Oak decline in a southern Finnish forest as affected by a drought sequence

ISSN 1897-1695 (online), 1733-8387 (p in ) © 2013 Silesian Uni e si y o Technology, Gliwice, Poland. All igh s ese ed. GEOCHRONOMETRIA 41(1) 2013: 92–103 DOI 10.2478/s13386-013-0137-2 A ailable online a www.sp inge link.com OAK DECLINE IN A SOUTHERN FINNISH FOREST AS AFFECTED BY A DROUGHT SEQUENCE KRISTINA SOHAR1, SAMULI HELAMA2, ALAR LÄÄNELAID1, JUHA RAISIO3 and HEIKKI TUOMENVIRTA4 1Depa men o Geog aphy, Ins i u e o Ecology and Ea h Sciences, Uni e si y o Ta u, Vanemuise 46, 51014 Ta u, Es onia 2Finnish Fo es Resea ch Ins i u e, No he n Uni , P.O. Box 16, 96301 Ro aniemi, Finland 3The Ci y o Helsinki, The Public Wo ks Depa men , S ee and Pa k Di ision, P.O. Box 1515, 00099 Helsinki, Finland 4Finnish Me eo ological Ins i u e, P.O. Box 503, 00101 Helsinki, Finland Recei ed 28 Ma ch 2013 Accep ed 21 Augus 2013 Abs ac : We in es iga ed he decline o a peduncula e oak (Que cus obu L.) o es g owing on shallow soil a he no he n dis ibu ional limi o he species in sou he n Finland, using he dend o- clima ic app oach. Abou 200-yea -old ees in h ee igou classes – heal hy, declining and dead – we e sampled in 2008. Annual ee- ing, ea lywood and la ewood wid hs we e measu ed and ch o- nologies we e es ablished. The ee- ing da a we e co ela ed wi h mon hly and seasonal clima e da a. Radial inc emen o oaks was posi i ely ela ed o he June and July p ecipi a ions. This was ex- p essed especially in o al ing wid h and la ewood wid h, whe eas he ea lywood was mo e in lu- enced by he wa me win e and sp ing. Fu he mo e, he co ela ion be ween he cu en yea ea - lywood wid h and he p eceding yea la ewood wid h was highe han be ween he ea lywood and la ewood o he same yea . The analyses showed ha he dead oaks and pa o he declining oaks had ceased g owing du ing 2005-2007 a e a decade-long summe d ough se ies. This indica es a ime lag in he oak dieback. The adial g ow h o he declining and he dead oaks had d opped al eady since he 1990s, while he heal hy oaks had be e long- e m g ow h and highe adap i e capaci y o clima e a ia ion. Keywo ds: dend oclima ology, mo ali y, ee ings, ea lywood, la ewood, Que cus obu L. 1. INTRODUCTION Fo es mo ali y has been epo ed wo ldwide ecen ly and has been a ibu ed o clima e change- ela ed d ough and hea s ess (Allen e al., 2010). Besides, se e al s ud- ies ha e shown a la ge-scale decline o Que cus species h oughou Eu ope (e.g. Füh e , 1998; Siwecki and U - nalski, 1998; Sonesson, 1999; Peñuelas e al., 2001; Landmann and D eye , 2006; D obyshe e al., 2007a, 2007b) as well as in No h Ame ica (e.g. S a kley and Oak, 1989; S a kley e al., 1989; Fabe -Langendoen and Tes e , 1993; S a kley e al., 2004, Ca on e al., 2007). As his ex ensi e mo ali y causes conce n o o es e s, ec ea ion manage s as well as o he wide communi y, he e is an in e es and demand o sea ch o he easons. Acco ding o McC acken (1985) h ee concep s o oak dieback can be dis inguished: (1) single causal ac o s (like an ex eme wea he e en o a pa hogen), (2) coho senescence (combina ion o age and en i onmen al s ess), and (3) decline synd ome complex. The la e in ol es long- e m p edisposing ac o s like ee age, clima e and soil condi ions, and long- e m con ibu ing Co esponding au ho : K. Soha e -mail: k is ina.soha[email p o ec ed] B ough o you by | Me la Finnish Fo es Resea ch Ins i u e Au hen ica ed Download Da e | 12/20/16 11:16 AM K. Soha e al. 93 ac o s such as pa hogens, as well as sho - e m inci ing ac o s like a single ex eme wea he o ai -pollu ion e en (Manion, 1981). Thus, he easons a e ela ed wi h se e al bio ic and abio ic ac o s, and a e equen ly a combina ion o di e en se e e de e minan s (e.g. Wa go, 1996; Gibbs and G eig, 1997; Thomas e al., 2002), pa - ly wi h si e-speci ic causes (Jung e al., 2000; Jönsson e al., 2005). Al hough oaks ha e bo h mo phological and physiolog- ical adap a ions o ole a e d ough (e.g. deep oo ing, con- ol o s oma al conduc ance, non-s oma al con ol o ca - bon ixa ion) (Dickson and Tomlinson, 1996), hese eac- ions lose impo ance wi h inc easing d ough s ess mak- ing ees mo e suscep ible o o he s ess ac o s (Thomas e al., 2002). I has been shown ha expe imen ally applied d ough educes s em and shoo g ow h, and whole- ee lea a ea (A end e al., 2011) while mode a e clima e change does no a ec nei he lea ni ogen no non- s uc u al ca bohyd a es (Li e al., 2013). All ha consid- e ed, d ough has been assumed o be one o he main ac o s causing oak decline h ough educ ion o wa e up ake (e.g. Siwecki and U nalski, 1998; D obyshe e al., 2007b) o weakening he ees agains o he s ess de e mi- nan s like pa hogenic ungi (e.g. mildew) and insec pes s (e.g. lea olle ). All his esul s in de olia ion which leads o he educ ion o pho osyn hesis and o dea h (e.g. Si- wecki and U nalski, 1998; Thomas e al., 2002). I is well known ha ees p o ide in o ma ion on he pas g owing condi ions in hei annual ings, and di e - en pa ame e s o he la e can be easily measu ed as a con inuous sequence o yea s and di ec ly calib a ed agains any p edisposing en i onmen al ime se ies, o example clima ic eco ds (F i s, 1976). Dend och ono- logical echniques allow us o ex ac ex e nal o in e nal g ow h signals om he o e all g ow h a iabili y (Cook, 1987) and o de e mine he ela i e impo ance o an en i onmen al ac o (e.g. a wea he a iable) o ee igou . A ee- ing wid h a chi e can be used in explain- ing mo ali y e ospec i ely, o example, iden i ying he p esence and spa io empo al pa e ns o p e-mo em g ow h dep essions and da ing he exac ime o dea h (D obyshe e al., 2007b). The dend och onological da a can be used o p edic ing he ime o ee dea h as well (Pede sen, 1998; Che ubini e al., 2002; Bigle and Bug- mann, 2004; Dobbe in, 2005). A decline in peduncula e oaks (Que cus obu L.) as well as o he ee species has been obse ed in he Hel- sinki me opoli an a ea in Finland du ing he ecen yea s, especially 2003 (Ympä is ö apo oinnin asian un i- ja yö yhmä, 2004; Holopainen e al., 2006). Finnish oaks g ow a he species’ no he nmos na u al egene a ion bo de (e.g. Axel od, 1983; Dahl, 1998). Cold ha diness du ing he au umn is conside ed as a main ac o limi ing oak seedling o sp ead u he no h (Repo e al., 2008), combined wi h he des uc ion by humans and he compe- i ion wi h sp uce (Rainio, 1977). On he o he hand, dend oclima ological analyses ha e e ealed ha adial g ow h is con olled especially by summe ain all, and mo ali y has been associa ed wi h d ough s, likewise wi h shallow soil dep h, which inc ease en i onmen al ee s ess (Helama e al., 2009). Ou wo k is a pa o a wide esea ch on oak mo ali- y in Finland, and he e o e i ollows simila s udy de- sign and he me hods used be o e (Helama e al., 2009). The goal was o de e mine he empo al mo ali y pa e n and o examine he decline o peduncula e oaks in he Tammis o o es using long- e m ee- ing se ies (ea - lywood, la ewood and o al annual ing wid h) in connec- ion wi h clima ic and soil da a. 2. MATERIAL AND METHODS T ee- ing da a The Tammis o Pa k is a 15.5 ha na u e ese e o na - u al o es loca ed in Van aa, Helsinki me opoli an a ea (60°16’35”N, 24°57’49”E; Fig. 1). A spo adically ex- posed bed ock idge makes up he cen e o he a ea, up o 38 m abo e sea le el, and is co e ed by Pinus syl- es is. The no he n slope is domina ed by Picea abies while he sou he n side is co e ed by b oadlea o es . In addi ion o Que cus obu , Tilia co da a and Ace pla - anoides a e ound while he unde s o y is occupied by Co ylus a ellana. Sp uce o e g ow h is s opped by he p o ec ion ac i i ies (Honkanen, 2009). We had a pe mission o co e no mo e han 30 pedun- cula e oaks o his s udy in July 2008. Following he c i e ia used ea lie (Helama e al., 2009), en heal hy ees de ined as li ing ees wi h g een oliage, en declin- ing ees as diagnosed by educed c own condi ion, mod- Fig. 1. Loca ion o he s udy si e. Dis ibu ion o oak in g ey (EUFORGEN, 2009). B ough o you by | Me la Finnish Fo es Resea ch Ins i u e Au hen ica ed Download Da e | 12/20/16 11:16 AM OAK DECLINE IN A SOUTHERN FINNISH FOREST AS AFFECTED BY A DROUGHT SEQUENCE 94 es oliage degene a ion and/o some b anch dea h, and en dead ees diagnosed by de olia ion and o e all b anch dea h we e andomly chosen. We ook ee- ing samples wi h an inc emen bo e om b eas heigh o- wa ds he pi h om he no he n side o he unk and su eyed he soil dep h by d i ing a soil p obe in o he g ound un il e usal (Tes a e al., 2009) a loca ions se- lec ed om ou di ec ions a a dis ance o wo me e s om he s em. The a i hme ic mean o he ou obse a- ions, and he shallowes obse a ion as minimum soil dep h, we e used in he da a analysis. Wi hin he ee ings we measu ed he ea ly- and la ewood wid hs sepa a ely o he nea es 1/100 mm using a Lin ab™ measu ing able (Heidelbe g, Ge many) equipped wi h a ligh mic oscope. We dis inguished he bounda y be ween he ea lywood and la ewood acco ding o a quali a i e aspec : he ea lywood con ains he la ge essel zone, while he la ewood comp ises he small po e zone (e.g. Schweing ube , 2007). The sum o he ea ly- and la ewood measu emen s was adop ed as he o al annual ee- ing wid h. The TSAP-Win so wa e (Rinn, 2003) was used o eco d he measu ed alues and plo he g aphs. The quali y con ol o he se ies was pe - o med isually om he g aphs and wi h he Co echa so wa e om he Dend och onology P og am Lib a y (Holmes, 1983). As he da e o co ing was known, he las annual ing o heal hy oaks could be coupled wi h he calenda yea 2008. The es o he samples we e c oss- da ed agains he mean ch onology o heal hy oaks using he dend och onological p og am CATRAS (Aniol, 1983) and he da ing esul s we e checked isually om he g aphs. This c oss-da ing esul ed wi h he calenda yea s o all ea lywood, la ewood, and annual ing wid hs. Dend och onological analysis A e age se ies o ea lywood, la ewood, and annual ing wid hs we e c ea ed om each igou class acco d- ing o he cambial age (e.g. B i a e al., 1992) whe e he ings we e aligned by hei age s a ing om he pi h ins ead o he calenda yea s a b eas heigh . Al hough he e we e only wo co es con aining he pi h, he majo i- y o he co es s ill had nea -pi h ee ings acco ding o he cu ed inne mos ing shape (on a e age, six inne - mos missing ings pe co e acco ding o a pi h indica o empla e). Only one ee had a oo la ge diame e o ca ching he pi h and one ee was o en inside. The ela i e g ow h a e (RGR) (B i a and Mel in, 2011; Helama e al., 2012) o each ee was calcula ed as ∑∑ == = n n xxRGR 11 (2.1) whe e x is he ee- ing wid h in yea , x is he mean wid h o all ings in yea , and n is he numbe o ings in he se ies. Thus, i is a a io o he ee adius a i s inal yea o he a e age o e all-sample adius a ha yea . The RGR alues we e a e aged ac oss each igou class. As ollows, we adop ed a s anda diza ion p ocedu e (F i s, 1976) and con e ed he measu emen se ies in o dimensionless indices. To emo e he ee-age- ela ed g ow h end and o educe non-clima ic noise (i.e. low- equency noise), he measu emen se ies we e de ended using a s i spline unc ion, which p ese es 50% o he a iance con ained in wo- hi ds o he indi idual se ies leng h (Cook and Pe e s, 1981). The spline cu e was i ed and he indices de i ed om he cu e as a ios using he A s an p og am (Cook, 1985). In o de o compa e he ee g ow h a iabili y wi h he soil da a, he s anda d de ia ion, he mean sensi i i y (MS), and he i s -o de au oco ela ion o each index se ies we e compu ed by he A s an p og am. While he s anda d de ia ion desc ibes he o al a ia ion in he se ies, he mean sensi i i y indica es he yea - o-yea a iabili y be ween he adjacen ing wid hs and can be de e mined as ∑ −= =+ + + − − = 1 11 1)(2 1 1n xx xx n MS (2.2) whe e x is he ee- ing wid h in yea , and n is he num- be o yea s/ ee- ings in he se ies (F i s, 1976). Also, we compa ed he RGR o each ee wi h he minimum soil dep h da a. As all hese a iables we e no mally dis ibu ed, we used he Pea son co ela ion coe icien ( ) o desc ibe hei ela ionships. Dend oclima ologial analysis Fo he dend oclima ological analysis, he index se ies we e p e-whi ened in he A s an o emo e he pe sis- ence due o au oco ela ion. The index se ies we e a e - aged using a bi-weigh obus mean es ima ion o educe ou lie e ec s (Cook, 1985). As a esul , we ecei ed nine esidual ch onologies acco ding o he h ee wood ypes and he h ee igou classes. These ch onologies we e used o calcula ing he c oss-co ela ions and o he dend oclima ological analysis. In addi ion, we co ela ed he ea lywood wid h ch onology wi h he la ewood wid h ch onology o he p eceding yea . The common pe iod 1894-2004 was used o all ch onologies. This ime ange con ained a leas i e ees in each igou class (Läänelaid, 2000). We compa ed ou s anda dised ee- ing da a wi h mon hly mean empe a u es and p ecipi a ion sums om he Helsinki Kaisaniemi wea he s a ion (60°10’48”N, 24°56’24”E, 4 m a.s.l.), loca ed ca. 11 km om Tammis- o. We used homogenised and de ended mon hly eco ds (Tuomen i a, 2004) and he Dend oClim2002 p og am (Biondi and Waikul, 2004) o examine he ela ionships be ween he ee- ing da a and he me eo ological a ia- bles. This p ocess uses boo s apped con idence in e als o es ima e he signi icance (p < 0.05) o he esponse B ough o you by | Me la Finnish Fo es Resea ch Ins i u e Au hen ica ed Download Da e | 12/20/16 11:16 AM K. Soha e al. 95 unc ion coe icien s, which a e mul i a ia e es ima es om a p incipal componen eg ession model (B i a and Cook, 1990). He e we used he 12-mon h window om he p e ious yea ’s Oc obe o he cu en yea ’s Sep em- be . Th ee-mon hly seasonal a e age empe a u es and p ecipi a ion sums om he p e ious Sep embe o he concu en Augus we e calcula ed and co ela ed wi h he ee- ing ch onologies. 3. RESULTS Age and dea h da es o ees Al oge he , we measu ed and da ed 30 oak ees equally dis ibu ed be ween he h ee igou classes. The dend och onological s a is ics o he aw da a as well as o he s anda dized ch onologies by igou class a e summa ised in Table 1. The sample size is shown in Fig. 2. Compa ing he igou classes, he declining oaks co - e ed he longes ime pe iod (1799-2008) while he dead and he heal hy oaks co e ed 1812-2006 and 1836-2008, espec i ely. Howe e , acco ding o he - es , mean ee age did no di e signi ican ly be ween he igou classes (p > 0.05), as well as mean annual ing wid h, s anda d de ia ion, mean sensi i i y and au oco ela ion. Thus, acco ding o he s a is ics he e is no ele an di e ence be ween igou classes. The oaks diagnosed as heal hy we e ound o ming he ing in he sampling yea 2008. Among he declining oaks, i e ees we e ound s ill g owing in he summe o 2008 ( wo o hem ha ing only ea lywood). The ou e mos pa s o wo ees we e dam- aged (da ed as 1950 and 1988), and wo ees had o med hei las ing in yea 2006 and one in 2007, a leas in he no he n pa o he s em. Among he dead ees, ou oaks had s opped g owing in he yea 2006 and one in he yea 2005 ( h ee o hem ha ing only ea lywood), he o he i e had damaged ou e mos ings and hese we e da ed as 1976, 1989, 1991, 1992 and 1993. T ee- ing g ow h The a e age ee- ing se ies o he declining and he dead ees e ealed clea p e-mo em g ow h educ ion du ing he las decades in all ing wid h pa ame e s (Fig. 2). Fo example, he mean annual ing wid hs di e ed signi ican ly be ween he wo pe iods ( - es , p < 0.05) being 1.3 and 1.2 mm o he declining and he dead ees, espec i ely, du ing he pe iod 1973-1990, and 0.8 and 0.7 mm, espec i ely, du ing he pe iod 1991-2008. A he same ime he heal hy oaks had also a signi ican bu s ill lowe g ow h educ ion (a di e ence 0.3 mm be ween wo pe iods). The lowe inc emen a he heal hy oaks’ ea ly li e ime was caused by he excep ionally small sample eplica ion (only one ee). The la ewood and he annual ing wid h cambial g ow h pa e n (Fig. 3) showed ha he heal hy and he dead ees had simila Table 1. S a is ics on he ee- ing measu emen s and on he indexed ch onology. Vi gou class T ee- ing measu emen s Residual ch onology Mean numbe o yea s Mean ing wid h (mm) S d. de ia ion (mm) Mean sensi i i y 1s o de au o- co ela ion S d. de ia ion Mean sensi i i y 1s o de au o- co ela ion Heal hy 118 1.77 0.77 0.21 0.73 0.16 0.18 0.02 Declining 126 1.60 0.73 0.23 0.69 0.23 0.24 -0.07 Dead 129 1.52 0.73 0.22 0.74 0.22 0.21 -0.03 Fig. 2. Annual sample eplica io n and a e age ea lywood, la ewood, and o al annual ing wid h se ies by igou class. B ough o you by | Me la Finnish Fo es Resea ch Ins i u e Au hen ica ed Download Da e | 12/20/16 11:16 AM OAK DECLINE IN A SOUTHERN FINNISH FOREST AS AFFECTED BY A DROUGHT SEQUENCE 96 g ow h s a egies o e hei li espan; howe e he dead oaks had a lowe ela i e g ow h a e (Eq. 2.1). The de- clining oaks appea ed o ha e a sligh g ow h ecession du ing he middle o hei li e ime. The cambial g ow h pa e ns o ea lywood se ies seemed o ha e a mo e he - e ogeneous wi hin he igou classes wi h a dec ease in he second hal o he dead ees’ inc emen becoming dis inguishable. The heal hy ees’ g ow h decline a hei age 30-40 was caused by he simila age o mos ees whe e o e he e ec o ad e se calenda yea s was no smoo hed ou om he ime se ies. O e all, acco ding o he - es , he di e ences in he ela i e g ow h a es be- ween he nine subse s we e no s a is ically signi ican (p > 0.05). The co ela ions be ween he p e-whi ened ch onolo- gies (da a om he same yea ) we e posi i e and s a is i- cally signi ican (p < 0.05; Table 2). Howe e , he co e- la ions wi hin he ea lywood ch onologies we e lowe han wi hin he la ewood and he o al ing wid h subse s. In addi ion, he simila i y be ween he ea lywood wid h o he cu en yea and he la ewood wid h o he p eced- ing yea was highe han be ween he la ewood and ea - lywood wid hs o he same yea in each igou class, especially wi hin he heal hy ees. The co ela ions be- ween he ea lywood wid h and he o al ing wid h ch o- nologies we e ema kably lowe han be ween he la e- wood wid h and he o al ing wid h ch onologies. Thus, la ewood wid h was mo e ele an in de e mining he annual ing wid h han ea lywood wid h, which a ied less be ween he yea s (Fig. 2). T ee- ing wid h and soil ela ionships Mean soil dep h anged be ween 29-66 cm, 28-66 cm, and 29-86 cm a ound he heal hy, he declining, and he dead ees, espec i ely. The shallowes soil obse a ions anged be ween 8-44 cm a ound bo h he heal hy and he declining oaks, and 14-76 cm a ound he dead oaks. No s a is ically signi ican di e ences in soil measu emen s be ween he igou classes we e iden i ied ( - es , p > 0.05). Mo eo e , none o he co ela ions be ween he g ow h a iabili ies o annual ing wid h indices, he ela- i e g ow h a e and minimum soil hickness a ound each Fig. 3. Ea lywood, la ewood, and o al annual ing wid h a iabili y by igou class du ing he 1 - 115 yea s cambial age being he common pe iod ha con ains a leas i e se ies in each igou class. A e age ela i e g ow h a es (RGR) a e calcula ed om indi idual ee cha ac- e is ics ac oss hei en i e li espan. Table 2. Pea son co ela ion coe icien s be ween he esidual ch onologies du ing 1894-2004: ea lywood wid h (EW), la ewood wid h (LW), p eced- ing yea ’s la ewood wid h (pLW), o al annual ing wid h (ARW) o heal hy (HE), declining (DE), and dead (DD) oaks. S a is ically signi ican ela io n- ships (p < 0.05) a e in bold. EWHE EWDE EWDD LWHE LWDE LWDD pLWHE pLWDE pLWDD ARWHE ARWDE ARWDD EWHE - EWDE 0.69 - EWDD 0.71 0.74 - LWHE 0.25 0.36 0.36 - LWDE 0.25 0.41 0.39 0.82 - LWDD 0.27 0.43 0.39 0.85 0.82 - pLWHE 0.62 0.42 0.46 0.01 0.06 0.09 - pLWDE 0.54 0.48 0.47 -0.01 0.01 0.02 0.82 - pLWDD 0.57 0.44 0.50 0.03 0.11 0.11 0.85 0.83 - ARWHE 0.39 0.41 0.44 0.94 0.79 0.80 0.03 -0.02 0.05 - ARWDE 0.37 0.60 0.50 0.79 0.95 0.83 0.07 0.03 0.11 0.79 - ARWDD 0.45 0.60 0.65 0.79 0.81 0.92 0.16 0.09 0.16 0.79 0.88 - B ough o you by | Me la Finnish Fo es Resea ch Ins i u e Au hen ica ed Download Da e | 12/20/16 11:16 AM K. Soha e al. 97 ee we e signi ican (p > 0.05; Fig. 4). The esul s we e alike wi h he ea lywood and he la ewood wid h cha ac e - is ics (no shown). Acco dingly, soil dep h was ound o ha e no signi ican e ec on g ow h a ia ion in his si e. T ee- ing wid h and clima ic ela ionships The esponse alues be ween he ee- ing da a and he mon hly clima e a iables e ealed ha oak g ow h was especially limi ed by June and July ain all in all igou classes based on he da a om 1894-2004 (p < 0.05; Fig. 5). This associa ion was posi i e and common o la ewood and annual ing wid h in all igou classes. Response o declining oak di e ed om o he igou classes in ha May empe a u e had a posi i e e ec on la ewood o ma ion and simila ly on annual ing wid h. Ea lywood wid h was less e iden ly con olled by he clima e, only he declining oaks showing an excep- ionally posi i e esponse o Ma ch p ecipi a ion. On he o he hand, he majo i y o win e and sp ing mon h em- pe a u es had posi i ely a ec ed ea lywood o ma ion, bu he ela ionships we e insigni ican on mon hly scale. On seasonal scale, we ound ha wa me win e and sp ing bene i ed he ea lywood dimension signi ican ly, while he la ewood was sensi i e o summe ain all (p < 0.05; Table 3). 4. DISCUSSION Speci ics o oak decline in Tammis o The c oss-da ing esul s showed ha Tammis o oaks ha e died mos ly du ing 2005-2007 o e en be o e ha ime, as he las ings we e uniden i iable in some sam- ples. The causes canno be associa ed wi h hese yea s igo ously because dieback can be a decade-long p ocess and dea h migh be lagging behind he iming o he de- cline-inducing ac o s (Che ubini e al., 2002; D obyshe e al., 2007b; Ande sson e al., 2011). The p e-mo em g ow h educ ion o he declining and dead ees was illus a ed o ha e occu ed al eady om he mid-1990s (Fig. 2). A simila deg ada ion pa e n has been iden i ied ea lie in he F amnäs oak si e in sou h-wes e n Finland (Helama e al., in p ep.) and in he Annala Pa k in Hel- sinki whe e an ab up oak g ow h educ ion appea ed Fig. 4. Rela ionships be ween g ow h a iabili y o annual ing wid h indices (s anda d de ia ion, mean sensi i i y, au oco ela ion), ela i e g ow h a e o each ee and minimum soil hickness a ound each ee, as desc ibed by Pea son co ela ion coe icien s ( ). Table 3. Pea son co ela ion coe icien s be ween he esidual ch o- nologies (ea lywood wid h (EW), la ewood wid h (LW), o al annual ing wid h (ARW) o heal hy (HE), declining (DE) dead (DD) oaks) and wea he cha ac e is ic s o ou seasons om he p e ious yea (small le e s) un il he cu en yea (capi al le e s) du ing he pe iod 1894 - 2004. S a is ically signi ican ela ionships (p < 0.05) a e in bold. Tempe a u e P ecipi a ion son dJF MAM JJA son dJF MAM JJA EWHE 0.06 0.33 0.34 0.08 0.19 0.01 0.00 0.11 EWDE 0.00 0.18 0.21 -0.07 0.07 -0.02 0.18 0.22 EWDD 0.01 0.20 0.20 -0.08 0.15 0.00 -0.03 0.15 LWHE -0.02 0.07 0.20 -0.10 0.03 0.08 0.01 0.46 LWDE -0.07 0.09 0.17 -0.15 0.03 0.09 0.01 0.55 LWDD -0.03 0.06 0.18 -0.14 0.03 0.05 0.07 0.47 ARWHE 0.01 0.09 0.26 -0.09 0.05 0.08 -0.01 0.42 ARWDE -0.06 0.12 0.21 -0.13 0.06 0.05 0.06 0.53 ARWDD -0.02 0.15 0.25 -0.13 0.08 0.06 0.00 0.44 B ough o you by | Me la Finnish Fo es Resea ch Ins i u e Au hen ica ed Download Da e | 12/20/16 11:16 AM OAK DECLINE IN A SOUTHERN FINNISH FOREST AS AFFECTED BY A DROUGHT SEQUENCE 98 some yea s la e , om 2002 o 2003 (Helama e al., 2009). Thus, i seems o be a common ea u e o he oaks g owing a he no he n dis ibu ional limi in Finland. La ewood wid h was mo e ele an in de e mining annual ing wid h han ea lywood wid h (Table 2), like i has been demons a ed be o e as well (Doležal e al., 2010; Ma isons and B ūmelis, 2012). Compa ing ou la ewood and annual ing wid h ch onologies and he co esponding cambial age g ow h pa e ns, he heal hy oaks showed be e long- e m g ow h han he declining and he dead ees (Figs. 2 and 3). I was u he shown ha he ela i e g ow h a e was no ela ed o soil hick- ness (Fig. 4). The be e long- e m g ow h may hus be ela ed o some long- e m s esses, such as in e - ee compe i ion (Dobbe in, 2005; Rozas and Ga cía Gonzá- lez, 2012) as he heal hy oaks ha e eco e ed om he g ow h descen supposedly due o hei a ou ed s a us in he s and hie a chy. Be e adial g ow h in he heal hy igou class o e he en i e li espan has been obse ed p e iously in o he Finnish oak si es as well (Helama e al., 2009; Helama e al., in p ep.). Fu he mo e, he a o emen ioned s udy conduc ed in an u ban pa k in Helsinki has shown ha a shallow soil laye can cause addi ional en i onmen al s ess on ee Fig. 5. Response alues be ween he nine ch onologies a nd he mon hly me eo ological a iables om he p e ious (small le e s) and cu en yea s (capi al le e s) du ing he pe iod 1894-2004. S a is ically signi ican ela ionships (p<0.05) a e indica ed as illed ba s. B ough o you by | Me la Finnish Fo es Resea ch Ins i u e Au hen ica ed Download Da e | 12/20/16 11:16 AM K. Soha e al. 99 g ow h. Con e sely, we ound ha soil dep h has no signi ican e ec on g ow h a ia ion in he conce ned si e (Fig. 4). Supposedly, soil condi ions a e equally poo on his spo adically exposed bed ock and he ees ha e ceased o esis o he wa e s ess du ing he d ough s. Insec pes s and pa hogenic ungi can con ibu e o dieback as well (Ha mann e al., 1989; Füh e , 1998; Siwecki and U nalski, 1998; Jung e al., 2000; Thomas e al., 2002). Fo example, de olia ion by insec s leads o ex ensi e losses o pho osyn ha es and deple ion o ca - bohyd a e ese es, oo pa hogens o Phy oph ho a spp. can des uc he ine oo sys em and, in a combined e - ec wi h se e e d ough , h ea en he heal h o ees. Mo eo e , oo pa hogens can dec ease he abili y o hos ees o cope wi h d ough and s op cambial ac i i y like i has shown on sp uces in he Eas e n Alps (Go i e al., 2013). A ou si e we did no examine he bio ic ac o s a all, hus hei con ibu ion in he analysed dieback canno be disca ded. On he o he hand, no single pa hogen o insec can cause oak decline on a la ge scale (Siwecki and U nalski, 1998), bu oak o es decline has epo ed wide- ly all o e sou he n Finland ecen ly (Ympä is ö apo - oinnin asian un ija yö yhmä, 2004; Holopainen e al., 2006; Helama e al., 2009). Thus, some disease could ha e con ibu ed o he decline in Tammis o bu mos p obably he weakening and dieback we e igge ed by a common la ge-scale ac o (e.g. clima e). Summe p ecipi a ion signal As g ow h was mos ly con olled by summe p ecipi- a ion (Fig. 5; Table 3) in all igou classes, he na ow ee ings can be coupled wi h d y g owing seasons and poo plan -wa e ela ions o e all (Thomas and Ha - mann, 1998). Summe d ough s ha e been sugges ed as a con ibu ing decline ac o o oak in Poland (Siwecki and U nalski, 1998), in combina ion wi h excep ional win e os s in G ea B i ain (Gibbs and G eig, 1997) and Ge - many (Ha mann e al., 1989). The geog aphically closes s udy o Finland de i es om sou he n Sweden, whe e D obyshe e al. (2007b) ha e sugges ed ha he peak in oak mo ali y a ound he yea 2000 was igge ed by he sp ing and summe d ough ha occu ed al eady in 1992. Ano he s udy om sou h-eas e n Sweden has shown p onounced peaks in oak mo ali y in 2004 and 2006 wi h educed g ow h a e a se e e d ough in 1992, oo (An- de sson e al., 2011). June and July 1992 we e excep ion- ally d y in Helsinki as well, in addi ion o se e al o he summe s om he 1990s like: 1991, 1994, 1995, 1997 and 1999 (Fig. 6). In o he wo ds, i was he pe iod when a p e-mo em g ow h educ ion o he declining and dead ees was obse ed in ou s udy si e. The yea 2003, wi h an excep ionally d y g owing season h oughou Eu ope (Fink e al., 2004) as well as in Finland (Silande and Jä inen, 2004), showed a nega i e peak in ou ee- ing se ies o each igou class (Fig. 2). The same happened in 2006, which he Finnish Me eo ological Ins i u e (2012) desc ibed as he ho es and d ies g owing season in he Helsinki-Van aa s a ion du ing he 1961-2006 pe iod. Thus, hese sequen ial d ough s migh ha e led o he dieback du ing 2005-2007. In addi ion o he 1990s, a mo e se e e d ough pe iod was expe ienced in Finland du ing he pe iod 1939-1942 (Hyd og a inen oimis o, 1944, 1948; Kuusis o, 2003, 2004). An inc emen d op in he 1940 and 1941 in he ee- ing ch onologies was ob- se able in e e y igou class bu he g ow h educ ion did no con inue (Fig. 2). This can be ela ed o he phys- iologically younge ee age eco e ing he o me s a us easie . In addi ion, he young oak ap- oo is known as well-de ined (Jones, 1959) and may be e each he low- e ed g oundwa e le el. Ou dend oclima ic indings (Fig. 5) a e suppo ed by he ea lie s udies which ha e e ealed a posi i e e- sponse o oak g ow h o summe p ecipi a ion in Finland (Helama e al., 2009; Hilas uo i and Be ninge , 2010) as well as in Sweden (D obyshe e al., 2008). This kind o ela ionship has been known o long (S ewa , 1913) and has been desc ibed o peduncula e oak widely a ound Eu ope ecen ly. Fo example in he Bal ic coun ies, June ain all has he s onges posi i e in luence on ee g ow h in Es onia (Läänelaid e al., 2008), Augus ain all in La ia (Ma isons e al., 2012), and mois summe s in he well-d ained si es in Li huania (Ruseckas, 2006). Also, a he eas in he middle Volga egion in Russia, a posi i e g ow h in luence o sp ing-summe p ecipi a ion has been obse ed (Askeye e al., 2005). Acco ding o Ced o (2007) as well as B onisz e al. (2012) oak g ow h in Poland is domina ed by summe p ecipi a ion while high empe a u e has nega i e in luence. Poo oak inc e- men is mainly asc ibed o he wa e de ici du ing he ea ly summe in he Czech Republic (Doležal e al., 2010) and in Ge many (F ied ichs e al., 2009; Scha n- Fig. 6. Reco ds o June and July p ecipi a ion sums om he Helsinki Kaisaniemi wea he s a ion. B ough o you by | Me la Finnish Fo es Resea ch Ins i u e Au hen ica ed Download Da e | 12/20/16 11:16 AM OAK DECLINE IN A SOUTHERN FINNISH FOREST AS AFFECTED BY A DROUGHT SEQUENCE 100 webe e al., 2011; Zang e al., 2012). Oak s udies in he B i ish Isles ha e shown ha high ain all du ing he g owing season and high empe a u e du ing he ea ly summe p omo e g ow h (Pilche and G ay, 1982; B idge e al., 1996). An enhancing e ec o p ecipi a ion is widely obse ed in he Medi e anean en i onmen s (San ini e al., 1994; Tessie e al., 1994; Ču a e al., 2008) and he A lan ic Spain (Rozas, 2001, 2005), whe e high p ecipi a ion c ea es a ou able and high empe a- u e un a ou able g ow h condi ions du ing he summe pe iod. On he o he hand, wa e excess can be limi ing o g ow h and is igge ing o es dieback in ainy em- pe a e deciduous o es (Rozas and Ga cía González, 2012). Anyway, he impo ance o summe p ecipi a ion o oak ee- ing wid h is e iden ac oss Eu ope. Rela ionships o win e and sp ing condi ions On he seasonal scale, ee- ings and especially ea - lywood wid hs we e posi i ely co ela ed wi h sp ing (Ma ch-May) empe a u e in e e y igou class (Table 3). On a mon hly scale, only he declining oaks showed a posi i e esponse o May empe a u e (Fig. 5). Ea lywood wid h was con olled by win e (Decembe –Feb ua y) a e age empe a u e as well (Table 3), which means ha mild a he han cool win e s a e mo e a ou able o ea lywood o ma ion. As his pa o he ee- ing in- ol es wa e -conduc ing essels o ees, he iming o g ound mel ing and g ound wa e accessibili y is ele an . Doležal e al. (2010) ha e associa ed ea lywood g ow h wi h p e ious au umn and win e empe a u es in he Whi e Ca pa hians in he Czech Republic. Vessel lumen a ea, ano he ea lywood cha ac e is ic, shows also he highes posi i e co ela ions wi h empe a u es om win e and sp ing mon hs in La ia (Ma isons and B ūmelis, 2012). E en i oak ea lywood essels enla ge in a sho ime in sp ing, be ween he bud b eak and he lea expansion (Sass-Klaassen e al., 2011), he p ocess can depend on he p e ious yea ’s la ewood (Nola, 1996). Mo eo e , acco ding o Ki in e al. (1999) in ing-po ous ha dwood ea lywood essel elemen s mos p obably o igina e om he o e win e ed cambial de i a i es. Nola (1996), Ga cía González and Ecks ein (2003) ha e demons a ed a highe co ela ion be ween he cu en yea ’s ea lywood and he p eceding yea ’s la ewood wid h han he same yea ’s co esponding po ions. Based on ou s udy we can admi a simila ela ionship: he co ela ion coe icien be ween ea lywood wid h o he cu en yea and he la ewood wid h o he p e ious yea we e 0.62, 0.48, and 0.50 in he heal hy, declining and dead oaks, espec i ely. The simila i y cha ac e is ics be ween he la ewood and ea lywood wid hs o he same yea we e 0.25, 0.41 and 0.39, espec i ely (Table 2). Thus, he heal hy ees showed he s onges ela ionship, which may e e o s onge biological dependence ins ead o en i onmen al s ess. The easons behind ha phenomenon we e beyond he aim o his s udy and should be analysed in mo e de ail in he u u e. A he same ime, he annual ing wid h showed no signi ican ela ionships o empe a u e du ing he deep do mancy pe iod (Fig. 5; Table 3). Thus, os damages, which can gene a e ba k nec osis and ini ia e decline, seem no o be associa ed wi h mo ali y a ou si e, as i has been ound in sou he n Sweden (Ba klund and Wahl- s öm, 1998). Mo eo e , p e ious s udy om Finland has shown signi ican nega i e co ela ion be ween ee- ing g ow h and win e empe a u es, which is explained h ough pe manen snow-co e appea ance du ing below ze o empe a u es and hus be e oo insula ion (Helama e al., 2009). Alike ela ionships we e no ound in ou s udy si e. Excep ionally, he declining oak ea lywood showed a posi i e esponse o Ma ch p ecipi a ion. Mos p obably his is andom coinciden al and no ela ed o oo insula ion by he snowpack because he snow co e o ms o e se e al mon hs and o he do mancy mon hs did no show a signi ican co ela ion be ween p ecipi a- ion and he inc emen . 5. CONCLUSIONS The esul s suppo he p e ious oak decline s udy om Finland (Helama e al., 2009) which showed d ough inci ed oak decline on he shallow soil. The com- pa ison o oak igou classes ( he li ing, he declining, and he dead) in he Tammis o o es e ealed no signi i- can di e ences in age, mean ing wid h, mean sensi i i- y, ela i e g ow h a e, no soil dep h a ound he ee, e en hough di e ences in he oliage and i al capaci y we e isible. We can assume he soil condi ions a e equally poo on he spo adically exposed bed ock. The dend oclima ic analysis showed a simila ly posi i e ‘summe esponse’ meaning he summe d ough s had an inhibi ing e ec on he oaks. Wa e s ess is equal o oaks in all igou classes, ob iously due o he shallow soil. Appa en ly, he sequen ial d ough s du ing he p eceding decade inci ed he dieback o he declining and he dead ees du ing 2005-2007. Thus, he e appea s o be a ime lag be ween he mo ali y-inducing ac o s and ac ual dea h. Ou esul s di e om he p e ious s udy in a lack o clea nega i e clima e ‘win e esponse’. On he sea- sonal scale, we can admi a a ou able sp ing wa m h e ec on bo h ea lywood and o al annual ing g ow h and a posi i e e ec o mild win e on ea lywood o - ma ion. ACKNOWLEDGEMENTS The ield wo k was ca ied ou unde he license om he Ci y o Van aa (Dno YM 2930/2008/516). The e- sea ch was suppo ed by he Eu opean Social Fund’s Doc o al S udies and In e na ionalisa ion P og amme DoRa, which is ca ied ou by Founda ion A chimedes, by he Es onian S a e Ta ge Founda ion B ough o you by | Me la Finnish Fo es Resea ch Ins i u e Au hen ica ed Download Da e | 12/20/16 11:16 AM