.13,251.270.31 (58.1)(67.7)(153.9)(62.9) Livebranches145.4A194.4A254.9AB386.7B481.43,253.250.04 (188.3)(171.7)(155.2)(187.9) Stemwood/bark355.3A812.6A771.6A1910.5B4338.13,258.16 0.001 (278.1)(493.6)(802.7)(1021.2) Total675.0A1219.6A1409.8A2705.8B5180.93,256.900.017 (493.1)(799.9)(1342.8)(1354.2) ANPP(g m 2 yr 1) Crowngrowth44.3A57.5A75.9AB111.7B191.53,253.740.02 (45.4)(37.2)(40.7)(54.0) Secondarygrowth50.2A93.0A82.0A156.1B606.83,253.830.02 (44.7)(64.3)(52.8)(96.7) Total94.5A150.4A157.9AB267.8B798.33,254.120.02 (89.4)(98.2)(92.5)(142.5) Pertree25.6A56.6A61.1A122.4B330.63,256.35 0.01 (18.2)(39.0)(49.2)(72.0) Snags(g/m2) Evergreen430.4238.2248.0274.643.83,250.230.88 (749.9)(395.3)(286.2)(351.9) Deciduous46.436.3109.4259.0289.63,251.440.25 (111.1)(66.3)(163.2)(382.8) Total476.7965.3357.4533.6333.43,250.640.59 (733.8)(1244.5)(200.3)(445.1) Downedwooddebris(g/m2) Finewoodydebris(FWD)1051.61018.21823.51238.3567.03,251.880.16 (626.4)(247.8)(792.4)(532.4) Coarsewoodydebris(CWD)600.0714.91131.7735.81323.43,250.450.72 (683.7)(592.8)(794.9)(878.0) Total1651.61733.12955.21974.11890.33,251.470.25 (1237.8)(777.3)(1343.3)(896.7) Notes: Valuesaremeans( 6 1SD).Differentsuperscriptlettersindicatesignificantdifferencesamongforesttypesforagiven forestparameter. Foresttypesrepresentedbyonlyasinglestandwereremovedfromstatisticalanalyses. Aseparateallometricequationwasusedtoestimatetotalabovegroundbiomasssotheindividualcomponentsdonotsum tothisnumber. v www.esajournals.org10May2012vVolume3(5)vArticle45ALEXANDERETAL.

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6 1354.2g/m2)stands.Crowngrowth,secondary growth,andtotalANPPwerethreetimeslessin blacksprucecomparedtoaspenstands,while ANPPexpressedonapertreebasiswas5times less.AlthoughweonlysampledasingleAlaska birchstandinthisagecategory,biomassand ANPPcomponentsofblacksprucestandswere alsoconsistentlylowerthanthoseinthisAlaska birchstand.Cstoredwithinbiomassofsnags anddownedwoodydebrisdidnotvaryamong standtypes. Amongolderstands(40–59yearssincefire), density,height,diameter,basalarea,LAI,abovegroundbiomass,andANPPofblackspruce standsoftendifferedfromstandscomprisedof aspen,Alaskabirch,oramixofblackspruceand Table3.MeanstandcharacteristicsandabovegroundCpoolsfor40–59-year-oldstandsdistributedacrossboreal foresttypeswithininteriorAlaska.Parameter Foresttype df FP Blackspruce ( n 4) Blackspruce Alaskabirch ( n 4) Blackspruce Aspen ( n 2) Aspen ( n 1 ) Alaskabirch ( n 2) Standcharacteristic Density(trees/m2)4.6A2.8B1.8B0.80.9b3,84.100.05 (2.1)(0.3)(0.8)(0.3) Treediameter(cm)1.6A2.8B2.7B5.26.0c3,833.47 0.001 (0.4)(0.3)(0.8)(0.9) Basalarea(cm2/m2)13.8A27.0B16.1A21.630.8c3,85.120.03 (8.9)(4.5)(2.4)(2.2) Height(m)2.7A7.9B8.1B10.613.8c3,814.86 0.01 (0.2)(3.2)(0.8)(0.3) Leafareaindex(m2/m2)0.7A2.3B1.1A1.82.9b3,78.79 0.01 (0.6)(0.5)(0.7) Abovegroundbiomass(g/m2)Foliage282.0A500.4B249.6A191.5291.7c3,85.710.02 (116.1)(85.2)(34.9)(4.3) Livebranches442.6A889.1BC546.8AB574.91284.7c3,84.430.04 (413.2)(221.8)(86.2)(11.3) Stemwood/bark1256.7A5273.8B2953.4A4897.79506.5c3,813.36 0.01 (683.3)(2430.2)(682.0)(334.6) Total2425.2A7534.9B4152.4A5869.411363.6c3,811.51 0.01 (1368.9)(2753.4)(644.8)(355.3) ANPP(g m 2 yr 1) Crowngrowth94.1A214.2BC135.7AB158.2344.0c3,87.66 0.01 (78.2)(62.1)(39.7)(21.5) Secondarygrowth109.1260.1158.3158.1311.73,82.480.14 (89.4)(13.6)(66.9)(229.7) Total203.2A474.2BC294.0AB316.3655.6c3,84.990.03 (167.1)(74.4)(106.6)(251.2) Pertree46.3A184.6B189.7B480.0720.7c3,856.25 0.001 (31.1)(47.7)(139.5)(24.2) Snags(g/m2) Evergreen79.8152.1126.38.40.33,80.680.59 (83.7)(163.5)(177.0)(0.5) Deciduous170.9434.8458.5229.9963.93,81.780.23 (243.9)(254.9)(265.7)(899.9) Total250.8586.9584.8238.3964.23,81.060.42 (320.2)(391.9)(442.7)(900.3) Downedwooddebris(g/m2) Finewoodydebris(FWD)319.3334.9632.7779.343.03,82.060.15 (176.5)(139.0)(125.9)(4.6) Coarsewoodydebris(CWD)282.3348.9258.6177.562.03,80.230.87 (461.3)(611.4)(155.1)(56.9) Total602.2683.8891.3896.8104.93,80.310.82 (626.0)(675.2)(29.2)(52.3) Notes: Valuesaremeans( 6 1SD).Differentsuperscriptlettersindicatesignificantdifferencesamongforesttypesforagiven forestparameter. Foresttypesrepresentedbyonlyasinglestandwereremovedfromstatisticalanalyses. Aseparateallometricequationwasusedtoestimatetotalabovegroundbiomasssotheindividualcomponentsdonotsum tothisnumber. v www.esajournals.org11May2012vVolume3(5)vArticle45ALEXANDERETAL.

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Alaskabirch,buttendedtobesimilartomixed standsofblackspruceandaspenstands(Table 3).Themostpronounceddifferenceswere betweenblackspruceandAlaskabirchstands. ComparedtoAlaskabirch,blacksprucestands werefivetimesdenser(4.6 6 2.1vs.0.9 6 0.3 trees/m2)andcomprisedofshortertrees(2.7 6 0.2vs.13.8 6 0.3m)withsmallerdiametersthat encompassedlessthanhalfthebasalarea(13.8 6 8.9vs.30.8 6 2.2cm/m2).Blacksprucestands alsohadfourtimeslowerLAIvaluesthanthese stands(0.7 6 0.6vs.2.9 6 0.7m2/m2).Total biomasswasapproximatelyfivetimeslowerin blackspruce(2425.2 6 1368.9g/m2)comparedto Alaskabirchstands(11,363.6 6 355.3g/m2),due tothreetimeslowerlivebranchbiomassand eighttimeslowerstemwood/barkbiomass.Althoughweonlysampledasingleaspenstandin thisagecategory,biomassandANPPcomponentsofblacksprucestandswerealsoconsistentlylowerthanthoseinthisaspenstand. Similartoyoungerstands,Cstoredwithin biomassofsnagsanddownedwoodydebris didnotvaryamongstandtypes. Changesinabovegroundbiomass,ANPP,and ANPPtreeasafunctionofyearssincefirevaried amongforesttypes(Tables4and5;Fig.3). Abovegroundbiomassexhibitedasignificant linearincreasewithyearssincefireinallforest types,exceptAlaskabirch,whichhadlow replication( n 3),buttherateofbiomass accumulationwas2–7timeslowerinblackspruce standscomparedtoallotherforesttypes.ANPP expressedatthestandleveldidnotvarywith yearssincefireinblackspruce,aspen,orAlaska birchstands,butexhibitedasignificantlinear increasewithyearssincefireinmixedstands. ANPPtreeincreasedwithyearssincefireinall foresttypes,butthistrendwasnotsignificantin blackspruce/aspenstandsandAlaskabirch stands.Therateofincreasewasslowestinblack sprucestandscomparedtoallotherforesttypes. Table4.Analysisofcovarianceresultscomparingtheregressionslopesofacrossforesttypesforaboveground biomass,abovegroundnetprimaryproductivity(ANPP),andANPPtree.Source Abovegroundbiomass (g/m2) ANPP (g m 2 yr 1) ANPPtree(g m 2 yr 1) df FP df FP df FP Foresttype5,311.870.125,313.350.025,310.750.59 Yearssincefire5,3117.29 0.0015,310.130.725,317.910.008 Foresttype 3 Yearssincefire5,314.360.0045,311.990.105,313.30.02 Contrasts Blacksprucevs.Alaskabirch/Blackspruce1,1822.77 0.0011,185.10.041,1812.910.002 Blacksprucevs.Aspen/Blackspruce1,163.160.091,160.810.381,163.130.1 Blacksprucevs.Aspen1,223.270.081,220.130.721,2211.750.002 Blacksprucevs.Alaskabirch1,138.630.011,134.330.061,1315.130.002 Alaskabirch/Blacksprucevs.Aspen/Blackspruce1,1010.510.0091,103.050.111,100.520.49 Alaskabirch/Blacksprucevs.Aspen1,165.360.031,163.720.071,161.430.25 Alaskabirch/Blacksprucevs.Alaskabirch1,70.050.841,79.560.021,72.660.15 Aspen/Blacksprucevs.Aspen1,140.440.521,140.920.351,142.710.12 Aspen/Blacksprucevs.Alaskabirch1,54.780.081,512.80.021,52.380.18 Aspenvs.Alaskabirch1,112.050.181,112.070.181,110.460.52 Table5.Parameterestimates,regressioncoefficient,andsignificanceoflinearmodelsforeachforesttypeand abovegroundbiomass,abovegroundnetprimaryproductivity(ANPP),andANPPtree.Foresttype Abovegroundbiomass(g/m2)ANPP(g m 2 yr 1)ANPPtree(g m 2 yr 1) intslope R2intslope R2intslope R2Blackspruce 65054.70.3*37.12.70.06 3.71.10.26 Blackspruce/Alaskabirch 8804.2345.70.8** 26715.20.63* 1276.50.66* Blackspruce/Aspen 1894.8115.10.95*** 33.76.50.77* 504.20.41 Aspen 1336.4150.60.47*2630.310 18811.90.48* Alaskabirch 3369.5311.60.76137916.60.72 13517.90.62 P 0.05,** P 0.01,*** P 0.001. v www.esajournals.org12May2012vVolume3(5)vArticle45ALEXANDERETAL.

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DISCUSSIONIffireintensitycontinuestoincreasewith climatewarminganddryingaspredicted(Turetskyetal.2010),manyareasoftheinterior Alaskanborealforestmayexperienceathreshold responsewherestandspreviouslyundergoing repeatedsuccessionalcyclesofblackspruceselfreplacementshifttonewtrajectoriescharacterized bygreaterdeciduousdominance(Johnstoneetal. 2010 a ).Becketal.(2011)suggestthatthisshiftis alreadyunderway,asforestedareaswhichburned atahighseveritysincethe1950scurrentlyexhibit agreaterfractionofdeciduousbiomassthan standswhichburnedatalowseverity.This transitionwilllikelyhavenumerousconsequences forwildlife(Chapinetal.2008),localclimate (Randersonetal.2006),andfutureforestflammability(Ruppetal.2002,JohnstoneandChapin 2006).Ourfindingsheresuggestthatatrajectory shiftwillalsoaltermid-successionalCdynamics, leadingtoincreasedCaccumulationratesand storageinabovegroundCpools.AbovegroundbiomassandANPPBothabovegroundbiomassandANPPoftrees andlargeshrubsincreasedwithincreasing deciduousIV.Theseincreaseswereinextricably linkedtoincreasesduetoyearssincefirebut wereunaffectedbystanddensity.Theymayalso bepartiallyassociatedwiththetypeofdeciduous species,asAlaskabirchstandsaccumulatedand storedmorebiomassthanaspenstands.However,becausewesampledonlythreeAlaskabirch stands,wecannotdeterminewhetherthese trendsholdtrueacrossthelandscapeorsimply reflectdifferencesinsitequalitybetweensampledaspenandAlaskabirchstands.Modeled estimatesofaspenvs.Alaskabirchbiomass acrossinteriorAlaskabasedonForestInventory Analysis(FIA)datasuggestthatlandscape-level abovegroundbiomassinAlaskabirchforestsis oftenmuchgreaterthanthatinaspenforests, especiallyduringmid-succession(Yarieand Billings2002).However,previousresearchhas clearlyshownthatsitequalitycanproduce substantialvariationsinANPPandbiomass, evenamongstandsofsimilarageandcomposition.Forinstance,Vierecketal.(1983)measured twoandthreetimeshigheraspenANPPand biomass,respectively,ona60-year-oldmesicsite comparedtoa50-year-olddrysitewithininterior Alaska,andWangetal.(1995)measured approximatelytwotimeshigherpaperbirch( B. papyrifera Marsh.)biomasson  good  vs.  poor  qualitysites.RegardlessofthefactorscontributingtohighlevelsofAlaskabirchANPPand biomass,ourdataclearlydemonstratethatthese parametersareconsistentlylowerinmid-successionalblacksprucestandscomparedtothose dominatedbyaspenorAlaskabirchandoften lowerthanstandsofmixedcomposition.Much Fig.3.(A)Abovegroundbiomassintreesandlarge shrubs,(B)abovegroundnetprimaryproductivity (ANPP)ofthestand,and(C)ANPPexpressedonaper treebasisformid-successionalstandsoffiveforest typeslocatedininteriorAlaska. v www.esajournals.org13May2012vVolume3(5)vArticle45ALEXANDERETAL.

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ofthebiomassaccumulationindeciduousstands canbeattributedtohighertree-levelANPP, allowingindividualstemsofdeciduousspecies toaccumulatemorestemwood/barkfasterthan blacksprucetrees.Asimilartrendofhighermidsuccessionalbiomassinmixed/deciduousstands comparedtoblacksprucestandswasalso observedbyMacketal.(2008),workingin interiorAlaska,andGoetzandPrince(1996)in northernMinnesota. Ourestimatesofabovegroundbiomassand ANPPformid-successionalstandsvarysomewhatfromthosereportedinotherstudies.Both biomassandANPPofblackspruceandaspen standsweresimilartothosemodeledbyYarie andBillings(2002)forinteriorAlaska,butour estimatesforAlaskabirchstandswere2–3times higherthantheirestimates.Ourdatafromonly threestandsmayrepresentanomalouslyhigh valuesassociatedwithsitequalityorother factorsunrepresentativeofthelargerlandscape. Ourbiomassestimatesformid-successional aspenstandswereaboutthreetimeslessthan thosequantifiedacrossachronosequenceof aspenstandsinBritishColumbia,Canada(Wang etal.1995),likelyreflectingregionaldifferences inclimateandlengthofgrowingseasonthat limitheightanddiametergrowthofindividual trees,asstanddensityinourstudywassimilarto orgreaterthanthatmeasuredintheCanadian study.Comparedtoanotherstudyininterior Alaska,ourestimateofabovegroundbiomassof a40-year-oldaspenstandwas12 % higherthan thatreportedfora50-year-oldaspenstand growingonadrysitebutwasfourtimeslower thana60-year-oldaspenstandgrowingona mesicsite;ourbiomassestimatefortwo40–59year-oldAlaskabirchstandswas11 % higher thana77-year-oldAlaskabirchstand(Vierecket al.1983,VanCleveetal.1983 b ).OurapproximationsofabovegroundbiomassandANPPfor blacksprucestandswereconsiderablylower thanthoseestimatedfor20and37-year-oldwelldrainedstandsinnorthernManitoba,Canada (Wangetal.2003,Bond-Lambertyetal.2004), likelybecausetreesinthepresentstudywere smallerwithlessbasalareaandlowerLAI.DownedwoodydebrisandsnagsWefoundlittleeffectofdeciduousIVon downedwoodydebrisorevergreensnagbiomass.Evergreensnagswerehighestinthe youngeststands,likelyduetoapulseofinputs followingthefiredisturbance,anddeclinedwith yearssincefire.Downedwoodydebrisdeclined withyearssincefireandincreasedwithstand density.Adecreasewithyearssincefiresuggests decreasedinputsfollowingtheinitialdisturbance,burialofdownedwoodydebrisbymosses andotherplants,and/orincorporationintothe soilCpool(Hagemannetal.2010).Increased downedwoodydebriswithstanddensitymay beindicativeofself-thinningleadingtowhole treeorbranchmortality(Sturtevantetal.1997). Or,thecloseproximityoftreesmaycause physicalabrasionandtransferofbranchesto theforestfloor. LittlecorrelationbetweendeciduousIVand downedwoodydebrisorevergreensnagbiomasssuggestthatthefactorsinfluencingdemographicprocessesandabovegroundbiomassof livingtrees/largeshrubs(i.e.,fireseverityand soilorganiclayerdepth)differfromthose affectingsnagandwoodydebrisCpools. Alternatively,theinfluenceoffireseverityon thesepoolsmaybemostpronouncedinearlysuccessionalstands.Forinstance,athickresidual soilorganiclayerinyoungstandsburnedata lowseveritymaykeepfire-killedtreesstanding, whileconsumptionoftheorganiclayerinstands burnedathighseveritymaycausetreestofall andenterthedownedwoodydebrispool. However,overtime,astherootsoffire-killed snagsdecayandsoilssubsidearoundtheroot ball,evenathicksoilorganiclayermaybe unabletokeepfire-killedtreesinthesnagCpool. DeciduoussnagbiomassincreasedwithincreasingdeciduousIV,butonlyintheolder stands.Becauselongevityofdeciduoustreesand largeshrubsisshorterthanblacksprucetrees, theseindividualsbegincontributingtothe deciduoussnagpoolinthelaterpartsofmidsuccession.Thus,asdeciduousIVincreases, thereappearstobetwodistinctperiodsoftree mortality:onecausedbytheinitialdisturbance andonerelatedtoindividualdeciduoustree mortalityasthestandages.Althoughsnagsare moreresistanttodecompositionthandowned woodydebris(WeiandKimmins1998,Kasischke andStocks2000),theearliermortalityofdeciduoustreescomparedtoblacksprucetrees suggeststhattheCaccumulatedinaboveground v www.esajournals.org14May2012vVolume3(5)vArticle45ALEXANDERETAL.

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biomassinthesestandscouldrepresentaless persistentCreservoir.Implicationsofincreased  deciduousness  onecosystemCdynamicsTheconsequencesofincreasedfireseverityon abovegroundCaccumulationratesandpools willdependontheproportionofthelandscape withpre-fireconditionsconducivetoatrajectory shift.Moderatetowell-drainedareas(Johnstone etal.2010 b )onsoutherlyslopesandmidelevation(Kurkowskietal.2008)orthosewith flattopography(Kasischkeetal.2000)aremost susceptibletofireseverityimpactsandsuccessionalshiftsbecausetheyaredrier,burnreadily, andarewithinthephysiologicaltolerancesof bothblackspruceanddeciduoustrees(Kurkowskietal.2008).Kurkowskietal.(2008)suggest thatabout30 % ofblacksprucestandscurrently onthelandscapetodayhavethepotentialtoshift fromaself-replacementtrajectorytoonewith greaterdeciduousdominance. Toillustratethepotentialimpactsofatrajectoryshiftonlandscape-levelCpools,weused ourabovegroundbiomassestimatestocalculate changesinabovegroundCpoolsassociatedwith ashiftfrommatureblackspruceforeststomidsuccessionalforestsofmixedordeciduous composition.Mature( 60-year-old)blackspruce forestscover ; 7.3millionhawithininterior Alaska,withanaverageabovegroundbiomassof 27.7Mg/ha(YarieandBillings2002).Basedon abovegroundbiomassestimatesassociatedwith therangeofforesttypesandconditionssampled inthisstudy,iftheseareassimultaneouslyshifted toaspenormixedblackspruce/aspenmidsuccessionalstands,abovegroundbiomass storedinborealforestsofinteriorAlaskawould remainunchangedbecausethesemid-successionalstandsareabletoaccumulateandstore biomassfasterthanmatureblacksprucestands. However,ifmatureblacksprucestandsconvertedtomixedblackspruce/AlaskabirchorAlaska birchstands,abovegroundbiomasswouldincreaseby35(58 % )or142(235 % )millionMg, respectively.TheforestbiomassofborealAlaska iscurrentlyestimatedat815millionMg(Yarie andBillings2002);assuch,atrajectoryshiftto Alaskabirchwouldrepresenta12 % and17 % increaseintotalabovegroundbiomassduring mid-succession,respectively.Mostbiomassin deciduousstandsaccumulatesasstemwood/ bark,whichdecomposesslowly(Trofymovet al.2002),sowouldlikelyprovidealong-term snaganddownedwoodydebrisCreservoir withinthesestandsevenasdeciduoustrees/large shrubsdie(Swift1977). Inadditiontoincreased  deciduousness  asa consequenceofincreasedfireseverity,compositionalshiftsmayalsooccurduetodirecteffects ofclimatewarminganddrying,potentially amplifyingtheeffectsofshiftingforesttrajectoriesonabovegroundCpools.Forinstance,Calef etal.(2005)usedalogisticregressionmodeling approachtoexaminet heeffectsofclimate warminganddryingonforestcoverandfound thatdeciduousforeststandsininteriorAlaska expandwithincreasedatmospherictemperatures anddecreasedprecipitation,andthatmuchof thisexpansionoccurredatthelossofwhite spruce.Theyalsofoundthatdeciduouscover increasedwithdecreasedfirereturninterval,but fire-vegetationfeedbacksmaynegatethiseffect. Althoughthefirereturnintervalininterior Alaskahasrecentlydecreasedfrom196to144 yearsduetoaprolongedfireseasonand increasedburnedarea(Kasischkeetal.2010),a continueddecreasemayonlyoccuraslongas highlyflammableblacksprucestandsremain dominantacrossthelandscape(Ruppetal.2002). Ashifttogreaterdeciduousdominancewould presumablyincreasefirereturnintervals(Chapin etal.2008)duetolowerfuelaccumulationinthe forestfloorandunderstory,higherleafmoisture (Johnson1992),andlowerflammability(Cummingetal.2000,Chapinetal.2008). TheroleofsuccessionaltrajectoriesinC balancealsodependsuponthefateofmixed anddeciduousstandsastheymature,yetstand dynamicsduringlatersuccessionalstagesare largelyunknown.Deciduoustreeslivebetween 80and150years(YarieandBillings2002),but mortalityincreasesandANPPdeclinesafter60 years(Vierecketal.1983).Deciduoustree regenerationmayproceedasexuallyviagap dynamics(Cummingetal.2000),butbecause aspenandpaperbircharegenerallyshadeintolerant(KobeandCoates1997),regeneration potentialmaybelimitedwithoutwind,ice,orotherdisturbancestofellmultipletreesand createlargecanopygaps.Becauseconiferabundanceisusuallylowinthedeciduousunderstory v www.esajournals.org15May2012vVolume3(5)vArticle45ALEXANDERETAL.

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(JohnstoneandChapin2006,Kurkowskietal. 2008),thesestandsarealsounlikelytoproceed alongarelaysuccessionalpathway,whereblack spruceemergefromunderstorysuppression followingmortalityofthedeciduouscanopy. Understorycompositionofmaturedeciduous standsisoftencomprisedofshrubsandgrasses (Vierecketal.1983);therefore,thesefunctional typesmaybecomethedominantcoverwhere treeregenerationislimited.Mixedstandsmay eventuallysucceedtoblackspruce(i.e.,relay succession)aslongthefirereturninterval remainssufficientlylongbecauserelaysuccessionmayrequireupto200years(Schulzeetal. 2005).Ifthisoccurs,blacksprucestandsoriginatingviarelaysuccessionmayhavelower density,abovegroundbiomass,productivity, andsoilCpoolsthanblacksprucestands originatingviaself-replacementbecauseoflegacyeffectsleftbehindbydeciduoustreesand largeshrubs(Macketal.2008). IfincreasedfireseverityalsoaltersbelowgroundbiomassorsoilCpools,thismayeither accentuateoroffsetthedifferencesinabovegroundCpoolsobservedbetweenforeststands undergoingdifferentsuccessionaltrajectories.In blacksprucestands,alargesoilCpooltendsto accumulateduetoathickmossunderstoryand lowdecompositionrates(VanCleveetal.1983 a ; Goweretal.1997).Deciduousleaflitterphysicallyimpedesmossdevelopment(Oecheland VanCleve1986)anddecomposesmorerapidly (FlanaganandVanCleve1983),loweringmean residencetimesofsoilC(FlanaganandVan Cleve1983).Forexample,VanCleveetal.(1983 b ) reportedthattheCstoredwithintheforestfloor andtop70cmofmineralsoilwithinmature blacksprucestands(120–150-year-old)was ; 8,000and7,000g/m2,respectively,compared toonly ; 6,000and5,000g/m2inmatureAlaska birchstands(70–130-year-old).However,abovegroundtreebiomasswasonly ; 5,000g/m2in theseblacksprucestandscomparedto ; 12,000 g/m2intheAlaskabirchstands.Whilethese findingsrepresentonlyonestudyareaand extremesinspeciescomposition,theydosuggest thatsoilCpoolsinblacksprucestandsmaynot alwaysbesufficienttooffsetthehighlevelsof abovegroundbiomassindeciduousstandsand thatconsiderationofbothabove-andbelowgroundCpoolsareessentialforestimatingthe effectsoftrajectoryshiftsonborealforestC dynamics.CONCLUSIONSOurdatasuggestthataclimate-drivenincrease infireseverityandconsequentshiftfromblack spruceself-replacementtosuccessionaltrajectorieswithgreaterdeciduousdominancewilllead toasubstantialincreaseinabovegroundC accumulationduringmid-succession.Thisincreasewillbedrivenbygreaterratesofabovegroundbiomassaccumulation,ANPP,and deciduoussnagbiomass.WhethertheseincreasesaresufficienttooffsetClosttotheatmosphere duringtheinitialhigh-severityfiredisturbanceis stillunknown,butwillultimatelydependupon theaccumulationofbelowgroundCpools,fire frequencyeffectsonthelengthofthesuccessionalcycle,andthefateofCpoolsasstandsmature. Increaseddeciduousdominanceduringmidsuccessionwillalsohaveregionalclimateimplications(Randersonetal.2006).Surfacealbedo willincreasebecausedeciduousspecieshave lighter-coloredfoliagethanblackspruce,and whenleafless,exposethesnow-coveredground (Goetzetal.2007,Becketal.2011).Surface conductanceandevapotranspirationwillalso increase,leadingtohigherlevelsofatmospheric watervapor(Amiroetal.2006). AlthoughthisstudyfocusedoninteriorAlaska,thepotentialforatrajectoryshiftexistsacross theborealforest,yettheeffectsonecosystemC balanceandclimatewilllikelyvarywith dominantvegetation,firefrequency,andpermafrostconditions.Vegetationfunctionaltype(tree vs.shrubvs.grass),growthform(deciduousvs. coniferous),regenerationstrategy(seedvs.vegetative),longevity,andflammabilityallinfluence Caccumulationpotential.FirefrequencydeterminesthetimeframeoverwhichCcanaccumulate,standagedistribution,andtheprevalenceof bare,charredsoils.PermafrostdepthanddistributionaffecttheamountofprotectedCandits susceptibilitytoatmosphericreleasefollowing firedisturbances.Thus,predictingtheeffectsof climate-drivenalterationstothefireregimeon long-termCbalanceandclimatefeedbacks requirestheintegrationofmultiplestudies assessingCpoolswithinvariousregionsofthe borealforestbiomedifferinginvegetation v www.esajournals.org16May2012vVolume3(5)vArticle45ALEXANDERETAL.

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composition,standage,andpermafroststability.ACKNOWLEDGMENTSWewouldliketothankKamalaEarl,CamiloMojica, NilsPederson,LeslieBoby,MindySun,andJennifer Stanleyfortheirinvaluablehelpinthefieldand laboratory.WeappreciatetheassistanceoftheUS Armyforallowingaccesstofirescarsonmilitarysites, andDr.AbbyBenninghoffforstatisticalassistance. FundingforthisresearchwasprovidedbyNASA EcosystemsandCarbonCycleGrantNNX08AG13G, NOAAGlobalCarbonCyclegrantNA080AR4310526 andtheBonanzaCreekLongTermEcologicalResearch SiteprogramfundedbyNSFDEB-0620579andUSDA ForestService,PacificNorthwestResearchStation, grantPNW01-JV11261952-231.LITERATURECITEDAlaskaClimateResearchCenter.2012.Climatological databaseforFairbanks,AK.http://climate.gi. alaska.edu/Climate/Locati on/Interior/Fairbanks. html Amiro,B.D.,A.Cantin,M.D.Flannigan,andW.J.D. Groot.2009.FutureemissionsfromCanadian borealforestfires.CanadianJournalofForest Research39:383–395. Amiro,B.D.,etal.2006.Theeffectofpost-firestand ageontheborealforestenergybalance.AgriculturalandForestMeteorology140:41–50. Balshi,M.S.,A.D.McGuire,P.Duffy,M.Flannigan, D.W.Kicklighter,andJ.Melillo.2009.Vulnerability ofcarbonstorageinNorthAmericanborealforests towildfiresduringthe21stcentury.GlobalChange Biology15:1491–1510. Beck,P.S.A.,S.J.Goetz,M.C.Mack,H.D.Alexander, Y.Jin,J.T.Randerson,andM.M.Loranty.2011. Theimpactsandimplicationsofanintensifyingfire regimeonAlaskanborealforestcompositionand albedo.GlobalChangeBiology17:2853–2866. BonanzaCreekLongTermEcologicalResearchSite. 2009.BNZ-LTERdatacatalog:http://www.lter.uaf. edu/data_detail.cfm?datafile_pkey 230 Bond-Lamberty,B.,S.D.Peckham,D.E.Ahl,andS.T. Gower.2007.Fireasthedominantdriverofcentral Canadianborealforestcarbonbalance.Nature 450:89–92. Bond-Lamberty,B.,C.Wang,andS.T.Gower.2002 a Abovegroundandbelowgroundbiomassand sapwoodareaallometricequationsforsixboreal treespeciesofnorthernManitoba.Canadian JournalofForestResearch32:1441–1450. Bond-Lamberty,B.,C.Wang,andS.T.Gower.2002 b Annualcarbonfluxfromwoodydebrisforaboreal blacksprucefirechronosequence.Journalof GeophysicalResearch:Atmospheres108D3:1–1. to1-10. Bond-Lamberty,B.,C.K.Wang,andS.T.Gower.2004. Netprimaryproductionandnetecosystemproductionofaborealblacksprucewildfirechronosequence.GlobalChangeBiology10:473–487. Brown,J.K.1974.Handbookforinventoryingdowned woodymaterial.INTGTR-16.USDAForestService,IntermountainForestandRangeExperiment Station,Ogden,Utah,USA. Burnham,K.P.andD.R.Anderson.2004.Multimodel inference:understandingAICandBICinmodel selection.SociologicalMethodsandResearch 33:261–304. Calef,M.P.,A.D.McGuire,H.E.Epstein,T.S.Rupp, andH.H.Shugart.2005.Analysisofvegetation distributioninInteriorAlaskaandsensitivityto climatechangeusingalogisticregressionapproach.JournalofBiogeography32:863–878. Chapin,F.S.,T.V.Callaghan,Y.Bergeron,M.Fukuda, J.F.Johnstone,G.Juday,andS.A.Zimov.2004. Globalchangeandtheborealforest:Thresholds, shiftingstatesorgradualchange?Ambio33:361– 365. Chapin,F.S.,etal.2008.IncreasingwildfireinAlaska  s borealforest:Pathwaystopotentialsolutionsofa wickedproblem.BioScience58:531–540. Cumming,S.G.,F.K.A.Schmiegelow,andP.J. Burton.2000.Gapdynamicsinborealaspen stands:Istheforestolderthanwethink?Ecological Applications10:744–759. Dyrness,C.T.andL.A.Viereck.1982.Controlof depthtopermafrostandsoiltemperaturebythe forestfloorinblackspruce/feathermosscommunities.PNWRN-396.USDAForestService,Pacific NorthwestForestandRangeExperimentStation, Portland,Oregon,USA. Flanagan,P.W.andK.VanCleve.1983.Nutrient cyclinginrelationtodecompositionandorganicmatterqualityintaigaecosystems.Canadian JournalofForestResearch13:795–817. Goetz,S.J.,M.C.Mack,K.R.Gurney,J.T.Randerson, andR.A.Houghton.2007.Ecosystemresponsesto recentclimatechangeandfiredisturbanceat northernhighlatitudes:observationsandmodel resultscontrastingnorthernEurasiaandNorth America.EnvironmentalResearchLetters2:1–9. Goetz,S.J.andS.D.Prince.1996.Remotesensingof netprimaryproductioninborealforeststands. AgriculturalandForestMeteorology78:149–179. Goldammer,J.G.andV.V.Furyaev.1996.Firein ecosystemsofborealEurasia:Ecologicalimpacts andlinkstotheglobalsystem.Pages1–20 in J.G. GoldammerandV.V.Furyaev,editors.Firein ecosystemsofborealEurasia.Kluwer,Dordrecht, TheNetherlands. Gower,S.T.,O.Krankina,R.J.Olson,M.Apps,S. v www.esajournals.org17May2012vVolume3(5)vArticle45ALEXANDERETAL.

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taigaforestwithinAlaska:presentandfuture. CanadianJournalofForestResearch32:757–767. Yarie,J.,E.Kane,andM.C.Mack.2007.Aboveground biomassequationsfortreesofInteriorAlaska. USFSAgriculturalandForestryExperimentStation Bulletin115.UniversityofAlaska,Fairbanks, Alaska,USA.SUPPLEMENTALMATERIALAPPENDIXA TableA1.Allometricequationsandstatisticsdevelopedtocalculatecomponentsoftreebiomass(g/tree)and annualcrowngrowth(g tree 1 yr 1)basedonbasaldiameter(trees 1.4mtall)anddiameteratbreastheight (DBH;trees 1.4mtall).Allequationsareexpressedintheformof y axb.Equationsfor Salix spp.and Larix laricina wereobtainedfromBond-Lambertyetal.2002 a .SpeciesTreecomponent BasaldiameterDBH abR2Obs abR2Obs Alnus spp.Totalbiomass32.172.540.96***10............ (Alderspp.)Stemwood/bark24.622.580.97***10............ Foliage1.302.370.65***10............ Livebranches5.382.400.83***10............ Livecrown6.772.400.81***10............ Deadtotalbiomass22.082.590.97***7............ Betulaneoalaskana Totalbiomass26.292.680.92***25164.182.290.98***19 (Alaskabirch)Stemwood/bark26.812.620.92***25147.962.250.98***19 Foliage0.632.650.86***256.392.100.89***19 Livecrown1.383.080.89***715.152.490.94***19 Crowngrowth3.452.380.47610.471.920.88***10 Piceaglauca Totalbiomass53.742.450.96***3796.772.400.98***58 (Whitespruce)Stemwood/bark39.132.440.95***3748.442.510.98***58 Foliage2.592.580.75***2925.222.040.78***49 Livecrown3.012.790.90***3729.342.240.83***58 Crowngrowth5.021.750.56***277.991.620.61***49 Piceamariana Totalbiomass37.292.400.92***119271.461.840.92***78 (Blackspruce)Stemwood/bark11.382.670.95***119117.911.990.95***78 Foliage14.611.860.50***9855.401.470.50***56 Livecrown21.642.120.72***12083.521.800.72***78 Crowngrowth1.831.750.50***565.261.550.50***56 Populustremuloides Totalbiomass56.832.490.96***23134.102.260.99***40 (Tremblingaspen)Stemwood/bark12.552.980.99***564.012.510.98***23 Foliage8.732.000.93***2318.981.530.91***41 Livecrown18.162.430.98***541.741.830.83***21 Crowngrowth3.452.380.98***510.241.760.91***23 Populusbalsamifera Totalbiomass58.292.440.96***17133.712.290.96***17 (Balsampoplar)Stemwood/bark43.352.470.95***1798.262.320.96***17 Foliage11.221.760.86***1722.161.620.84***17 Livecrown7.362.460.93***1717.242.300.94***17 Crowngrowth14.731.720.86***1527.141.600.85***15 *** P 0.001. v www.esajournals.org20May2012vVolume3(5)vArticle45ALEXANDERETAL.

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APPENDIXB TableB1.Characteristicsof44mid-successional(20–56yearsincefire)foreststandssampledacrossborealforests ofinteriorAlaska.Firename Burn year Year sampled Since fire (yr) Elev (m) Slope ( 8 ) Aspect ( 8 ) Dec IV Treeage(yr) } LAI|| Forest type SlateCreek19872009225508680.21PM:8–11(27)*0.09PM WickershamDome197120083752612030.49BN:27–34;PM:24–31(51)*ndPM ChenaDome195820085022931450.94PM:32–420.34PM MurphyDome19582009512006111.1PM:31–451.64PM WickershamDomeExp1978200931509101081.17BN:13–22;PM:20–280.14PM MintoFlats198320092642891031.18PM:19–222.36PM WickershamDome1971200837403112351.19BN:24–35;PM:23–32ndPM BN WickershamDomeExp197820093152181081.2PM:19–261.36PM SawtoothMountain195720095248942131.28PM:37–420.67PM BigDenver196920083912502341.39PM:27–34;PT:30–350.62PM PT RosieCreek198320082521051281.53BN:12–22;PM:19–220.52PM Healy195820085064261491.53nd2PM BN SlateCreek198720092255351091.55BN:14–18;PM:13–160.34PM BN Bolio198120102950723011.58PM:11–23(39)*;PT:27–281.69PT SawtoothMountain195720095250242861.7PM:31–43;PT:36–401.05PM PT Goldstream19662009431441381.7BN:35–37;PM32–372.9PM BN WickershamDomeExp197820093149491241.73BN:17–24;PM:20–250.67PM BN TokRiver19902010205040Flat1.82PM:14–170.27PM Goldstream196620094314432761.87PM:33–371.55PM RosieCreek1983200825212101541.88BN:14–21;PM:22–241.35PM WickershamDomeExp197820093152281301.97PM:21–31;PT:28–320.39PT GraniteCreek19872009224361201.98PM:11–180.13PM PT MurphyDome19582009511976531.99BN:42–48(57)*;PM:33–470.12PM BN BigDenver196920083913601922.04PM:25–31;PT:30–370.9PT BigDenver196920094026972552.13BN:31–35;PM:27–321.83PM BN WickershamDomeExp197820093151512872.16PM:19–250.92PM PT TokRiver19902010205030Flat2.23PM:15–18;PT:10–170.33PM PT MintoFlats1983200926473151222.4PM:16–20;PT:25–260.19PT Bolio198120102949623012.41PM:19–270.29PM GraniteMountain19542010564190612.45PM:27–38;PT:51–53ndPM PT WickershamDomeExp197820093153771052.46PM:14–23;PT:28–310.43PM BN Bolio198120102949613082.49PT:24–280.76PT RosieCreek198320082519961542.57BN:13–21;LL:14–17; PM:15–20 0.27PM GraniteCreek19872008213851542.59PM:15–18;PT:15–191.68PT BigDenver196920104134552062.59PT:39–41;PM:26–321.78PT WickershamDome1971200837483111762.61PM:22–29;PT:34–361.27PT WickershamDomeExp197820093149491402.66BN:22–29;PM:20–250.22BN GraniteCreek19872008213820452.71PM:7–16;PT:8–170.26SA AL GraniteCreek19872008213830452.72PM:14–17;PT:10–150.22PT TokRiver199020102050012972.89PT:15–171.56PT MintoFlats198320092641711992.91PM:16–22;PT:25–272.39PT MurphyDome19582009512087452.99BN:39–480.2BN SlateCreek19872009225725643BN:17–20;PT:19–221.97PT BigDenver196920094022181603BN:29–343.43BN Notes: nd Nodata.BN Alaskabirch,PG whitespruce,PM blackspruce,PT tremblingaspen,AL alder,SA willow.Flat Standswereonflatgroundsodonothaveslopeoraspect. Yearssincefirebasedonsamplingyearminusyearburned. DeciduousImportanceValue(IV).See MaterialsandMethods forcalculation. } Treeagerangesforcanopyspecies. *Agesinparenthesesexceededyearssincefire.Standswithtwoormoretreesolderthanageoffirescarwereremovedfrom analyses. jj Leafareaindex(LAI). Standcompositionbasedonrelativecontributionofspeciestototalstandingbiomass.See MaterialsandMethods v www.esajournals.org21May2012vVolume3(5)vArticle45ALEXANDERETAL.