Dendroclimatology in Fennoscandia

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H. W. Linderholm et al. Dendroclimatology in Fennoscandia – from past accomplishments to future Title Page potentials Abstract Introduction Conclusions References H. W. Linderholm1, J. A. Bjorklund¨ 1, K. Seftigen1, B. E. Gunnarson2, Tables Figures I. Drobyshev3, J.-H. Jeong1, P. Stridbeck1, and Y. Liu4

1 Regional Climate Group, Department of Earth Sciences, University of Gothenburg, J I Gothenburg, Sweden 2Department of Physical Geography and Quaternary Geology, Stockholm University, J I Stockholm, Sweden Back Close 3Southern Swedish Forest Research Centre, SLU, Alnarp, Sweden 4 The State Key Laboratory of Loess and Quaternary Geology, The Institute of Earth Full Screen / Esc Environment, Chinese Academy of Sciences, Xi´an, China

Received: 29 April 2009 – Accepted: 5 May 2009 – Published: 19 May 2009 Printer-friendly Version

Correspondence to: H. W. Linderholm ([email protected]) Interactive Discussion Published by Copernicus Publications on behalf of the European Geosciences Union.

1415 Abstract CPD Dendroclimatology, i.e. using tree-ring data to reconstruct past climates, in Fennoscan- dia has a strong tradition. Due to the high-latitude location of the region, trees are 5, 1415–1461, 2009 sensitive to climate; in general to temperatures during summer. However, a strong 5 gradient from the oceanic west to the continental east, makes it possible to ﬁnd trees Dendroclimatology in that respond to other parameters, such as precipitation and drought. Situated in a Fennoscandia sparsely populated part of the Boreal belt, Fennoscandia with its large areas of old- growth forests is suitable for constructing tree-ring chronologies reaching far back in H. W. Linderholm et al. time. Indeed, some of the world longest tree-ring chronologies are found in the re- 10 gion, covering all, or most of, the Holocene. In addition to providing valuable information about regional climate variability during the Holocene, tree-ring data have played Title Page

signiﬁcant roles in recent reconstructions of hemispheric and global temperatures as Abstract Introduction well as large-scale circulation patterns. Here we review the ﬁeld of dendroclimatology in Fennoscandia, showing the wealth of climate information obtained from various Conclusions References 15 tree-ring parameters (ring widths, density and stable isotopes), and look in to future Tables Figures possibilities.

J I 1 Introduction J I

In order to better understand climate change both from a natural and anthropogenic Back Close induced origin, analyzing the historical climate and its behaviour can give indications 20 of what to expect in the future (Bradley, 1985). Instrumental measurements of tem- Full Screen / Esc perature and other climate variables have been done with mixed quality since the 18th century but a century later the set of observing sites attained sufficient geographical Printer-friendly Version coverage to calculate a global average (Jones and Moberg, 2003). Obviously this short time period is not enough to understand past climate variability, and other climate indi- Interactive Discussion 25 cators need to be utilized (e.g., Mann et al., 1999). High-resolution reconstructions of past climate are mainly based on corals, tree rings, sediments and ice cores. These 1416 reconstructions are calibrated against instrumental records, but the reliability of a reconstruction may also be inferred from comparisons with modelling results from runs of CPD coupled atmosphere-ocean circulation models (Collins et al., 2002). Analyses of long 5, 1415–1461, 2009 proxy records can emphasize distinctions between natural long-term modes and an- 5 thropogenic induced climate variability. Presently, a large number of hemispheric and global temperature reconstructions are available (see Mann, 2002). However, these do Dendroclimatology in not provide information about regional-scale variations, where regions with key climatic Fennoscandia features may be masked in hemispheric or global reconstructions (Luterbacher et al., 2004). Consequently, studies of regional climate variability are very important when H. W. Linderholm et al. 10 impacts of climate change on the environment are evaluated (Shindell et al., 1999). Reconstructions of Holocene climate variability in Scandinavia have mainly been Title Page based on relatively low-resolution climate proxies such as: fluctuations of glaciers (e.g., Karlen´ , 1976), tree-limit variations (e.g., Kullman, 1995), pollen/macrofossil anal- Abstract Introduction ysis (e.g., Barnekow, 2000), speleothems (e.g., Lauritzen and Lundberg, 1999), peat Conclusions References 15 humification (e.g., Nilssen and Vorren, 1991), and lake sediments (e.g., Yu and Harri-

son, 1995). However, if records with yearly resolution are required, the most suitable Tables Figures method available is to examine tree-rings (Gouirand et al., 2008). This review will focus on dendroclimatology, the study of the relationships between J I climate and tree-growth parameters and their use in the reconstruction of past cli- 20 mates, in Fennoscandia. Fennoscandia, constituting of Norway, Sweden and Finland, J I is a region where remote virgin forests are common, and is thus highly suitable for Back Close dendrochronological studies. This paper will begin with a brief summary of the dendroclimatological achievements in the twentieth century, before the science “came into Full Screen / Esc fashion”. Then, recent accomplishments will be described in more detail. We ﬁnish 25 with an outlook of future prospects in the ﬁeld of Dendroclimatology in Fennoscandia: Printer-friendly Version what more can we do? Interactive Discussion

1417 2 Dendroclimatology CPD In extra tropical tree species which are exposed to seasonal variations, defined annual growth bands, tree rings, are produced. Different features of these tree rings can reveal 5, 1415–1461, 2009 details about in what conditions or climate they were formed (Schweingruber, 1987). 5 The annual growth of trees represents a combined record of different environmental Dendroclimatology in forcing factors, one of which is climate (Briffa et al., 1998a). Factors that have large Fennoscandia impact on tree growth are temperature, soil-moisture, and availability of nutrients etc. (Raven, 1999). When temperature is explored, the sampling sites of the trees need to H. W. Linderholm et al. fulfil certain criteria that eliminate or marginalise the effect of other variables that usu- 10 ally affect them. Typically, these locations are found near the altitudinal or latitudinal limits of a certain species (Cook and Kairiukstis, 1989). However, non-climatic factors Title Page

such as stand-competition or silvicultural intervention can influence tree growth, so in Abstract Introduction order to obtain an optimized climate signal from the trees, the selection of sampling-site is crucial. Carefully chosen trees can thus be used as proxies for local-to-regional dom- Conclusions References 15 inating climate features. Furthermore, tree growth is a function of multiple influences, Tables Figures and e.g. Linderholm and Chen (2005) have found that it is also possible to extract information of less dominant climate features that may affect growth even outside the vegetation period. J I

Tree rings have played an important role in establishing how climate has varied in J I 20 the recent past (Jones et al., 2009). The strength of tree-ring records is the annual resolution and the spatial extent of climatically sensitive chronologies. Today there exist Back Close

vast networks of tree-ring chronologies at mid-to-high latitudes, and these networks Full Screen / Esc have been utilized to reconstruct spatiotemporal climate variability, mainly temperature, in the last centuries to millennia. Printer-friendly Version

25 2.1 Tree-ring parameters Interactive Discussion

When analyzing tree-rings, a variety of parameters are studied. The most commonly used parameter is radial measurements of tree-ring widths, followed by density mea- 1418 surements of the radial cell growth and chemical analysis of stable isotopes. For the beneﬁt of the reader, short descriptions of annual ring formation and the diﬀerent tree- CPD ring parameters are given in the following paragraphs (for a more in-depth description 5, 1415–1461, 2009 we refer to Fritts, 1976, and references there).

5 2.1.1 The annual growth ring Dendroclimatology in Fennoscandia The growth of woody plants can be divided into primary and secondary growth. The primary growth in a plant occurs in the apical meristem and forms primary tissues, and H. W. Linderholm et al. the primary growth involves shoots, leaves and root and is mainly lengthwise. The secondary growth takes place in the cork and vascular cambium inside the bark. Here 10 secondary phloem (outwards) and xylem (inwards towards the pith) are formed and Title Page produce radial growth of the stem. The cells formed on the outside of the cambium Abstract Introduction become phloem which conducts the nutrients of the tree, and the cells formed on the inside of the cambium become xylem and conducts water and minerals. The xylem Conclusions References is also the woody tissue in the tree. The xylem consists of tracheids, parenchyma Tables Figures 15 and resin ducts in conifers. The tracheids are vertically oriented elongated cells with ligniﬁed wall which transport water and mineral salts. These cells grow in diﬀerent rate throughout the growing season with large and thin-walled cells in the early spring, J I called early wood. Gradually smaller and more thick-walled cells are formed towards J I the end of the growing season called late wood. Before the winter, radial growth 20 abruptly ends and the tree enters the dormant period where little or no growth oc- Back Close curs. Together the lighter early wood and the darker late wood form an annual growth Full Screen / Esc ring. When the growth is resumed in the spring large cells are formed again creating a stark contrast between late wood and early wood resulting in a clearly visible annual ring (Raven, 1999). Printer-friendly Version Interactive Discussion

1419 2.1.2 Tree-ring widths CPD From living trees cores, 5 mm in diameter, are usually extracted at a height of ∼1.3 m from the base of the trunk using an increment borer. When dead wood is concerned, a 5, 1415–1461, 2009 disc from the tree is usually taken, if possible at the same stem height. Annual tree-ring 5 widths of each sample are then measured, usually with a precision of 1/100 mm, and Dendroclimatology in visual inspection as and basic statistics are used to cross date all samples in order to Fennoscandia assign to each ring the correct calendar year. A tree-ring series can be expressed as a linear composition of several sub-series (e.g., Cook, 1990). These sub-series include H. W. Linderholm et al. an age-related growth trend, a climatic signal common to all trees at the site, inﬂuence 10 of endogenous (single tree) and exogenous (common to most trees) disturbances in the stand. In order to remove the non-climatic growth-trend caused by changing age Title Page

and geometry of the tree, all individual tree-ring width series are standardised by fitting Abstract Introduction an approximate growth-function to it, e.g. a negative exponential curve or a straight line etc. (Fritts, 1976). The tree-ring series are then divided by the fitted curve, and the re- Conclusions References 15 sulting so called indices are all centred around 1 and can thus be averaged to produce Tables Figures the standardised chronology. The method of standardization significantly affects which frequencies of variability can be recovered from the tree-ring data. Presently, Regional Curve Standardization (Briffa et al., 1992) is the most widely used method to preserve J I

low-frequency (>100 years) information. Although the standardization issue is far from J I 20 settled, it will not be further explored in this paper. Back Close 2.1.3 Density Full Screen / Esc While measuring ring width variations is the most common and least complicated approach in dendroclimatology, an increasing number of works utilizes density variations Printer-friendly Version in the annual rings. Densitometric analysis of wood-tissue is an integrated measure- Interactive Discussion 25 ment of all the components of the annual rings, e.g. thickness of cell walls, proportion of fibres etc. (Polge, 1970). Since the early and late wood differ in characteristics (see above), the detailed distinction of cell-density can be quantified. Thin slices of 1420 increment cores are exposed to narrow and high energy X-ray beams at short intervals (∼20 µm) and a high-resolution digital image is produced. The radiographic images are CPD then evaluated with an image analysis system, e.g. WINDENDRO (Guay, 1992), which 5, 1415–1461, 2009 provides several parameters: maximum and minimum density and the mean densities 5 of the early wood, late wood and of the whole ring. One advantage with density variations, compared to ring-widths, is the much weaker Dendroclimatology in age trend, demonstrated by Grudd (e.g., 2008), which suggests that standardization Fennoscandia of density values potentially recover more low frequency variability. Maximum density is most frequently used in dendroclimatology, and this parameter has been shown to H. W. Linderholm et al. 10 be highly correlated to warm-season (April–September) temperatures in high altitude and latitude areas (e.g., Briffa et al., 1988a, 2002b; Schweingruber et al., 1993). Since Title Page tree-ring widths may incorporate information from before the actual growing season (Fritts, 1976) as well as having a shorter optimum seasonal signal (Briffa et al., 2002b), Abstract Introduction density data is to be preferred when reconstructing warm season temperatures. Conclusions References

15 2.1.4 Stable isotopes Tables Figures Stable isotopes (mainly δ18O and δ13C and δD) from tree-ring species, such as cellulose, can provide information of temperature, precipitation, humidity and soil moisture J I depending on the growth environment of the trees (Burk and Stuiver, 1981; McCar- J I roll and Loader, 2004). Also it is likely that the annual variations in isotope ratios in 20 tree rings are lacking long-term (>50 years) age related trends, and thus will reﬂect Back Close climate variations on century time scales. One additional advantage with assembling Full Screen / Esc a number of annually resolved isotope chronologies is that they are comparable to and can be used to calibrate isotope series from other climate proxies, such as ice cores, speleothems and lake sediments. Printer-friendly Version Interactive Discussion

1421 3 Early Fennoscandian dendroclimatological studies CPD The link between the annual growth of trees and climate has under a long time been the subject of scientific investigations. Already in the 18th century, the annual growth 5, 1415–1461, 2009 of trees was related to favorable summer conditions (Carl Fredric Broocman, 1709– 5 1761) or winter severity (Carl von Linne,´ 1707–1778). Finnish Ulric Rudenschold¨ Dendroclimatology in (1704–1765) compared pine trees in Northern and Southern Finland trees finding both Fennoscandia differences and similarities, where he accounted the similarities to regional climate- conditions and the differences to both climate and soil-conditions. The Danish states- H. W. Linderholm et al. man Christian Ditlev Reventlow (1748–1827) made tree-ring investigations on oaks and 10 beeches in the early 19th century, not only sampling the base of the tree but also at different heights of the stem. Dendroclimatology thus has strong traditions in the Nordic Title Page

countries, but it was the American astronomer Andrew Ellicott Douglass (1867–1962), Abstract Introduction who in the beginning of the 20th century established a statistically valid relationship between climate and tree-ring widths (Douglass, 1909, 1914). He set out to investigate Conclusions References 15 the inﬂuence of sunspots on precipitation in America and chose tree-rings as a suitable Tables Figures drought indicator. During his studies he noted that tree-rings growing in similar environments showed homogenous growth patterns, despite the sites being far apart. He also made investigations of European data, e.g. from southern Sweden and Norway were J I

sunspots was the focal point (Douglass, 1919). Douglass’ sunspot-curve was found to J I 20 be in good agreement with these localities except for the inner coast of Norway, where the agreement practically was inverted. Back Close

In some key investigations during the early decades of the 20th century, most of the Full Screen / Esc features of modern Fennoscandian dendroclimatology were outlined. In a thorough in- vestigation, Erlandsson (1936) established the strong dependence of Scots pine (Pinus Printer-friendly Version 25 Sylvestris L.) to summer temperatures in the northern region of Fennoscandia, which had also previously been noted by the Norwegian Eide (1926). Erlandsson noted that Interactive Discussion July temperatures speciﬁcally were well correlated with tree-ring widths and that precipitation regardless of when it occurred had a weak, if existent, relationship with tree

1422 growth. Erlandsson also developed a predecessor of the Regional Standardization Curve (RCS) method, later improved by Briffa et al. (1992). The growth-trends of the CPD trees from each locality included in Erlandsson’s study were “Summed up from a com- 5, 1415–1461, 2009 mon starting-point (the inner-most ring) independent of the chronological age...”, and 5 an average of each annual ring was calculated resulting in a mean growth-trend. The mean growth-trend was then used to standardize every tree-ring series. Dendroclimatology in Other important works in the development of a Fennoscandian network of tree-ring Fennoscandia chronologies were made in Norway in the mid 20th century, where tree-ring chronologies from both Scots pine and Norway spruce (Picea Abies L. Karst) were constructed. H. W. Linderholm et al. 10 Eidem (1953) found that the chronologies from sites in the interior of Norway as well as from the coastline showed considerable differences, and that Norway spruce corre- Title Page lated better with June but Scots pines with July temperatures. The difference between pine and spruce was also manifested by the pronounced autocorrelation in pine, which Abstract Introduction seem to be absent in spruce, where the difference depends on the different develop- Conclusions References 15 ment and assimilation-rates in the needles of pine and spruce (Ording, 1941). Jonsson

(1969) however, argued that pines showed stronger autocorrelation than spruce. Re- Tables Figures moving the autocorrelation from the direct or indirect inﬂuence of climate in pine- or spruce chronologies is however not without problems as low-frequent variations might J I also be removed (Hustisch and Elfving, 1944). 20 Schove (1954) stated that tree-ring analysis involves three initial limitations: “only J I one species should be considered at the time, the trees should be from a small climat- Back Close ically uniform region and the growth measurements for each year should be corrected for age and standardized by a single objective method”. Furthermore, when deriving cli- Full Screen / Esc matic evidence from such a diverse region as Fennoscandia, more than one species of 25 tree is needed, these trees should be sampled from a large area and diﬀerent methods Printer-friendly Version of standardization should be used to investigate the sensitivity of the data to correction methods. Interactive Discussion

1423 4 Spatial variability of tree growth/climate relationships in Fennoscandia CPD Despite the high-latitude setting of Fennoscandia, the proximity to the North Atlantic Ocean in the western part of the region results in a gradient from oceanic climate 5, 1415–1461, 2009 in the west to continental in the east. Also, Fennoscandia spans over several de- 5 grees of latitude, which means large diﬀerences in seasonal climate, e.g. the length of Dendroclimatology in the growing season which is of importance for tree growth. Thus, it is not expected Fennoscandia that trees will respond similarly to climate across the region. Makinen¨ (2002), investigating growth/climate relationship of Norway spruce across latitudinal and altitudinal H. W. Linderholm et al. gradients in Central and Northern Europe, noted that the temperature inﬂuence on 10 tree-growth increased with altitude and latitude, while the opposite was found for precipitation. Helama et al. (2005) examined four tree-ring width chronologies in a north- Title Page

south transect in Finland and found evidence of an eco-geographical gradient where Abstract Introduction the northern chronologies showed the typical mid-summer temperature response and all chronologies had positive correlation with early-summer precipitation but more so Conclusions References 15 in the south. The low-frequency variability common to all chronologies was related to Tables Figures the North Atlantic Oscillation (NAO), and a weakening of common high-frequency variability from north to south was accounted to increased growth-competition (Helama et al., 2005). Andreassen et al. (2006), studying Norway spruce in Norway, found that J I

summer temperature was the limiting factor for tree growth at northern and high alti- J I 20 tudes sites, but in the southeastern lowlands summer precipitation was growth limiting. Andreassen et al. (2006) also found that below a threshold of June temperatures of Back Close ◦ 12–13 C, spruce was expected to be sensitive to temperature and above it to precipita- Full Screen / Esc tion. Analysing 21 Scots pine chronologies along a westeast gradient in northernmost Fennoscandia, Macias et al. (2004) found a common climate signal (July temperatures Printer-friendly Version 25 and May precipitation) across the Scandinavian Mountains. Still, differences between coastal and more continental sites were observed, and this difference increased from Interactive Discussion the late 1800s/early 1900s to the late 1900s, suggesting a differentiation in regional climate occurring in the twentieth century. In a study of nine Scots pine tree-ring width

1424 chronologies along a gradient at 62–64◦ N in central Fennoscandia, Linderholm et al. (2003) noticed higher growth variability and stronger response to climate in the oceanic CPD area west of the Scandinavian Mountains, compared to the more continental areas fur- 5, 1415–1461, 2009 ther east. Moreover, Pine growth responded positively to summer temperatures in the 5 western areas, and positively to summer precipitation in the east. Dendroclimatology in Fennoscandia 5 Climate reconstructions H. W. Linderholm et al. 5.1 Temperature

Although the strong relationship between conifer growth and summer temperatures Title Page at the latitudinal tree-line in Northern Fennoscandia had previously been established, 10 (e.g., Erlandsson, 1936; Schove, 1954; Siren´ , 1961; Jonsson, 1969; Bartholin and Abstract Introduction Karlen´ , 1983), the ﬁrst reconstruction of summer temperatures using tree-ring data Conclusions References was made in the 1980s. Still, already in the 1960s, Siren´ claimed that his chronologies would suﬃce directly as summer temperature records (Siren´ , 1961). Aniol and Eckstein Tables Figures (1984) used chronologies from 4 locations around Tornetrask,¨ sampled and described

15 by Bartholin and Karlen´ (1983), to reconstruct July temperatures from 1680–1983. It J I is not clear if they use the calibration-verification methods described by Fritts (1976) but they claim a correlation coefficient of 0.58 for the period 1880–1979. The Bartholin J I and Karlen´ (1983) chronology went back to AD 436 but Aniol and Eckstein (1984) hes- Back Close itated to reconstruct July temperatures this far because of “...the striking deviations 20 from average temperatures around 1750–1830” (Aniol and Eckstein, 1984). A more Full Screen / Esc transparent reconstruction of summer temperatures (July–August) back to AD 1700 was made by Briffa et al. (1988b). They used a network of tree-ring width chronologies Printer-friendly Version from Northern Fennoscandia and a few density chronologies from Northern Sweden Interactive Discussion and reduced them to principal component predictors. Over the calibration-verification 25 periods 1852–1925 and 1926–1964, the tree-ring data explained 69 and 32% of the variance respectively in observed temperatures. Briffa et al. (1988b) concluded to re- 1425 mark that although the year-to-year variability was reproduced with fidelity, the long timescale fluctuations above 50 years could not be reconstructed because of limita- CPD tions in the standardization technique. 5, 1415–1461, 2009 The summer temperature reconstruction was further developed by Briffa et al. 5 (1990), extending the reconstructed period to April–August, using tree-ring data from Tornetrask.¨ The reconstructed period spanned AD 500–1975 where tree-ring widths Dendroclimatology in and density together explained 51% of the variance in observed temperatures over Fennoscandia the 1876–1975 calibration period. Based on the reconstruction, Briffa et al. (1990) found no evidence of a homogenous Medieval Warm Epoch, and a short Little Ice Age H. W. Linderholm et al. 10 was interpreted to have occurred around 1570–1650. However, the weaknesses in the standardization method made it difficult to retrieve centennial to multi-centennial Title Page variation in the reconstruction, why the claim of a short Little Ice Age could have been a bit premature. An effort to enhance the low-frequency variation was made by Briffa Abstract Introduction et al. (1992), where a “new” standardization method was applied to the same dataset Conclusions References 15 used Briffa et al. (1990), although with some additional tree-ring width series. Instead

of standardizing every tree-ring series individually all series were arranged starting at Tables Figures their cambial age, and every year was averaged into a grand mean curve of the whole population. This method was called Regional Curve Standardization (RCS), and was J I developed from the method used by Erlandsson (1936) (see above). A consequence 20 of using the RCS method was a lowered common signal among the trees, but the low- J I frequency variability was greatly enhanced and a longer more pronounced Little Ice Back Close age emerged around 1570–1750. Up to now, most eﬀorts in building tree-ring chronologies and making climate re- Full Screen / Esc constructions had mainly been restricted to northern Fennoscandia. However, Kalela- 25 Brundin (1999) expanded the network of reconstructions in Fennoscandia by investi- Printer-friendly Version gating climate from a Scots pine tree-ring series sampled in Femundsmarka, in the eastern part of central Norway. In addition to whole tree-ring widths, Kalela-Brundin Interactive Discussion (1999) measured the width of early wood and late wood separately and made a reconstruction of July and July–August temperatures 1500–1985, based on combinations

1426 of these parameters. The reconstructions explained 52 and 49% of the variance in observed July and July–August temperatures for the period 1872–1993, respectively. CPD Another eﬀort to expand the diversity of locations in Fennoscandia was made by 5, 1415–1461, 2009 Kirchhefer (2001), who investigated Scots pine tree-ring widths from three coastal 5 sites in Northern Norway; Vikran (northernmost), Stonglandseidet and Forfjordsdalen (southernmost). Due to the dramatic landscape in the area, diﬀerences in tree re- Dendroclimatology in sponses to climate were evident, where tree-growth at Vikran was associated with a Fennoscandia shorter growing period than at the two more southern sites. Based on these chronologies, a set of July–August temperature reconstructions were made, where the longest, H. W. Linderholm et al. 10 based on data from Forfjordsdalen, spanned 1358–1989. In a new approach to develop a summer temperature reconstruction for the whole Title Page of Fennoscandia, Gouirand et al. (2008) selected, from a network of tree-ring width and density chronologies, those having the strongest temperature information a priori, Abstract Introduction to obtain a strong common climate signal suitable for a regional-scale reconstruction. Conclusions References 15 Depending on the number of tree-ring chronologies available through time, seven sep-

arate reconstructions were created. They showed that it was possible to get a good Tables Figures spatial representation of the reconstructions back to around AD 1700, with correlations of >0.7 with observed summer temperatures for nearly the whole of Fennoscandia, J I and even higher correlations (>0.85) over much of central-northern Fennoscandia. 20 One important conclusion was that relatively few (12) chronologies could represent J I Fennoscandian summer temperatures equally well as a large number (30) of chronolo- Back Close gies. Furthermore, it was stated that in order to extend a spatially strong reconstruction back in time (and to update it to the present), only a few key sites are needed. Thus, Full Screen / Esc rather than sampling a large number of new sites, work should be focuses on improving 25 existing sites included in the reconstruction as well as ﬁnding a few more key sites. Printer-friendly Version

5.2 Precipitation Interactive Discussion

Reconstructions of precipitation in Fennoscandia are sparse, but some eﬀorts have been done to extract information about rainfall. Due to the high latitude and the proxim- 1427 ity to the North Atlantic Ocean, trees do not in general show such strong precipitation signals that may be found in more continental regions or at the mid-latitudes. More- CPD over, dendroclimatological reconstructions of precipitation are further complicated by 5, 1415–1461, 2009 the high local variability (compared to temperature) and the fact that most precipitation 5 during summer falls as convective precipitation, so that large amounts of precipitation may fall in very short times. Thus, the importance of selecting a suitable site is highly Dendroclimatology in important when seeking precipitation information from Fennoscandian tree-ring data. Fennoscandia The precipitation signal in tree-ring data is usually secondary to the temperature forcing, especially in the west-central and northern parts of Fennoscandia (Linderholm et H. W. Linderholm et al. 10 al., 2003; Macias et al., 2004). The strongest association between precipitation and tree growth is generally found in summer, where it, in general, is positive in southern Title Page and eastern Fennoscandia and negative in the northern and western parts, (Lindholm et al., 1997; Linderholm et al., 2002, 2004; Seftigen et al., 2009a), although positive Abstract Introduction responses to May precipitation were found in northernmost Fennoscandia (Macias et Conclusions References 15 al., 2004). In the central Scandinavian mountains, late winter/early spring precipitation

was found to have a significant influence on tree-ring widths from Scots pines growing Tables Figures ca 200 m below the tree-line (Linderholm, 2001). Due to this lack of strong and consistent precipitation signals in Fennoscandian J I conifer tree-ring data, few attempts have been made to reconstruct this parameter. 20 Nevertheless, Helama and Lindholm (2003) reconstructed annual variability of rainfall J I and drought in southeastern Finland. May–June precipitation explained 36 and 19% of Back Close the variance in the tree-rings over the calibration/verification periods 1918–1951 and 1952–1985 respectively, and although these statistics are not entirely satisfactory, a Full Screen / Esc reconstruction of summertime rainfall back to AD 874 was made. Reconstructed dry 25 periods occurred in the 12th, 14th and 17th centuries and wet spells occurred in the Printer-friendly Version 11th, 15th and 18th centuries. Scots pine tree-ring widths data from east-central Swe- den was used to reconstruct the Standardised Precipitation Index (SPI), which is a Interactive Discussion drought indicator, back to 1750 (Linderholm and Molin, 2005). Results were similar to the Finnish study, where around 25% of the variance in the observed SPI data could

1428 be obtained using tree-ring data. The most pronounced dry period was found in the beginning of the 19th century, between 1810 and 1835, and this was corroborated CPD by evidence from a farmer’s diary. An attempt to reconstruct the regional precipita- 5, 1415–1461, 2009 tion pattern back to 1735 for the region was also made by Seftigen et al. (2009a). 5 Based on 5 Scots pine chronologies the model explained 43 and 24% of the variance in the precipitation over the calibration/veriﬁcation periods (Fig. 2). Using the same tree Dendroclimatology in species, Linderholm and Chen (2005) developed a 400-year long winter (September– Fennoscandia April) precipitation reconstruction for the west-central Scandinavia. Over the period of 1500–1990, winter precipitation in central Scandinavia showed an increasing trend dur- H. W. Linderholm et al. 10 ing 20th century (within the frame of 400 years). This centurial variability was imposed over variability at shorter frequencies, expressed as periods of negative precipitation Title Page anomalies occurred in early 1500s, 1590s, 1620s, 1710s, 1870s, 1900s, and around 1920s. Several of these periods were also present in the reconstruction of Helama Abstract Introduction and Lindholm (2003) (early 1500s, 1900s, 1920s), however, overall comparison of two Conclusions References 15 reconstructions over the common period shows large diﬀerences between reconstruc-

tions, sources of which can be both local differences in precipitation dynamics as well Tables Figures as site-specific effects. In the southern parts of Fennoscandia, pedunculate oak (Quercus robur L.) is an- J I other tree species which could potentially be promising for precipitation reconstruc- 20 tions. Oak growth is strongly influenced by early growing season precipitation and the J I spatial pattern of growth anomalies have been shown to follow spatial pattern of major Back Close regional weather anomalies (Drobyshev et al., 2008). Multi-century oak chronologies developed in this region (Bartholin, 1975) may therefore prove to be a useful proxy for Full Screen / Esc historical growing season precipitation. Currently, existing and recently developed oak 25 chronologies are analyzed within the frame of an ongoing project aimed at the recon- Printer-friendly Version struction of precipitation dynamics in southern Fennoscandia over the last 500 years (see www.ekskog.org/proxy500). Interactive Discussion

1429 6 Multi millennial temperature reconstructions CPD As described earlier, tree-ring records play a prominent role in attempts to establish how climate has varied in the past. In Fennoscandia hundreds of tree-ring chronolo- 5, 1415–1461, 2009 gies exist, of which some have been utilized for climate reconstructions. However to 5 further utilize large-scale average and spatial pattern of climate variability on millennial Dendroclimatology in timescales, a dense network of tree ring reconstructions is needed (e.g., Jones et al., Fennoscandia 2009). Globally, only few tree-ring chronologies extend back over the past 1000 years (Jansen et al., 2007). One main challenge in constructing millennial-long chronologies H. W. Linderholm et al. is to ﬁnd trees that covers such a long period of time, either living or dead trees that 10 have been preserved through time. No living trees exceeding 1000 years can be found in Fennoscandia. In order to develop “supra long” tree-ring chronologies, data from Title Page

living, recently dead and preserved wood from protective environments such as peat Abstract Introduction bogs or river and lake sediments, so called subfossil wood, is needed, and potential sites with known subfossil material are limited (Jones et al., 2009). Although the num- Conclusions References 15 ber of available >millennial long tree-ring chronologies is increasing, it still remains Tables Figures small and the eﬀorts of developing these are fully justiﬁed. Northern Scandinavia has been shown to be suitable for developing long tree-ring chronologies, because of its richness of natural forests with of old living trees, dead trees (snags) which may resist J I

decomposition for centuries, and subfossil trunks and stumps found in small mountain J I 20 lakes and peat bogs. Presently three multi-millennial tree-ring width chronologies exist, spanning more than 6000 years, which will be described below (Fig. 1). Back Close

Full Screen / Esc 6.1 Tornetrask¨

The area is located in the northernmost part of Sweden, where the Scandinavian Printer-friendly Version mountain range acts as a barrier to warm and moist air masses from the Norwegian Interactive Discussion 25 Sea and consequently forms a strong east-west climate gradient in precipitation and seasonal temperature variability. The Scots pine material consists of living trees, dry dead trees (snags) preserved on land and subfossil wood preserved in lakes and lake 1430 sediments. As mentioned above, extensive sampling in the area occurred in 1970s and 1980s (Bartholin and Karlen´ , 1983; Bartholin, 1984) and the data was later used CPD to make a reconstruction of summer temperature spanning AD 540–1980 (Briffa et al., 5, 1415–1461, 2009 1990, 1992). In the mid 1990s, the Tornetrask¨ chronology was updated and extended 5 within the ADVANCE-10K project (Briffa et al., 2002a) to cover the last 7400 years (Grudd et al., 2002). The Tornetrask¨ chronology is one of the most well known multi- Dendroclimatology in millennial tree-ring chronologies, and has been extensively used in large-scale climate Fennoscandia reconstruction (Jones et al., 1998; D’Arrigo et al., 1999; Mann et al., 1999; Briffa et al., 2002b; Moberg et al., 2005). A selection of the material collected by Bartholin was used H. W. Linderholm et al. 10 to produce a maximum density chronology covering AD 443–1980 (Schweingruber et al., 1988). This density chronology has recently been updated by Grudd (2008) within Title Page the Millennium project, so that it now covers the period AD 500–2004, and is presently one of the longest density records in the world. The updated MXD chronology show Abstract Introduction generally higher temperature estimates than previously reconstructed from the region Conclusions References 15 and also that the late-twentieth century is not exceptionally warm in a 1500-year con-

text. Moreover, Grudd (2008) argues that the “Medieval Warm Period” in northern Tables Figures Fennoscandia was warmer than previously predicted.

6.2 Jamtland¨ J I

J I In the late 1990s, the province of Jamtland,¨ Sweden, was selected as a suitable lo- 20 cation for building a multi-millennial tree-ring chronology which could reflect climate Back Close variability in central Scandinavia and possibly also provide a link between Tornetrask¨ Full Screen / Esc and chronologies from central Europe. The selected area, east of the main dividing line of the Central Scandinavian Mountains, seemed ideal: close to the tree line Scots pines of up to 700 years can be found and areas of old-growth forests, virtually untouched Printer-friendly Version 25 by man still exist. But most importantly, large quantities of old pines, preserved for Interactive Discussion centuries to millennia, were frequently found in small mountain lakes (Gunnarsson, 2001; Gunnarsson and Linderholm, 2002). During the last decade, large efforts have been made to collect Scots pine tree-ring data from living and subfossil wood at a 1431 number of sites in the area, (Gunnarsson, 2001, 2008), and tree-ring width data has been used to reconstruct summer temperatures back to 1600 BC (Linderholm and CPD Gunnarsson, 2005). Furthermore, the temporal distribution patterns of subfossil wood 5, 1415–1461, 2009 found in various lakes has been used as a high resolution proxy for lake levels fluctu- 5 ation, which in turn could be coupled to changes in precipitation and changes in the atmospheric circulation (Gunnarsson et al., 2003; Gunnarsson, 2008). In the 1970s, Dendroclimatology in Schweingruber and colleagues collected tree-ring data from Jamtland,¨ producing ring- Fennoscandia width and density series covering large parts of the last millennium (Schweingruber et al., 1993; Schweingruber, 1987). This data has been included in reconstructions H. W. Linderholm et al. 10 of large-scale temperatures by Esper et al. (2002); D’Arrigo et al. (2006). However, part of this material has likely been collected from historical buildings, not necessarily Title Page from high elevation sites, and consequently the climate information in the data set may be ambiguous. However, progress is being made on updating the density data from Abstract Introduction tree-line sites (Linderholm et al., 2009; Gunnarsson et al., 2009). Conclusions References

15 6.3 Finnish Lapland Tables Figures In northern Finnish Lapland, an abundance of subfossil pines were collected in the beginning of 1970s in order to study pine forests history and tree-line variability (e.g., J I Eronen, 1982). The tree-ring width chronology contain series from a rather wide re- J I gion approximately between 68–70◦ N and 20–30◦ E, with altitudes ranging between 20 75–515 m a.m.s.l. The chronology was extended and updated through the Finnish Re- Back Close search Programme on Climate Change (SILMU) and later through the ADVANCE-10K Full Screen / Esc project, with the goal to build a more than 7000-year long tree-ring width chronology for dendroclimatological purposes (Zetterberg et al., 1994; Eronen et al., 1999, 2002). In the early stages of the work, the chronology was not continuous, with an absolutely Printer-friendly Version 25 dated younger part and a “floating” older part, due to difficulties in bridging a 300-year Interactive Discussion long gap prior to 160 BC. The two chronologies were finally connected in 1999 (Ero- nen et al., 2002) and is presently is the longest conifer tree-ring chronology in Eurasia extending back to 5634 BC (Helama et al., 2008). This data has been used in several 1432 summer temperature reconstructions over different time periods (Linholm et al., 1996; Lindholm et al., 1997; Lindholm and Eronen, 2000; Helama et al., 2002, 2008). How- CPD ever, due to the wide range of sample collection it may be that the homogeneity and 5, 1415–1461, 2009 the robustness of the data, especially in the older part of the record, is weak (Eronen 5 et al., 2002). Dendroclimatology in 6.4 A regional comparison Fennoscandia

The fact that dendrochronological cross-dating is possible between Jamtland¨ and Tor- H. W. Linderholm et al. netrask,¨ more than 600 km apart, indicates a common, regional, representative of the high- frequency variability of the two records. However, on longer timescales, peri- 10 odical differences are evident. A comparison of the two long Swedish tree-ring width Title Page chronologies is shown in (Fig. 3). Several synchronous periods have occurred in the Abstract Introduction nearly 6000 years when the records co-exist. Dramatic shifts from relative warm periods to cold and vice versa (e.g. 3400 BC, 2900 BC and 3600–3400 BC) seem to show Conclusions References better correspondence between the two tree-ring records than complacent growth pe- Tables Figures 15 riods. Generally dry and warm summer conditions prevailed in Fennoscandia between 1800 and 1400 BC. This period was interrupted by a brief period of harsh climate conditions. A marked climate deterioration between 1700–1500 BC is indicated in both J I datasets. The period 600–1 BC is characterized by low sample replication (not shown J I here) and high ring-width variability, especially in the Tornetrask¨ record (Grudd et al., 20 2002). The warm “Roman” period in the first century AD seems to be synchronous Back Close in both records. This is interrupted by exceptionally severe summer conditions that Full Screen / Esc occurred around AD 540 and resulted in reduced growth. The “AD 540 event” has been also been found in other paleoclimate data in the Northern Hemisphere (Larsen et al., 2008). The warm period around AD 1000 corresponds well to the “Medieval Printer-friendly Version 25 Warm Period” and is followed by colder conditions, which seems to have started earlier Interactive Discussion in Jamtland.¨ Both records show a fairly synchronous growth decrease in AD 1550– 1700, corresponding to the “Little Ice Age”, but this period seems to have been more pronounced and of longer duration in Tornetrask.¨ 1433 7 The atmospheric circulation CPD Climate variability over the North Atlantic region has been closely associated with the NAO (Visbeck et al., 2001), a mode described as a measure of the strength of west- 5, 1415–1461, 2009 erly winds blowing across the North Atlantic Ocean (e.g., Hurrel, 1995; Slonosky et 5 al., 2000). The NAO, which can be defined as a see-saw of atmospheric mass be- Dendroclimatology in tween the Icelandic low and the Azores high, is most dominant during winter due to Fennoscandia the large north-south temperature, and hence pressure, gradients (Hurrel, 1995). Dur- ing high NAO-index winters, westerly winds are stronger than normal, causing climate H. W. Linderholm et al. conditions over Fennoscandia to be warmer and wetter than usually. During low in- 10 dex winters the westerlies are relatively weak and the influence of cold continental air masses increases in Fennoscandia. Naturally, there has been an interest to extend this Title Page

large scale feature, with such a pronounced inﬂuence on climate in the region, beyond Abstract Introduction the observational record. Using Scots pine tree-ring data from Norway and Finland, D’Arrigo et al. (1993) found associations between above/below average ring-width de- Conclusions References 15 partures and positive/negative December–February sea-level pressure (SLP) anoma- Tables Figures lies related to the Icelandic low. Thus, after cold winters, associated with negative NAO, the trees responded with low growth, and vice versa. This was a ﬁrst indication that tree-rings could be used to reconstruct winter NAO. Consequently, (Cook et al., 1998) J I

used an extensive tree-ring data set from eastern North America and north-western J I 20 Europe, including Fennoscandia, to reconstruct the winter NAO. The reconstruction spanned 1701–1980 and explaining 41% of the variance in the observed NAO over Back Close

the calibration period. Using 30 ring-width chronologies from Fennoscandia, Russia Full Screen / Esc and Estonia, Lindholm et al. (2001) analyzed the sensitivity of Scots pine to seasonal NAO indices. They found that pines in the northern, forest limit, part of the boreal belt Printer-friendly Version 25 were sensitive to summer (June–August) NAO, while pines in the southern boreal belt were responded to winter (December–February) NAO variability. Interestingly, several Interactive Discussion of the northern chronologies showed as strong, or stronger, correlations with summer NAO as the southern chronologies with winter NAO. Despite that, only winter NAO was

1434 attempted to be reconstructed, a reconstruction that explained 25% of the variance in winter NAO. Further improvements of the winter NAO reconstructions were made in CPD subsequent years, combining tree rings and ice core data, eventually extending it back 5, 1415–1461, 2009 to AD 1400 (Glueck and Stockton, 2001; Cook et al., 2002). 5 Although most interest in NAO has been in its winter phase, leading atmospheric circulation patterns similar to that of winter NAO are also found in summertime (Hurrel Dendroclimatology in et al., 2003), although the summer pattern has a smaller spatial extent than in winter, Fennoscandia and is located further north, with the southern node over North West Europe and a northern node over Greenland. In a recent study, Folland et al. (2009) found strong as- H. W. Linderholm et al. 10 sociations between July–August climate over the western north Atlantic region and the summer (here July–August) NAO (SNAO) on interannual to interdecadal timescales. Title Page Positive SNAO is associated with warm and dry conditions over North West Europe but cooler and wetter conditions over southern Europe and the Mediterranean. Using Abstract Introduction tree-ring data from Great Britain and western Norway, Folland et al. (2009) made a Conclusions References 15 reconstruction of the SNAO back to 1706, explaining 38% of the variance in observed

SNAO 1850–1976. The skill of the reconstruction was supported by a strong relation- Tables Figures ship with Central England temperature remaining strong through the nearly 150 years before 1850. The SNAO reconstruction was further extended back to AD 1441 by Lin- J I derholm et al. (2008), based on 37 tree-ring width and density chronologies, mainly 20 from North-Western Europe. On inter-annual timescales, the reconstruction explained J I at most, with the maximum number of tree-ring series, 46% of the variance in the in- Back Close strumental SNAO. On periods longer than 10 years, the reconstruction showed more skill, where up to 86% of the variance could be explained. Over the last 550 years Full Screen / Esc the SNAO has been in a negative phase during the majority of the record. Periods 25 of relatively low SNAO in 1650–1750 and around 1800 coincides with periods of very Printer-friendly Version low solar activity (Linderholm et al., 2009), and the strong positive of the SNAO around 1970–1995 seems unprecedented in the last 550 years. Interactive Discussion

1435 8 Isotope dendroclimatology CPD As noted previously, climatic information can be extracted not only from widths and density of annual tree-rings, but also from stable carbon, oxygen and hydrogen isotope 5, 1415–1461, 2009 compositions within the tree rings. However, while various non-climatic factors may in- 5 fluence the growth of a tree (Fritts, 1976), the isotopic ratios in wood are influenced by Dendroclimatology in a limited range of relatively simple physiological controls (McCarroll and Loader, 2004). Fennoscandia Thus, there is a large potential for stable isotopes in tree-rings as paleoclimate proxies. Dendroclimatological studies in Fennoscandia, using isotopic techniques, have up to H. W. Linderholm et al. now almost exclusively been restricted to stable isotopes of carbon and focused on 10 trees growing at high-latitude sites in Finland (Fig. 1). The main goal of these studies has to this point been to identify the climate parameters influencing the isotope vari- Title Page

ability in tree-rings, and to understand the physiological and biochemical processes Abstract Introduction that influence isotope ratios within a tree. So far, few attempts have been made to use isotopes for climatic reconstruction purposes. Conclusions References 15 It has been demonstrated that summer sunshine and moisture stress are the domi- Tables Figures nant controlling factors for 13C/12C ratios in late wood cellulose of Scots pine growing in northern Finland (McCarroll and Pawellek, 2001; McCarroll et al., 2003), reflecting the influence of photon flux (sunshine) on photosynthetic rate and moisture stress on J I

reduced stomatal conductance. Gagen et al. (2007) observed high correlations be- J I 13 12 20 tween summer temperatures and the C/ C ratio in pine, and an attempt was made to reconstruct July–August temperatures. The reconstruction captured a well known Back Close

cold period in the region and was in agreement with a recent reconstruction of summer Full Screen / Esc temperature from Northern Sweden. In the Central Scandinavian Mountains, Sweden, Seftigen et al. (2009b) have showed that the δ13C variability in Scots pine is strongly Printer-friendly Version 25 associated to summer temperatures (Fig. 4). As temperature controls the rate at which photosynthetic enzyme is produced, which Interactive Discussion is closely involved in the removal of CO2 from the stomatal chambers during the pho- tosynthesis (Beerling, 1994), it may explain the observed positive relationship between

1436 δ13C and temperature. Robertson et al. (1997) investigated the influence of climate upon the 13C/12C ratio in tree-ring cellulose from oak growing at sites with different CPD hydrological characteristics near its northern distribution limit in southwestern Fin- 5, 1415–1461, 2009 land. Results demonstrated a strong negative relationship between summer precip- 13 5 itation/relative humidity and δ C, and a positive relationship for summer temperature. No intra-site variability in the response of tree-ring δ13C to climate could however be Dendroclimatology in observed. Fennoscandia Tree-ring carbon isotope values often show a declining trend from the start of the in- 13 H. W. Linderholm et al. dustrial period (∼1850), due to incorporation of C depleted CO2 primarily released by 10 the burning of fossil fuels (Freyer and Wiesberg, 1973; McCarroll and Loader, 2004). 13 A correction procedure that attempts to calculate δ C values that would have been Title Page obtained under pre-industrial conditions was proposed by McCarroll et al. (2009). The procedure, which essentially is a nonlinear de-trending of the low frequency changes Abstract Introduction 13 in the δ C based on the physiological response of trees to rising CO2, was tested 13 Conclusions References 15 on the δ C cellulose data obtained from Northern Finland and North-West Norway. In each case the correction improved the correlation with local meteorological records Tables Figures (McCarroll et al., 2009). Using highly replicated δ13C chronologies of Scots pine from a number of sites in Northern Finland, Gagen et al. (2008) investigated non-climatic J I trends in the tree-ring δ13C series. After a correction for changes in δ13C of atmo- J I 20 spheric CO2, the chronologies, comprised of 32 trees of several age classes, displayed 13 juvenile trends in absolute δ C values lasting for approximately the first 50 years of Back Close the tree-growth. The authors showed that Regional Curve Standardization (RCS) approach could be used to identify and remove the juvenile phase, as an alternative to Full Screen / Esc simply omitting the juvenile portion of the stable carbon isotope series. McCarroll and 13 Printer-friendly Version 25 Pawellek (1998) measured the variability and signal strength of δ C in late wood cellulose of Scots pine trees growing at sites located along a latitudinal transect, extending Interactive Discussion from the Arctic Circle to the northern limit of pine distribution, in northern Finland. A strong within and between site signal was observed. However, different trees yielded absolute δ13C values offset by >2‰, which is similar in magnitude to the variability 1437 within individual series that includes the effect of climate. The authors thus concluded that the common strategy of pooling four cores from four trees is inadequate for paleo- CPD climate research. 5, 1415–1461, 2009

9 The temporal strength of climate-signals in Fennoscandian trees Dendroclimatology in Fennoscandia 5 One of the premises to dendroclimatology is that if a growth-climate relationship can be established, this relationship is assumed to be linear over time so that climate H. W. Linderholm et al. can be reconstructed back in time (Fritts, 1976). However, in a high-latitude North- ern Hemisphere network of tree-ring density chronologies of conifers exhibiting strong temperature-growth associations, Briﬀa et al. (1998a) discovered that the chronologies Title Page 10 appeared to loose some of their sensitivity to temperature towards the end of the 20th century. Such a “divergence” in the calibration period could lead to reconstructions Abstract Introduction

of temperatures being overestimated, but also that estimates of future atmospheric Conclusions References CO2 levels, based on carbon-cycle models which are sensitive to high-latitude warming, could be too low (Briffa et al., 1998a). No conclusive evidence of the cause of Tables Figures 15 this divergence has been found, but factors such as increasing CO2, increased nitrogen fertilization and increased UV-B levels (Briffa et al., 1998b), an increasing trend in J I winter precipitation in subarctic regions (Vaganov et al., 1999), or increased drought stress due to increased temperatures (Barber et al., 2000) have been suggested. Still, J I divergence at the end of a chronology may be a methodological problem, either as Back Close 20 an effect of standardization (e.g., Linderholm and Gunnarsson, 2005), or the sampling strategy when developing a chronology, where in general old trees are used (see Full Screen / Esc Linderholm and Linderholm (2004), and references therein). Updating the Tornetrask¨ density chronology (see above), Grudd (2008) included relatively young trees in the Printer-friendly Version most recent period, and by doing so the apparent loss of temperature sensitivity in an Interactive Discussion 25 earlier version was eliminated. Instead, a loss of temperature sensitivity in tree-ring with data in the early nineteenth century was found, likely related to changes in stand dynamics due to a strong regeneration period at that time (Grudd, 2008). 1438 In the Jamtland¨ tree-ring data an anomalous growth increase, which is not seen in the temperature record, occurred around the 1950s (Linderholm, 2002; Linderholm and CPD Gunnarsson, 2005). This growth “surge” was more prominent in the eastern, wetter, 5, 1415–1461, 2009 part of the sampled region. Despite moderate summer temperatures the pines attained 5 growth levels unprecedented for 320 years. Around this time a nearby glacier experienced a substantial volume loss and Scots pine germination peaked, indicating an Dendroclimatology in extended growing season and likely drier conditions. Similar growth increases around Fennoscandia the 1950s were found at sites with a oceanic influence in Norway (e.g., Kirchhefer, 2001; Linderholm et al., 2003), but not at more continental sites, suggesting that only H. W. Linderholm et al. 10 relatively wet sites benefited from a longer and drier growing season. In a study of conifer chronologies from northern Sweden, Bjorklund¨ (2009) noted Title Page that periodical drops in the correlation between tree-ring with chronologies and observed summer temperatures coincided with drops in correlations among the individual Abstract Introduction chronologies (Fig. 5). The implication of this is that the validity of multi-chronology re- Conclusions References 15 construction could be evaluated with running inter-correlation analysis of the included

chronologies the entire length of the regional reconstruction. Tables Figures

10 Some future prospects J I

10.1 Isotopes J I Back Close While dendroclimatological investigations have utilizing tree-ring width and density to 20 interpret past climate variability, the use of isotope dendroclimatology is more recent. Full Screen / Esc Earlier, work on stable isotopes in tree-rings has been prevented by laborious and time consuming technical procedures, which have constrained a wider application of Printer-friendly Version this methodology. Recent advances in mass spectrometry and sample preparation techniques have however increased the rate at which samples are analyzed, enabling Interactive Discussion 25 a larger number of samples to be processed and consequently permitting the development of chronologies with a longer temporal range. Improvement has also been 1439 made in understanding how climate and other environmental factors aﬀect the isotopic ratios in trees, which have enhanced the ability to interpret the stable isotope CPD archive (McCarroll and Loader, 2004). This progress had led to an increasing num- 5, 1415–1461, 2009 ber of isotope studies conducted worldwide, and in Scandinavia several EU-funded 5 projects (e.g. PINE, ISONET and Millennium) as well as national projects (e.g. Swe- DenClim) contributes/have contributed to an increased number of isotope records for Dendroclimatology in the late Holocene, which will provide valuable information of climate parameters other Fennoscandia than temperature. H. W. Linderholm et al. 10.2 Spruce

10 As shown from this review, Norway spruce has been extensively studied in Fennoscan- Title Page dia, and, as shown by Gouirand et al. (2008), spruce is also highly useful as a re- Abstract Introduction gional temperature indicator, possibly superior to Scots pine when tree-ring widths are concerned. In fact, new ﬁndings by Bjorklund¨ (2009) indicate that tree-ring widths Conclusions References from Norway spruce contain a stronger spatial temperature signal than tree-ring widths Tables Figures 15 (Fig. 6). Since pines are in general better preserved than spruce, Norway spruce has been a neglected species in Fennoscandian dendroclimatology aiming at millennial-scale J I climate reconstructions. Presently, eﬀorts are being made to create a long Norway J I spruce tree-ring width and maximum late wood density chronology from the central 20 Scandinavian mountains. New data from Jamtland¨ includes a more than 1000-year Back Close long tree-ring width chronology (Gunnarsson, unpublished) which gives a unique op- Full Screen / Esc portunity to enhance climate change signal by multi- analysis of coniferous trees and better assess the impact of climate variability on forest ecosystems in the region. Printer-friendly Version 10.3 Teleconnections Interactive Discussion

25 Teleconnections, i.e. strong statistical relationships between weather in different parts of the world, in the climate system are associated with the large-scale circulation of 1440 the atmosphere and the oceans. In the atmosphere, climate signals can be transferred to regions far removed from the physical source of the variability, while in the oceans CPD teleconnections are associated with the global thermohaline circulation (e.g., Chase et 5, 1415–1461, 2009 al., 2005). Using high-resolution climate proxies such as tree rings, the spatiotemporal 5 stability of observed climate patterns and associated teleconnections can be studied. As an example, in a study of two millennium-long tree-ring records from Jamtland¨ and Dendroclimatology in Tibet, Linderholm and Brauning¨ (2006) noted that both records showed similar evolu- Fennoscandia tion in tree-growth variability, on decadal and longer time scales, between ca. 1100 and 1550, but around 1550 the two chronologies diverge, possibly due to a strengthening H. W. Linderholm et al. 10 of the Asian monsoon. Feng and Hu (2008) suggested that Temperatures on the Ti- betan plateau, which affects the strength of the Asian monsoon, are associated with Title Page the Atlantic Multidecadal Oscillation, i.e. sea surface temperature variability in the At- lantic Ocean (Kerr, 2000). The AMO affects climate in the North Atlantic region (Sutton Abstract Introduction and Hodson, 2005; Knight et al., 2005), and this may account for the long term associa- Conclusions References 15 tions in tree growth between Jamtland¨ and Tibet. Moreover, preliminary results suggest

links between the summer NAO, which also is associated with the AMO (Folland et al., Tables Figures 2009), and climate in south-east Asia (Linderholm et al., 2009). This may be related to teleconnections between the NAO and the East Asian jet (Branstator, 2002), where the J I East Asian jet the strength of the East Asian jet stream aﬀects weather downstream 20 in East Asia (Yang et al., 2002). A new 2.5 millennia long tree-ring record from Tibet J I (Liu et al., 2009) will facilitate the study of possible teleconnections between Asia and Back Close Fennoscandia during the late Holocene. Full Screen / Esc 10.4 Combining tree-ring data and climate models

By virtue of great advances in climate modelling science for recent a few decades, Printer-friendly Version 25 understanding of climate change has been greatly improved. As well as to simulate Interactive Discussion present-day climate realistically and predict future climate change exactly, a challenging topic of climate modelling is to simulate climate variation in the past. This enables us to better estimate the sensitivity of climate system to the external forcings and to 1441 understand profound mechanism of climate change. The only surrogate to validate the past climate simulation of global climate models (e.g., PMIP2 results) is proxy records CPD from various sources. Tree-ring records provide synthetic information of climate con- 5, 1415–1461, 2009 ditions in particular for last a few thousand years with relatively high density over the 5 world, so it has been often utilized to validate climate model’s performance in simulat- ing multi-decadal (e.g., PDO , decadal ENSO-monsoon variability) to multi-centennial Dendroclimatology in climate variation (e.g., MWP, LIA ). Although there are much uncertainties both in tree- Fennoscandia ring records and climate models, it has been suggested that if tree-ring based reconstruction and instrumental records are applied to constrain climate model simulation, H. W. Linderholm et al. 10 we can reduce uncertainties in predicting future climate change (Hegerl et al., 2006). Evolving toward earth system model, most climate models are getting to include fully Title Page interactive vegetation models. Tree-ring record can be the most suitable information to validate those model’s vegetation dynamics. Abstract Introduction

Conclusions References 11 Some ﬁnal remarks Tables Figures

15 As has been shown in this review, the dendroclimatological research in Fennoscandia has contributed to better understanding of late Holocene climate variability in the re- J I gion. Moreover, some of the data has been used to reconstruct past temperature variability on hemispheric and global scale. Still, Fennoscandia oﬀers large possibilities J I for future exciting paleoclimate climate research, e.g. regarding precipitation, humidity Back Close 20 and the role of the large-scale circulation on regional to local climate variability. Finally, it should be noted that this review is by no means comprehensive; several works have Full Screen / Esc not been included. This is not because of these works being of less importance, but rather a constraint of time and space. Printer-friendly Version

Acknowledgements. This work is a result of the workshop on Holocene Climate variability Interactive Discussion 25 over Scandinavia, hosted by the Bert Bolin Centre for Climate Research, Stockholm Univer- sity, in April 2008. This research has been funded by the Swedish Research Council (VR), The Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning 1442 (Formas), the Swedish International Development Cooperation Agency (Sida/SAREC) and EU project Millennium (Contract No. 017008 GOCE). Finally, we would like to acknowledge all den- CPD droclimatologists working in Fennoscandia, who have contributed to the science throughout the centuries. Thanks goes to Hakan˚ Grudd for providing the Tornetrask¨ data. 5, 1415–1461, 2009

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D., Hulme, M., and New, M.: A comprehensive set of high-resolution grids of monthly climate for Europe and the globe: the observed record J I (1901–2000) and 16 scenarios (2001–2100), Tyndall Centre for Climate Change Research, Working Paper 55, University of East Anglia, Norwich, 25 pp., 2004. 1457, 1461 J I Makinen,¨ H., Nojd,¨ P., Kahle, H. P., Neumann, U., Tveite, B., Mielikainen,¨ K., Rohle,¨ H., and Back Close 25 Spiecker, H.: Radial growth variation of Norway spruce (Picea abies L. Karst.) across latitudinal and altitudinal gradients in central and northern Europe, Forest Ecol. Manag., 171, Full Screen / Esc 243–259, 2002. 1424 Nilssen, E. and Vorren, K.-D.: Peat humification and climate history, Norsk Geografisk Tidskrift, Printer-friendly Version 71, 215–217, 1991. 1417 ˚ 30 Ording, A.: Arringsanalyser pagran˚ og furu. Medd. Fra det norske skogsforsoksvesen, 7(2), pp. Interactive Discussion 354, 1941. 1423 Polge, H.: The use of X-ray densitometric methods in dendrochronology, Tree Ring Bulletin, 30, 1–10, 1970. 1420 1452 Raven, P. H.: Biology of Plants, Sixth Edition, WH Freeman and Company/Worth Publishers, USA, 1999. 1418, 1419 CPD Robertson, I., Rolfe, J., Switsur, V. R., Carter, A. H. C., Hall, M. A., Barker, A. C., and Wa- terhouse, J. S.: Signal Strength and Climate Relationships in 13C/12C Ratios of Tree Ring 5, 1415–1461, 2009 5 Cellulose from Oak in Southwest Finland, Geophys. Res. Lett., 24, 1487–1490, 1997. 1437, 1455 Shindell, D. T., Miller, R. L., Schmidt, G. A., and Pandolfo, L.: Simulation of recent northern Dendroclimatology in winter climate trends by greenhouse-gas forcing, Nature, 399, 452–455, 1999. 1417 Fennoscandia Schove, J.: Summer temperatures and tree-rings in North-Scandinavia A.D. 1461–1950, Ge- H. W. Linderholm et al. 10 ografiska Annaler, 36, 40–80, 1954. 1423, 1425 Schweingruber, F. H.: Tree Rings: Basics and Applications of Dendrochronology, Dordrecht, The Netherlands: D. Reidel Publishing Company, 276 pp., 1987. 1418, 1432 Schweingruber, F. H., Bartholin, T., Schar, E., and Briffa, K. R.: Radiodensitometric- Title Page dendroclimatological conifer chronologies from Lapland (Scandinavia) and the Alps (Switzer- 15 land), Boreas, 17, 559–566, 1988. 1431 Abstract Introduction Seftigen, K., Linderholm, H., Drobyshev, I., and Niklasson, M.: The association between reconstructed precipitation extremes and forest fire activity in east-central Sweden since A.D. Conclusions References 1700, in preparation, 2009. 1428, 1429, 1457 Tables Figures Seftigen, K., Linderholm, H., Liu, Y., and Loader, N.: The influence of climate on 13C/12C and 18 16 20 O/ O ratios in tree rings of Pinus sylvestris growing central Scandinavian Mountains, in preparation, 2009. 1436, 1459 J I Schweingruber, F. H., Briffa, K. R., and Nogler, P.: A tree-ring densitometric transect from J I Alaska to Labrador, Int. J. Biometeorology, 37, 151–169, 1993. 1421, 1432 Siren,´ G.: Skogsgranstallen¨ som indikator for¨ klimatfluktuationerna i norra Fennoskandien un- Back Close 25 der historisk tid, Comm. Inst. Forest. Fenn., 54, pp. 1–66, 1961. 1425 Slonosky, V. C., Jones, P.D., and Davies, T. D.: Variability of the surface atmospheric circulation Full Screen / Esc over Europe, 1774–1995, Int. J. Climatol., 20, 1875–1897, 2000. 1434 Sonninen, E. and Jungner, H.: Stable carbon isotopes in tree rings of a Scots pine (Pinus Printer-friendly Version sylvestris L.) from northern Finland, Palaoklimaforschung,¨ 15, 121–128, 1995. 1455 30 Sutton, R. T. and Hodson, D. L. R.: Atlantic Ocean forcing of the North American and European Interactive Discussion summer climate, Science, 309, 115–118, 2005. 1441 Vaganov, E. A., Hughes, M. K., Kirdyanov, A. V., Schweingruber, F. H., and Silkin, P. P.: Influ- ence of snowfall and melt timing on tree growth in subarctic Eurasia, Nature, 400, 149–151, 1453 1999. 1438 Visbeck, M. H., Hurrell, J. W., Polvani, L., and Cullen, H. M.: The North Atlantic Oscillation: CPD Past, present, and future, PNAS, 98, 12876–12877, 2001. 1434 Yang, S., Lau, K.-M., and Kim, K.-M.: Variations of the East Asian Jet stream and Asian-Pacific- 5, 1415–1461, 2009 5 American winter climate anomalies, J. Climate, 15, 306–325, 2002. 1441 Yu, G. and Harrison, S. P.: Holocene changes in atmospheric circulation patterns as shown by lake status changes in northern Europe, Boreas, 24, 260–268, 1995. 1417 Dendroclimatology in Zetterberg, P., Eronen, M., and Briffa, K. R.: Evidence on climatic variability and prehistoric Fennoscandia human activities between 165 B.C. and A.D. 1400 derived from subfossil Scots pine (Pinus 10 sylvestris) found in a lake in Utsjoki, Northernmost Finland, Bulletin of the Geological Society H. W. Linderholm et al. of Finland, 66, 102–124, 1994. 1432

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1454 CPD 5, 1415–1461, 2009

Dendroclimatology in Table 1. Fennoscandian stable isotope (δ13C) studies. Fennoscandia

Reference Species Site Age-range Wood component Signal H. W. Linderholm et al. Gagen et al. (2007) 12 pines (PS) 2 sites, 1612–2002 Annual, Summer temperature N Finland LW cellulose Gagen et al. (2008) 32 pines (PS) 3 sites, 1780–2002 Annual, Summer precipitation, N Finland cellulose summer temperature, Title Page precipitation deﬁcit (sitedependent) Abstract Introduction Linderholm et al. (2009) 14 pines (PS) 1 site, 1736–2006 Annual, pooled, Temperature W Sweden EW+LW >precipitation α-cellulose Conclusions References McCarroll and Pawellek (1998) 25 pines (PS) 4 and 1 sites from ∼1960–1995 Annual, N and W Finland LW cellulose Tables Figures McCarroll and Pawellek (2001) 36 pines (PS) 2 sites 1961–1995 Annual, Summer sunshine, N Finland LW cellulose precipitation (sitedependent) McCarroll et al. (2003) 10×3 pines (PS) 3 site, Annual, Summer sunshine J I N Finland LW cellulose >summer precipitation >RH McCarroll et al. (2009) 7 Finnish and N Finland, 1820–2002, Annual, J I 6 Norweigan N Finland 1895–1995 LW cellulose pines (PS) NW Norway 1910–2001 Back Close Robertson et al. (1997) 5×2 Oaks (QR) 2 sites, 1895–1994 Annual, LW Precipitation α > > SW Finland -cellulose RH temperature Full Screen / Esc Sonninen and Jungner (1995) 1 pine (PS) Finland 1841–1990 Annual, July temperature cellulose

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1455 H W Linderholm et al.: Dendroclimatology in Fennoscandia 5 to-year variability was reproduced with fidelity, the long timescale fluctuations above 50 years could not be recon- CPD structed because of limitations in the standardization technique. 5, 1415–1461, 2009 The summer temperature reconstruction was further developed by Briffa et al. (1990), extending the reconstructed pe- Dendroclimatology in riod to April-August, using tree-ring data from Tornetrask.¨ Fennoscandia The reconstructed period spanned A.D. 500–1975 where tree-ring widths and density together explained 51% of the H. W. Linderholm et al. variance in observed temperatures over the 1876–1975 calibration period. Based on the reconstruction, Briffa et al. (1990) found no evidence of a homogenous Medieval Warm Epoch, and a short Little Ice Age was interpreted to have oc- Title Page curred around 1570–1650. However, the weaknesses in the Abstract Introduction standardization method made it difficult to retrieve centennial to multi-centennial variation in the reconstruction, why Conclusions References the claim of a short Little Ice Age could have been a bit premature. An effort to enhance the low-frequency variation Tables Figures was made by Briffa et al. (1992), where a “new” standardization method was applied to the same dataset used Briffa et al. J I (1990), although with some additional tree-ring width series. Instead of standardizing every tree-ring series individually J I all series were arranged starting at their cambial age, and every year was averaged into a grand mean curve of the whole Back Close population. This method was called Regional Curve Stan- dardization (RCS), and was developed from the method used Full Screen / Esc by Erlandsson (1936), (see above). A consequence of using the RCS method was a lowered common signalFig.1. amongMap the of FennoscandiaFig. 1. Map of with Fennoscandia chronologies with chronologies of multi-millennial of multi-millennial extension, Jamtland,¨ Tor- Printer-friendly Version trees, but the low-frequency variability was greatlynetrask¨ enhanced and Finnishextension, Lapland, Jamtland,¨ (grey shaded Tornetrask¨ areas), and Finnish and sites Lapland, sampled (grey shaded for isotopic chronologies and a longer more pronounced Little Ice ageand emerged test aroundstudies (redareas), dots). and sites sampled for isotopic chronologies and test studies, Interactive Discussion 1570–1750. (red dots). Up to now, most efforts in building tree-ring chronologies and making climate reconstructions had mainly been restricted to northern Fennoscandia. However, Kalela- In a new approach to develop1456 a summer temperature re- Brundin (1999) expanded the network of reconstructions in construction for the whole of Fennoscandia, Gouirand et al. Fennoscandia by investigating climate from a Scots pine tree- (2008) selected, from a network of tree-ring width and den- ring series sampled in Femundsmarka, in the eastern part sity chronologies, those having the strongest temperature in- of central Norway. In addition to whole tree-ring widths, formation a priori, to obtain a strong common climate signal Kalela-Brundin (1999) measured the width of early wood suitable for a regional-scale reconstruction. Depending on and late wood separately and made a reconstruction of July the number of tree-ring chronologies available through time, and July-August temperatures 1500–1985, based on combi- seven separate reconstructions were created. They showed nations of these parameters. The reconstructions explained that it was possible to get a good spatial representation of the 52 and 49% of the variance in observed July and July-August reconstructions back to around A.D. 1700, with correlations temperatures for the period 1872–1993 respectively. of >0.7 with observed summer temperatures for nearly the Another effort to expand the diversity of locations in whole of Fennoscandia, and even higher correlations (>0.85) Fennoscandia was made by Kirchhefer (2001), who inves- over much of central-northern Fennoscandia. One important tigated Scots pine tree-ring widths from three coastal sites conclusion was that relatively few (12) chronologies could in Northern Norway; Vikran (northernmost), Stonglandsei- represent Fennoscandian summer temperatures equally well det and Forfjordsdalen (southernmost). Due to the dramatic as a large number (30) of chronologies. Furthermore, it was landscape in the area, differences in tree responses to climate stated that in order to extend a spatially strong reconstruction were evident, where tree-growth at Vikran was associated back in time (and to update it to the present), only a few key with a shorter growing period than at the two more south- sites are needed. Thus, rather than sampling a large number ern sites. Based on these chronologies, a set of July-August of new sites, work should be focuses on improving existing temperature reconstructions were made, where the longest, sites included in the reconstruction as well as finding a few based on data from Forfjordsdalen, spanned 1358–1989. more key sites. CPD 5, 1415–1461, 2009 6 H W Linderholm et al.: Dendroclimatology in Fennoscandia

5.2 Precipitation Dendroclimatology in Fennoscandia Reconstructions of precipitation in Fennoscandia are sparse, but some efforts have been done to extract information about H. W. Linderholm et al. rainfall. Due to the high latitude and the proximity to the North Atlantic Ocean, trees do not in general show such strong precipitation signals that may be found in more conti- Title Page nental regions or at the mid-latitudes. Moreover, dendrocli- Abstract Introduction matological reconstructions of precipitation are further complicated by the high local variability (compared to tempera- Conclusions References ture) and the fact that most precipitation during summer falls as convective precipitation, so that large amounts of precip- Tables Figures itation may fall in very short times. Thus, the importance of selecting a suitable site is highly important when seeking J I precipitation information from Fennoscandian tree-ring data. J I The precipitation signal in tree-ring data is usually secondary to the temperature forcing, especially in the west-central and Back Close northern parts of Fennoscandia (Linderholm et al.,Fig. 2003; 2. Spatial Ma- correlationFig. 2. Spatial between correlation tree-ring between PC1 scores, tree-ring obtained PC1 scores, from obtained 5 chronologies in the Stockholm area, and gridded mean May-June precipitation data (CRU TS2.1) (Mitchell et al., cias et al., 2004). The strongest association between precip- from 5 chronologies in the Stockholm area, and gridded mean May- Full Screen / Esc 2004) over the AD 1901–1995 period (Seftigen et al., 2009a). itation and tree growth is generally found in summer, where June precipitation data (CRU TS2.1)(Mitchell et al., 2004) over the it, in general, is positive in southern and eastern Fennoscan- A.D. 1901-1995 period (Seftigen et al., in prep.a). Printer-friendly Version dia and negative in the northern and western parts, (Lind- Interactive Discussion holm et al., 1997; Linderholm et al., 2002, 2004; Seftigen et al., in prep.a), although positive responses to May precipitation were found in northernmost Fennoscandia (Macias et al., 2004). In the central Scandinavian mountains, late win- Chen (2005) developed a 400–year long winter (September- 1457 ter/early spring precipitation was found to have a significant April) precipitation reconstruction for the west-central Scan- influence on tree-ring widths from Scots pines growing ca dinavia. Over the period of 1500–1990, winter precipitation 200 m below the tree-line (Linderholm, 2001). in central Scandinavia showed an increasing trend during th Due to this lack of strong and consistent precipitation sig- 20 century (within the frame of 400 years). This centurial nals in Fennoscandian conifer tree-ring data, few attempts variability was imposed over variability at shorter frequen- have been made to reconstruct this parameter. Nevertheless, cies, expressed as periods of negative precipitation anomalies Helama and Lindholm (2003) reconstructed annual variabil- occurred in early 1500s, 1590s, 1620s, 1710s, 1870s, 1900s, ity of rainfall and drought in southeastern Finland. May- and around 1920s. Several of these periods were also present June precipitation explained 36 and 19% of the variance in in the reconstruction of Helama and Lindholm (2003) (early the tree-rings over the calibration/verification periods 1918– 1500s, 1900s, 1920s), however, overall comparison of two 1951 and 1952–1985 respectively, and although these statis- reconstructions over the common period shows large differ- tics are not entirely satisfactory, a reconstruction of summer- ences between reconstructions, sources of which can be both time rainfall back to A.D. 874 was made. Reconstructed dry local differences in precipitation dynamics as well as site- periods occurred in the 12th, 14th and 17th centuries and specific effects. wet spells occurred in the 11th, 15th and 18th centuries. Scots pine tree-ring widths data from east-central Sweden In the southern parts of Fennoscandia, pedunculate oak was used to reconstruct the Standardised Precipitation Index (Quercus robur L.) is another tree species which could (SPI), which is a drought indicator, back to 1750 (Linder- potentially be promising for precipitation reconstructions. holm and Molin, 2005). Results were similar to the Finnish Oak growth is strongly influenced by early growing sea- study, where around 25% of the variance in the observed SPI son precipitation and the spatial pattern of growth anomalies data could be obtained using tree-ring data. The most pro- have been shown to follow spatial pattern of major regional nounced dry period was found in the beginning of the 19th weather anomalies (Drobyshev et al., 2008). Multi-century century, between 1810 and 1835, and this was corroborated oak chronologies developed in this region (Bartholin, 1975) by evidence from a farmers diary. An attempt to reconstruct may therefore prove to be a useful proxy for historical grow- the regional precipitation pattern back to 1735 for the region ing season precipitation. Currently, existing and recently de- was also made by Seftigen et al. (in prep.a). Based on 5 Scots veloped oak chronologies are analyzed within the frame of an pine chronologies the model explained 43 and 24% of the ongoing project aimed at the reconstruction of precipitation variance in the precipitation over the calibration/verification dynamics in southern Fennoscandia over the last 500 years periods (Fig 2). Using the same tree species, Linderholm and (see www.ekskog.org/proxy500). CPD 5, 1415–1461, 2009

8 H W Linderholm et al.: Dendroclimatology in Fennoscandia Dendroclimatology in Fennoscandia rather wide region approximately between 68–70◦ N and 20– 30◦ E, with altitudes ranging between 75–515 m a.m.s.l. The H. W. Linderholm et al. chronology was extended and updated through the Finnish Research Programme on Climate Change (SILMU) and later through the ADVANCE—10K project, with the goal to build Title Page a more than 7000–year long tree-ring width chronology for dendroclimatological purposes (Zetterberg et al., 1994; Ero- Abstract Introduction nen et al., 1999, 2002). In the early stages of the work, the chronology was not continuous, with an absolutely dated Conclusions References younger part and a “floating” older part, due to difficul- Tables Figures ties in bridging a 300–year long gap prior to 160 B.C. The two chronologies were finally connected in 1999 (Eronen et al., 2002) and is presently is the longest conifer tree-ring J I chronology in Eurasia extending back to 5634 B.C. (Helama et al., 2008). This data has been used in several summer J I Fig. 3. A comparison between the¨ Tornetrask¨ (blue) and¨ Jamtland¨ temperature reconstructions over different timeFig. periods 3. A (Lin- comparison between the Tornetrask (blue) and Jamtland (red) tree-ring width Back Close holm et al., 1996; Lindholm et al., 1997; Lindholmchronologies. and Ero- Both record(red) tree-ring has been width RCS chronologies. standardized Both record (see has text) been and RCS transformed stan- into Z-score nen, 2000; Helama et al., 2002, 2008). However,values, due showing to the centennialdardized (seevariability. text) and Interpreted transformed synchronous into Z-score values, periods showing are shown in grey. Full Screen / Esc wide range of sample collection it may be that the homogene- centennial variability. Interpreted synchronous periods are shown ity and the robustness of the data, especially in the older part in grey. Printer-friendly Version of the record, is weak (Eronen et al., 2002). netrask.¨ Interactive Discussion 6.4 A regional comparison

The fact that dendrochronological cross-dating is possible 7 The Atmospheric circulation between Jamtland¨ and Tornetrask,¨ more than 600 km apart, indicates a common, regional, representative of the high- fre- Climate variability over the1458 North Atlantic region has been quency variability of the two records. However, on longer closely associated with the NAO, (Visbeck et al., 2001), a timescales, periodical differences are evident. A compari- mode described as a measure of the strength of westerly son of the two long Swedish tree-ring width chronologies is winds blowing across the North Atlantic Ocean (e.g., Hur- shown in (Fig. 3). Several synchronous periods have oc- rel, 1995; Slonosky et al., 2000). The NAO, which can be curred in the nearly 6000 years when the records co-exist. defined as a see-saw of atmospheric mass between the Ice- Dramatic shifts from relative warm periods to cold and vice landic low and the Azores high, is most dominant during versa (e.g. 3400 B.C., 2900 B.C. and 3600–3400 B.C.) winter due to the large north-south temperature, and hence seem to show better correspondence between the two tree- pressure, gradients (Hurrel, 1995). During high NAO-index ring records than complacent growth periods. Generally dry winters, westerly winds are stronger than normal, causing and warm summer conditions prevailed in Fennoscandia be- climate conditions over Fennoscandia to be warmer and wet- tween 1800 and 1400 B.C. This period was interrupted by a ter than usually. During low index winters the westerlies brief period of harsh climate conditions. A marked climate are relatively weak and the influence of cold continental air deterioration between 1700–1500 B.C. is indicated in both masses increases in Fennoscandia. Naturally, there has been datasets. The period 600–1 B.C. is characterized by low sam- an interest to extend this large scale feature, with such a pro- ple replication (not shown here) and high ring-width variabil- nounced influence on climate in the region, beyond the ob- ity, especially in the Tornetrask¨ record (Grudd et al., 2002). servational record. Using Scots pine tree-ring data from Nor- The warm “Roman” period in the first century A.D. seems way and Finland, D’Arrigo et al. (1993) found associations to be synchronous in both records. This is interrupted by ex- between above/below average ring-width departures and pos- ceptionally severe summer conditions that occurred around itive/negative December-February sea-level pressure (SLP) A.D. 540 and resulted in reduced growth. The “A.D. 540 anomalies related to the Icelandic low. Thus, after cold win- event” has been also been found in other paleoclimate data ters, associated with negative NAO, the trees responded with in the northern hemisphere (Larsen et al., 2008). The warm low growth, and vice versa. This was a first indication that period around A.D. 1000 corresponds well to the “Medieval tree-rings could be used to reconstruct winter NAO. Conse- Warm Period” and is followed by colder conditions, which quently, (Cook et al., 1998) used an extensive tree-ring data seems to have started earlier in Jamtland.¨ Both records show set from eastern North America and north-western Europe, a fairly synchronous growth decrease in A.D. 1550–1700, including Fennoscandia, to reconstruct the winter NAO. The corresponding to the “Little Ice Age”, but this period seems reconstruction spanned 1701–1980 and explaining 41% of to have been more pronounced and of longer duration in Tor- the variance in the observed NAO over the calibration pe- CPD 5, 1415–1461, 2009

10 H W Linderholm et al.: DendroclimatologyDendroclimatology in Fennoscandia in Fennoscandia constructed back in time (Fritts, 1976). However, in a high-latitude Northern HemisphereH. W. Linderholm network of ettree-ring al. density chronologies of conifers exhibiting strong temperature- growth associations, Briffa et al. (1998a) discovered that the chronologies appeared to loose someTitle of Page their sensitivity to temperature towards the end of the 20th century. Such a “divergence” in the calibrationAbstract period couldIntroduction lead to reconstructions of temperatures being overestimated, but also that Conclusions References estimates of future atmospheric CO2 levels, based on carbon- cycle models which are sensitiveTables to high-latitudeFigures warming, could be too low (Briffa et al., 1998a). No conclusive evidence of the cause of this divergence has been found, but J I factors such as increasing CO2, increased nitrogen fertilization and increased UV-B levels (Briffa et al., 1998b), an J I 13 increasing trend in winter precipitation in subarctic regions Fig. 4. The relationship between δ C values from Scots13 pine growing in the Central Scandina- Fig. 4. The relationship between δ C values from Scots pine (Vaganov et al., 1999), or increased drought stress due to vian Mountains and mean monthly June–July temperature over the 1901–2000 period (Seftigen Back Close growing in the Central Scandinavian Mountains and mean monthly increased temperatures (Barber et al., 2000) have been sug- et al., 2009b). June-July temperature over the 1901-2000 period (Seftigen et al., in gested. Still, divergence at theFull end Screen of a / chronologyEsc may prep.b). be a methodological problem, either as an effect of standardization (e.g., LinderholmPrinter-friendly and Gunnarsson, Version 2005), or the sampling strategy when developing a chronology, where in δ13C based on the physiological response of trees to rising general old trees are used (seeInteractive Linderholm Discussion and Linderholm CO , was tested on the δ13C cellulose data obtained from 2 (2004), and references therein). Updating the Tornetrask¨ northern Finland and north-west Norway. In each case the density chronology (see above), Grudd (2008) included rel- correction improved the correlation with local meteorolog- atively young trees in the most recent period, and by doing ical records (McCarroll et al., 2009). Using highly repli- so the apparent loss of temperature sensitivity in an earlier cated δ13C chronologies of1459 Scots pine from a number of sites version was eliminated. Instead, a loss of temperature sen- in northern Finland, Gagen et al. (2008) investigated non- sitivity in tree-ring with data in the early nineteenth century climatic trends in the tree-ring δ13C series. After a correction was found, likely related to changes in stand dynamics due for changes in δ13C of atmospheric CO , the chronologies, 2 to a strong regeneration period at that time (Grudd, 2008). comprised of 32 trees of several age classes, displayed juvenile trends in absolute δ13C values lasting for approximately In the Jamtland¨ tree-ring data an anomalous growth in- the first 50 years of the tree-growth. The authors showed crease, which is not seen in the temperature record, occurred that Regional Curve Standardization (RCS) approach could around the 1950s (Linderholm, 2002; Linderholm and Gun- be used to identify and remove the juvenile phase, as an al- narsson, 2005). This growth “surge” was more prominent in ternative to simply omitting the juvenile portion of the stable the eastern, wetter, part of the sampled region. Despite mod- carbon isotope series. McCarroll and Pawellek (1998) mea- erate summer temperatures the pines attained growth levels sured the variability and signal strength of δ13C in late wood unprecedented for 320 years. Around this time a nearby cellulose of Scots pine trees growing at sites located along a glacier experienced a substantial volume loss and Scots pine latitudinal transect, extending from the Arctic Circle to the germination peaked, indicating an extended growing season northern limit of pine distribution, in northern Finland. A and likely drier conditions. Similar growth increases around strong within and between site signal was observed. How- the 1950s were found at sites with a oceanic influence in Nor- ever, different trees yielded absolute δ13C values offset by way (e.g., Kirchhefer, 2001; Linderholm et al., 2003), but not >2‰, which is similar in magnitude to the variability within at more continental sites, suggesting that only relatively wet individual series that includes the effect of climate. The au- sites benefited from a longer and drier growing season. thors thus concluded that the common strategy of pooling four cores from four trees is inadequate for paleoclimate re- In a study of conifer chronologies from northern Sweden, search. Bjorklund¨ (2009) noted that periodical drops in the correlation between tree-ring with chronologies and observed sum- 9 The temporal strength of climate-signals in mer temperatures coincided with drops in correlations among Fennoscandian trees the individual chronologies (Fig. 5). The implication of this is that the validity of multi-chronology reconstruction could One of the premises to dendroclimatology is that if a growth- be evaluated with running inter-correlation analysis of the in- climate relationship can be established, this relationship is cluded chronologies the entire length of the regional recon- assumed to be linear over time so that climate can be re- struction. H W Linderholm et al.: Dendroclimatology in Fennoscandia 11

Table 1. Fennoscandian stable isotope (δ13C) studies

Reference Species Site Age-range Wood component Signal (Gagen et al., 2007) 12 pines (PS) 2 sites, 1612–2002 Annual, Summer temperature N Finland LW cellulose (Gagen et al., 2008) 32 pines (PS) 3 sites, 1780–2002 Annual, Summer precipitation, N Finland cellulose summer temperature, precipitation deﬁcit (sitedependent) (Linderholm in prep., 2009) 14 pines (PS) 1 site, 1736–2006 Annual, pooled, Temperature W Sweden EW+LW >precipitation α-cellulose (McCarroll and Pawellek, 1998) 25 pines (PS) 4 and 1 sites from ∼1960–1995 Annual, N and W Finland LW cellulose (McCarroll and Pawellek, 2001) 36 pines (PS) 2 sites 1961-1995 Annual, Summer sunshine, N Finland LW cellulose precipitation (sitedependent) (McCarroll et al., 2003) 10×3 pines (PS) 3 site, Annual, Summer sunshine N Finland LW cellulose >summer precipitation >RH (McCarroll et al., 2009) 7 Finnish and N Finland, 1820–2002, Annual, 6 Norweigan N Finland 1895–1995 LW cellulose pines (PS) NW Norway 1910-2001 (Robertson et al., 1997) 5×2 Oaks (QR) 2 sites, 1895–1994 Annual, LW Precipitation SW Finland α-cellulose CPD>RH >temperature (Sonninen and Jungner, 1995) 1 pine (PS) Finland 1841–1990 Annual, July temperature 5, 1415–1461, 2009 cellulose

Dendroclimatology in ability, the use of isotope dendroclimatologyFennoscandia is more recent. Earlier, work on stable isotopes in tree-rings has been prevented by laborious andH. time W. Linderholm consuming et technical al. procedures, which have constrained a wider application of this methodology. Recent advances in mass spectrometry and sample preparation techniques haveTitle Page however increased the rate at which samples are analyzed, enabling a larger number Abstract Introduction of samples to be processed and consequently permitting the development of chronologiesConclusions with a longerReferences temporal range. Improvement has also been made in understanding how climate and other environmentalTables factors affectFigures the isotopic ratios in trees, which have enhanced the ability to interpret the stable isotope archive (McCarrollJ and Loader,I 2004). This progress had led to an increasing number of isotope studies conducted worldwide, andJ in ScandinaviaI several EU- Fig. 5. Running correlations of July temperature vs. a regional TRW pine chronology for North- Fig. 5. Running correlations of July temperature vs. a regional funded projects (e.g. PINE, ISONET and Millennium) as ern Fennoscandia plotted against the mean inter-correlation of the 7 included chronologies of Back Close TRW pine chronology for Northern Fennoscandia plotted against the regional TRW chronology, with a base-length of 11 years and lag of 1 year forwell the as period national projects (e.g SweDenClim) contributes/have the mean inter-correlation of the 7 included chronologies of the re- 1901–1980 (Bjorklund¨ , 2009). contributed to an increased numberFull Screen of isotope / Esc records for the gional TRW chronology, with a base-length of 11 years and lag of late Holocene, which will provide valuable information of 1 year for the period 1901–1980 (Bjorklund,¨ 2009). climate parameters other thanPrinter-friendly temperature. Version

10 Some future prospects 10.2 Spruce Interactive Discussion

10.1 Isotopes As shown from this review, Norway spruce has been extensively studied in Fennoscandia, and, as shown by Gouirand et While dendroclimatological investigations have utilizing al. (2008), spruce is also highly useful as a regional tempera- 1460 tree-ring width and density to interpret past climate vari- ture indicator, possibly superior to Scots pine when tree-ring 12 H W Linderholm et al.: Dendroclimatology in Fennoscandia widths are concerned. In fact, new ﬁndings by Bjorklund¨ (2009) indicate that tree-ring widths from Norway spruce contain a stronger spatial temperature signal than tree-ring widths (Fig. 6). CPD Since pines are in general better preserved than spruce, 5, 1415–1461, 2009 Norway spruce has been a neglected species in Fennoscan- dian dendroclimatology aiming at millennial-scale climate reconstructions. Presently, efforts are being made to create a long Norway spruce tree-ring width and maximum Dendroclimatology in late wood density chronology from the central Scandinavian mountains. New data from Jamtland¨ includes a more than Fennoscandia 1000–year long tree-ring width chronology (Gunnarsson unpublished) which gives a unique opportunity to enhance climate change signal by multi- analysis of coniferous trees H. W. Linderholm et al. and better assess the impact of climate variability on forest ecosystems in the region.

Title Page 10.3 Teleconnections

Abstract Introduction Teleconnections, i.e. strong statistical relationships between weather in different parts of the world, in the climate system are associated with the large-scale circulation of the atmo- Conclusions References sphere and the oceans. In the atmosphere, climate signals can be transferred to regions far removed from the physical Tables Figures source of the variability, while in the oceans teleconnections are associated with the global thermohaline circulation (e.g., Chase et al., 2005). Using high-resolution climate proxies such as tree rings, the spatiotemporal stability of observed J I climate patterns and associated teleconnections can be studied. As an example, in a study of two millennium-long J I tree-ring records from Jamtland¨ and Tibet, Linderholm and Brauning¨ (2006) noted that both records showed similar evo- Fig. 6. A comparison of the regionalFig. 6. A comparison signal of in the pine regional and signal spruce in pine and chronologies spruce for Fennoscandia. Back Close lution in tree-growth variability, on decadal and longer time chronologies for Fennoscandia. (On top), spatial correlations of scales, between c. 1100(On and top), 1550, butspatial around 1550correlations the two a of regional a regional TRW spruce TRWchronology spruce consisting chronology of 1 chronology of consisting of 1 chronology ◦ ◦ chronologies diverge,of possibly 20 series due to a strengthening from Arjeplog of the Lat.20 series 66 from◦ N, Arjeplog Long. Lat. 17 66◦ N,E, Long. in Northern 17 E, in Northern Fennoscandia against June– Full Screen / Esc Asian monsoon. Feng and Hu (2008) suggested that Tem- Fennoscandia against June-July temperature(CRU TS2.1) (Mitchell ◦ peratures on the TibetanJuly plateau, temperature(CRU which affects the strength TS2.1)et al.,(Mitchell 2004) from et theal. area, above2004 Lat.) from 63◦ N, during the area the period above Lat. 63 N, during the of the Asian monsoon,period are associated 1901–1980 with the Atlantic (Bj Multi-orklund¨ 1901-1980, 2009 (Bj).orklund,¨ (On the 2009). bottom), (On the bottom), spatial spatial correlations correla- between a regional decadal Oscillation, i.e. sea surface temperature variability in tions between a regional TRW pine chronology, averaged from 7 Printer-friendly Version the Atlantic Ocean (Kerr,TRW 2000). pine The AMO chronology, affects climate averaged in chronologies from from 7 Northern chronologies Fennoscandia, and from July temperatures Northern Fennoscandia, and July (CRU TS2.1) (Mitchell et al., 2004) from the area above Lat. 63◦ N, the North Atlantic region (Sutton and Hodson, 2005; Knight ◦ temperatures (CRU TS2.1) (duringMitchell the period et 1901-1980 al., 2004 (Bjorklund,¨ ) from 2009) the area above Lat. 63 N, during the Interactive Discussion et al., 2005), and this may account for the long term associations in tree growth betweenperiod Jamtland¨ 1901–1980 and Tibet. Moreover,(Bjorklund¨ , 2009). preliminary results suggest links between the summer NAO, which also is associated with the AMO (Folland et al., 2009), 10.4 Combining tree-ring data and climate models and climate in south-east Asia (Linderholm in prep., 2009). This may be related to teleconnections between the NAO and By virtue of great advances in climate modelling science for the East Asian jet (Branstator, 2002), where the East Asian recent a few decades, understanding1461 of climate change has jet the strength of the East Asian jet stream affects weather been greatly improved. As well as to simulate present-day downstream in East Asia (Yang et al., 2002). A new 2.5 mil- climate realistically and predict future climate change ex- lennia long tree-ring record from Tibet (Liu et al., 2009) will actly, a challenging topic of climate modelling is to simulate facilitate the study of possible teleconnections between Asia climate variation in the past. This enables us to better esti- and Fennoscandia during the late Holocene. mate the sensitivity of climate system to the external forcings