Testing long-term summer temperature reconstruction based on maximum density chronologies obtained by reanalysis of tree-ring data sets from northernmost Sweden and Finland

Clim. Pas , 10, 1473–1487, 2014 www.clim-pas .ne /10/1473/2014/ doi:10.5194/cp-10-1473-2014 © Au ho (s) 2014. CC A ibu ion 3.0 License. Tes ing long- e m summe empe a u e econs uc ion based on maximum densi y ch onologies ob ained by eanalysis o ee- ing da a se s om no he nmos Sweden and Finland V. V. Ma sko sky1and S. Helama2 1Ins i u e o Geog aphy, Russian Academy o Sciences, Moscow, Russia 2Finnish Fo es Resea ch Ins i u e, No he n Uni , Ro aniemi, Finland Co espondence o: V. V. Ma sko sky (ma sko sk[email p o ec ed]) Recei ed: 13 Augus 2013 – Published in Clim. Pas Discuss.: 16 Oc obe 2013 Re ised: 1 June 2014 – Accep ed: 11 June 2014 – Published: 5 Augus 2014 Abs ac . He e we analyse he maximum la ewood densi y (MXD) ch onologies o wo published ee- ing da a se s: one om To ne äsk egion in no he nmos Sweden (TORN; Mel in e al., 2013) and one om no he n Fennoscan- dia (FENN; Espe e al., 2012). We paid pa icula a en- ion o he MXD low- equency a ia ions o econs uc summe (June–Augus , JJA) long- e m empe a u e his o y. We used published me hods o ee- ing s anda diza ion: e- gional cu e s anda diza ion (RCS) combined wi h signal- ee implemen a ion. Compa isons wi h RCS ch onologies p oduced using single and mul iple (non-clima ic) ageing cu es ( o be emo ed om he ini ial MXD se ies) we e also ca ied ou . We de elop a no el me hod o s anda diza- ion, he co ec ion implemen a ion o signal- ee s anda d- iza ion, ailo ed o de ec ion o pu e low- equency signal in ee- ing ch onologies. In his me hod, he e o in RCS ch onology wi h signal- ee implemen a ion is analy ically assessed and ex ac ed o p oduce an ad anced ch onology. The impo ance o co ec ion becomes ob ious a lowe e- quencies as smoo hed ch onologies become p og essi ely mo e co ela i e wi h co ec ion implemen a ion. Subsam- pling he FENN da a o mimic he lowe ch onology sam- ple size o TORN da a shows ha he ch onologies bi u - ca e du ing he 7 h, 9 h, 17 h and 20 h cen u ies. We used he wo MXD da a se s o econs uc summe empe a u e a ia ions o e he pe iod 8 BC h ough AD 2010. Ou new econs uc ion shows mul i-decadal o mul i-cen ennial a i- abili y wi h changes in he ampli ude o he summe empe a- u e o 2.2◦C on a e age du ing he Common E a. Al hough he MXD da a p o ide palaeoclima e esea ch wi h a highly eliable summe empe a u e p oxy, he bi u ca ing dend o- clima ic signals iden i ied in he wo da a se s imply ha u- u e esea ch should aim a a mo e ad anced unde s anding o MXD da a on dis inc issues: (1) in luence o pas popu- la ion densi y a ia ions on MXD p oduc ion, (2) po en ial biases when calib a ing di e en ly p oduced MXD da a o p oduce one p oxy eco d, (3) in luence o he biological age o MXD da a when in oducing young ees in o he ch onol- ogy o e he mos ecen pas and (4) possible ole o wa e - logging in MXD p oduc ion when analysing ee- ing da a o ipa ian ees. 1 In oduc ion Dend och onology is one o he mos common me hods o ob aining he la e Holocene econs uc ions o pas clima e a iabili y because ee- ing ch onologies a e high- esolu ion indica o s o exac ly da ed paleoclima e in o ma ion (F i s, 1976). Mo eo e , ee- ing ch onologies a ound he ex a - opical No he n Hemisphe e con ain clea clima ic signals o summe empe a u es (B i a e al., 2002). This dend ocli- ma ic associa ion is no ably high o di e en ee species g owing nea hei pola and alpine ange o dis ibu ion. Ye he s eng h o his dend oclima ic esponse depends on he measu ed ee- ing pa ame e . While he use o ee- ing wid h ch onologies as indica o s o pas clima e a i- abili y has a long adi ion in dend oclima ology, he de el- opmen o X- ay-based es ima es o wood densi y luc ua- ions (Schweing ube e al., 1978) has inc eased he alue o dend och onology as an in eg al pa o paleoclima ology. Published by Cope nicus Publica ions on behal o he Eu opean Geosciences Union. 1474 V. V. Ma sko sky and S. Helama: Tes ing summe empe a u e econs uc ion Se e al s udies ha e shown ha he measu emen s o maxi- mum la ewood densi ies (MXD) yield ch onologies wi h an imp o ed clima ic signal (B i a e al., 2002). Consequen ly, he MXD ch onologies om se e al egions a ound Eu asia and No h Ame ica ha e been used o summe empe a u e econs uc ions (Hughes e al., 1984; Da i e al., 2003; Luck- man and Wilson, 2005; Bün gen e al., 2008). Simila s udies ha e been conduc ed in no he n Fennoscandia whe e a ew econs uc ions o summe empe a u es (JJA) o e he Common E a ha e been based on MXD eco ds o li ing and sub ossil pines (B i a e al., 1988, 1990, 1992, 1996; B i a and Schweing ube , 1992; G udd, 2008; Bün gen e al., 2011; Espe e al., 2012; McCa oll e al., 2013; Mel in e al., 2013). While he ea ly s udies concen a ed on ma e ials om no he nmos Sweden, pa icula ly he To ne äsk egion (Schweing ube e al., 1988; B i a e al., 1990, 1992, 1996; G udd, 2008), he ee- ing ma e ials om no he n Finland (E onen e al., 2002) ha e ecen ly been exploi ed o MXD analyses. These ma e ials ha e no ably inc eased he a ailable MXD sample size and empo ally ex ended he ch onology o he egion o 138 BC (Bün gen e al., 2011; Espe e al., 2012). In a ecen dend oclima ic analysis Mel in e al. (2013) used upda ed and adjus ed sub ossil (Schweing ube e al., 1988) and li ing- ee (G udd, 2008) MXD da a om To ne äsk. A b ie li e a u e e iew exempli ies se e al inconsis en- cies in he published palaeoclima e eco ds. While he i s o hese econs uc ions (B i a e al., 1992) s o e o de- ine he deg ee o he 20 h-cen u y wa m h in he con ex o he las 1500y , he subsequen s udy (G udd, 2008) demon- s a ed a posi i e empe a u e anomaly o abou 0.7–1.5◦C (30y mean) o he Medie al Wa m Pe iod (MWP) ela- i e o he 20 h-cen u y condi ions (1951–1970 mean). I also showed a cooling phase (up o 1.1◦C nega i e anomaly o 30y mean) o he Li le Ice Age (LIA). Howe e , Bün gen e al. (2011) did no ind e idence o a cool pe- iod du ing he LIA in e al. Espe e al. (2012) s a ed ha he p e iously es ima ed wa m h o he MWP may be no- ably unde es ima ed. Ano he de ailed dend oclima ic anal- ysis (Mel in e al., 2013) p esen ed MWP empe a u es a a le el simila o he mode n ones in he egion sugges ing an o e es ima ion o he medie al wa m h published by G udd (2008). Mel in e al. (2013) showed ha he G udd (2008) s udy con ained sys ema ic bias and should no be consid- e ed sui able o use as a clima e econs uc ion. The esul s o McCa oll e al. (2013) sugges ed ha he 20 h and 17 h cen u ies we e wa mes and coldes in he egion, while sim- ila ly wa m spells we e expe ienced du ing he 11 h cen u y. While he pos -1700 inconsis encies in a pa o he MXD da a (B i a e al., 1992; G udd, 2008) ha e been cla i ied (Mel in e al., 2013), he wo da a se s (Mel in e al., 2013; Espe e al., 2012) ha e no been analysed collec i ely o e- cons uc an MXD-based long- e m empe a u e a iabili y o e he whole o no he n Fennoscandia. This si ua ion is e en mo e unsa is ac o y, i no wo isome, conside ing he impo an ole o he pa icula MXD da a in se e al hemi- sphe ic and equen ly ci ed paleoclima e econs uc ions o he Common E a (e.g. Jones e al., 1998; Espe e al., 2002; Osbo n and B i a, 2006; Mann e al., 2009; Ljungq is e al., 2012). We assume ha he clima e a iabili y wi hin he egion is likely oo uni o m o ha e caused he epo ed de ia ions in he published empe a u e econs uc ions om adjacen egions o no he nmos Sweden (To ne äsk) and Finland. Ins ead, he easons o hese de ia ions may mo e ealis i- cally s em om he ee- ing s anda diza ion and mic oden- si ome ic me hods, as well as non-clima ic noise inhe en o he a ia ions in a ailable samples and hei si e cha ac- e is ics. A p oblem associa ed wi h he dend oclima ic im- p o emen , consis ing o he de elopmen om he adi ion o simply egis e ing he wid hs o he ings o he mo e so- phis ica ed MXD, is he complexi y o me hods equi ed o p oduce he high- esolu ion wood densi y p o iles. As a pi - all, e en ma ginal changes in he mic odensi ome y me h- ods may p oduce signi ican ly al e ed MXD da a (Helama e al., 2010d, 2012). T ee- ing s anda diza ion is a necessa y p ocess whe e he ini ial measu emen se ies a e ans o med in o dimensionless indices o emo e he biological (i.e. non- clima ic) g ow h a ia ions p io o dend oclima ic in e p e- a ions (F i s, 1976). Di e en s anda diza ion me hods p o- duce ch onologies wi h de ia ing cha ac e is ics, and pa ic- ula ly he long- e m (i.e. low- equency) es ima ion o den- d oclima ic signals is sensi i e o he applied me hod (B i a e al., 1992; Espe e al., 2002; Helama e al., 2004). In- deed, he ecen eanalysis o he To ne äsk MXD da a de- ailed a se o es ima ion biases o igina ing om changes in he mic odensi ome y p ac ices and ee- ing s anda d- iza ion (Mel in e al., 2013). I is essen ial o no e ha all he discussed s udies used he same ype o s anda diza ion me hod, he egional cu e s anda diza ion (RCS; B i a e al., 1992), o p oduce he MXD ch onologies as a p oxy o empe a u e econs uc ions, bu selec ed di e en op- ions which migh explain di e ences be ween he esul - ing ch onologies. While his pa icula me hod is capable o p oducing ch onologies wi h p ese ed low- equency a ia- ions (B i a e al., 1992; Espe e al., 2002; Helama e al., 2004), i is also mo e sensi i e o da a he e ogenei y and he e o e equi es highe sample eplica ion han o he com- monly applied s anda diza ion me hods (B i a and Mel in, 2011). Mo eo e , ecen yea s ha e seen a di e si ica ion o he o iginal RCS me hod, wi h de elopmen o i s signal- ee implemen a ion (Mel in and B i a, 2008) and i s subsequen co ec ion (Ma sko sky, 2011), as well as mul iple cu e ap- p oaches o RCS (Helama e al., 2005b; Nicaul e al., 2010; Mel in e al., 2013). The ecen e sions o he To ne äsk and Fennoscandian ch onologies ha e used he RCS along wi h i s a ia ions (Espe e al., 2012; McCa oll e al., 2013; Mel in e al., 2013) bu so a no sys ema ic s udy exis s o p esen he cons uc ion o he esul ing MXD ch onologies using consis en ee- ing s anda diza ion. Clim. Pas , 10, 1473–1487, 2014 www.clim-pas .ne /10/1473/2014/ V. V. Ma sko sky and S. Helama: Tes ing summe empe a u e econs uc ion 1475 Figu e 1. Sub ossil and li ing-pine si es loca ion o FENN and TORN da a se s. He e we eanalyse and compa e wo MXD da a se s om To ne äsk (Mel in e al., 2013) and om a la ge egion o Fennoscandia (Espe e al., 2012) using di e en s anda d- iza ion me hods. MXD a ia ions in he da a we e iden i ied in de ail and he same ypes o RCS me hod we e applied o bo h da a se s. We calib a es anda dized MXD da a o ins u- men al JJA empe a u es o he To nedalen clima e eco d lo- ca ed in no he n Sweden and co e ing he las 200y (Kling- bje and Mobe g, 2003). Then we con inue o econs uc e- gional summe empe a u e in he Common E a. 2 Ma e ials and me hods 2.1 Geog aphical se ing This s udy is based on X- ay-based mic odensi ome ic da a o mode n and sub ossil Sco s pine (Pinus syl es is L.) ee ings. We use published and online a chi ed MXD da a om no he nmos Sweden (i.e. To ne äsk egion) and no he n Fennoscandia. These da a se s a e e e ed o as TORN (Schweing ube e al., 1988; G udd, 2008; Mel in e al., 2013) and FENN (o iginally named N-Scan; Espe e al., 2012). O iginally, he collec ion o he TORN ma e ial was ca - ied ou in i e indi idual si es a ound Lake To nes äsk (68.17–68.33◦N, 19.75–20.75◦E; 400–470ma.s.l.; Fig. 1). Th ee o hese locali ies p o ided sub ossil ee ings o ex- ending he ch onology back in ime o AD 436 (Ba holin and Ka lén, 1983; Ba holin, 1987). The MXD ch onology o TORN was p oduced by Schweing ube e al. (1988) and upda ed wi h mo e ecen ma e ial o li ing ees by G udd (2008) and Mel in e al. (2013). The TORN da a now co e he pe iod AD 441–2010 and can be downloaded ia h p://www.c u.uea.ac.uk/c u/pape s/mel in2012holocene/. The sub ossil FENN ma e ial o E onen e al. (2002) o igina es om he egion o no he n Finland. A po ion o his ma e ial, co e ing oughly he pas wo millennia (since 138 BC) was la e used o build a new MXD ch onol- ogy o he egion (66.80–69.50◦N, 23.00–29.00◦E; 190– 340ma.s.l.; Fig. 1). The sub ossil ma e ial was upda ed wi h li ing- ee ma e ial om no h-wes Finnish Lapland and no he nmos Sweden (Espe e al., 2012). In ac ual ac , one o he li ing- ee si es included in he FENN da a se is om he To ne äsk egion. As a esul , he mod- e n pa o he FENN da a se is weigh ed owa ds he wes , while he eas e n hal o egion is ep esen ed only by sub ossil si es. Mo eo e , wo o he h ee li ing- ee si es (67.90◦N, 20.10◦E; 68.20◦N, 19.80◦E) a e a ou side he ne wo k o sub ossil si es. The FENN da a can be down- loaded ia h p://www.blogs.uni-mainz.de/ b09clima ology/ iles/2012/03/Da a.pd . Fo subsequen analyses FENN da a we e ans o med in o “mean ee” se ies, i.e. mul iple mea- su emen s o each ee we e a e aged in o one se ies. This was done o a oid a i icial educ ion o e o ma gins and o b ing he wo da a se s in o co espondence. 2.2 Sampling si es The ee- ing ma e ial o igina es om ou di e en ypes o sampling si es: mode n samples o li ing pines ep esen ing d y (inland) en i onmen s and lake ipa ian habi a s, sub os- sil samples p ese ed in subae ial condi ions and sub ossils p ese ed in lacus ine sedimen s. I is no able ha he sub- ossil ma e ial o he TORN da a se o igina es me ely om subae ial condi ions (Ba holin and Ka lén, 1983; Schwein- g ube e al., 1988). In o he wo ds, he e is no sub ossil ma e ial om lacus ine sedimen s included in he Swedish MXD da a (G udd, 2008). On he o he hand, he FENN assemblage consis s o sub ossil samples collec ed a lakes only (E onen e al., 2002). The mode n pa o FENN MXD da a ( om li ing ees) was p oduced by using ee- ing sam- ples om only lake ipa ian si es (Espe e al., 2012).Thus, he p incipal di e ence be ween he TORN and FENN da a se s s ems om he s udied habi a : he TORN da a include d y si es whe eas he FENN da a a e ep esen ed by lake sho e en i onmen s. 2.3 Mic odensi ome y X- ay-based mic odensi ome y p oduces adial wood den- si y p o iles o each ee- ing sample. Each ing exhibi s an annualdensi y cyclewhe e he woodo low densi yis o med du ing he ea ly g owing season (i.e. ea lywood), wi h in- c easingly high densi ies owa ds he la e summe and au- umn (i.e. la ewood). The in a-annual poin whe e he den- si y eaches i s highes alue is aken as he maximum densi y www.clim-pas .ne /10/1473/2014/ Clim. Pas , 10, 1473–1487, 2014 1476 V. V. Ma sko sky and S. Helama: Tes ing summe empe a u e econs uc ion (MXD) o ha ing. The consecu i e MXD alues o each sample ep esen he indi idual MXD se ies. These se ies a e o special in e es in dend oclima ology because o hei s ong co ela ions wi h g owing season empe a u es as ob- se ed b oadly ac oss he ci cumpola bo eal o es s oday (B i a e al., 2002). Commonly, li ing ees a e co ed a b eas heigh , and he inc emen co es exposed o mic odensi ome ic analy- ses. Sub ossil pinewood ma e ial can ei he be ound lying on d y g ound o p ese ed in he sedimen o small lakes. In he ield, he unea hed logs a e sawn in o disks. Subse- quen ly, in he wo kshop, adial la hs a e sawn pe pendicula o he ing bounda ies o be analysed using mic odensi ome- y. I is impo an o no e ha e en sligh di e ences in he labo a o y p o ocol du ing he X- ay p ocess may p o oundly al e he esul ing MXD da a (G udd, 2008; Helama e al., 2010d, 2012). MXD da a may become al e ed depending on he me hods ha a e used o emo e he wood ex ac i es (o - ganic, non-cell-wall componen s) p io o analyses (Helama e al., 2010d), he wood mois u e con en du ing he mea- su emen (Helama e al., 2012) and he adial s ep size o he densi y scanning (G udd, 2008; Helama e al., 2012). The wo MXD da a se s analysed he e (TORN and FENN) we e p oduced using di e en X- ay me hods and da a analy- sis. The TORN da a se consis s o da a p oduced in di e en decades and labo a o ies (G udd, 2008; Mel in e al., 2013). I has been shown o TORN da a ha he old MXD alues (Schweing ube e al., 1988) exhibi lowe s anda d de ia ion o densi y a iabili y han he MXD alues p oduced using he e ol ed echnical p ope y o he X- ay scanne (G udd, 2008). In o de o deal wi h his bias, G udd (2008) educed he a iance in new MXD da a o ma ch wi h he old a i- ance. Mo e ecen ly, Mel in e al. (2013) added mo e da a o he TORN da a se . These da a we e p oduced in a di e en labo a o y. Compa isons wi h he di e en ly p oduced MXD da a showed a need o addi ional adjus men s o mean and s anda d de ia ion o he da a because o he di e en labo a- o y p o ocols (Mel in e al., 2013). In his s udy, we a e us- ing hese adjus ed TORN da a (S88G1112A.mxd da a om h p://www.c u.uea.ac.uk/c u/pape s/mel in2012holocene/). 2.4 S anda diza ion me hods 2.4.1 Regional cu e s anda diza ion Ou analyses a e based on egional cu e s anda diza ion (RCS) (B i a e al., 1992, 1996; B i a and Mel in, 2011) wi h he signal- ee implemen a ion (Mel in and B i a, 2008; B i a and Mel in, 2011) and RCS wi h a p oposed co ec ion (Ma sko sky, 2011). In he o iginal RCS ech- nique p esen ed by B i a e al. (1992), he same s anda d- iza ion cu e is used o de end each se ies o ee- ing mea- su emen s. The s anda diza ion cu e is assumed o display egionally ep esen a i e cambial ( ha is, biological and hus non-clima ic) ends in he o al ee- ing a iabili y (B i a e al., 1992, 1996). The e o e, his end can be de i ed as a single mean cu e o he da a aligned acco ding o hei cam- bial ages. The p ocess o a e aging he se ies e eals ee- ing a ia ions ela ed o ageing and he expec ed age- ela ed componen can be de ec ed as he mean cu e. As is ypical o he con en ional RCS me hod, his cu e is assumed o be no a ec ed by clima e. Subsequen o a e aging, he mean cu e is commonly smoo hed o educe he e ec o an- dom luc ua ions. Time- a ying esponse smoo hing (TVRS; Mel in e al., 2007) wi h he ou ine o Mel in e al. (2007; see hei Appendix A) was used whe e SSY (spline s i ness o each yea ) equalled he cambial age in yea s plus 15y . We used a cubic spline wi h 50% a iance cu -o (Cook and Pe e s, 1981) on he equency pe iod o SSY yea s. Using his me hod, he RCS is ca ied ou wi h a single egional cu e ( e e ed he ea e as RC1) o s anda dize all ee- ing se ies and he ee- ing indices a e de i ed as a ios be ween he obse ed ee- ing alue and he alue expec ed by he RC1. This p ocess is expec ed o emo e a la ge po ion o age- ela ed a ia ions p esen ed in he ini ial se ies o mea- su emen s. Pa s o each ee- ing se ies con ibu ing o he egional cu e o e he weakly eplica ed old cambial ages (wi h eplica ion less han 10) we e emo ed be o e using he RCS me hod. 2.4.2 Signal- ee app oach The con en ional RCS me hod can be combined wi h he signal- ee (SF) app oach ( hus, RC1SF) as sugges ed by B i a and Mel in (2011) and as al eady applied in se e al s udies (Helama e al. 2010b; Bjö klund e al., 2013; Mel in e al., 2013; Coope e al., 2013; Wilson e al., 2013). In con- as wi h con en ional RCS, his me hod assumes ha he es ima e o he s anda diza ion cu e as jus an a e age o all a ailable se ies is biased by clima e because he ini ial ee- ing se ies hemsel es con ain a signi ican clima ic compo- nen . The eby he mean cu e may e ain a la ge p opo ion o clima ic in luence, which becomes e ec i ely emo ed om ee- ing indices because he g ow h componen ep- esen ed by he cu e is emo ed om he ini ial se ies. As an al e na i e, a mo e non-clima ic mean cu e could be p o- duced om signal- ee measu emen s whose clima ic signal is emo ed p io o es ima ion o he s anda diza ion cu e. Following Mel in and B i a (2008), he emo al o he cli- ma ic a ia ions is done h ough a p ocess whe e he ini ial ee- ing measu emen s, aligned by hei calenda yea s, a e i s di ided by he index alues o he con en ional RCS ch onology. The esul ing SF measu emen s a e a e aged, aligned by hei cambial yea s, ollowing he ypical RC1 p ocedu e (see abo e). The new mean cu e is hen emo ed om he ini ial ee- ing measu emen s o p oduce SF indices and a new (RC1SF) ch onology. The p ocess is epea ed un- il no imp o emen in ch onology es ima ion is achie ed (see Mel in and B i a, 2008, 2013 o de ails). Clim. Pas , 10, 1473–1487, 2014 www.clim-pas .ne /10/1473/2014/ V. V. Ma sko sky and S. Helama: Tes ing summe empe a u e econs uc ion 1477 2.4.3 Co ec ion p ocedu e An imp o emen o he ch onology es ima ion, subsequen o RCS and SF me hods, was p e iously a ained by applying a co ec ion (C) me hod ( hus, RC1SFC; Ma sko sky, 2011). This me hod, RC1SFC, is a no el applica ion o he RCS and SF me hods, ailo ed o de ec ion o a pu e low- equency signal in ee- ing ch onologies. The e o in RC1SF ch onol- ogy is assessed and subsequen ly ex ac ed om he da a in o de o p oduce an RC1SFC ch onology. Fo ou pu poses ee g ow h can be hough o as a p oduc o clima ic and non-clima ic componen s. When a e aging non-clima ic componen s om di e en samples ( ees) we ge non-clima ic componen s in he ch onology. These ypes o ch onology a ia ions o igina e om he non- uni o m dis ibu ion o indi idual se ies in ime and uncom- mon indi idual ee g ow h peculia i ies and depend on he da a se p ope ies; hus he e o associa ed wi h hese a i- a ions is deno ed as “da a se e o ”. In p ac ice i is di icul o ob ain an ideal da a se wi hou such inhomogenei ies. SF and “co ec ion” a e used o emo e his non-clima ic com- ponen om he ch onology, ocusing on di e en equen- cies. SF deals only wi h high equencies. “Co ec ion” is designed o deal wi h lowe equencies. The co ec ion p o- cedu e consis s o h ee s eps: (1) building a ch onology om smoo hed SF measu emen s (o SF cu es – ini ial se ies o measu emen s wi hou clima ic signal and high- equency a ia ions): his ch onology ep esen s he da a se e o ; and (2) sub ac ing his e o om he ini ial ch onology, hus co ec ing i . See he de ailed algo i hm o he co ec ion p o- cedu e in he Supplemen . 2.4.4 Mul iple-RCS me hod Ou ou h ype o s anda diza ion me hod u ilizes mul i- ple RCS cu es (ins ead o a single cu e used in he RC1, RC1SF and RC1SFC me hods) o emo e he non-clima ic a ia ions (B i a and Mel in, 2011; Mel in e al., 2013). Following he p e ious sugges ions (Mel in e al., 2013), we used wo egional cu es ha we e buil using he SF ou ine ( hus, RC2SF) and RC2SF wi h Ma sko sky’s (2011) co ec- ion ( hus, RC2SFC). The p ocess o de ining he mul iple RCS cu es began wi h calcula ing he mean o SF measu e- men s o he cambial ages 1–100y o each ee. These al- ues we e di ided by he mean o he i s 100y o a single RC1SF cu e (c ea ed using all ees) o yield ela i e g ow h a e (sensu B i a and Mel in, 2011) o MXD p oduc ion o each ee. All ees we e a anged by his ela i e g ow h a e and he ull se o ees was acco dingly di ided in o wo equally la ge g oups (in p ac ice, one g oup was la ge by one ee i he o al numbe o se ies in ha ch onology was odd). The egional cu e was p oduced o each g oup o de- ending each MXD se ies in he co esponding g oup. He e again we unca ed all he se ies o ee- ing measu emen s o achie e eplica ion o a leas 10 samples o e e y egional cu e alue and TVRS spline was used o smoo hing o e- gional cu e. Compa isons be ween he ch onologies we e ca ied ou only o he pa s o ch onologies wi h eplica- ion o a leas six se ies. The e o e, he common pe iod was limi ed by eplica ion o TORN da a se and was se o AD 542–2006. 2.5 Design o expe imen s Fi e ypes o s anda diza ion me hods we e used o com- pa ison unde he assump ions ha TORN and FENN da a se s a e p oxies o mean summe empe a u e (JJA) and ha empe a u e is spa ially homogenous o he whole egion. Co ela ion be ween he JJA empe a u es (AD 1959–2007) as obse ed a Abisko and Sodankylä me eo ological s a ions is 0.83. Unde hese assump ions, imp o ed co ela ion be- ween any pai o di e en ly s anda dized TORN and FENN ch onologies demons a es a highe common (i.e. empe a- u e) signal. We compa ed i e ypes o ch onologies (RCS, RC1SF, RC1SFC, RC2SF and RC2SFC) o TORN and FENN da a se s and also used low-pass il e ed ch onologies wi h N-yea smoo hing splines (Cook and Pe e s, 1981) o ocus on low- equency signals (N=50,100,200 and 300y ). De ailed compa ison o low- equency signals be ween he TORN and FENN da a se s was ca ied ou using hei RC2SFC ch onologies. We also used subsampled FENN da a because o hei highe eplica ion. Subsampling was supposed o highligh any di e ence a ising in he FENN da a se when a i icially educed o he sample dep h o he TORN da a se . Fo de ails o he subsampling algo i hm see he Supplemen . 2.6 Tempe a u e econs uc ion me hods While he FENN da a se bene i s om i s g ea e sample size and eplica ion o e he common pe iod (since ca. AD 700), he bene i s o TORN da a lie in hei homogenei y and hei s onge co ela ion o empe a u e. Bo h ma e ials ha e hei bene i s and ollowing hei high o e all co ela i i y, i was jus i iable o combine he da a in o an enhanced MXD- based paleoclima e econs uc ion. The use o a combined da a se could be seen o bene i om se e al o he ci cum- s ances (see McCa oll e al., 2013). Including a highe num- be o si es p oduces a mean se ies ha is s onge and yields an unce ain y es ima e ha inco po a es he in o ma ion o changes in bo h da a se s o e ime. This app oach also e- duces he e ec s o ecological o cul u al (i.e. non-clima ic) dis u bance e en s in he ch onology. Combining he da a om si es o e a la ge a ea also p oduces a econs uc ion ha is ep esen a i e o he clima e o he ex ended a ea (Mc- Ca oll e al., 2013). The empe a u e econs uc ion was p o- duced om a combina ion o TORN and FENN da a se s (called FULL da a se ). The FULL da a se includes 430 se- ies and co e s he pe iod 216 BC–AD 2010 (8 BC–AD 2010 wi h eplica ion o mo e han i e se ies). The combined www.clim-pas .ne /10/1473/2014/ Clim. Pas , 10, 1473–1487, 2014 1478 V. V. Ma sko sky and S. Helama: Tes ing summe empe a u e econs uc ion da a se has exp essed popula ion signal (EPS; Wigley e al., 1984) alues consis en ly la ge han 0.85 a e AD 535, mo e han o he TORN and FENN da a se s indi idually. We used AD 1802–2010 as he common pe iod o compa - isons be ween he MXD p oxy and ins umen al da a (Kling- bje and Mobe g, 2003). I is no able ha he ins umen al eco d used he e was co ec ed o inhomogenei ies (Klingb- je and Mobe g, 2003, as ci ed in he ex ). Mo eo e , a ecen s udy (Helama e al., 2013) did no show inc easing inhomo- genei ies in he 19 h-cen u y pa o his eco d, in compa i- son o he 20 h-cen u y pa . The da a o he yea AD 1815 (wi h missing ins umen al da a) we e excluded om all he dend oclima ic compa isons. The common pe iod o ins u- men al and MXD da a was di ided in o wo pe iods (AD 1802–1905 and AD 1906–2010) o be used as calib a ion and e i ica ion pe iods and ice e sa o p oducing a JJA empe a u e econs uc ion om he FULL-RC2SFC MXD ch onology. We used he ollowing commonly used s a is- ics o assess he quali y o he econs uc ion: Pea son co - ela ion coe icien ( ), coe icien o de e mina ion (R2), e- duc ion o e o (RE), coe icien o e iciency (CE) and oo mean squa e e o (RMSE) (Cook e al., 1994). The s a is ics o R2, and RMSE we e calcula ed o calib a ion, e i ica- ion and common (1802–2010) pe iod wi h ins umen al da a (no e ha CE is ac ually R2 o e i ica ion pe iod). MXD-based JJA empe a u es we e econs uc ed using non-smoo hed and smoo hed a iance adjus men and linea eg ession me hods (Lee e al., 2007, de ails in he Supple- men ). 2.7 Unce ain y es ima es o he econs uc ion The unce ain ies o he econs uc ion a ising om h ee in- dependen sou ces we e es ima ed as ollows: 1. The unce ain y o eplica ion by he egional cu e (RC): his ype o unce ain y a ises om he RC eplica ion unce ain y ha gene ally inc eases owa ds highe cambial ages. This unce ain y will a ec he un- ce ain y o e e y MXD index, depending on i s cambial age. Since we used wo RCs, his ype o unce ain y is also dependan on he RCs we used and hence on he mean MXD o he i s 100y . 2. The unce ain y o eplica ion by he mean ch onology: his unce ain y p o ides he con idence in e al o he a e age o all MXD indices o each calenda yea ( ak- ing in o accoun he unce ain y 1) 3. The unce ain y om calib a ion: he unce ain y ac- coun s o he unce ain y o mean and a iance (as we use mean and a iance adjus men econs uc ion ech- nique) in he calib a ion pe iod ( aking in o accoun un- ce ain ies 1 and 2). As he h ee ypes o unce ain iesa e assumed obe indepen- den , he unce ain y ha a ises om each sou ce can be de- Figu e 2. Sample eplica ion o FENN and TORN ull da a se s, dense and less dense coho s ( ees wi h highe and lowe a e age MXD alues o he i s 100y ) (a) and a ios be ween eplica ions o dense and less dense coho s o TORN and FENN da a se s (b). i ed by sub ac ing p e ious unce ain ies on he same con i- dence le el. Fo his eason, he esul s in Fig. 7 show ela i e alues o all he h ee unce ain ies. Fo de ails o unce ain y es ima ion, see he Supplemen . 3 Resul s 3.1 Sample size eplica ion The FENN da a se con ains 1.5–3 imes mo e samples han he TORN da a se h ough mos o he common pe iod (Fig. 2a). The eplica ion o TORN ch onology emains be- low 20 samples o e he sub ossil pe iod. The sample size a ia ion o e he pas 200y is explained by he high numbe o co e samples om li ing ees du ing his pe iod. O e he 20 h cen u y, he absolu e inc ease o young pines is highe o FENN da a in compa ison o TORN da a (Fig. 2a). TORN and FENN ull sample se s we e spli in o ees wi h dense (“ ees wi h good g ow h”) and less dense (“ ees wi h poo g ow h”) MXD alues. The e seem o be almos equal amoun s o ees g owing well and poo ly g owing ees h oughou he s udy pe iod (Fig. 2). The isual inspec- ion shows ha he a ios be ween he numbe o dense and less dense ee samples do no co ela e in he FENN and TORN da a se s ( =0.08; Fig. 2b). Since he da a se s o igi- na e om nea by egions, he desc ibed asynch ony indica es ha he examined g ow h componen ( ela i e g ow h a e) is no ela ed o low- equency clima e a iabili y. The e o e we assume ha his inding suppo s he use o mul iple RCS ha will emo e his g ow h componen om he esul ing ch onology. Clim. Pas , 10, 1473–1487, 2014 www.clim-pas .ne /10/1473/2014/ V. V. Ma sko sky and S. Helama: Tes ing summe empe a u e econs uc ion 1479 Figu e 3. Regional cu es (a) and ch onologies (b) o TORN, FENN and FULL da a se s and di e en ypes o s anda diza ion. All he ch onologies a e smoo hed wi h 100y splines. Va iance and mean s anda dized o he pe iod AD 1802–2006, 15y smoo hed da a a e used o a iance adjus men . S88G1112A (Mel in e al., 2013) and N-SCAN (Espe e al., 2012) a e o iginal ch onologies o he TORN and FENN da a se s, espec i ely. 3.2 MXD ch onologies Subsequen ly, he ini ial MXD alues we e ans o med in o RC1, RC1SF, RC1SFC, RS2SF and RS2SFC indices us- ing he co esponding egional cu es (Fig. 3a) and a - e aged in o a co esponding se o mean MXD ch onolo- gies (Fig. 3b). The co ela ions be ween he di e en ly s an- da dized ch onologies decline owa ds lowe equencies o g ow h a iabili y (Supplemen Table S1). On a e age, he non-smoo hed ch onologies ag ee wi h a mean co ela ion coe icien o =0.66, whe eas he 50y , 100y , 200y and 300y smoo hing esul s in mean co ela ion coe icien s o =0.54, =0.49, =0.46 and =0.49, espec i ely. The obse ed endency o lowe co ela i i y a lowe equencies indica es he inc easing unce ain ies in es ima ing pa icu- la ly he long- e m empe a u e a ia ions by MXD p oxy da a. In a iably, he highes co ela ions be ween he ch onolo- gies o he same s anda diza ion me hod we e hose ob- ained by RC1SFC p ocedu e (Supplemen Table S1), ha is, using a single egional cu e bo h wi h signal- ee and Ma sko sky’s (2011) co ec ion implemen a ions. This e- sul becomes e en mo e ob ious o co ela ions calcu- la ed a lowes equencies (Supplemen Table S1). I be- comes e iden ha he applied Ma sko sky’s (2011) co ec- ion (C) subsequen o SF implemen a ion imp o es he ac- ual SF es ima ion, and in pa icula he es ima ion o he low- equency ee- ing a ia ions, as al eady sugges ed in he o iginal me hodological s udy (Ma sko sky, 2011). Ap- plying mul iple-RCS echnique (RC2SF and RC2SFC) p o- duces an imp o emen in FENN ch onologies, bu no in TORN ch onologies. This may be, a leas in pa , owing o a smalle sample size o he TORN da a se ha may no be su icien o obus es ima ion o mul iple RCs. This may be he eason o highes co ela ions ob ained be ween TORN- RC1SFC and FENN-RC2SFC on a ying imescales (Sup- plemen Table S1). Di e en ly p oduced FENN and TORN ch onologies show consis en bu also bi u ca ing dend oclima ic signals (Fig. 3b). Apa om many common ea u es wi h syn- ch onously amelio a ed and de e io a ed g ow h pe iods, i is ound ha he FENN ch onologies exhibi no ably highe g ow h p io o AD 900 and du ing he 17 h cen u y. Rega d- less o he s anda diza ion me hod, he TORN ch onologies indica e highe g ow h du ing he ecen decades and a end inc easing owa ds he p esen day (Fig. 3b). Since he TORN da a se is cha ac e ized by lowe sample eplica ion h ough he common pe iod (Fig. 2a), an expe - imen was pe o med o es whe he he obse ed g ow h bi u ca ions (Fig. 3b) could be explained by his p ope y. The FENN da a se was andomly subsampled o mimic he TORN eplica ion (Fig. 4a). We ound ha he di e ence in sample eplica ion be ween he wo da a se s was no likely a eason o he de ia ing MXD luc ua ions in FENN and TORN o he abo e-men ioned pe iods (Fig. 4b). The low- equency luc ua ions could be de ailed in he con ex o oscilla o y modes domina ing he mul i-decadal o cen ennial MXD a ia ions o FENN and TORN da a (Fig. 5). Bo h ch onologies showed nea ly iden ical 54–56y and 67–70y pe iodici ies, as well as p ecisely iden ical 113 and 133 pe iodici ies. Howe e , he TORN ch onology was also cha ac e ized by he 33y , 244y and 488y pe iodici- ies, whe eas he FENN ch onology exhibi ed an addi ional sub-cen ennial oscilla ion o 86y pe iodici y (Fig. 5). 3.3 Clima e-p oxy compa isons Compa isons be ween he ch onologies and ins umen al cli- ma e eco ds demons a e ha bo h ch onologies co ela e wi h summe empe a u es wi h a ange o coe icien s = 0.76−0.79 (Table S2). A low equencies, howe e , he co - ela ions a e highe when using TORN ch onologies han wi h FENN ch onologies ( =0.95 and =0.84, espec- i ely). This is no an issue o s anda diza ion me hod since he dend oclima ic co ela ions a e in a iably highe in he case o TORN, ega dless o he me hod used (Table S2). Ra he , he FENN ch onologies show wa ming du ing he pe iod 1802–1815 when he TORN and ins umen al da a www.clim-pas .ne /10/1473/2014/ Clim. Pas , 10, 1473–1487, 2014 1480 V. V. Ma sko sky and S. Helama: Tes ing summe empe a u e econs uc ion Figu e 4. Replica ion o TORN, FENN and subsampled FENN da a se s (a) and compa ison o RC2SFC ch onologies o di e en da a se s (b). Red shading shows he a ea be ween 2.5 h and 97.5 h pe - cen iles o FENN subsampled da a se s. Ve ical ed line shows h eshold o well- eplica ed da a se s (mo e han i e se ies). All se ies smoo hed wi h 100y splines. Va iance and mean s anda d- ized o he pe iod AD 1802–2006; 15y smoo hed da a a e used o a iance adjus men . indica e cooling (Fig. 6). Mo eo e , he FENN ch onologies unde es ima e empe a u e a ound 1900 and since he 1950s. In addi ion, no malized FENN ch onologies show in la ed alues ela i e o TORN and ins umen al eco d z-sco es du ing he ea ly-20 h-cen u y wa ming ha occu ed in he egion in he cou se o he 1920s and 1930s (Fig. 6). The FULL ch onologies’ co ela ions wi h ins umen al empe a u es each 0.8 a e mul iple RCs and signal- ee im- plemen a ion (Table S2). He e, he FULL-RC2SFC ch onol- ogy was used o econs uc ing he JJA empe a u e because successi e implemen a ion o mul iple RCS and signal- ee and Ma sko sky’s (2011) co ec ion ou ines leads o e- mo al o high- and low- equency non-clima ic signals om ee- ing ch onology (Sec . 2.4) and because i was indica ed (Sec . 3.2, Supplemen Table S1) ha co ec ion p ocedu e imp o es p oxy quali y on longe imescales. 3.4 Summe empe a u e econs uc ion Ou new empe a u e econs uc ion is shown wi h h ee sep- a a e ypes o unce ain ies since 216 BC (Fig. 7). EPS is usu- ally used as a heo e ical limi o ex ension o econs uc ion Figu e 5. TORN-RC1SFC (a) and FENN-RC2SFC (b) spec um. Red line shows powe spec um o ed noise wi h lag 1 au oco ela- ion es ima ed om he se ies. Pe iod: AD 542–2006. in o he pas . Fo he FULL da a se EPS is consis en ly la ge han0.85 only a e AD 535. We don’ es ic ou econs uc- ion o his yea because he unce ain y es ima es p o ided could be used o he same pu pose mo e easonably. Clea ly, he unce ain ies a ising om MXD da a a e conside ably na owe han he clima e-p oxy calib a ion unce ain y (un- ce ain y 3), especially o e he pas hi een cen u ies. O e he ea lie imes, he unce ain ies o he MXD da a in- c ease, consis en ly wi h he dec ease in he sample eplica- ion (Fig. 2a). A low equencies, he Common E a o he e- cons uc ion is go e ned by he wa m clima ic phases culmi- na ing in he 10 h and 20 h cen u ies. The peak empe a u es du ing hese pe iods a e compa able wi h each o he , whe eas he wa m h o he 1s cen u y BC shows sligh ly highe em- pe a u es. Ne e heless, he empo al leng h o he medie al wa m h om he 8 h o 11 h cen u ies o e shadows he o he wa m pe iods. The cooles clima ic phases p e ailed du ing he 6 h and 7 h, as well as he 13 h h ough 19 h cen u ies, he la e o e lapping wi h he iming o he Li le Ice Age. The wa mes and cooles 30y pe iods occu ed AD 27–56 and AD 536–565, espec i ely, when JJA empe a u es a e - aged 15 and 12.8 ◦C, ansla ing in o maximal mul i-decadal empe a u e ampli ude o 2.2◦C. The econs uc ion exhibi s pe iodic oscilla ions on dis inc ly mul i-decadal (55 o 66y ), cen ennial (86 o 128y ), mul i-cen ennial (171 o 513y ) and millennial (1027y ) imescales (Fig. 8). Clim. Pas , 10, 1473–1487, 2014 www.clim-pas .ne /10/1473/2014/ V. V. Ma sko sky and S. Helama: Tes ing summe empe a u e econs uc ion 1481 Figu e 6. TORN, FENN and FULL ch onologies wi h To nedalen JJA empe a u es (a) and esiduals o he ch onologies om To nedalen JJA empe a u es (b). Va iance and mean adjus ed o AD 1802–2006 using 15y smoo hed da a. All he da a smoo hed wi h 50y splines. S88G1112A (Mel in e al., 2013) and N-SCAN (Espe e al., 2012) a e o iginal ch onologies o he TORN and FENN da a se s, espec i ely. 4 Discussion 4.1 Compa ison o da a se s Focusing on low- equency luc ua ions, he TORN (Schwe- ing ube e al., 1988; G udd, 2008; Mel in e al., 2013) and FENN (Espe e al., 2012) da a se s show se e al common anomalies o synch onously high and low MXD alues, as well as se e al common pe iodici ies. Despi e high co ela- ion wi h summe empe a u es and o e lapping p o enance o he o iginal ee- ing ma e ial, he MXD da a do no show ully consis en g ow h a ia ions h oughou he s udy pe- iod. The mos ob ious pe iods wi h he bi u ca ing dend o- clima ic luc ua ions occu ed in he 7 h, 9 h, 17 h and 20 h cen u ies (Fig. 3b). Ye , he esul s om spec al analysis in- dica e dispa a e dend oclima ic signals in he o m o sig- ni ican oscilla o y modes (Fig. 5). I is easonable o a gue ha hese dissocia ions likely ep esen MXD a ia ions no closely ela ed o he egional clima ic signal. The e o e, be- o e in e p e ing he ob ained paleoclima e in o ma ion on he summe empe a u es, i may be use ul o discuss a. Samples o he wo ch onologies o igina e om adjacen and pa ially o e lapping geog aphical se ings in no he n Fennoscandia and we sugges ha esul s showing di e ging MXD luc ua ions be ween he ch onologies a e no d i en by spa ial anomalies in egional clima e. Indeed, one con- lic ing esul was iden i ied o he la e 20 h cen u y when bo h da a se s con ain ee- ing ma e ials om li ing ees o he same (To ne äsk) egion and hus o he pe iod o supposedly enhanced homogenei y o p o enance. Ins ead o clima e, he e a e six likely sou ces o he e ogenei y: (i) da a quan i y, (ii) biogeog aphical di e ences in sou ce ma e ials, (iii) a ying mic odensi ome ic and (i ) ee- ing s anda d- iza ion me hods, ( ) cambial age s uc u e o he ch onology and ( i) habi a o ipa ian and inland pines. 4.1.1 Da a quan i y Da a quan i y is a ac o ha dis inc ly di ides he o igi- nal ee- ing ma e ials acco ding o ch onologies wi h high (FENN) and low (TORN) eplica ions (Fig. 2a). The sample eplica ion and he law o la ge numbe s should no be o e - looked in he cou se o any dend och onological p ocedu e (F i s and Swe nam, 1989). Mo eo e , i may be gene ally com o able o ely on massi e sample eplica ion in den- d och onology (Bün gen e al., 2012). E en so, an inc easing sample eplica ion may no necessa ily emo e he sys em- a ic e ec s i p esen in he da a. Subsampling o he mo e deeply eplica ed da a se (FENN) e ealed, howe e , ha he di e ging MXD luc ua ions could no be ep oduced by an- domly educing he quan i y o ha da a se (Fig. 4b). Thus, we a gue ha he da a quan i y may no be a p ima y ac- o p oducing he di e ences as obse ed be ween he TORN and FENN ch onologies. 4.1.2 Biogeog aphical aspec s The p ima y di e ence be ween he TORN and FENN da a se s s ems om hei spa ial cha ac e is ics (Fig. 1). While he TORN was collec ed om a es ic ed locali y o To ne äsk in no he nmos Sweden (Ba holin and Ka - lén, 1983; Schweing ube e al., 1988; G udd, 2008), he sub ossil sampling si es o FENN a e sp ead o e no h- e n Finnish Lapland (E onen e al., 2002). Two o he h ee mode n FENN si es a e loca ed in no he n Sweden, in- cluding To ne äsk (Espe e al., 2012). Despi e his p ox- imi y o he mode n sampling si es in he wo collec ions, he wo ch onologies in ac di e ged in iguingly a e AD 1950 wi h no ably highe g ow h obse ed o he TORN ch onologies (Fig. 6). Since he ee- ing ma e ial o FENN and TORN o his pe iod comes om neighbou ing si es, i canno be he biogeog aphical aspec s which play a c u- cial ole in he obse ed di e gence. I is in e es ing, how- e e , ha he p eceding pe iods o di e gence (du ing he 7 h, 9 h and 17 h cen u ies) showed an opposed misma ch (in espec o he 20 h cen u y) whe e he FENN ch onologies come wi h highe g ow h. In a biogeog aphical con ex , he MXD may no be insensi i e o s and densi y a ia ions, as shown by hinning expe imen s. These s udies indica e ha www.clim-pas .ne /10/1473/2014/ Clim. Pas , 10, 1473–1487, 2014