Modeling of bud break of Scots pine in northern Finland in 1908-2014

ORIGINAL RESEARCH ARTICLE published: 05 Ma ch 2015 doi: 10.3389/ pls.2015.00104 Modeling o bud b eak o Sco s pine in no he n Finland in 1908–2014 Hannu Salminen* and Ris o Jalkanen Na u al Resou ces Ins i u e Finland (Luke), Ro aniemi, Finland Edi ed by: Heikki Hänninen, Uni e si y o Helsinki, Finland Re iewed by: C is ina Nabais, Uni e si y o Coimb a, Po ugal Ignacio Ga cía-González, Uni e sidade de San iago de Compos ela, Spain *Co espondence: Hannu Salminen, Na u al Resou ces Ins i u e Finland (Luke), E elä an a 55, Ro aniemi, FI-96300, Finland e-mail: hannu.salminen@luke.fi Bud b eak and heigh -g ow h o Sco s pine (Pinus syl es is L.) in he no he n bo eal zone in Lapland, Finland, was ollowed h ough he en i e g owing seasons in he pe iods 2001–2003 and 2008–2010 in sapling s ands in wo di e en loca ions in no he n Finland se some 250 km apa along a la i udinal ansec . Field measu emen s con inued a he sou he n si e also in 2011–2013. Ai empe a u e was eco ded hou ly a he si es. A simple op imiza ion algo i hm (GA) was used o adjus pa ame e s o he models p edic ing he iming o bud b eak o Sco s pine in o de o minimize he di e ence be ween obse ed and p edic ed da es. The models gi ing he bes pe o mance and cen u y- long daily empe a u es we e used o econs uc bud-b eak ime se ies.The empe a u e obse a ions we e eco ded o he pe iod 1908–2014 in Sodankylä, which is loca ed in- be ween he sapling s ands in he no h–sou h di ec ion and o he pe iod 1877–2014 in Ka asjok, which is in No way abou 145 km no h–wes om he no he nmos s and o his s udy. On a e age buds began o ex end in he beginning o May in he sou he nmos s and and in mid-May in he no he nmos s ands, and he a ia ion be ween yea s was in he ange o 3 weeks. A simple day-leng h- igge ed (fixed da e) model p edic ed mos accu a ely he da e o bud b eak; oo mean squa e e o (RMSE) was 2 and 4 days in he no he n and sou he n si e, espec i ely.The econs uc ed bud-b eak se ies indica ed ha based on empe a u e obse a ions om Sodankylä, g ow h onse o Sco s pine has clea ly ad anced since he 1960s, hough i cu en ly ma ches ha o he ea ly 1920s and ea ly 1950s. The empe a u e eco d om Ka asjok indica ed a simila a ia ion, hough he e was a weak linea end ad ancing bud b eak by abou 3–4 days o e a 100-yea pe iod. Keywo ds: Pinus syl es is L., bud bu s , phenology, clima e, high la i ude, ee line INTRODUCTION Bud b eak in ees can be seen om di e en pe spec i es. The iming o bud b eak in ees is influenced by bo h gene ic (Beuke , 1994) and clima ic ac o s (Chuine, 2000;Heide, 2003;Linkos- alo e al., 2006). Due o he la e , bud b eak can be linked o changes in sp ing empe a u es, and bud b eak and o he phenophases such as flowe ing ha e been used as a bioindica- o o clima e change. In ega d o ee g ow h, he iming o bud b eak plays an impo an ole in compe i ion o sca ce esou ces; ea ly g ow h onse adds leng h o an o he wise sho g owing season, al hough i also exposes he new g ow h o os damage o en b ough on by he a ying sp ing empe - a u es. In gene al, bud b eak has been o in e es because i can be isually assessed and is he e o e ela i ely simple o obse e. Acco ding o Hänninen (1990), any model o bud b eak can be p esen ed using h ee submodels; he es b eak, he de elopmen o g ow h compe ence, and he ac ual bud de elop- men o bu s ing. The mos salien ac o in bud-b eak models is he accumula ion o empe a u e in sp ing, i.e., o cing. Fo cing is ei he a empe a u e sum o an equa ion d i en by empe a u e accumula ion. Accumula ion is e ec i e a e i s p e- equisi e, g ow h compe ence, is me . The simple app oach uses day-leng h o d i e g ow h compe ence, while he mo e sophis ica ed one inco po a es he e ec o chilling o bo h (see Chuine, 2000). The de elopmen o g ow h compe ence and he ac ual on ogene ic de elopmen can be modeled as sequen- ial o simul aneous p ocesses (Hänninen, 1990). The downside o bud-b eak models is ha hey a e usually d i en by daily empe a u e da a, and a ailable con inuous daily empe a u e se ies om no he n Finland a e usually ela i ely ecen , ypi- cally s a ing om he ea ly 1960s. In his s udy, obse a ions o p o incial empe a u e co e ing a pe iod o o e 100 yea s we e applied. Recen epo s on he how he phenology o ees is impac ed by clima e change a e qui e unanimous; phenological e en s ha e ad anced and he g owing season has become longe o e ecen decades (e.g., Chmielewski and Rö ze , 2001;Doi and Ka ano, 2008;Kaye and Wagne , 2014) o e en o e he las cen u y (Linkosalo e al., 2009). Empi ical da a on phenology exis , bu he se ies a e usually sho , las ing 7–30 yea s [see a e iew by Linde holm (2006)] and wi hou econs uc ion o inco po a ing da a on flowe ing o lea bud bu s o b oad-lea ed ee species. No way sp uce (Picea abies (L.) H. Ka s .) and Sco s pine (Pinus syl es is L.), he wo main coni e species in he no he n bo eal zone in Fennoscandia, ha e a di e en s a egy in hei g ow h phenology. Suscep ible o la e os , No way sp uce delays i s flush o he leade bud beyond he beginning o he accumula ion o he www. on ie sin.o g Ma ch 2015 |Volume 6 |A icle 104 |1 Salminen and Jalkanen Bud b eak o Sco s pine empe a u e sum. The e o e he final leng h o he leade shoo is mainly de e mined by he cu en -yea empe a u e. In con as , he final leade -shoo leng h in Sco s pine is p e-de e mined by he p e ious-yea summe empe a u e (Lanne , 1968;Jun ila, 1986;Salminen and Jalkanen, 2005). As he empe a u e o he cu en season canno be o ecas ed, i is a he isky o pine o be de e mined o g ow much in a cold season. To minimize he isk o ea ly os damage on he leade shoo and bud, pine’s s a egy is o flush as quickly as he day leng h and empe a u e de elopmen alone will allow. Ne e heless, i is e y necessa y o bo h species o p og ess h ough hei phenological phases, including bud b eak, in a clea ly sho e ime in no he n han mo e sou he n la i udes simply o su i e in he ha sh no he n ee-line condi ions. To ou knowledge, bud b eak o Sco s pine a high la i- udes has no been s udied p e iously using a ime-scale o se e al decades. The aims o his s udy a e (1) o calib a e p e iously published bud-b eak models (Hänninen, 1990) wi h field obse a ions and me eo ological da a, (2) o es he di - e en ypes o bud-b eak models, and (3) o econs uc a 100-yea -long bud b eak ime-se ies o selec ed si es using a ail- able empe a u e da a in o de o es ablish how econs uc ed bud-b eak da es in no he n Finland eflec he supposed clima e change. MATERIALS AND METHODS FIELD OBSERVATIONS Bud b eak was ollowed al oge he in fi e di e en Sco s pine sapling plo s ep esen ing wo di e en loca ions in he no he n bo eal zone in Lapland, Finland, h ough nine g owing seasons in 2001–2003, 2008–2010, and 2011–2013 (Table 1). The num- be o obse a ion yea s was six in one and nine in he o he si e, and he same ees we e ollowed o h ee consecu i e yea s. Thus he empi ical ma e ial consis s o fi e di e en ee g oups. The heigh and age o he sample ees in he begin- ning o he s udy pe iod we e 1.2–3.0 me e s and 8–20 yea s (Table 1). Fo accu a e measu emen o g ow h accumula ion, a pe manen pin was inse ed h ough he main s em o he p e ious-yea shoo . Heigh g ow h o he leade shoo o he s em was measu ed usually a leas once a week. The bud b eak o a plo was defined o ake place when mo e han hal o he sample ees had gained a leas 1 mm o leng h and he mean inc emen o a plo was a leas 1% o he o al heigh g ow h o ha season. I hose c i e ia we e ulfilled be ween he once-a-week measu emen in e als, which was usually he case, linea in e pola ion was used o es ima e he da e o bud b eak. Ai empe a u e was eco ded hou ly a he si e ei he du - ing he whole yea o h oughou he g owing season using a Tiny ag Ul a da a logge (Gemini Da a Logge s L d.) equipped wi h an ex e nal senso ( ange –40 o +125◦C and esolu- ion 0.4◦Ca +25◦C). Me eo ological measu emen s aken a 3-h in e als ( ans o med in o mean daily alues) we e also a ailable om he nea es o ficial clima e s a ions (Table 2). The da a we e calib a ed using mon hly mean empe a u e di e ences be ween he s udied s and and he espec i e cli- ma e s a ion. I onsi e measu emen s we e missing, as was he case ou side he g owing season in some yea s, espec i e cal- ib a ed alues om he nea es o ficial clima e s a ion we e used. Table 1 |Field obse a ions. S and Plo Yea s Loca ion Heigh abo e see le el (m) Si e ype Mean heigh o sample ees (cm) Numbe o sample ees (1) Laanila, Ina i 1 2001–2003 N 68◦30E27 ◦28215 D yish hea h 169 5 2 2008–2010 N 68◦30E27 ◦28215 D yish hea h 178 101 (2) Van auskoski, Ro aniemi 1 2001–2003 N 66◦22E26 ◦43150 D yish hea h 149 15 2 2008–2010 N 66◦22E26 ◦43150 D yish hea h 188 5 3 2011–2013 N 66◦22E26 ◦44140 Mesic hea h 202 10 1Due o mo ali y, h ee new sample ees we e measu ed in 2010. Table 2 |Loca ion o he me eo ological s a ions. S a ion S a ion numbe Owne Loca ion Heigh abo e see le el (m) Daily empe a u e da a Apukka, Ro aniemi, Finland 7502 Finnish Me eo ological Ins i u e N 66◦35E26 ◦01106 1961–2014 Sodankylä, Finland 7501 Finnish Me eo ological Ins i u e N 67◦22E26 ◦39179 1908–2014 I alo, Ina i, Finland 9601 Finnish Me eo ological Ins i u e N 68◦40E27 ◦34 123 1958–2014 Ka asjok, No way 97250 No wegian Me eo ological Ins i u e N 69◦47E25 ◦48 155 1876–2004 Ka asjok, No way 97251 No wegian Me eo ological Ins i u e N 69◦46E25 ◦50 131 2004–2014 F on ie s in Plan Science |Func ional Plan Ecology Ma ch 2015 |Volume 6 |A icle 104 |2 Salminen and Jalkanen Bud b eak o Sco s pine METHODS The basic idea o model calib a ion was simple: a selec ion o bud-b eak models we e d i en by daily empe a u e da a and esul s we e compa ed o ac ual obse a ions. Fi s , he daily em- pe a u e da a o he calib a ion pe iods we e impo ed and he bud-b eak models we e implemen ed in Mic oso Excel (2010). Second, we used an e olu iona y sol ing me hod (gene ic op i- miza ion algo i hm) add-in included wi h Excel o selec hose model pa ame e s ha esul ed in he smalles oo mean squa e e o (RMSE) compu ed om modeled and obse ed bud-b eak da es. Bud b eak was defined as he da e when mo e han hal o sample ees had hei leade -shoo bud elonga ed a leas 1 mm and he mean inc emen o he plo was a leas 1% o he o al heigh g ow h o ha season. The ollowing op ions we e se o he e olu iona y me hod: con e gence 0.0001, mu a- ion a e 0.1, popula ion size 250, and maximum ime wi hou imp o emen 60 s. Op imiza ion was es ic ed by se ing he allowed bounds o each pa ame e (Table 3). The op imiza- ion algo i hm sc eened o he lowes RMSE wi hin he gi en pa ame e ange. Each model was op imized sepa a ely, and he numbe o pa ame e s depended on he model in ques ion. Fo example, when fi ing he day-leng h- igge ed model, op imiza- ion adjus ed he pa ame e s o he o cing model as well as he day-leng h h eshold o o cing in o de o minimize RMSE. A compa ison be ween he models was based on he ou come o he op imiza ion: he lowes RMSE and he espec i e pa ame e alues. Bud-b eak models gene ally include componen s o es b eak, a ainmen o g ow h compe ence and he ac ual on ogene ic de elopmen (Hänninen, 1990,1995).Thefi s modelsca ch he e ec o g ow h condi ions on de elopmen om ull do - mancy o g ow h compe ence, and he la e desc ibes how ac ual o cing e en ually esul s as a bud b eak. The de el- opmen om es o g ow h compe ence is usually exp essed as a unc ion o accumula ed chilling o pho ope iod o bo h. The e a e a ious ways in which he de elopmen o g ow h compe ence can be inco po a ed in bud-b eak models (see Hän- ninen, 1990;Chuine, 2000). We chose o es he ou models by Hänninen (1990), which desc ibe how accumula ed chilling a ec s he po en ial a e o o cing. The models a e deno ed acco ding o he o iginal a icle by IA,I B, II, III, and IV (Hänninen, 1990;Table 2). As a compa ison, we applied a simple model whe e a fixed day o yea eleases he accumu- la ion o o cing uni s (FUs). Tha model is deno ed by DL (day-leng h). Besides modeling he on ogene ic compe ence, o cing can also be calcula ed using di e en ypes o equa ions ha a e usually d i en by empe a u e. In his s udy, he FU equa ion based on da a by Sa as (1974) and o mula ed by Hänninen (1990, Eq. 8) was applied (Eq. 1). m c ( )=⎧ ⎨ ⎩ 0FU day−1,T( )≤0◦C a·FU day−1 1+e−b·◦C−1·(T( )−c◦C)T( )>0◦C (1) whe e m c( ) is he po en ial a e o o cing in FUs pe day, and T( ) is he p e ailing ai empe a u e. The o iginal pa ame e s (Table 3) o his sigmoidal model a e based on pooled da a o se e al bo eal species (Sa as, 1974; Hänninen and K ame , 2007). The e o e, we used he o iginal pa ame e s as s a ing alues and eleased hem o be selec ed by he op imiza ion algo i hm wi hin a p edefined ange (Table 3). The models gi ing he bes pe o mance in RMSE we e used o econs uc a bud-b eak da e when d i en by he longes a ailable daily empe a u e eco d measu ed since 1908 a he Sodankylä me eo ological s a ion o he Finnish Me eo ological Ins i u e in no he n Finland. As a compa ison, ano he ime se ies was con- s uc ed based on empe a u e eco ds om Ka asjok, no he n No way a ailable since 1877. The long daily empe a u e da a we e impo ed o he same Excel implemen a ion o he bud-b eak models as cons uc ed o he calib a ion phase. The s a is ical significance o he end and au oco ela ion o he econs uc ed bud-b eak se ies we e assessed wi h he ime-se ies analysis p o- cedu e AUTOREG, which is included wi h he SAS s a is ical so wa e (SAS Use ’s Guide, 2002). RESULTS BUD-BREAK DATES The obse ed bud-b eak da es du ing he pe iods 2001–2003 and 2008–2013 showed la ge a ia ion in g ow h onse (Table 4). E en du ing he sho 3-yea pe iod ha each plo was measu ed, he ange o bud-b eak da es wi hin he same plo co e ed one week Table 3 |The bounds o he model pa ame e s. Model Op ion De aul Minimum Maximum Fo cing Fo cing equi emen (FUc i )∗50 0 250 ain Eq. 1∗28.361 15 40 bin Eq. 1∗0.185 0.01 1.0 cin Eq. 1∗18.431 5 30 Chilling Chilling equi emen (CUc i )∗20 0 120 Res pe iod s a da e∗(fixed) Sep embe 1s Sep embe 1s Sep embe 1s Day-leng h Quiescence pe iod s a da e Ma ch 15 h Janua y 1s May 31 h Fo a de ailed pa ame e desc ip ion, see Eq. 1 and Hänninen (1990). *de aul alues a e aken om Hänninen (1990). www. on ie sin.o g Ma ch 2015 |Volume 6 |A icle 104 |3 Salminen and Jalkanen Bud b eak o Sco s pine Table 4 |The obse ed bud-b eak da es. S and Plo Yea s Bud-b eak da es (1) Laanila, Ina i 1 2001–2003 17.05.2001, 01.05.2002, and 13.05.2003 2 2008–2010 26.05.2008, 14.05.2009, and 18.05.2010 (2) Van auskoski, Ro aniemi 1 2001–2003 06.05.2001, 30.04.2002, and 09.05.2003 2 2008–2010 21.05.2008, 10.05.2009, and 13.05.2010 3 2011–2013 11.05.2011, 12.05.2012, and 14.05.2013 Bud b eak was defined as he da e when mo e han hal o sample ees had hei leade -shoo bud elonga ed by a leas 1 mm and he mean inc emen o he plo was a leas 1%o he o al heigh g ow h o ha season. Table 5 |Pa ame e s o he bud-b eak models and hei pe o mance (RMSE) a e op imiza ion. S and (numbe o obse a ions) Model Da e o CU-c i e ia FU-c i e ia Pa ame e s o Eq. 1 RMSE (1) Laanila, Ina i (n=6) DL Ma ch 30 h 124.2 a=19.8; b=0.73; c=6.1 1.58 IA15.4 127.9 a=31.9; b=0.53; c=7.8 1.58 (2) Van auskoski, Ro aniemi (n=9) DL Ma ch 7 h 196.2 a=21.5; b=0.47; c=6.0 3.12 IA21.7 112.4 a=15.8; b=0.95; c=6.2 3.45 on a e age, he la ges ange being 16 days. T ees in Laanila s a ed hei g ow h a ew days la e han hose g owing abou 300 km sou h in Van auskoski. MODEL CALIBRATION A simple day-o -yea - igge ed model (DL) ga e equal pe o - mance o he simples chilling model (Model IA, Eq. 2) used in connec ion wi h on ogene ic compe ence. Bo h o hese a e sequen ial; o cing may s a a e he s a e o quiescence is a ained. Model DL and Model IA esul ed in an RMSE o 1.6 and 3.1–3.4 o S ands 1 and 2, espec i ely, (Table 5). The o he ou models o on ogene ic compe ence add essed by Hänninen (1990) we e in e io o IA, so hei esul s a e no epo ed. In Laanila in 2009, bud b eak ook place 3 days be o e p edic ed (Figu e 1). In Van auskoski, he yea s 2003 and 2012 inc eased he RMSE alue he mos , pa ly due o ea lie - han-p edic ed bud b eak (Figu e 2). Mchl ( )= ⎧ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩ 0CU day−1,T( )≤−3.4 ◦C 0.159 CU day−1◦C−1·−3.4 ◦C<T( )≤3.5 ◦C T( )+0.506 CU day−1, −0.159 CU day−1◦C−1·3.5 ◦C<T( )≤10.4 ◦C T( )+1.621 CU day−1, 0CU day−1,T( )>10.4 ◦C (2) whe e Mchl( ) is he a e o chilling in chilling uni s (CU) pe day, and T( ) is he p e ailing ai empe a u e. The pa ame e s o he FU equa ion (Eq. 1) we e changed as a esul o op imiza ion. The de aul alues (a=28.361, b=0.185, FIGURE 1 |P edic ed and obse ed bud b eak in plo s 1 and 2 in Laanila, Model IAwi h a chilling componen and Model DL wi h day-leng h- igge ed o cing. c=18.431) a e based on pooled da a o se e al bo eal species (Sa as, 1974) and hus a e no species-specific. The pa ame e alues ob ained in his s udy a e qui e di e en compa ed o he de aul alues. Pa ame e b ha defines he slope o he empe a- u e esponse cu e ecei ed a high alue compa ed o he de aul alue, which esul ed in a much s eepe cu e (Figu e 3). Pa am- e e cdefines he shape o he cu e, i.e., he empe a u e a he u ning poin o he esponse cu e. The pa ame e alues a e dependen on each o he and he change in he slope has an e ec also on he shape. BUD-BREAK RECONSTRUCTION Model IA(includes a chilling e ec ) and he day-o -yea Model DL we e used o econs uc bud-b eak da es based on he daily F on ie s in Plan Science |Func ional Plan Ecology Ma ch 2015 |Volume 6 |A icle 104 |4 Salminen and Jalkanen Bud b eak o Sco s pine FIGURE 2 |P edic ed and obse ed bud b eak in plo s 1, 2, and 3 in Van auskoski, Model IAwi h a chilling componen and Model DL wi h day-leng h- igge ed o cing. FIGURE 3 |Fo cing uni (FU) equa ion esponse o empe a u e on h ee di e en pa ame e se s; de aul s (dashed line), pa ame e s sol ed o Laanila (solid line), and pa ame e s sol ed o Van auskoski (do ed line). empe a u e eco d om Sodankylä and Ka asjok since he yea 1908 and 1877, espec i ely. The di e ence be ween Ap il and May mean empe a u es in each o he si es du ing he model calib a- ion yea s was used o scale Sodankylä and Ka asjok empe a u e eco ds. Hence, when using Sodankylä da a wi h pa ame e s om Laanila, daily empe a u e alues we e educed by 1.4◦C and when using pa ame e s omVan auskoski inc eased by 0.7◦C. Likewise, when bud b eak in Laanila and Van auskoski we e econs uc ed wi h da a om Ka asjok, empe a u e was educed by 0.4◦C and inc eased by 1.7◦C, espec i ely. Thus, he o m o he econ- s uc ed bud-b eak da es emained in ac bu hei le el was in line wi h he mean empe a u es o he calib a ion da a. Due o he missing obse a ions in Ap il–May o empe a u es in Sodankylä and Ka asjok, he espec i e yea s 1918 and 1877–1889 we e omi ed om he esul s (Figu es 4 and 5). In Laanila, Model IA(includes a chilling-e ec ) p edic ed la e bud b eak han he Model DL wi h empe a u e da a om Sodankylä da a, while da a om Ka asjok he o de was e e sed (Table 6). The di e ence be ween he Sodankylä and Ka asjok empe a u e eco ds is s ongly a ec ed by he adjus men s based on he Ap il–May empe a u es. Hence, he absolu e alues p oduced based on hese wo empe a u e eco ds canno be di ec ly compa ed. Ins ead, he ocus should be on ends and a ia ion. A ime-se ies analysis e ealed ha none o he cons uc ed se ies included s a is ically significan au oco ela ion. A possible linea end in bud-b eak da es was analyzed by fi ing an au o e- g essi e model o he da a. Se ies based on empe a u e eco ds om Sodankylä did no include a significan linea end. Tem- pe a u e eco ds om Ka asjok ga e sligh ly con adic o y esul s. Model IAbased on pa ame e s o Van auskoski p oduced a se ies wi h no s a is ically significan end, bu he h ee o he se ies, Model DL o Van auskoski and Models IAand DL o Laanila, included a end ha was s a is ically significan acco ding o he maximum likelihood (ML) es ima es bu insignifican based on o dina y leas squa es (OLS) es ima es. The o me a e known o o e es ima e esidual a iance while he la e unde es ima es i (SAS Use ’s Guide, 2002). The e o e, he conclusion is ha he e is a linea end ha is bo de line s a is ical significan (p=0.05). Coe ficien s o he ime- a iable in he ML es ima ion in he Ka asjok da a anged om –0.028 o –0.045, which co esponds o a 3–4 day ad ance in bud b eak in he las 107 yea s. DISCUSSION Cla k e al. (2014) poin ou se e al sou ces o e o in phenolog- ical modeling. Ou s udy is no ee o hem. We se a minimum equi emen o measu ed heigh g ow h a e bud b eak was obse ed and applied linea in e pola ion when u ning weekly obse a ions in o a p ecise da e. The weekly measu emen in e al is a comp omise be ween cos s and da a p ecision. Fu he mo e, he chilling and o cing models we e d i en by daily mean em- pe a u e da a, which fla ens ou he e ec o minimum and maximum alues, while we did no pay a en ion o sola adia ion, soil empe a u e, g ound os , o snow mel . These simplifica- ions we e due o he ela i ely sho and incomple e da a eco ds a ailable. Compa ison o he modeled and obse ed bud-b eak da es e ealed ha due o he defini ion o bud b eak, he models pe - o med be e when he empe a u e change om cool o wa m was clea and s eep. In con as , when he empe a u e change was slow, he models ga e la ge RMSE alues. A ypical example is seen in 2008 in he sou he n loca ion, when hal o he ees began o g ow ollowed by a b eak o o e a week, a e which he es o he ees also b oke. Acco ding o he chosen c i e ia, he ime when he la e g oup o ees s a ed o elonga e was selec ed as he bud-b eak da e. Daily obse a ions and longe con inuous field measu emen s would enable mo e eliable calib a ion o he models. Bud-b eak models by Hänninen (1990) we e calib a ed wi h field obse a ions om wo di e en loca ions in no he n Finland. Empi ical ma e ial consis s o fi e di e en ee g oups, each ollowed o h ee consecu i e yea s. These sample da a we e ed in o an op imiza ion p ocedu e ha selec ed pa ame e s o chilling and o cing models and which ga e he lowes RMSE. www. on ie sin.o g Ma ch 2015 |Volume 6 |A icle 104 |5 Salminen and Jalkanen Bud b eak o Sco s pine FIGURE 4 |Recons uc ed bud b eak using adjus ed empe a u e eco ds om Sodankylä and pa ame e s o wo models, Model IAwi h a chilling componen and Model DL wi h day-leng h- igge ed o cing, sol ed o Laanila (A) andVan auskoski (B).Yea 1918 was omi ed due o he missing empe a u e obse a ions. FIGURE 5 |Recons uc ed bud b eak using adjus ed empe a u e eco ds om Ka asjok and pa ame e s o wo models, Model IAwi h a chilling componen and Model DL wi h day-leng h- igge ed o cing, sol ed o Laanila (A) andVan auskoski (B).Yea s 1877–1889 we e omi ed due o missing empe a u e obse a ions. Table 6 |Mean bud-b eak da es (day o yea ) wi h wo empe a u e eco ds, Sodankylä and Ka asjok, and wo models (IAand DL) pa ame e ized o Laanila and Van auskoski. Laanila, Model IA Laanila, Model DL Van auskoski, Model IA Van auskoski, Model DL Sodankylä 144 142 138 135 Ka asjok 146 155 137 150 Bo h pu e nume ical pa ame e s and fixed-day-o -yea , when applicable, we e analyzed simul aneously. Ou app oach exem- plifies how young ees o oday would ha e eac ed o he chilling and o cing empe a u es o he pas clima e. The size o he da a does no jus i y ex ensi e gene aliza ion and a he han ocusing on he pa ame e s o he models, he emphasis is on he ends shown by he econs uc ed bud-b eak se ies. The e a e isible cyclic changes bu no clea long- e m ends ha can be dis in- guished in he se ies based on Sodankylä empe a u e da a; he bud-b eak da es o he 1920s and he 1950s a e as ea ly as hose seen in he beginning o he 2000s. This is no consis en wi h he esul s o ecen a icles in he field o phenology, which ha e usu- ally confi med a long- e m empe a u e ise (e.g., Linkosalo e al., 2009). This s udy is based on obse a ions om no he n Finland and ocus on ea ly sp ing empe a u e, which may explain why findings a e no he same when analyzing sou he n loca ions and F on ie s in Plan Science |Func ional Plan Ecology Ma ch 2015 |Volume 6 |A icle 104 |6 Salminen and Jalkanen Bud b eak o Sco s pine empe a u e la e in he g owing season. On he o he hand, da a om Ka asjok indica ed ha he e is a linea end, al hough i s s a is ical significance was no e y clea . Al hough obse a ions om Ka asjok and Sodankylä a e simila o a la ge deg ee, heAp il and May mean empe a u es om Ka asjok in pa icula ha e a small inc easing end o e he cen u y ha makes a di e ence. Recen findings, o example, on empe a u e de elopmen based on dend oclima ological s udies o pine in he uppe Fennoscan- dia and NW Russia (McCa oll e al., 2013;Lindholm e al., 2014) a e in line wi h ou esul s on he mal de elopmen . Ou esul s o a end owa ds ea lie bud bu s in pine a e no en i ely in keeping wi h esul s o downy bi ch (Be ula pubescens Eh h.) om he same a ea, which had p edic ed he po en ial g owing season would inc ease conside ably (Bennie e al., 2010). Simila ly Heikinheimo and Lappalainen (1997) and Pudas e al. (2008) see ha sp ings seasons a high la i udes ha e ad anced. The eason o he dispa i y cen e s on he leng h o he ime span; pu simply, one is unable o sugges sp ing ad ance wi h jus da a co e ing 7–15 yea s. E en gi en he leng h o ou econ- s uc ion pe iod, he maximum ins umen al pe iod since 1908 in Sodankylä may be oo sho , since i canno indica e low equencies in he se ies a all. ACKNOWLEDGMENTS We acknowledge Ta mo Aal o and Pekka Nä hi o echnical assis- ance. This wo k was pa ly unded by he EU p ojec PINE (EVK2-2001-00182) and Academy o Finland (SA 138937). REFERENCES Bennie, J., Kubin, E., Wil shi e, A., Hun ley, B., and Bax e , R. (2010). 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(2005). Modelling he e ec o empe a u e on heigh inc emen o Sco s pine a high la i udes. Sil a Fenn. 39, 497–508. doi: 10.14214/s .362 Sa as, R. (1974). In es iga ions on he annual cycle o de elopmen o o es ees. Au umn do mancy and win e do mancy. Comm. Ins . Fo . Fenn. 84, 1–101. SAS Use ’s Guide. (2002). SAS/ETS Use ’s Guide, Vol. 1, 2. Ca y, NC: SAS Publishing. Con lic o In e es S a emen : The au ho s decla e ha he esea ch was conduc ed in he absence o any comme cial o financial ela ionships ha could be cons ued as a po en ial conflic o in e es . Recei ed: 15 Sep embe 2014; accep ed: 08 Feb ua y 2015; published online: 05 Ma ch 2015. Ci a ion: Salminen H and Jalkanen R (2015) Modeling o bud b eak o Sco s pine in no he n Finland in 1908–2014. F on . Plan Sci. 6:104. doi: 10.3389/ pls.2015.00104 This a icle was submi ed o Func ional Plan Ecology, a sec ion o he jou nal F on ie s in Plan Science. Copy igh © 2015 Salminen and Jalkanen. This is an open-access a icle dis ibu ed unde he e ms o he C ea i e Commons A ibu ion License (CC BY). The use, dis- ibu ion o ep oduc ion in o he o ums is pe mi ed, p o ided he o iginal au ho (s) o licenso a e c edi ed and ha he o iginal publica ion in his jou nal is ci ed, in acco dance wi h accep ed academic p ac ice. No use, dis ibu ion o ep oduc ion is pe mi ed which does no comply wi h hese e ms. www. on ie sin.o g Ma ch 2015 |Volume 6 |A icle 104 |7