Tree Rings As Sensitive Proxies of Past Climate Change

Home , Common year, Dendroclimatology, Keith Briffa

Tree rings as sensitive proxies of past climate change

Håkan Grudd

Department of Physical Geography and Quaternary Geology Stockhom University Stockholm 2006 © Håkan Grudd 2006 ISSN 1653-7211 ISBN 91-7155-251-0 pages 1-17

Paper I ©Arnold 2002; Paper III ©American Geophysical Union 2000; Paper IV ©Macmillan Publishers Ltd 2001

Layout: Håkan Grudd Cover: Tjuoltadalen, Kvikkjokk Printed by PintCenter US-AB ii Doctoral dissertation 2006 Håkan Grudd Department of Physical Geography and Quaternary Geology Stockholm University

Abstract In the boreal forests of the Northern Hemisphere, time series of tree-ring width (TRW) and maximum density in the latewood (MXD) are highly correlated to local instrumental summer-temperature data and are thus widely used as proxies of temperature in high-resolution reconstructions of past climates. Hence, much of our present knowledge about temperature variability in the last millennium is based on tree-rings. However, many tree-ring records have a lack of data in the most recent decades, which severely hampers our ability to place the recent temperature increase in a longer-timescale perspective of natural variability. The main objective of this thesis is to update and extend the Torneträsk TRW and MXD records in northern Sweden by collecting new samples. Lo- cal instrumental climate-data is used to calibrate the new tree-ring records. The results show that TRW is mainly forced by temperature in the early growing season (June/July) while MXD has a wider response window (June – August) and has a higher correlation to temperature. Two reconstructions of summer temperature are made for (i) the last 7,400 years based on TRW, and (ii) the last 1,500 years based on a combination of TRW and MXD. The reconstructions show natural variability on timescales from years to several centuries. The 20th century does not stand out as a notably warm period in the long timescale perspective. A medieval period from AD 900 – 1100 is markedly warmer than the 20th century. The environmental impact from a large explosive volcanic eruption in 1628/1627 BC is analysed in the tree rings of 14C-dated bog pines in south- central Sweden and in absolutely-dated subfossil pines from Torneträsk. The results show an impact in the southern site at approximately this time but no detectable impact in the North. Hence, the results indicate that the effect is confined to more southerly areas. Subfossil trees of Fitzroya cupressoides in southern Chile were 14C-dated to approx. 50,000 years BP and amalgamated into a 1,229-year TRW chronology. This tree-ring record is the oldest in the world. The variability in this Late-glacial chronology is similar to the variability in present-day living trees of the same species. These results suggest that the growth–forcing mechanisms 50,000 years ago were similar to those at present.

Keywords: Tree ring, width, density, climate, temperature, change, variability, sensitivity, Torneträsk, Tornetrask, Sweden, Fennoscandia, explosive, volcanic, eruption, Santorini, Thera, subfossil, Fitzroya, Chile.

iii iv Thesis Contents This doctoral thesis consists of a Summary and four appended Papers which will be referred to in the text by their respective Roman numerals.

List of Papers I. Grudd, H., Briffa, K.R., Karlén, W., Bartholin, T.S., Jones, P.D. and Kromer, B. 2002. A 7400-year tree-ring chronology in northern Swedish Lapland: natural climatic variability expressed on annual to millennial timescales. The Holocene, 12, 657-665. Reprinted by permission from Hodder Arnold Journals.

II. Grudd, H. Manuscript. Torneträsk tree-ring width and density AD 500 – 2004: A test of climatic sensitivity and a new 1500-year reconstruction of north Fennos- candian summers. To be submitted before the defence

III. Grudd, H., Briffa, K.R., Gunnarson, B.E. and Linderholm, H.W. 2000. Swedish tree rings provide new evidence in support of a major, widespread environmental disruption in 1628 BC. Geophysical Research Letters, 27(18), 2957-2960. Reprinted by permission from American Geophysical Union.

IV. Roig, F.A., Le-Quesne, C., Boninsegna, J.A., Briffa, K.R., Lara, A., Grudd, H., Jones, P.D. and Villagran, C. 2001. Climate variability 50,000 years ago in mid- latitude Chile as reconstructed from tree rings. Nature, 410, 567-570. Reprinted by permission from Macmillan Publishers Ltd.

The co-authorship of Papers I – IV I. This paper was planned in collaboration with Wibjörn Karlén and Thomas Batho- lin. They also collected a majority of the samples and made most of the ring-width measurements. I made 33% of the ring-width measurements, about 75% of the analyses and 100% of the writing after comprehensive discussions with Keith Briffa.

II. I made 100% of the work in this paper.

III. I planned and outlined this study together with Keith Briffa. I collected the samples together with Björn Gunnarson and Hans Linderholm. Björn Gunnarson did the major part of the ring-width measurements and crossdating. I made the Wiggle match and all statistical analyses for this paper. I did 100% of the writing.

IV. I initiated this project by making the first collection of samples, the first ring width measurements and the first analyses. Fidel Roig expanded the project and made all the writing.

v Contents of the Summary

Introduction ...... 1

Objectives ...... 2

Field areas ...... 3

Materials and methods ...... 4

Results ...... 7

General conclusions ...... 11

Future perspectives ...... 12

Acknowledgements...... 13

References ...... 14

vi Introduction et al., 2002; Nicolussi & Schießling, 2002; Friedrich In the last century, our planet has experienced a cli- et al., 2004; Kirchhefer, 2005; Linderholm & Gun- matic change that is partly a result of natural vari- narson, 2005; Luckman & Wilson, 2005; Boswijk ations in the Earth’s systems and partly attributed et al., 2006). to Man’s alteration of the geosphere, biosphere and In northern Fennoscandia, Scots pine (Pinus syl- atmosphere (IPCC, 2001). Natural climate change vestris) can attain ages of more than 600 years (Engel- operates on all timescales, it has occurred in the past mark & Hofgaard, 1985) while subfossil pine-wood and it will continue to occur in the future, while the can be preserved on dry grounds and in cold-water anthropogenic influence on global climate is related lakes and sediments for thousands of years (Karlén, to expanding human activities after about AD 1800. 1976; Eronen et al., 2002; Kullman, 2005). Using To predict future climate change we use complex such materials from Torneträsk in northern Sweden, computer models that are calibrated to emulate the Bartholin and Karlén (1983) established a continu- Earth’s climate system in the instrumental period ous and absolutely dated tree-ring width (TRW) of the last c. 150 years and it is only when both chronology for the period AD 436 to 1981. Small natural and anthropogenic factors are included in lakes in the area provided a rich source for subfos- the models that they are able to accurately simulate sil pine and radiocarbon dating showed that these the patterns of climatic variability over the entire samples were spread out over most of the Holocene calibration period and especially the warming in the period. Many of the older subfossil samples could late-20th century (Mitchell et al., 2001). However, be individually cross matched and amalgamated into the climate models still have large uncertainties in several chronologies that were dated by radiocarbon key factors and to improve our understanding of but otherwise “floating” in time (Bartholin, 1987). natural climate variability, the relatively short in- Using a selection of 65 absolutely dated samples, strumental period that is used for model calibration Schweingruber et al. (1988) succeeded in producing must be extended into the past by using high-resolu- a chronology of maximum latewood density (MXD) tion proxy data that can capture the magnitude and which covered the period AD 445 to 1980. This rate of temperature change (Osborn & Briffa, 2004; MXD record was, to date, the longest density record von Storch et al., 2004). in the world. Trees in temperate zones of the Earth offer an Temperatures in this northern region are repre- excellent opportunity for developing networks of sentative of a large area in Fennoscandia (Gouirand high-resolution proxy data. In areas where the an- et al. ; Holmgren & Tjus, 1996; Alexandersson, nual growth is limited by the amplitude in warm- 2002) and the absolutely dated TRW and MXD season temperature a new layer of wood is formed chronologies from Torneträsk have been used in re- each year – a “tree ring” which has specific physi- constructions of regional summer temperature for cal and chemical characteristics, e.g. ring width, the past 1,400-years (Briffa et al., 1990; Briffa et al., maximum density or isotopic composition and by 1992). The Torneträsk record has also been widely measuring such characteristics in consecutive rings used in high-resolution large-scale reconstructions it is possible to produce annually resolved time se- of Northern Hemisphere temperature (Jones et al., ries that are correlated to the mean temperature of 1998; D’Arrigo et al., 1999; Mann et al., 1999; Briffa, the growing season (Fritts, 1976; Schweingruber, 2000; Briffa et al., 2002a, 2002b; Esper et al., 2002; 1988; McCarroll et al., 2003). Carefully replicated Mann & Jones, 2003; Cook et al., 2004; Luterbach- measurement series from many trees in an area can er et al., 2004; Moberg et al., 2005). However, the then be averaged, calendar year by calendar year, to lack of data after AD 1980 in the Torneträsk records construct a mean time series (a “chronology”) that hampers the calibration with instrumental data. This can be used for climate reconstruction and that will lack of data in the most recent period is a common extend into the past for as long as there is sufficient problem in many tree-ring records throughout the material. Some tree species can attain very high ages, Northern Hemisphere (Briffa et al., 2004). Further- e.g. Fitzroya cupressoides (alerce) in southern Chile more, there is increasing evidence for a change in the can live for more than 3,500 years (Lara & Villa- sensitivity of tree growth to temperature in the most lba, 1993). In other areas, there may be an abundant recent period (Jacoby & D’Arrigo, 1995; Briffa et supply of dead (“subfossil”) wood that can be used al., 1998b; Briffa et al., 1998c; Briffa et al., 2004; to extend the living-tree series to past times (Eronen Waterhouse et al., 2004; Wilmking et al., 2004; Dr-

1 Håkan Grudd

iscoll et al., 2005; Wilmking et al., 2005; D’Arrigo growing under Last Glacial (interstadial) conditions et al., 2006). The phenomenon is not fully under- with trees growing under Late Holocene (stadial) stood and the evidence is sometimes contradictory climatic conditions and to draw conclusions about (Myneni et al., 1997; Briffa et al., 2004; Waterhouse the robustness of climatic growth-forcing processes et al., 2004). A similar change in sensitivity has been through time. reported also for the Torneträsk data (Briffa et al., 1992). Hence, there is an urgent need to update the Torneträsk and other TRW and MXD records and Objectives to analyse the data for recent changes in sensitivity The objectives of this thesis are: to climate. Large explosive volcanic eruptions can emit 1. to construct a continuous tree-ring width (TRW) massive amounts of aerosols into the stratosphere. record from Torneträsk that covers most of the This creates a dust veil that will reduce solar energy Holocene period (Paper I), and cool down the Earth’s climate for a period of 2. to update the existing record of maximum late- 1-4 years (Angell & Korshover, 1985; Rampino et wood density (MXD) at Torneträsk (Paper II), al., 1988). Reconstructing the history of explosive 3. to reconstruct the past climate of northern Swe- volcanic eruptions is, therefore, important for our den using the updated and extended TRW and understanding of past natural climate variability. MXD chronologies from Torneträsk (Papers I Tree-ring chronologies have annual resolution and and II), hence offer the ability to detect short-lived extreme 4. to analyse tree-ring data from the 17th century events such as explosive volcanic eruptions in the BC in south/central Sweden for extreme events past (Scuderi, 1990; Jones et al., 1995; Briffa et al., (Paper III) and examine possible links to the Tor- 1998a). The relatively short-lived nature of these neträsk data, stratospheric dust veils also means that the cooling 5. to create a floating TRW chronology from effect may have large regional differences that can 50,000-year old Fitzroya cupressoides in south- be studied within a network of tree-ring sites. The ern Chile and use it in a comparison with the cataclysmic volcanic eruption of Santorini (Thera) in present-day tree growth (Paper IV). the Mediterranean in the 17th century BC seems to have affected a large part of the mid-latitudes of the

Northern Hemisphere (LaMarche & Hirschboeck, -90˚ -60˚ -30˚ 0˚ 30˚ 60˚ 1984; Hammer et al., 1987; Baillie & Munroe, 1988; Baillie, 1990; Zielinski et al., 1994; Kuniholm et al., 1996; Clausen et al., 1997). In Sweden, tree-ring Torneträsk data from this period now exist from Torneträsk in Hanveds- mossen the north (Paper I) and from Hanvedsmossen in the 60˚ 60˚ south part of the country (Gunnarson, 1999) thus offering an opportunity to study this Bronze Age event at higher latitudes. When tree-ring reconstructions extend far into 30˚ 30˚ the past, we have to assume that the current relation between growth forcing processes and patterns of tree growth, established in the “modern” 0˚ 0˚ calibration period, is valid also for the past. This is commonly referred to as the Uniformitarian Princi- ple in dendroclimatology and we usually have very -30˚ -30˚ few possibilities to quantitatively check the validity Pelluco of these assumptions. It does, however, emphasize the importance of data quality in the calibration km 0 10002000 period and data equality in the reconstruction pe- -60˚ -60˚ riod. The discovery of c. 50,000 year old subfossil -90˚ -60˚ -30˚ 0˚ 30˚ 60˚ Fitzroya cupressoides in the south of Chile (Paper Figure 1. Map of field areas. IV) presents a unique possibility to compare trees

2 Tree Rings as Sensitive Proxies of Past Climate Change

Field areas The field work for this thesis was carried out at three sites: Torneträsk in northern Swedish Lapland, Hanvedsmossen in the southeast of Sweden, and Pel- luco in the Lake District of southern Chile (Fig. 1). The Torneträsk site represents an area of about 100x100 km centred on the eastern side of Lake Torneträsk (19.45 E; 68.15 N) (Papers I and II). The area is characterized by mountains of alpine type, reaching about 1,500 m a.s.l. in the western part of the area and gradually decreasing towards the east. The large Lake Torneträsk cuts through the area at 341 m a.s.l. in the east/west direction. The climate is continental subarctic (Andersson et al., 1996) with July as the warmest month (+12 °C) and a mean an- Figure 2. View of the tree-limit area north of Torneträsk nual amplitude of 22 °C. Annual precipitation is in early September about 300 mm with the highest values usually occurring in July. The treeline is formed by mountain birch (Betula pubescens ssp. tortuosa) at about 700 m a.s.l. (Fig. 2). The pine (Pinus sylvestris) limit is found at an altitude of about 440 m a.s.l. and remnant (subfossil) pine can be found up to about 600 m a.s.l. Living trees were sampled at five different localities close to the pine treeline while subfossil trees were sampled from a larger spatial range of localities, both from within the present day pine forests and from above the present pine limit. The altitude of these intra-site localities varies from 341 up to about 600 m a.s.l. Hanvedsmossen (59° 08’ N; 17° 55’ E) is a peat bog 50 km south of Stockholm where there have been periods of extensive peat mining in the 20th century 2 (Paper III). The bog originally covered about 1 km . Figure 3. View of the remaining part of Hanvedsmossen Today, less than 25% of the peat remains (Fig. 3). and the excavated subfossil trees. The landscape has a low relief where the bog itself has an altitude of c. 60 m a.s.l. and the surrounding areas are just a few tens of metres higher. Climate is continental temperate but with an influence from the more maritime Baltic. July is the warmest month with a mean temperature of +16 °C. The mean amplitude between the warmest month and the coldest month is 19 °C and annual precipitation is 600 mm. The bog is situated in a mosaic of farmlands and forests with pine (Pinus sylvestris) and spruce (Picea abies) as the dominating tree species. Pelluco (41° 30’ S; 72° 45’ W) is a point (punta) on the coastline east/southeast of the town of Puerto Montt in the Lake District of southern Chile (Paper IV). The field site at Punta Pelluco is at sea level with the high mountains of the Andes rising a few tens of Figure 4. The Puerto Montt area in southern Chile. View kilometres to the East (Fig. 4). The data from Pel- towards the Andes.

3 Håkan Grudd

luco are compared to other data from adjacent sites branches. Whole disc samples from living pine trees to the west in the coastal range and to the east in the were obtained at Torneträsk railway station. foothills of the Andes. The area has a maritime tem- Subfossil material was sampled at all three study perate climate which to some degree is influenced by sites: The subfossil material from Torneträsk (Papers the more mediterranean climate to the north. Janu- I and II) originates from two sources: Relic tree-re- ary and February are the warmest months with a mains on dry grounds, e.g. snags, stumps and logs mean temperature of about +15 °C. The mean an- (Fig. 6), and tree-remains recovered from the surface nual amplitude in temperature is about 8 °C. An- sediments of small mountain lakes (Fig. 7). Branch- nual precipitation varies between about 1,200 and es and/or roots could be identified in most of the 3,000 mm with the highest values in winter. The dry material and in about 50% of the lake mate- native forest type in this area is the Valdivian rain rial. The subfossil material is in a varying state of forest, which is one of a small number of temperate degradation. For the dry material this degradation rain forests in the world. The Valdivian forest is a is more related to resin content than to age, whereas mixed type including broadleaf and coniferous trees. for the wet material it is clearly related to age. The It includes stands of huge trees, especially Fitzroya dry material is rich in resins, in contrast to the lake cupressoides, which can live to a great age. This material where resins are washed out completely in rain-forest type is, as so many others, endangered wood older than about 3,000 years. Subfossil trees by extensive logging and it has been reduced by ap- are not present in all lakes and I have used a vari- proximately half already. ety of methods to locate and pull out the material, including helicopters and divers. The subfossil material was sampled with a chainsaw to obtain a disc Materials and methods or a wedge from the stem. I tried to sample disks or wedges where I could get a minimum of two radii Living pine trees (Fig. 5) were sampled at the Tor- from each stem. However, this was sometimes dif- neträsk site (Papers I and II) using 5 and 10 mm in- ficult since the subfossil trees were too degraded. It crement borers. A minimum of two cores were sam- was in some cases also difficult to determine which pled from each tree. To avoid compression wood part of the stem the samples represent since roots and other types of “disturbed” growth within the and branches were missing and the remaining trunk tree, I excluded injured trees and sampling from of wood was, at best, semi-circular. the up and down hill side of trees growing on steep At Hanvedsmossen (Paper III), logs and stumps slopes. I also strived to sample from a part of the of Pinus sylvestris have been excavated by the peat tree where the stem was straight and without large mining industry and disposed of in large log jams

Figure 5. A 500-year old living Figure 6. Dry subfossil pine at Figure 7. A subfossil pine from pine at Torneträsk. Torneträsk. 6,000-years ago.

4 Tree Rings as Sensitive Proxies of Past Climate Change

(Fig. 3). These mounds of logs provided a conven- of varying thickness. Multiple sec- ient source for the sample material. A few trees were tions were sampled from this rim also sampled in situ in the so called “forest layer” using a chain saw. in the lower section of the bog (Gunnarson, 1999). Ring widths were measured to The material is well preserved with pith and sap- the nearest 1/100 of a millimetre wood remaining in almost all samples. A majority using standard dendrochrono- of the trees had roots preserved. Sample disks were logical equipment (Schweingru- cut from the lower part of the stem of trees using a ber, 1988). The material from chainsaw. Torneträsk (Papers I and II) was In 1960, an earthquake in southern Chile shifted measured at laboratories in Lund, the coastline around Puerto Montt several metres Stockholm, and Abisko. The ma- and new, previously protected areas became subject terial from Hanvedsmossen (Paper to ocean erosion. At Punta Pelluco (Paper IV), well III) was measured at the tree-ring preserved remains of Fitzroya cupressoides (alerce) laboratory of Stockholm Universi- eventually became exposed in the inter-tidal zone ty. The subfossil material from Pel- (Fig. 8). The trees appear to be in situ, i.e. trunks luco (Paper IV) was measured at are standing upright and are probably still rooted at the tree-ring laboratory of Stock- some unknown depth. They are embedded in a sedi- holm University and at collaborat- ment composed of volcaniclastic materials (lahar) in- ing laboratories in Mendoza (Ar- dicating that the trees died in one single catastrophic gentina), and Valdivia (Chile). event. Earlier work have indicated that these trees Maximum density in the late- could be 50,000 years old (Klohn, 1976; Heusser, wood was measured at the tree- 1981; Porter, 1981). The trees are inundated by the ring laboratory of Abisko Scientif- ocean at high tide and thus only accessible at low ic Research Station using an Itrax tide. The centre of the protruding tree stems have WoodScanner (Lindeberg, 2004). Figure 9. in most cases been eroded, leaving a rim of wood The sample preparation required X-ray image for this system is identical to the preparation that was used by Schweingruber et al. (1988) when they produced the first density data for Torneträsk. This includes cutting laths from the samples using a twin-blade saw, followed by resin extraction in a Soxhlet apparatus (Schweingruber et al., 1978). The WoodScanner uses X-rays to produce digital radiographs (Fig. 9) that were subsequently analysed in the commercial software WinDendro. The year-to-year variation in tree-ring width in all individual sample series from each site was cross matched against all other series from within the same site. This standard procedure in dendrochronology is termed “crossdating” and gives all individual tree rings a year-datum relative to all other tree rings at that site (Schweingruber, 1988). By averaging all data year by year it is then possible to construct a time series of tree-ring data, a chronology. Cross- dating is also a method to check the data for missing rings and measurement error. The quality of the crossdating, as well as a number of statistical properties of the data, was determined by using the software Cofecha (Holmes, 1983). Figure 8. Punta Pelluco with 50,000-year old subfossil Fitzroya cupressoides. A tree-ring chronology can be absolutely dated as long as there are tree rings of known age (i.e. from

5 Håkan Grudd

living trees) included in the crossdating process. This al-to-decadal timescale signals can thus be empha- is the case for the Torneträsk data (Papers I and II). sized by fitting a flexible spline to the data that has In the data from Hanvedsmossen and Pelluco (Pa- a frequency cut-off length proportional to the trends pers III and IV) there are no tree rings of known age that need to be removed. The residuals or the ra- included (e.g. all samples are subfossil) and, hence, tio between the observed value and the spline-curve the precise calendar age can not be determined by value for each year will give a new “index” value for crossdating. However, all included ring-width meas- each year. Variability on longer timescales (decadal- urements can still be crossdated on a timescale that to-centennial) may differ more between trees within is rigid but “floating” in time. The approximate a site because growth variations on these timescales calendar age of a floating chronology can then be are more dictated by the status of individual trees at determined by 14C-dating. This was done in the ini- any specific time. One of the main factors effecting tial stages of chronology building in the Torneträsk the status of a tree is its age (Fritts, 1976; Schwe- material (Paper I) and in the material from Hanv- ingruber, 1988; Cook & Kairiukstis, 1990). Hence, edsmossen (Paper III) and Pelluco (Paper IV). In Pa- removing the “age-effect” from tree-ring data is cen- pers I and III, I also used the technique of “wiggle tral when climate, i.e. external low-frequency forc- matching” a number of 14C determinations with a ing of tree growth, is the objective of interest. This known separation in “dendro-years” to get a higher is also the reason why we need large numbers of precision in the radiocarbon dating. replicate series from a site to bring out the common The year-to-year variability in ring width is a very long-term variability in a set of tree-ring data. It is, strong common feature among all trees (young and however, problematic to remove un-wanted trends old) within a site and sometimes between sites over on time scales equal to or longer than the length of very large areas. This common year-to-year growth individual segments (tree series) without also remov- pattern is governed by the annual variation in cli- ing a large part of the signal of interest. This prob- mate and it is the foundation for all dendrochrono- lem is known as the “segment-length curse” (Cook logical crossdating. On longer timescales, however, et al., 1995). Attempts to circumvent this problem other slowly evolving biological and environmental are usually made by making a priori assumptions of factors superimpose the climatic forcing. Hence, the shape of the age-related curves, e.g. negative ex- tree-ring series are commonly treated with a filtering ponential (Cook et al., 1990). The curves can also technique known as “standardization” and designed be empirically determined using the Regional Curve to remove unwanted “noise” or trend in the data Standardization (RCS) approach (Erlandsson, 1936; series, leaving the signal of interest (Fig. 10). Annu-

Figure 10. Left hand panels show an example of the effect of Regional Curve Standardization. The right hand panels illustrate how the more flexible Spline standardization method removes a larger portion of the low-frequency variation in the ring-width data.

6 Tree Rings as Sensitive Proxies of Past Climate Change

Mitchel, 1967; Briffa et al., 1992; Esper et al., 2003; Results D’Arrigo et al., 2006). The following is a short summary of the four papers In Papers I and II, the climatic signals in the tree- included in this thesis. ring data are used to reconstruct past climate change on short-to-long term timescales. The high-frequency climatic component in the data is preserved by Paper I using standardization with flexible splines. In Paper Grudd, H., Briffa, K.R., Karlén, W., Bartholin, T.S., I this method also involves autoregressive modelling Jones, P.D. and Kromer, B. 2002. A 7400-year (Cook, 1985) to account for autocorrelation in the tree-ring chronology in northern Swedish Lap- ring width data. The low-frequency climatic com- land: natural climatic variability expressed on ponent in the Torneträsk tree-ring data is preserved annual to millennial timescales. The Holocene, by using RCS standardization. Hence, the different 12, 657-665. standardization techniques produce various index The aim of this study is to build a continuous ring- chronologies which show climatic variability on width chronology from living and subfossil pine short and long timescales. These chronologies are (Pinus sylvestris) that covers most of the Holocene used together with overlapping instrumental data period in northern Sweden, and to use this chronol- from Abisko Scientific Research Station (Andersson ogy to infer changes in the past climate. The study is et al., 1996) in a response function analysis to find a continuation of previous work, which resulted in the strength and direction of climatic forcing on tree a continuous ring width chronology from AD 445 growth (Fritts, 1976). Once this is established, the – 1980 and with older material amalgamated into regression equations are reversed and the climatic three floating chronologies, approximately dated by variable, in this case the mean summer temperature, 14C. is reconstructed for the full length of the chronol- New field work was carried out with a focus on ogy. finding material to fill in the gaps between the float- In Paper III, the focus is on detecting severe ing chronologies. A persistent gap in the first three growth conditions on timescales much shorter than centuries BC proved to be most difficult to bridge. the lifetime of individual trees. The data are, there- Hundreds of new samples were needed before the fore, standardized to show growth changes on yearly last remaining gap could be bridged. Six new 14C to decadal timescales. This is done in two steps: first determinations aided in this process. Three of these a transformation of the ring-width data into loga- 14C determinations were from the same tree and rithms to normalise the effect from high variability were used in a wiggle-match exercise to improve in the juvenile phase of tree growth. The second step the radiocarbon dating. All 14C dates could be veri- is a filtering of each individual series with a flexible fied by subsequent dendrochronological crossdat- smoothing spline function in order to remove lowing. Hence, the ring-width data from 880 individual frequency variation on timescales longer than 50- trees could be amalgamated into a continuous and years. The index chronology constructed from the precisely dated 7,400-year chronology. At present, standardized data series is then transformed to Z- this is the longest tree-ring record in Sweden and one scores, i.e. with mean = 0 and a normalized standard of the longest in the world. deviation. Values outside 3 standard deviations from Non-climatic trends in the data were removed the mean are considered as major anomalies. by RCS and spline standardisation, thus producing In Paper IV, the tree-growth variability in an an- two chronologies that show climatic forcing on tree cient (subfossil) population of Fitzroya cupressoides growth on annual to century timescales. A response is compared to the variability in modern (living) function analysis showed high positive correlation populations of the same species. Tree-ring widths in between ring width and summer temperature over both the ancient and the modern series are standard- a common period (1869-1997) and the regression ized using a spline with a fixed 50% frequency re- weights from this period were used to reconstruct sponse of 200 years, which preserves variability on a record of summer temperature for the last 7,400 timescales up to a few centuries. Spectral properties years. The record displays significant temperature in the subfossil index chronology are then examined changes on timescales from years to several cen- using Fourier analysis and compared to the cyclic turies, which is representative for a large area in behaviour of tree growth in modern chronologies of northern Sweden, possibly also for a wider area. The the same length.

7 Håkan Grudd

confidence limits around the expressed temperature trend over the last 200 years is adjusted to fit with changes are largest in the pre-Christian period re- the “expected” TRW values as calculated from a re- flecting the much lower sample replication for this gression with MXD data. Reconstruction 4 uses the period. RCS standardized MXD chronology and the spline standardized TRW chronology. Thus, reconstruc- Paper II tion 4 is based on the strong high-to-low frequency temperature signal in the MXD data and the high- Grudd, H. Manuscript. Torneträsk tree-ring width frequency signal in TRW data (where the bias in the and density AD 500 – 2004: A test of climatic last two centuries has been filtered out by the spline sensitivity and a new 1500-year reconstruction standardization). Reconstruction 5 uses the two of north Fennoscandian summers. (to be Submit- spline standardized chronologies of MXD and TRW ted before defence) and, thus, retains only the high-frequency climatic The objective of this paper is to update the exist- variation. ing tree-ring width (TRW) and maximum latewood The results show that the previously reported density (MXD) chronologies at Torneträsk, north- loss in sensitivity of MXD to temperature in the late ern Sweden, to analyse the data for recent changes 20th century was an artefact of the age structure in sensitivity of tree growth to climate, and to make in the data. The reconstructed temperatures show a new multi-proxy reconstruction of summer tem- higher values in the pre-instrumental period as com- perature for the last 1,500 years. pared to earlier temperature reconstructions based Existing Torneträsk TRW and MXD data are upon Torneträsk TRW and MXD data. The Medieval dated to AD 2004 using samples from 35 living trees. Warm Period around AD 1000 is notably warmer The new data are derived from a mixture of young than temperatures in the 20th century. Other nota- and old trees. This has significantly changed the age bly warm but shorter periods occur around AD 750, structure of the MXD data in the 20th century and 1400 and 1750. The coldest temperatures occur the previously reported decline in density after about around AD 1900, which is consistent with the maxi- AD 1950 is no longer present in the data. The data mum extent of glaciers in Swedish Lapland in the were standardized using both RCS and spline stand- last 1,500 years. ardization to produce various index chronologies of TRW and MXD. These chronologies are then used Paper III in a multiple regression and reconstruction of the Grudd, H., Briffa, K.R., Gunnarson, B.E. and Lind- summer climate for the last 1,500 years. Response erholm, H.W. 2000: Swedish tree rings provide function analyses shows that tree growth is forced by new evidence in support of a major, widespread temperatures during June – August and that MXD is environmental disruption in 1628 BC. Geophysi- a much stronger proxy for temperature than TRW. cal Research Letters, 27(18), 2957-2960. The new MXD data show no indication of a change in sensitivity of tree growth to tempera- Large quantities of subfossil pine logs have been un- ture in the recent period. However, TRW show a earthed in conjunction with peat mining at Hanv- positively diverging trend to the instrumental data edsmossen c. 50 km south of Stockholm. This that can be understood as an increased sensitivity ancient forest is 14C-dated to the 16th and 17th to temperature. A comparison of the TRW and the centuries BC. Hence, the trees are contemporaneous MXD chronologies shows that this diverging trend with a major pre-historic frost event in 1627/1628 most likely started around AD 1800. This incorpo- BC, documented in the tree rings of bristlecone pine rates a bias in the calibration equations that affects in the western USA and in Irish, English, and Ger- the long-term trend over the reconstruction period. man oaks. A very large growth anomaly in tree-rings Therefore, the reconstruction is made in five “steps”, from Anatolia (Turkey) has also been dated to this producing five time series of temperature variation time. This environmental disruption, possibly a ma- for the last 1,500 years. In the first reconstruction jor volcanic eruption in 1628 BC, had widespread only raw data are used (i.e. without any standardi- climatic effects in the mid-latitude regions of the zation or statistical pre-treatment). Reconstruction Northern Hemisphere and it has been tentatively at- 2 uses the RCS standardized MXD and TRW chro- tributed to the cataclysmic Bronze Age eruption of nologies. In reconstruction 3, a “correction” is made Santorini (Thera) in the Aegean Sea. In this paper, to the RCS standardized TRW chronology where the our aims were twofold: 1) to increase the precision

8 Tree Rings as Sensitive Proxies of Past Climate Change

of the radiocarbon dating of the tree material from Our results reinforce the evidence of a major Hanvedsmossen by using the technique of “wiggle environmental event around 1628 BC, probably a matching” and 2) to analyse the tree ring widths for severe cooling of the climate that lasted for 3 – 4 extreme events, i.e. abrupt short term changes that years, and extends the geographical area of effect to could be related to large volcanic explosive erup- south-central Sweden. The 1628 BC tree-ring event tions in the Northern Hemisphere. has the signature of a cataclysmic volcanic eruption Six 14C determinations, each representing a and Santorini is the largest known explosive erup- defined number of tree rings and with a precisely tion in the 17th century BC. known separation in years, were wiggle matched to the radiocarbon calibration curve. The wiggle match Paper IV shows that the trees date to the period 1695 – 1496 Roig, F.A., Le-Quesne, C., Boninsegna, J.A., Briffa, BC with an error estimate of ±65 years on the 95% K.R., Lara, A., Grudd, H., Jones, P.D. and Villa- significance level (±23 years with p=68%). gran, C. 2001. Climate variability 50,000 years One major growth anomaly can be identified ago in mid-latitude Chile as reconstructed from where four consecutive years have Z-scores below tree rings. Nature, 410, 567-570. -3, the two most extreme, at 1637 and 1636BC (±65 years) have Z-scores around -4. Hence, this anoma- The discovery of c. 50 000 year old Fitzroya cupres- lous event seems to be contemporaneous with the soides (alerce) in southern Chile presents a unique growth anomaly starting in 1628 BC in central Eu- opportunity to study climatic variability in a time rope and the British Isles, and with the severe frost period from which very few annually resolved proxy event in 1627 BC recorded in the White Mountains records exist. Hence, the objectives for this study are of California. to construct a time series of ring-width variation for In our discussion, we argue that the unusual these trees using dendrochronological techniques, to growth anomaly in our data is caused by an explo- date the trees by using 14C AMS, and to compare the sive volcanic eruption. Our data show a distinct 4- spectral properties of ring-width variation in these year reduction in growth, presumably because of ice-age trees with modern (living) Fitzroya cupres- severe cooling. Large explosive volcanic eruptions soides from the same area. in historic times have caused detectable cooling in The materials have been collected by an inter- the instrumental records over a period of 1-3 years, national team and ring widths were independently even on temperatures averaged across the Northern measured and crossdated by tree-ring laboratories Hemisphere. Environmentally sensitive trees from a in Sweden, Chile and Argentina. A total of 30 trees wide range of geographical areas have been shown were sampled at Punta Pelluco. Long term trends in to display this cooling effect, caused by increased the data associated with tree age and forest dynam- amounts of aerosols in the stratosphere which re- ics were removed from the individual series using duce solar radiation to the Earth. spline standardization. From this standardized data Even though there is circumstantial evidence that a 1229-year index chronology was compiled. To our trees in Europe and North America were severely ef- knowledge, this is the oldest tree-ring chronology fected by a short but very cold climatic event caused developed to date. by explosive volcanism somewhere in the North- Three wood samples, each containing five rings ern Hemisphere in 1628/1627 BC, the attribution were dated with 14C AMS and gave consistent dates to a specific volcano is problematic since tree rings of around 50,000 years BP. This age is in agreement record a secondary climatic effect. Chemical analy- with previous 14C dates from Punta Pelluco. How- ses of volcanic ash accumulated in the Greenland ice ever, the high 14C age is also on the very limit of the sheet could potentially identify a specific source for radiocarbon method and the dates must therefore the eruption. Indeed, the ice cores from GRIP, GISP2 be taken as a minimum age of the subfossil wood. and Dye3 have several layers in the 17th century BC A date of c. 50,000 years BP places the subfossil that can be attributed to volcanic eruptions (major Fitzroya in the early part of the middle Llanquihue peaks are at: GRIP 1636 BC; GISP2 1623 and 1695 interstadial. This relatively warm period was brack- BC; Dye3 1644 BC). However, the uncertainty in eted by the two major glacier advances: Llanquihue the dates of these layers makes it difficult to corre- I (c. 70-65 kyr BP) and Llanquihue II (28-18 kyr late them to events in the absolutely dated tree-ring BP). The Fitzroya is restricted to wet environments records in the British Isles and continental Europe. and it is a sensitive indicator of climate, primarily

9 Håkan Grudd

responding to variations in summer (December to ly 1.5°C (using a lapse rate of 0.65°C/100 m). Trees March) temperatures. Hence, we interpret the sub- at the extreme altitudes will have different growth fossil Fitzroya to have established during an interval responses to temperature. Hence, mixing data from of humid and cool climate. this rather wide altitude range will increase the error Spectral analyses were performed on the subfos- bars around the reconstructed temperatures. This is sil Pelluco chronology and on a set of three millen- one reason for the generally depressed temperatures nium-long living Fitzroya chronologies for a similar in the early part of the 7,400-year reconstruction in time span and standardized in a similar way as the Paper I. subfossil chronology. Our results suggest that com- This problem can, possibly, be circumvented by parable cycles in the radial growth of Fitzroya oc- using specific Regional Curves for different altitude curred between an interstadial of the last glaciation bands in the RCS standardization procedure. At and today. Considering that the Earth’s atmosphere present, however, low sample replication obstructs and the ocean/atmosphere system were significantly the calculation of robust Regional Curves for the different during the glacial cycles of the Late Pleis- most extreme altitudes. tocene, the notable similarity in the spectral prop- Tree-ring data from the 17th century BC exist erties of the Pelluco subfossil and modern Fitzroya from two places in Sweden: Torneträsk in the north cupressoides chronologies suggests that the forcing (Paper I) and Hanvedsmossen in the south (Paper mechanisms from climate were not dramatically al- III). The Torneträsk chronology is dated by den- tered during the interstadial compared to the present drochronological crossdating and is, thus, fixed in interglacial. time while the chronology from Hanvedsmossen is dated by 14C which means that it is floating in time Additional results with an uncertainty of about 65 years (p=95%). The chronology from Hanvedsmossen has a ma- The Torneträsk tree-ring data originate from many jor growth anomaly that was tentatively attributed localities within a relatively large area and with sig- to the cataclysmic volcanic eruption of Santorini nificant differences in altitude. Each data point in (Thera) in the Mediterranean in 1628 BC (Paper Figure 11 represents the altitude of a dendrochrono- III). The corresponding Torneträsk data, treated logically dated subfossil sample. The range between with exactly the same statistical methods, show no the highest and the lowest altitudes is about 225 m, evidence of an event in 1628 BC (Fig. 12). There which represents a temperature difference of rough-

Cal. BP -8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 0 600

550

500 . l . s . a

m 450

400

350

-7000 -6000 -5000 -4000 -3000 -2000 -1000 BC/AD0 1000 2000 Calendar years

Figure 11. Each point represents the altitude (vertical scale) and the age (horisontal scale) for a dendro-dated subfossil sample at Torneträsk.

10 Tree Rings as Sensitive Proxies of Past Climate Change

Years BC The last-glacial trees (Fitzroya cupressoides) from southern Chile were dated with three AMS radio- 1700 1600 1500 carbon determinations which gave consistent ages of about 47,000-52,000 years (Paper IV). These dates

k 14 s are compatible with earlier C dates from this site ä r

t including samples from soil profiles and wood re- e

n mains (Klohn, 1976; Heusser, 1981; Porter, 1981). r n o 14 e In this thesis, two additional C dates are presented T s

s that support these ages (Tab. 1) o

m An age of around 50,000 years is indeed on the s

d limit of the radiocarbon dating method and, hence, e

v must be taken as a minimum age. However, the n

a 51,900 BP date (Tab. 1) resulted from an extremely H long counting time under optimal conditions and 1700 1600 1500 was reported to be a “finite” date (Minze Stuiver, Years BC pers. communication 1994).

Figure 12. The TRW chronology from Hanvedsmossen, with the anomalous growth event lined up at 1628 BC is General conclusions compared to the absolutely dated TRW chronology from • The tree ring width chronology from Torneträsk Torneträsk. Both series are standardized in a similar way. in northern Sweden is absolutely dated and annu- The two series do not crossdate and there is no evidence ally resolved from 5407 BC to AD 2004 and the of a severe growth anomaly in 1628 BC in the Torneträsk strong association with July temperature allows data. the possibility to reconstruct summer temperatures for the last 7,400 years. The large numbers are, however, earlier notable growth reductions in of replicate samples provide narrow confidence the Torneträsk record: at 1633, 1646, and around limits around the reconstructed temperatures in 1665 BC. These events do not correspond to the the last two millennia while the much lower sam- frost event observed in other absolutely dated tree- ple replication in the pre-Christian period results ring chronologies (LaMarche & Hirschboeck, 1984; in significantly larger errors in the early part of Baillie & Munroe, 1988; Baillie, 1990). They could, the record. however, correspond to the less precisely dated acid • Over the last two centuries, tree-ring width shows peaks observed in the Greenland ice cores (GRIP a diverging trend with respect to maximum late- and Dye 3) around 1636 and 1644 BC (Clausen et wood density and temperature, implying an in- al., 1997) and around 1695 BC in GISP2 (Zielinski creasing productivity in response to temperature. et al., 1994). Hence these events could be related to If not attended to, this phenomenon introduces other (undefined) explosive eruptions that affected a bias in the climatic reconstruction, resulting in more northerly areas in the Northern Hemisphere. depressed temperatures in the pre-instrumental period. • The maximum latewood density data from Tor- Table 1. Radiocarbon dates for the Pelluco site. neträsk are continuous from AD 441 to 2004. By 14C date 1σ including data from young trees in the most re- BP probability Lab ID Reference cent period, it is demonstrated that the previously Grudd, this reported reduced sensitivity in density to temper- 45490 -2900 +4600 ST13580 thesis ature in the late 20th century was an artefact from Grudd, this the age structure in the old data. 51900 -1600 +2000 QL4738 thesis • The Torneträsk density record is the longest in the world and with a temperature signal that is exceptionally strong: The correlation to instrumental summer (June – August) temperature is 0.79.

11 Håkan Grudd

• A new multi proxy reconstruction for the last replication will change through time in long tree-ring 1,500 years, based on maximum latewood density records and this will affect the signal-to-noise ratio and ring width, shows significantly higher temper- so that low replication will lead to wide error bars atures in the pre-industrial period as compared to around the reconstructed climate signal. Further- previous reconstructions from Torneträsk. Several more, the age of the trees that constitutes a tree-ring periods during the past 1,500 years were warmer chronology will change through time: Some periods than the 20th century. The most striking feature in the record will be dominated by relatively young is a long and warm medieval period centred on trees while other periods will be dominated by rela- AD 1000. The coldest period occurred around AD tively old trees and the climatic growth response may 1900 which is consistent with evidence of maxi- be very different. For example, in nearly all existing mum glacier expansion over this period. records, data for the most recent period is based on • The severe environmental disruption that caused tree rings that on average have a high cambial age frost rings and growth anomalies in tree rings in as compared to the earlier parts of the record. This North America, Central Europe and Turkey in causes an in-homogeneity in the age structure that 1628 BC probably also affected trees at Hanv- may misrepresent the calibrated growth response edsmossen in the south of Sweden at this time. and lead to spurious reconstruction results. The The lack of evidence in the Torneträsk record widely reported change in the sensitivity of recent suggests that the geographical area of effect in tree-growth to temperature could, potentially, be ex- Sweden is confined to the southern parts of the plained as an artefact from using inadequate data in country. the most recent period. Hence, a major task in the near future will be to update and renovate existing • The example from Hanvedsmossen suggests that tree-ring collections throughout the Northern Hemi- floating tree ring chronologies may be precisely sphere. The new tree-ring data from Torneträsk is calendar dated by using wiggle-matching of 14C- just one step in this work. dates in combination with a teleconnection of ex- Maximum density in the latewood (MXD) is a treme growth events caused by explosive volcanic superior proxy for summer temperature as com- eruptions. pared to ring width (TRW) and, indeed, to most oth- • The floating chronology from Pelluco in south- er climate proxies. It is, however, laborious to obtain 14 ern Chile is C dated to around 50,000 years BP. the MXD data and it requires expensive analytical The error bars for this date are large and the lack equipment. Therefore, a few well equipped labora- of a high-precision radiocarbon calibration curve tories should be supported that have the knowledge from this period makes it impossible to improve and the ambition to act as a resource for the whole the accuracy by means of wiggle-match dating. paleo-climate community. The novel state-of-the-art • The cyclic behaviour of tree growth in the 1,229- X-ray facility at Stockholm University will, undoubt- year floating chronology from Pelluco resembles edly, be one of these laboratories and I have the am- those in modern tree-ring chronologies of the bition to be in the forefront of producing new densi- same species from the same area. This suggests ty data in collaboration with scientists from Europe that the growth-forcing mechanisms from climate and elsewhere. In the near future, new 1,000-year 50,000 years ago were similar to those at present chronologies will be produced from northern Scot- and gives further support for the so called Uni- land; Fennoscandia; and the Alps. The new MXD formitarian Principle relied upon in dendroclima- data will greatly improve our understanding of the tology. climate over the last millennium and place the recent temperature increase in a longer-term perspective of natural variation. A more homogeneous age struc- Future perspectives ture, particularly in the most recent period, and an Our present knowledge about natural climate vari- improved replication of the data, will significantly ability in the last 1,000 years is to a very large extent increase the credibility and reduce the error bars for based on tree-ring proxies. However, many existing reconstructions of past climate, and allow the com- tree-ring records have problems with age structure, puter models to better emulate the natural climate with changing sample replication, and with a lack variation in projections of future climate change. of data in the most recent period. Inevitably, sample The Regional Curve Standardization (RCS) method is capable of preserving a large portion of the

12 Tree Rings as Sensitive Proxies of Past Climate Change

low-frequency climatic variation in tree-ring data. ing a thesis. It was Wibjörn Karlén who, in 1991, However, the method is sensitive to low replication first drew my attention to the fascinating world of and to inhomogeneity in the age composition of the tree rings when we were standing in the backyard data. Hence, the RCS method should be carefully of a small sawmill in southern Chile, looking at a used and reserved for particularly well-replicated, pile of 1,000-year old logs. I already had an interest millennium-long tree-ring records. For other tree- in paleoclimatology, and here I discovered the per- ring records, alternative standardization methods fect tool for studying past climates. Back in Sweden, need to be employed, e.g. Age Band Decomposition Wibjörn and Thomas Bartholin brought me on my (Briffa et al., 2001), or the more advanced approach- first field trip to the pristine north side of Lake Tor- es to the RCS method: Multiple RCS and Size-Ad- neträsk to sample subfossil pine. Wibjörn became justed RCS (Melvin, 2004). However, the basic RCS my supervisor, and Thomas took on my basic train- method is both simple and elegant and, whenever ing in dendrochronology during an intense period possible, dendroclimatologists should strive to im- when I literary lived in the basement of his lab in prove the quality of data rather than elaborate the Lund. Without these two gentlemen, I would never standardization methods. have entered the field of dendroclimatology. Wib- Methodological differences in the development jörn and Thomas, thank you for your support and of local dendroclimatological reconstructions some- friendship over the years! times hampers our ability to assess climate variabil- Keith Briffa from the Climatic Research Unit in ity on a regional scale (Wilson et al., 2005). It is, Norwich, UK, has been involved in my work from therefore, imperative to continue to supply open-ac- the very start. Without his wisdom and help I would cess databases with raw data, e.g. the International probably have drowned in the vast amounts of data. Tree-Ring Data Bank (http://www.ncdc.noaa.gov/ Keith’s enthusiasm and friendship has kept me going paleo/treering.html) and the Dendrochronologi- and this thesis owes much credit to him. I am also cal Database (http://www.wsl.ch/dendro/dendrodb. very grateful to Peter Kuhry, first of all for helping html), and to develop public tools to extract and me structure the thesis and secondly for reading and standardize the data in a consistent way, e.g. AR- commenting on the manuscripts, and for not giving STAN (Cook & Krusic) (http://www.ldeo.columbia. up on me! edu/res/fac/trl/public/publicSoftware.html). I would also like to thank my close colleagues One of the most exiting future prospects is the Björn Gunnarson and Hans Linderholm, especially excellent opportunity to develop a network of multi- for their supportive attitude and helpfulness in my millennial tree-ring records from Norway, Sweden finalizing the thesis work. I am looking forward to and Finland (Fennoscandia) that will cover most of much exiting collaboration with you guys! Björn the Holocene period. I will continue my work in the also helped me with the fieldwork at Torneträsk, Torneträsk area by collecting new samples and I will and so did many other colleagues and friends: Björn, work in close collaboration with colleagues from Mart Nyman and Per Klingbjer spent hours in the northern Norway and Finland where there is ongo- cold lake waters diving for subfossil logs, with me ing work on multi-millennial tree-ring records. yelling from the shores. I am also grateful for the Together with colleagues from Norway and Swe- very important help Mart and Per gave me in the den I will also work on developing two new multi- lab. My good friend Robert Kjessel helped me bring millennial chronologies in south/central Scandina- some of the largest subfossil trees to shore and also via. taught me “safari-style” fieldwork. Another very good friend, Mats Nilsson, came along and taught me how to dive – no more yelling from the shores Acknowledgements from my side! Much of this field work could not have been done First, I would like to express my deep gratitude to without the aid from helicopters. Lars Lidström (Li- my beloved family: To my dear wife Ylva – I could das) is more then just a helicopter pilot! He always not have done this without you! And to my parents kept an eye open for subfossil trees and he discov- for always supporting my choice of life. ered many new exiting sites in the area. Lidas has There are so many colleagues and friends that become a good friend and I thank him for invalu- have helped, inspired and encouraged me during this able help with the “dendrocopter”. Flying out to a long and exciting journey towards, finally, present- remote area and spending a week in tent with good

13 Håkan Grudd

company, lots of good food and interesting fieldwork australis) chronology to 1724 BC. The Holocene, 16(2), is what makes up for the hundreds of hours in front 188-199. of a computer. Thanks, to Thomas Schneider for the week we had in Tjuoltadalen (see front cover), and Briffa, K.R. (2000) Annual climate variability in the Holocene: intepreting the message of ancient trees. to Ninis Rosqvist, Urban Axelsson and Juan Car- Quaternary Science Reviews, 19, 87-105. los Aravena for adventurous fieldwork in Chile. My thoughts also go to the late Dr. Carolina Villagran, Briffa, K.R., Bartholin, T.S., Eckstein, D., Jones, P.D., Kar- lén, W., Schweingruber, F.H., & Zetterberg, P. (1990) A with whom I spent weeks on fieldwork in Chile and 1400-year tree-ring record of summer temperatures in who first showed me the exiting Pelluco site. Fennoscandia. Nature, 346, 434-439. I am grateful for much help and support from Briffa, K.R., Jones, P.D., Bartholin, T.S., Eckstein, D., Sch- the personnel at Abisko Research Station and at weingruber, F.H., Karlén, W., Zetterberg, P., & Eronen, DendroLab in Kiruna: Håkan Samuelsson, Anders M. (1992) Fennoscandian summer from AD 500: tem- Hedefalk and Kjell Eriksson helped me both in the perature changes on short and long timescales. Climate field and in the lab. Thanks also to my co-supervi- Dynamics, 7, 111-119. sors Per Holmlund and Peter Jansson for help and Briffa, K.R., Jones, P.D., Schweingruber, F.H., & Osborn, inspiration, and to my friend and colleague Christer T.J. (1998a) Influece of volcanic eruptions on North- Jonasson who taught me that scientific work should ern Hemisphere summer temperature over the past 600 be “lustfyllt och värdigt”. years. Nature, 393, 450-455. Last but not least, I would like to acknowledge Briffa, K.R., Osborn, T.J., & Schweingruber, F.H. (2004) my financial supporters: The Environment and Space Large-scale temperature inferences from tree rings: a re- Research Institute in Kiruna (MRI); The Swedish view. Global and Planetary Change, 40(1-2), 11-26. Agency for Research Cooperation (SAREC), and Briffa, K.R., Osborn, T.J., Schweingruber, F.H., Harris, EU-projects: ADVANCE-2K (EV5V-CT94-0500); I.C., Jones, P.D., Shiyatov, S.G., & Vaganov, E.A. (2001) ADVANCE-10K (ENV4-CT95-0127); PINE (EVK2- Low-frequency temperature variations from a northern tree ring density network. Journal of Geophysical Re- CT-2002-00136), and Millennium (CONTRACT search, D106(3), 2929-2941. No 017008 GOCE). Briffa, K.R., Osborn, T.J., Schweingruber, F.H., Jone, P.D., Shiyatov, S.G., & Vaganov, E.A. (2002a) Tree-ring width and density data around the Northern Hemisphere: Part References 1, local and regional climate signals. The Holocene, 12, Alexandersson, H. (2002). Temperature and precipitation 737-757. in Sweden 1860-2001. SMHI Meteorologi,104. Briffa, K.R., Osborn, T.J., Schweingruber, F.H., Jone, P.D., Andersson, N.Å., Callaghan, T.V., & Karlsson, P.S. (1996) Shiyatov, S.G., & Vaganov, E.A. (2002b) Tree-ring width The Abisko Scientific Research Station. Ecological Bul- and density data around the Northern Hemisphere: Part letins, 45, 11-14. 2, spatio-temporal variability and associated climate patterns. The Holocene, 12, 759-789. Angell, J.K. & Korshover, J. (1985) Surface temperature changes following the six major volcanic episodes be- Briffa, K.R., Schweingruber, F.H., Jones, P.D., Osborn, tween 1780 and 1980. J. Climatol. Appl. Meteorol., 24, T.J., Harris, I.C., Shiyatov, S.G., Vaganov, E.A., & 937-951. Grudd, H. (1998b) Trees tell of past climates: but are they speaking less clearly today? Philosophical Transac- Baillie, M.G.L. (1990). Irish tree rings and an event in tions of the Royal Society of London, B353, 65-73. 1628 BC. In Thera and the Aegean world III. D.A. Har- dy & A.C. Renfrew eds. TheThera Foundation, Lon- Briffa, K.R., Schweingruber, F.H., Jones, P.D., Osborn, don. 160-166 pp. T.J., Shiyatov, S.G., & Vaganov, E.A. (1998c) Reduced sensitivity of recent tree-growth to temperature at high Baillie, M.G.L. & Munroe, M.A.R. (1988) Irish tree rings, northern latitudes. Nature, 391(6668), 678-682. Santorini and volcanic dust veils. Nature, 332, 344- 346. Clausen, H.B., Hammer, C.U., Hvidberg, C.S., Dahl- Jensen, D., & Steffensen, J. (1997) A comparison of Bartholin, T.S. (1987) Dendrochronology in Sweden. An- the volcanic records over the past 4000 years from the nales Academiae Scientiarum Fennicae, 145(A), 79-88. Greenland Ice Core Project and Dye 3 Greenland ice Bartholin, T.S. & Karlén, W. (1983). Dendrokronologi i cores. Journal of Geophysical Research, 102(C12), Lappland AD 436-1981. Dendrokronologiska Sällska- 26,707-26,723. pets Meddelanden. Rep. No. 5. Cook, E., Briffa, K.R., Shiyatov, S., & Mazepa, V. (1990). Boswijk, G., Fowler, A., Lorrey, A., Palmer, J., & Ogden, Tree-ring standardization and growth-trend estimation. J. (2006) Extension of the New Zealand kauri (Agathis In Methods of Dendrochronology. Applications in the

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Paper I

The Holocene 12,6 (2002) pp. 657–665

A 7400-year tree-ring chronology in northern Swedish Lapland: natural climatic variability expressed on annual to millennial timescales Håkan Grudd,1,2* Keith R. Briffa,3 Wibjorn Karle´n,2 Thomas S. Bartholin,4,5 Philip D. Jones3 and Bernd Kromer6 (1Climate Impacts Research Centre, Royal Swedish Academy of Sciences, S-981 07 Abisko, Sweden; 2Department of Physical Geography and Quaternary Geology, Stockholm University, S-106 91 Stockholm, Sweden; 3Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ, UK; 4Department of Quaternary Geology, Lund University, Tornav. 13, S-223 63 Lund, Sweden; 5Natural Science Research Institute, National Museum of Denmark, NY Vestergade II, DK-1471 Copenhagen, Denmark; 6Heidelberg Academy of Sciences, Inst. f. Umweltphysik, INF 366, D-69120 Heidelberg, Germany)

Abstract: Tree-ring widths from 880 living, dry dead, and subfossil northern Swedish pines (Pinus sylvestris L.) have been assembled into a continuous and precisely dated chronology (the Tornetra¨sk chronology) covering the period 5407 bc to ad 1997. Biological trends in the data were removed with autoregressive standardization (ARS) to emphasize year-to-year variability, and with regional curve standardization (RCS) to emphasize variability on timescales from decades to centuries. The strong association with summer mean temperature (June–August) has enabled the production of a temperature reconstruction for the last 7400 years, providing information on natural summer-temperature variability on timescales from years to centuries. Numerous cold episodes, comparable in severity and duration to the severe summers of the seventeenth century, are shown throughout the last seven millennia. Particularly severe conditions suggested between 600 and 1 bc correspond to a known period of glacier expansion. The relatively warm conditions of the late twentieth century do not exceed those reconstructed for several earlier time intervals, although replication is relatively poor and con dence in the reconstructions is correspondingl y reduced in the pre-Christian period, particularly around 3000, 1600 and 330 bc. Despite the use of the RCS approach in chronology construction, the 7400-year chronology does not express the full range of millennial-timescale temperature change in northern Sweden.

Key words: Dendroclimatology, tree rings, climate, summer temperature, Pinus sylvestris, northern Scandinavia, Holocene.

Introduction to be affected by anthropogenic greenhouse-gas emissions. There- fore, natural climate variability can be better assessed by combining Climatic change studies generally recognize both a natural and an the instrumental record with other indirect or inferred records of cli- anthropogenic component in uencing the climate in the twentieth mate in the pre-industrial era. These so-called proxy records should century (e.g., Mitchell et al., 2001). However, the relative importance preferably have high resolution and good dating control to allow of these two factors is debated and requires a better capability to precise identi cation of short (i.e., yearly) anomalies as well as multi- estimate the role of natural variability in the climate system. Instru- decadal and longer variability, but also to allow rigorous calibration mental observations cover only the last 100–200 years and are likely against recent instrumental records. Trees growing close to their thermal limit of distribution, such *Author for correspondence (e-mail: [email protected]) as at high-latitude or high-altitude tree-lines, react to variations in

Ó Arnold 2002 10.1191/0959683602hl578rp 658 The Holocene 12 (2002) the growing-season temperature through its in uence on cambial activity (Tranquillini, 1979). In such localities, the widths of the annual tree-rings will tend to be proportional to the overall warmth of the summer. Hence, the interannual pattern of such tree-ring sequences can be used as a tool to reconstruct annually resolved histories of summer-temperature variability in areas where instrumental records are sparse, potentially for periods long before the instrumental observations began. Averaging the data derived from numerous tree-ring samples into mean chronologies, continuously linked to its present day, ensures absolute dating control and an enhanced expansion of common underlying growth forcing by reducing the random ‘noise’ contained in individual tree series (Fritts, 1976). Scots pines (Pinus sylvestris L.) in northern Fennoscandia commonly reach an age of over 300 years, and may on some rare occasions grow for more than 600 years (Sireń, 1961; Bartholin and Karleń, 1983; Engelmark and Hofgaard, 1985). In northern Sweden, the juxtaposition of such old-living trees and dead trees from even older times, preserved on dry ground and in lake sediments, provides a rare opportunity to construct a temperature- sensitive ring-width chronology representing much of the Holocene. Extensive sampling in the Tornetra¨sk area in northern Figure 1 The eld area is located close to the tree-limit area on the eastern Swedish Lapland in the 1970s and 1980s (Bartholin and Karleń, slopes of the Scandinavian mountain range in the extreme north of 1983; Bartholin, 1987) yielded a 1440-year reconstruction of Sweden. annually resolved summer temperatures, covering the period ad 540–1980 (Briffa et al., 1990; 1992). The Tornetra¨sk tree-ring- based reconstruction has made an important contribution to the glaciation were rapid over a wide area in northernmost Swedish understanding of natural climate variability over the last millen- Lapland, indicating that the ice sheet over the mountain range was nium (Jones et al., 1998; Briffa and Osborn, 1999; Mann et al., thin when deglaciation started (Melander, 1980). Thus, the high 1999) and, in recent years, considerable effort has been made to alpine areas may have been ice-free well before the nal deglaci- extend this record into early-Holocene time within the framework ation. These ice-free areas probably experienced early coloniz- of the European Community (the ADVANCE-10K project; see ation of plants and trees while there was still ice in the valley Briffa, 1999; Briffa et al., 1999). oors (Barnekow, 1999; Kullman, 1999). The margin of the dim- In this paper we present an updated, extended and considerably inishing ice sheet retreated towards the east, thus obstructing improved tree-ring-width chronology from the Tornetra¨sk area water drainage in that direction (Melander, 1977). Therefore, covering the last 7400 years, from which we are able to deduce meltwater in the Tornetra¨sk basin had to drain towards the new information on natural temperature variability, on annual to Norwegian fjords in the west through a canyon 80 m above millennial timescales, in the climate system of northern Sweden. today’s lake level. When drainage towards the east eventually started, the lake surface was lowered successively, washing the glacial tills free of ne material and leaving a patchwork of Area description washed tills and accumulations of ne material. The different stages of the proto-Lake Tornetra¨sk are evident today from Our study area is located in the northwest of Sweden (Figure 1). numerous erosional beach features and deltas located high up on Though this is well north of the Arctic Circle, the climate of this the slopes surrounding the lake (Gretener and Stro¨mquist, 1981). area is relatively mild and classed as Subarctic (Sonesson, 1974; The altitudinal tree-line, de ned as a line connecting the highest Sonesson and Lundberg, 1974) in uenced by the ameliorating patches of forest within a given slope or series of slopes of similar effect of the Atlantic Ocean circulation, which conveys warm sur- exposure (for terminology conventions, see Ko¨rner, 1998), con- face waters to the Norwegian Sea. The Scandinavian mountain sists of mountain birch (Betula pubescens ssp. tortuosa) at 600– range forms an effective barrier to the warm and moist air masses 800 m a.s.l. The present-day continuous boreal forest extends to originating off the Norwegian coast, giving rise to a strong the eastern end of Lake Tornetra¨sk, where a distinct pine tree-line east/west gradient in continentality, i.e., in seasonal temperature can be identi ed at 440 m a.s.l., while the pine distribution further variability and total precipitation. to the west is of a disjunct relic type. The geology in this area is characterized by two main bedrock During the twentieth century there has been a major establish- units: the Caledonian mountain range in the west with alpine ment of young trees in the tree-line forests and, presently, new relief, and the Precambrian basement in the east with rocks of pine germination is extensive throughout the region; for example, low relief (Lindstro¨m, 1987). The majority of our eld sites are about 50% of the pine population at Abisko is less than 20 years located within the transition zone between these two main geo- old and about 75% is less than 50 years old (Sto¨cklin et al., 1996). logical units, in particular in the Tornetra¨sk area. The dominating However, few pine trees have established above the mid-twentieth physical feature of our study area is the large Lake Tornetra¨sk at century tree-line and, thus, the recent expansion has been in tree 341 m a.s.l., surrounded by mountains typically 1000–1500 m density rather than in altitude. high. Lake Tornetra¨sk covers an area of 340 km2 and, although it is only some tens of kilometres from the Atlantic waters in the west, it drains to the Gulf of Bothnia about 300 km to the Chronology construction southeast. Radiocarbon dated lake sediments in the area show that de- Material glaciation took place at about 10 000 cal. years BP (Sonesson, The sampled pine trees that constitute the basis of the current 1974; Karleń, 1979; Barnekow, 1999). The nal stages of the de- chronology number 880 and can be divided into three distinct ADVANCE-10K Special Issue: Long pine chronologies from northern Fennoscandia 659 groups. We use the following terminology: living trees, dry dead in most stem discs, were measured to the nearest 0.01 mm and trees (i.e., preserved on dry ground) and ‘subfossil’ trees found the data were stored on computer les using standard tree-ring in lakewaters and lake sediments. measurement systems. Within-tree radii were cross-checked for We have sampled living trees from sites around Lake measurement error and averaged to produce a mean ring-width Tornetra¨sk. Two of these sites are situated at the limit of the series for each individual tree. present-day boreal zone at the eastern end of Lake Tornetra¨sk, while three sites outside (i.e., west of) the boreal zone represent Cross-dating pine stands of a relic type. In sampling, we avoided coring close The interannual ring-width variability within each mean-tree to branches, wounds or wedging at the base of the stem. We also series was compared against all other series using both statistical avoided taking samples from the upslope or downslope side of and graphical programs, e.g. CATRAS (Aniol, 1983) and TSAP trees growing on sloping ground where reaction wood may form. (Rinn, 1996). This procedure is known as ‘cross-dating’ (a It is common dendrochronological practice to sample trees at bre- description of ‘European style’ cross-dating methodology is given ast height (e.g., Schweingruber, 1988). However, in many trees in Schweingruber, 1988). In order to identify locally absent rings, at the tree-line, the lowest branches grow at about, or even below, eliminate measurement errors and ensure relative and absolute 1 m above the ground and sampling was therefore done at a vari- dating accuracy, the quality of the cross-dating was repeatedly able height between 0.4 and 1.4 m up the stem. Two cores were checked with program COFECHA (Holmes, 1983). Problematic sampled from each living tree using a standard increment borer. samples, such as samples with missing rings or a distorted growth Pines growing in this environment commonly produce large pattern, were dismissed from the data set. amounts of resin, which preserves the wood by inhibiting the The outermost ring in the current sample of living trees was growth of fungi for a long time after a tree has died. In recent produced during the summer of 1997. By including the dry dead decades, it has been possible to nd dry dead wood on the ground and the subfossil trees, progressively older material could be over a wide area in northern Sweden. Unfortunately, this wood cross-dated and, thus, the record of precisely dated tree rings has also been collected for rewood, so that this source of wood could be extended back to 5407 bc. Thus, the data are now con- has virtually disappeared from all but a few remote areas. This tinuous over the last 7400 years; incorporating measurements material has a wide age range, including trees that died more than from a total of 880 calendrically aligned individual trees 1000 years ago. The oldest dry dead wood remains we have found (Figure 2). were from a tree that germinated a few years before ad 447, i.e., Dif culties in nding datable material from the rst centuries about 1560 years ago. This sample contains 365 rings and the tree bc resulted for many years in a persisting ‘gap’ in the data around must therefore have been preserved on dry ground for about 1200 330 bc. Trees from around this period show extremely suppressed years. This source of wood was sampled near to, or inside, growth, often associated with distorted ring patterns and the present-day pine forests in the Tornetra¨sk area by cutting stem occurrence of reaction wood. The ‘gap’ around 330 bc was nally disks with a chainsaw. bridged by dendrochronological cross-dating using samples from Trees that grow on or close to, lake shorelines may at some point fall into the water, where they can be preserved under near anaerobic conditions for thousands of years. Many small lakes, 9 00 i.e., with an area less than about 0.1 km2, have clearly visible tree remnants lying on top of, or in the surface layers of, the sediments. This source of subfossil wood in small northern Swedish lakes in 8 00 the tree-line zone has been known to exist since the late nineteenth century (Gavelin, 1909) and it is this material that has served as one of the most important assets for reconstructing changes in 7 00 Holocene tree-lines (Karleń, 1976; Kullman, 1995; Karleń and Kuylenstierna, 1996). 6 00 On clear calm days, subfossil logs may be easily identi ed from

the air. We have undertaken systematic helicopter surveys of lakes r e over a wide area, from Keinovoupio in the north to Kvikkjokk in b

m 5 00 the south (Figure 1), and up to an altitude of about 1200 m. We u n

discovered a wealth of subfossil pines in lakes up to a height of e l about 600 m a.s.l. However, we found no evidence of subfossil p

m 4 00 trees in lakes above this altitude. Only a limited number of the a numerous lakes that contain subfossil logs have been sampled to S date, and the majority of these sampled sites are concentrated in 3 00 the Tornetra¨sk area. In the lakes that have been sampled, we attempted to collect material from all existing, sizable subfossil pines located. With the aid of divers, it is probable that the large 2 00 majority of trees have been found. The subfossil logs were recovered using manpower or a winch, and from each a stem disc was cut at about 0.5–2 m up the trunk. However, since the trees 1 00 were in a varying state of disintegration, i.e., in most cases with roots and branches missing, it was not always possible to determine the exact sampling height of the sample. After sampling, the 0 trees were restored to their approximate original position and -6000 -40 00 -20 00 0 20 00 marked on a detailed map. Y e ar Clean and sharp surfaces for measuring were prepared using a Figure 2 A total of 880 sampled trees was used to construct the razorblade knife and chalk powder. All samples were prepared by 7400-year Tornetra¨sk chronology. Each sample is represented by a this method, which provides very good detail. Tree-ring widths horizontal bar, with length equal to the number of annual rings, and in two cores from each living tree, and a minimum of two radii position corresponding to the dendrochronologica l cross-dating. 660 The Holocene 12 (2002)

Table 1 Dendro age and corresponding radiocarbon dates of some subfossil samples

Dendro Subsample Dendro age 14C 14C Error 14C age sample ID rings no. (cal. BP) lab. ID age BP (1 sigma) (cal. BP) (95%)

25045 1–22 6970–6991 Lu2306 6050 70 6720–7160 TB111Z 1–10 6366–6375 – 5648 18 6350–6490 TB111Z 81–90 6286–6295 – 5512 21 6200–6400 TB111Z 111–120 6256–6265 – 5362 18 5990–6280 25075 1–60 6168–6227 Lu2576 5440 60 5990–6400 19006 1–60 5548–5607 Lu2577 4750 70 5310–5610 22117 5–30 4733–4758 Lu2254 4100 60 4440–4830 22038 100–139 4579–4618 Lu2253 4030 60 4300–4850 22137 70–140 3762–3832 Lu2303 3540 60 3640–3990 22028 180–352 3631–3803 Lu2298 3570 60 3690–4080 22176 35–95 3590–3650 Lu2305 3390 60 3470–3830 22095 50–80 3465–3495 Lu2302 3150 50 3250–3480 22029 100–220 2867–2987 Lu2299 2910 50 2880–3220 22040 1–20 2760–2779 Lu2300 2750 50 2760–2950 TE0879 26–29 2164–2166 St13722 2115 15 2000–2150 22072 1–25 1929–1953 Lu2301 2000 50 1820–2110 TE0689 30–32 1862–1864 St14410 1880 15 1730–1880 22089 1–30 1697–1726 Lu2782 1650 50 1410–1700 22173 1–20 1586–1605 Lu2304 1760 50 1550–1820 TD0019 1–232 1266–1497 St14021 1395 55 1180–1410

a subfossil tree located at Vettasja¨rvi east of the main investi- matching’ a set of high-quality 14C determinations from one gation area (Figure 1), i.e., well inside the present-day boreal individual sample, i.e., with precisely known separation in calen- zone. The precise dating of this period was later con rmed by dar years (Pearson, 1986). The wiggle-match provided a 14C date independent cross-dating of overlapping tree-ring data from with error limits of 6 10 years (at 95% probability) (Figure 4) northern Finland (Eronen et al., this issue), and from Ja¨mtland in which is precisely consistent with the dendrochronological dating. central Sweden (Gunnarson and Linderholm, this issue). The precise dendrochronological dating is supported by 14C Standardization determinations of a selected set of 20 samples (Table 1). The 14C Tree-ring widths are the expression of a number of environmental dates agree with the dendrochronological dating within the and physiological factors interacting in a complex manner to con- uncertainty provided by the radiocarbon method (Figure 3). To trol tree growth. These factors can be conceptually modelled by minimize the relatively large errors associated with the radio- regarding each individual ring width (RW) as a linear aggregate carbon dating method, we also applied the technique of ‘wiggle- of a restricted number of hypothetical ‘signals’ (Cook, 1990), the

5700

1-10 P

B 6000 s e t a P d 81-90

n B 5500 o e b r g a c A o i 4000 d a r d e t a r b

i 111-120 l

a 5300 C 2000 -4500 -4400 -4300 -4200 Ca l a g e B C 2000 4000 6000 Figure 4 A sequence of three 10-year blocks of wood (rings 1–10, D endro dates BP 81–90 and 111–120) from one individual tree (TB111Z; Table 1) was Figure 3 Each box represents one sample, which was dated with both 14C dated and ‘wiggle-matched’ to t the decadal radiocarbon calibration dendrochronolog y and radiocarbon (data from Table 1, columns 3 and 7). curve (Stuiver and Becker, 1993). The sequence happens to fall on a dis- The width represents the number of rings in the sample and the height tinct anomaly of the calibration curve. Thus, the 95% probability range represents the 95% con dence range of the radiocarbon date. Ideally, all of the match is just 6 one decade. This date corresponds perfectly with boxes should be centred on the diagonal line. the dendro date of the same sample. ADVANCE-10K Special Issue: Long pine chronologies from northern Fennoscandia 661

most important ones for our purpose being the product of climatic Following standardization, each individual index series (It) may forcing (C) and biological age (A). Thus: be regarded as the product of climate forcing (Ct) and an error (e t):

RW 5 C + A + e It 5 Ct + e t where e represents growth-controlling factors other than climate where the error is regarded as largely random. Consequently, and biological age. Our main concern in this study is to isolate averaging overlapping index series into a mean chronology will the climatic signal (C), which we may think of as a regional signal reduce the error at a proportion that is directly related to the acting similarly on the whole population. The age signal (A) is sample size for each year. controlled by the ageing process within each individual tree which The 880 precisely calendar-dated indices series were averaged tends to decrease the thickness of the annual rings as the tree gets to produce two site index chronologies for the Tornetra¨sk area: older. This decline in ring width is partly due to a decline in (1) an ARS chronology representing the year-to-year variability, vigour and partly due to a geometrical effect, i.e., in each year a and (2) an RCS chronology representing year-to-year and decadal new layer of wood is formed circumferentially outside that of the to century, and possibly longer, timescale variability. previous year, thus, around an increasingly larger perimeter. The process of removing the age-related growth (A) from the Temperature reconstruction observed tree-ring growth (RW) is commonly termed ‘standardiza- Meteorological data from Abisko Scienti c Research Station tion’ in dendrochronological studies and produces a set of (ANS), where daily records of temperature and precipitation dimensionless index values (I). In one speci c tree, we may extend back to ad 1913, were used to explore the relationship express the index value for year t as: between climate and tree growth. A simple correlation analysis shows that the annual mean tree-ring index is positively correlated RWt It 5 with monthly mean temperatures, notably in July (Figure 6a), At while there is no apparent correlation with monthly sums of pre- In this study we have standardized the data with two principally cipitation (Figure 6b). The strong positive in uence of July mean different methods. (i) The rst is autoregressive standardization temperature on pine growth in the tree-line ecotone in northern (ARS), where a smoothing spline function is tted to each Fennoscandia is well known from earlier studies (e.g., Aniol and Eckstein, 1984; Briffa et al., 1990; 1992; Kirchhefer, 2001; Lind- measurement series and regarded as the age function (arsAt) for each individual tree (Cook et al., 1990). This technique removes holm et al., 1996). Hence, indexed tree-ring-width chronologies slowly evolving trends in the data, leaving the high-frequency, from these northern areas are commonly regarded as qualitative i.e., year-to-year, variability in the index series. To adjust for auto- proxies of high-summer temperatures. The main vegetation period correlation in the series, ARS also involves autoregressive model- in the Tornetra¨sk area is June, July and August. Temperatures of ling. However, the ARS technique is not capable of distinguishing these three summer months all show a clear positive correlation between low-frequency biological trends and those that result against tree growth (Figure 6a). Therefore, we regard the from climate forcing, which leads to a loss of climatic signals on Tornetra¨sk tree-ring indices to be an approximation of the mean timescales longer than a few decades (Cook et al., 1995; Briffa summer (Jun–Aug) temperature, but dominated by July. et al., 1996). Therefore, we also use (ii) an alternative standardiz- Complementary data from neighbouring meteorological stations ation method, originally adopted by Erlandsson (1936) in his early have enabled the record of mean summer (Jun–Aug) temperature work on dendroclimatology in the Tornetra¨sk region. The method at ANS to be extended a further 45 years back in time, i.e., to has later been termed regional curve standardisation (RCS), and ad 1869 (Holmgren and Tjus, 1996). Hence, we may use linear proposed as a technique to preserve long-timescale variability in regression over a common 129-year calibration period, i.e., from tree-ring data (Briffa et al., 1992; 1996). In RCS, the age function ad 1869 to 1997 (Figure 7). The regression coef cients were then

(rcsAt) is de ned as an empirical time-independent growth function, i.e., controlled by biological age rather than calendar 1 25 age. Thus, rcsAt is an approximation of the common age function a )

in the population, regardless of calendar time (Figure 5). C s

15 e e r g e d (

0 5 e r

Hugershoff model: y=a*x^b*exp(-c*x)+d Coefficient data: u t a

a = 0.642 r e

b = 0.088 -5 p = 0.017 m ) c e T m 1 d = 0.445 n m o ( i

t -1 -15 h a t l d e

i 1 60 r r w

o b g C

n 50 i ) R m

40 m ( n o i 0 30 t a t i p

0 i 20 c e 0 100 200 300 r P Biological age 10 Figure 5 The shape of the age-dependent growth curve was found by, -1 0 rst, averaging the initial ring widths in all 880 samples (biological age J F M A M J J A S O N D 5 1), then averaging the widths of the second rings (biological age 5 2), etc.The shaded area represents one standard error around the average Figure 6 Bars show correlation between RCS tree-ring index and mean values. A ‘Hugershoff’ function (Warren, 1980) was then tted to the data monthly temperature (a) and precipitation (b) at Abisko Scienti c to nd a common age function (rcsAt) for the trees in our sample data set. Research Station over the period 1913–97. 662 The Holocene 12 (2002)

S tandardiz ed tem p. A RS rec onst ructed

r = 0 .5 8 2

a -2 C s e e r

Ac tual tem p. g RCS rec ons tructed e D

r = 0 .6 0 12

8 b 6 1860 1900 1940 1980 Y e ar

Figure 7 The tree-ring indices were calibrated against summer mean temperature (Jun–Aug) over the common period 1869–1997. The temperature data in (a) have been ltered to be comparable to the high-frequency variation displayed in the ARS-standardized tree-ring data. In (b), actual temperature is compared to the RCS-standardized tree-ring data. Scatter plots show correlation between tree-ring indices and temperature data. Through linear regression, the tree-ring data were transformed into proxies of summer-temperature change on both high-frequency (a) and medium- to low-frequency (b) timescales. used to reconstruct summer temperature back to 5407 bc. The Furthermore, the expression of underlying forcing in a chron- ARS record (Figure 8a) shows summer temperature variability on ology is a function of sample replication, i.e., the number of a year-to-year timescale over the last 7400 years, while the RCS samples making up the mean index in each year. This is often record (Fig. 8b) is used here to display temperature variability on referred to in tree-ring studies as the sample depth. The largest timescales from decades to several centuries. numbers of samples are in the most recent part of our data set, because we have a virtually unlimited capability to sample living trees (Figure 8c). Further back in time, the chronology is depen- Discussion dent on whatever dry dead and subfossil trees can be found and, consequently, the sample depth decreases notably in the pre- The positive correlation between summer temperature and Christian period. These changes in sample depth through time tree-ring widths of Scots pine in the Tornetra¨sk area has enabled undoubtedly affect the strength with which the climatic signal is various reconstructions of past temperature variability as far back expressed. The large sample depth for each year in the last 2000 as ad 540 (Briffa et al., 1990; 1992). Here, we extend the years in conjunction with the strong temperature association trans- Tornetra¨sk ring-width chronology back to 5407 bc and update it lates into narrow con dence limits around the expressed tempera- to ad 1997, allowing temperature inference over a continuous ture signal (Figure 8b). In the pre-Christian era, the errors are 7400-year period. However, our calibration period (129 years) is signi cantly larger. However, while we stress that the potential less than 2% of the reconstructed period, and we simply have to errors are large at these times, a major part of the temperature assume that the environmental processes that force current pat- signal may still be correctly expressed. Thus, we regard the terns of tree growth have equally been in operation over the last Tornetra¨sk tree-ring reconstructions as a valid record of summer- seven millennia. This is most likely not true. Certainly, the rep- temperature variability over the last 7400 years, at least at interan- resentation of error (e ) in our conceptual linear aggregate model nual, decadal and century timescales. will change through time over the 7400-year timespan. Further- Tree-ring records notoriously lack long-term trends, i.e., the more, it will change on many different timescales. Hence, the series are usually restricted in the low-frequency domain by the quality of the temperature reconstruction may also be related to relatively short length of the individual series from which the our choice of calibration period. Since the mid-twentieth century, chronology is constructed (Cook et al., 1995; Briffa et al., 1996). there has been a documented change in temperature forcing in the Theoretically, this problem can be solved by using very large growth of northern-latitude boreal forests (e.g., Briffa et al., numbers of overlapping individual series uniformly spread over 1998a; 1998b). This effect is not strongly evident in our data. In time. This is rarely possible in practice, and we usually have to this study we utilize the full length of the 129-year period to retain accept that tree-ring chronologies are demonstrably of great value as much low-frequency information as possible in the reconstruc- for reconstructing climate variation on short to medium-length tion. timescales, i.e., from interannual to a few hundred years, and that ADVANCE-10K Special Issue: Long pine chronologies from northern Fennoscandia 663

Figure 8 Summer-temperature variability over the last 7400 years in northern Sweden expressed as anomalies from the base period 1869–1997. (a) High- frequency variability, i.e., on a year-to-year timescale. (b) Medium- to low-frequency variability, i.e., on decadal to century timescales, with one standard error banding. (c) The sample size making up the mean index for each year. the millennium-scale variability is often lost. The RCS technique cene maximum position (Karle´n, 1976; 1988). It includes the does have the potential to express century and longer timescale classic biostratigraphic division between Sub-Boreal and Sub- variability to a greater extent than is possible in some earlier Atlantic (Berglund, 1986) and, in northern Europe, it also includes approaches (e.g., Briffa et al., 1990), though this method ideally the archaeological transition from Bronze Age to Iron Age – a requires large sets of sample data, evenly distributed over a time- time that brought about major changes to human societies span that is substantially longer than the individual series (Briffa (Burenhult, 1999; van Geel et al., 1996). Especially severe con- et al., 1992; 1996). The Tornetra¨sk data are indeed spread over a ditions occurred in 330 bc, which is the most extreme year in the very long period, but they are far from evenly distributed through Tornetra¨sk data indicating a short-term negative change in mean time. Nevertheless, there is some millennial-timescale variation summer temperature of about 3–4°C (Figure 8a). Around this expressed in the RCS record which can be compared to other time, several volcanic eruptions occurred (see Baillie, 1995; 1996; proxy records of low-frequency Holocene climate variation: and references therein) that may have played an important role pollen and macrofossil records from the Tornetra¨sk area indicate in forcing some of the high variability we see in the Tornetra¨sk optimal conditions for pine from about 4300 bc to about 2500 chronology. The record of the last 2000 years displays features bc (Barnekow, 1999), and records of tree-line change in northern of century-timescale climatic variation known from other proxy Sweden show high tree-lines around 4000 bc (e.g., Karle´n and and historical sources, including a warm ‘Roman’ period in the Kuylenstierna, 1996). These data indicate a broad period in early- rst centuries ad and a generally cold ‘Dark Ages’ climate from to mid-Holocene times when summer temperatures were 1.5–2°C about ad 500 to about ad 900 (Lamb, 1982; Baillie, 1995). Excep- higher than present. However, the Tornetra¨sk tree-ring record does tionally severe summer conditions occurred around ad 540, which not indicate notably high temperatures at this time. Thus, although resulted in frost-damaged rings and reduced growth, contempor- the RCS record displays some millennial-scale variation, it appar- aneous with independent evidence of severe climatic conditions ently expresses only a portion of the actual millennial scale trends from other parts of the world (Stothers, 1984; Austin, 1985; in Holocene summer temperature. Baillie, 1994). The warm period around ad 1000 may correspond The fact that dendrochronological cross-dating is possible to a so-called ‘Mediaeval Warm Period’, known from a variety between Tornetra¨sk and such distant areas as northern Finland and of historical sources and other proxy records (Hughes and Diaz, Ja¨mtland in central Sweden indicates a regional representativity of 1994). In the Tornetra¨sk region, this period ends at about ad 1100 the high-frequency temperature variation displayed in the Torne- when a shift to a colder climate occurred. The climatic deterio- tra¨sk record. Comparing the record to other independent evidence ration in the twelfth century can be regarded as the starting point of low-frequency climatic change in northern Europe indicates of a prolonged cold period that continued to the rst decade of that some of the century-timescale variability is representative for the twentieth century. This so-called ‘Little Ice Age’ culminated a wider area. The Tornetra¨sk data demonstrate particularly severe in the seventeenth century and is known from instrumental, climatic conditions in the period 600–1 bc, a period that is charac- historical and proxy records (Grove, 1988). terized by low sample replication and high ring-width variability. It includes both the highest and the lowest ring-width values over Conclusions the last 7400 years, indicating a highly variable but generally cold (1) The Tornetra¨sk tree-ring chronology is annually resolved climate. This period is contemporary with a major glacial expansion in Scandinavia when many glaciers advanced to their Holo- and includes continuous data from 5407 bc to ad 1997, providing a precisely dated record of summer-temperature variability. 664 The Holocene 12 (2002)

(2) The strong association with summer temperature in conjunc- Briffa, K.R., Jones, P.D., Bartholin, T.S., Eckstein, D., Schweingruber, tion with the large numbers of samples for each year translates F.H., Karleń, W., Zetterberg, P. and Eronen, M. 1992: Fennoscandian into narrow con dence limits around the expressed temperature summer from ad 500: temperature changes on short and long timescales. signal in the last 2000 years. The errors are signi cantly larger in Climate Dynamics 7, 111–19. Briffa, K.R., Jones, P.D., Schweingruber, F.H., Karleń, W. and the pre-Christian era, mainly due to reduced sample depth. Shiyatov, S. G. 1996: Tree-ring variables as proxy-climate indicators: (3) The record displays signi cant temperature changes on problems with low frequency signals. In Jones, P.D., Bradley, R.S. and timescales from years to several centuries, which is representative Jouzel, J., editors, Climate variations and forcing mechanisms of the last for a large area in northern Sweden, possibly also for a wider area. 2000 years, NATO ASI, Berlin: Springer-Verlag, 9–41. (4) The record does not express the full range of millennial- Briffa, K.R., Schweingruber, F.H., Jones, P.D., Osborn, T.J., Harris, timescale temperature variation in the Tornetra¨sk area. I.C., Shiyatov, S.G., Vaganov, E.A. and Grudd, H. 1998a: Trees tell of past climates: but are they speaking less clearly today? Philosophical Transactions of the Royal Society of London B 353, 65–73. Acknowledgements Briffa, K.R., Schweingruber, F.H., Jones, P.D., Osborn, T.J., Shiyatov, S.G. and Vaganov, E.A. 1998b: Reduced sensitivity of recent tree growth Over the last c. 25 years, numerous eld assistants, colleagues, to temperature at high northern latitudes. Nature 391, 678–82. close friends and family have been involved in the eldwork – Burenhult, G. 1999: Arkeologi i Norden. Stockholm: Natur & Kultur, 540 pp. none mentioned by name, but none forgotten! Without the kind Cook, E.R. 1990: A conceptual linear aggregate model for tree rings. help from all these people, we would not have succeeded in nally In Cook, E. and Kairiukstis, L., editors, Methods of dendrochronology : completing the Tornetra¨sk chronology. We are also grateful to the applications in the environmental science, Dordrecht: Kluwer Academic colleagues who gave valuable comments on the manuscript. This Publishers, 98–104. work was nanced by EU research projects EV5V-CT94-500 and Cook, E.R., Briffa, K.R., Meko, D.M., Graybill, D.A. and Funkhouser, ENV4-CT95-0127, and by the EU-funded Environment and Space G. 1995: The ‘segment-length curse’ in long tree-ring chronology develop- Research Institute in Kiruna, Sweden. ment for palaeoclimatic studies. The Holocene 5, 229–37. Cook, E.R., Briffa, K.R., Shiyatov, S. and Mazepa, V. 1990: Tree-ring standardization and growth-trend estimation. In Cook, E. and Kairiukstis, References L., editors, Methods of dendrochronology : applications in the environmental science, Dordrecht: Kluwer Academic Publishers, 104–23. Engelmark, O. and Hofgaard, A. 1985: The oldest Scots pine, Pinus Aniol, R.W. 1983: Tree-ring analysis using CATRAS. Dendro- sylvestris, in Sweden. Svensk Botanisk Tidskrift 79, 415–16. chronologia 1, 45–53. Erlandsson, S. 1936: Dendro-chronologica l studies. PhD thesis, Aniol, R.W. and Eckstein, D. 1984: Dendroclimatologica l studies at the University of Uppsala. northern timberline. In Mo¨rner, N.-A. and W. Karleń, editors, Climatic changes on a yearly to millennial basis, Dordrecht: D. Reidel, 273–79. Fritts, H.C. 1976: Tree rings and climate. London: Academic Press. Gavelin, A. 1909: Om tradgransernas nedgång i den svenska fjalltrakterna. 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Ecology 76, 2490–502. 2000: Analysis of dendrochronologica l variability and associated natural —— 1999: Early Holocene tree growth at a high elevation site in the climates in Eurasia – the last 10,000 years (ADVANCE-10K). In Proceed- northernmost Scandes of Sweden (Lapland). A paleobiogeographica l case ings of the European Climate Science Conference, Vienna, October 1998 study based on megafossil evidence. Geogra ska Annaler 81, 63–74. (CD-ROM available from Biodiversity and Global Change Unit, Research Lamb, H.H. 1982: Climate, history and the modern world. London: DG, European Commission, Brussels). Methuen. Briffa, K.R., Bartholin, T.S., Eckstein, D., Jones, P.D., Karleń, W., Lindholm, M., Timonen, M. and Merilaïnen, J. 1996: Extracting mid- Schweingruber, F.H. and Zetterberg, P. 1990: A 1400-year tree-ring summer temperatures from ring-width chronologies of living pines at the record of summer temperatures in Fennoscandia. 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Lindstro¨m, M. 1987: Northernmost Scandinavia in the geological Sire´n, G. 1961: Skogsgra¨nstallen som indikator fo¨r klimat uktuationerna perspective. Ecological Bulletin 38, 17–37. i norra Fennoscandien under historisk tid. Communicationes Instituti Mann, M.E., Bradley, R.S. and Hughes, M K. 1999: Northern Forestalis Fenniae 54, 1–66. Hemisphere temperatures during the past millennium: Inferences, uncer- Sonesson, M. 1974: Late Quaternary forest development of the Torne- tainties, and limitations. Geophysical Research Letters 26, 759–62. tra¨sk area, North Sweden. 2. Pollen analytical evidence. Oikos 25, Melander, O. 1977: Tornetra¨skområdetsissjo¨strandlinjer, Stockholm: 288–307. Naturgeogra ska institutionen, 71 pp. Sonesson, M. and Lundberg, B. 1974: Late quaternary forest develop- —— 1980: The disintegration of the inland ice sheet in northwestern ment of the Tornetra¨sk area, North Sweden. 1. Structure of modern forest Lapland, Sweden. Department of Physical Geography, Stockholm ecosystems. Oikos 25, 121–33. University, 89 pp. Sto¨cklin, J., Jacomet, S. and Ko¨rner, C. 1996: Structure and vitality of Mitchell, J.F.B., Karoly, D.J., Hegerl, G.C., Zwiers, F.W., Allen, M.R., a Swedish Pinus sylvestris population near the arctic tree line. Botanical Marengo, J. and 28 contributing authors 2001: Detection of climate Institute, University of Basel, Switzerland, 17 pp. change and attribution of causes. In Houghton, J.T., Ding, Y., Griggs, Stothers, R.B. 1984: Mystery cloud of ad 536. Nature 307, 344–45. D.J., Noguer, M., van der Linden, P.J. and Xiaosu, D., editors, Climate Stuiver, M. and Becker, B. 1993: High-precision decadal calibration of change 2001: the scienti c basis. Contribution of Working Group I to the the radiocarbon time scale, ad 1950–6000 bc. Radiocarbon 35, 35–65. Third Assessment Report of the Intergovernmenta l Panel on Climate Tranquillini, W. 1979: Physiological ecology of the alpine timberline. Change (IPCC), Cambridge: Cambridge University Press. Berlin: Springer-Verlag. Pearson, G.W. 1986: Precise calendrical dating of known growth-period van Geel, B., Buurman, J. and Waterbolk, H.T. 1996: Archaeological samples using a ‘curve tting’ technique. Radiocarbon 28, 292–99. and palaeoecologica l indications of an abrupt climate change in The Rinn, F. 1996: Time series analysis and presentation. Heidelberg: Nederlands, and evidence for climatological teleconnections around 2650 Frank Rinn. BP. Journal of Quaternary Science 11, 451–60. Schweingruber, F.H. 1988: Tree rings: basics and applications of Warren, W.G. 1980: On removing the growth trend from dendro- dendrochronology. Dordrecht: Kluwer Academic Publishers. chronological data. Tree-Ring Bulletin 40, 35–44.

Paper II

Torneträsk tree-ring width and density ad 500 - 2004: A test of climatic sensitivity and a new 1500-year reconstruction of north Fennoscandian summers

Håkan Grudd

Abisko Scientific Research Station, the Royal Swedish Academy of Sciences, and Dept. of Physical Geography and Quaternary Geology, Stockholm University

Abstract This paper presents updated tree-ring width (TRW) and maximum latewood density (MXD) from Tor- neträsk in northern Sweden, now covering the period AD 500 – 2004. By including new data also from young trees, a previously noted decline in recent MXD is eliminated. Non-climatological growth trends in the data are removed using two different techniques: Spline standardization and Regional Curve Standardi- zation (RCS) thus producing chronologies of TRW and MXD that are used in a multiple regression with local instrumental records. A bootstrapped response function analysis shows that tree growth is forced by June – August temperatures and that the regression weights for MXD are much stronger than for TRW. The robustness of the reconstruction equation is verified by independent temperature data. Trends in TRW and MXD start to diverge at about AD 1800, apparently caused by an increased sensitivity of TRW to temperature. This trend in TRW is removed in an attempt to “correct” the RCS chronology. Five different June – August temperature reconstructions are presented based on various combinations of the TRW and MXD data, including raw data chronologies, and “corrected” and “un-corrected” standardized data. These reconstructions capture up to 65% of the variance in the instrumental data. Compared to previously published reconstructions from Torneträsk, the new data show generally higher temperatures, particularly in a 200-year period centred on AD 1000. The temperature of the late 20th century is exceeded in at least four earlier periods in the last 1,500 years.

Introduction The extensive compilation of Scots pine (Pinus sylvestris L.) tree-ring data in the Torneträsk area of northern Sweden (Karlén, 1976; Bartholin, 1987; Briffa et al., 1990) has resulted in one of the longest continuous tree-ring width (TRW) chronologies in the world, covering the period 5407 BC to AD 1997 (Grudd et al., 2002). This 7,400-year precisely dated record was amalgamated from 880 individual tree samples, collected from living trees and remnant Figure 1. The new maximum latewood density (MXD) (subfossil) wood preserved on dry ground and in data (thick curves) updates the Torneträsk database to AD small lakes. From this material, Schweingruber and 2004. The number of overlapping trees for each year (the co-workers (1988) used a selection of 65 samples sample depth) is shown in the lower part of the graph. to produce a record of maximum latewood density The former MXD data for Torneträsk (thin curves) were (MXD), covering the period AD 441 to 1980. This produced and published by the Swiss Federal Institute for 1,540-year density record is, to date, the longest in Forest, Snow and Landscape Research (WSL, Dendro Da- the world. tabase; Schweingruber et al., 1988) using the analytical Tree growth in the north of Fennoscandia has a instrument DENDRO2003. The new data were produced strong correlation to summer temperature: TRW is using the Itrax Density Scanner, a novel analytical instru- predominantly correlated with a short peak period ment from Cox Analytical Systems (www.coxsys.se).

1 Håkan Grudd

of summer warmth (Lindholm & Eronen, 1995; urgent need to update existing tree-ring collections Kirchhefer, 1999; Grudd et al., 2002) while MXD is throughout the Northern Hemisphere and to further related to a longer summer period and the correla- develop networks of high-resolution proxy data that tion is usually much higher (Briffa et al., 1990; Mc- can capture the magnitude and rate of temperature Carroll et al., 2003). Thus, density is the more pow- change over the last millennium. erful proxy for summer temperature and, as a result, There is also a growing concern for a change in sen- multi proxy temperature reconstructions based on sitivity of recent tree-growth to temperature at high these two tree-ring variables will be greatly domi- northern latitudes where trends in TRW and MXD nated by latewood density (Briffa et al., 2002a). have been reported to increasingly diverge from the The Torneträsk data have been used in a number instrumental records during the second half of the of high resolution large-scale climate reconstructions 20th century (Jacoby & D’Arrigo, 1995; Briffa et (Jones et al., 1998; D’Arrigo et al., 1999; Mann et al., 1998b). The diverging trends are not consistent al., 1999; Briffa, 2000; Briffa et al., 2002b; Esper throughout the widely-distributed circumpolar sites et al., 2002; Mann & Jones, 2003; Luterbacher et (Briffa et al., 2004; Driscoll et al., 2005; Wilmking et al., 2004; Moberg et al., 2005) with the objective al., 2005; D’Arrigo et al., 2006) and the mechanisms to place the documented late-20th century tempera- behind this phenomenon are poorly understood. The ture increase (IPCC, 2001) in a longer time-scale per- apparent change in climate sensitivity has been at- spective. The amplitude of this recent temperature tributed to changes in the seasonal cycle of CO2 in change is, however, debated in context of the last the atmosphere (Myneni et al., 1997); to changes in millennium (Jones et al., 2001; Mann et al., 2002; winter precipitation and melt-timing in permafrost Cook et al., 2004; Luterbacher et al., 2004; Esper et environments (Vaganov et al., 1999); to tempera- al., 2005; Moberg et al., 2005; D’Arrigo et al., 2006) ture-induced drought stress (Barber et al., 2000); and and it is also recognized that existing millennia-long to falling ozone concentrations in the stratosphere climate reconstructions rely on proxy data that may (Briffa et al., 2004). underestimate climatic variability (Briffa & Osborn, This paper presents new data from Torneträsk 2002; Osborn & Briffa, 2004; von Storch et al., which update the existing tree-ring records to AD 2004). A majority of the proxy reconstructions carry 2004. The sensitivity of recent tree growth to tem- a strong weight from MXD. However, they are also perature is examined using the alternative hypothesis seriously hampered by the lack of data in the most that apparent changes are related to the data and/or recent period (Briffa et al., 2004; Esper et al., 2005; to the methods used in chronology construction, and D’Arrigo et al., 2006). The problem can be illus- not to climatic or environmental factors. Finally, a trated by the Torneträsk MXD data which terminate new summer temperature reconstruction is presented in AD 1980, i.e. prior to the “extreme” temperature for the last 1,500 years in northern Fennoscandia. increase in the most recent decades. Thus, there is an

Figure 2. The updated Torneträsk tree-ring data for the last two millennia now constitutes (a) 100 series of maximum latewood density (MXD); and (b) 620 series of tree-ring width (TRW). The number of replicate series for each year (the sample depth) is shaded in grey, and the average age of the constituent tree rings for each year is illustrated by the top curve in both graphs.

2 Torneträsk tree-ring width and density AD 500 – 2004

The tree-ring data produce radiographic (X-ray) images from thin laths that are cut from the samples. Before X-raying, the The tree-ring widths (TRW) and maximum latewood laths are treated with alcohol in a Soxhlet apparatus densities (MXD) used in this study originate from to extract resins and other movable compounds in the Lake Torneträsk area, northern Sweden (19° 45’ the wood that are not related to annual wood tissue E; 68° 15’ N). The sample material is derived from production (Schweingruber et al., 1978). living and subfossil Scots pine (Pinus sylvestris L.). In the DENDRO2003 system, laths are placed on The subfossil material comes from two sources: dead an X-ray film and exposed to X-rays (similar to the wood found on dry ground, and submerged logs re- technique traditionally used in hospitals). Subse- trieved from small mountain lakes. In this study, all quently, light intensity (grey levels) in the X-ray film material is regarded as samples from the same popu- is analysed using a manually operated photo-sensor. lation. The dimensions of the photo-sensor slit varies with MXD data from Torneträsk were first published the choice of magnifying lens, resulting in varying di- by Schweingruber et al. (1988) and this original data mensions from 0.004 x 1 mm to 0.05 x 5 mm. Grey- set of 65 individual tree-series is hereafter denoted level light intensity is calibrated to wood density us- SEAL88 (WSL, Dendro Database). In SEAL88, data ing a standard calibration wedge which is X-rayed at for the most recent period originated entirely from the same time as the sample laths. very old-aged trees and, as a result, tree rings from In the Itrax WoodScanner, the laths are passed in the 20th century had a very high cambial age as front of an X-ray beam while a digital sensor detects compared to the earlier part of the record. Here, the the amount of radiation that passes through the sam- SEAL88 data is updated to AD 2004 using new sample. Hence, the output from the Itrax WoodScanner ples from 35 relatively young trees (Fig. 1), which is a 16 bit digital image with pixel dimensions 20x20 has significantly reduced the average cambial age of µm and pixel depth of 65,536 grey levels. The dig- the tree-ring data in the 20th century. Thus, the new ital X-ray image is subsequently analysed using the Torneträsk MXD data-base includes samples from commercial software WinDendro (www.regentin- a total of 100 trees and covers the period AD 441 struments.com) where light intensity is analysed in a – 2004. Dry dead wood covers the period AD 441 user-defined path in the longitudinal direction of the – 1789 and living trees cover the period AD 1336 sample by means of a “virtual” slit scanning process – 2004. No subfossil lake material is included in the (Guay et al., 1992). The dimensions of the virtual MXD data. The representation of samples through slit are 0.02 x 2 mm. Grey-level light intensity is cali- time (the sample depth) and the average cambial age brated to wood density using the same type of stand- of the constituent rings for each year are shown in ard calibration wedge as used by the DENDRO2003 Figure 2a. system. The TRW data are part of a much larger data set Figure 1 shows that the output data from the two than the one used for density (Grudd et al., 2002). X-ray systems are compatible: the 35 new data series Here, the TRW data are updated to AD 2004 using produced by the Itrax WoodScanner have an overall the same 35 trees as for density and a total of 620 average maximum latewood density of 0.57 g cm-3 trees is now used to cover the period AD 500 − 2004. which is roughly equivalent to 0.58 g cm-3 in the The source of these data is living trees, dead dry SEAL88 data over the same period and based on 21 wood, and subfossil logs that were recovered from series. The variance, however, is lower in the data small lakes. The sample depth and the average age produced by the Itrax WoodScanner (standard devi- of the constituent rings for each year are shown in ation 0.08) as compared to the data produced by the Figure 2b. DENDRO2003 system (standard deviation 0.12).

X-ray densitometry Standardization The new MXD and TRW data were produced using The annual growth of a tree, manifest as TRW or the Itrax WoodScanner, which is a novel instrument MXD in the stem, may in theory be attributed to from Cox Analytical Systems (www.coxsys.se), while a restricted number of different growth-control- the original SEAL88 data were produced using the ling factors (Cook, 1990). At Torneträsk, trees live DENDRO2003 X-ray analysis instrumentation from close to their climatological limit of distribution and, Walesch Electronic (www.walesch.ch). Both systems

3 Håkan Grudd

hence, annual production of wood tissue is prima- the new MXD data have a difference in variance as rily controlled by the climate (Tranquillini, 1979). A compared to SEAL88 (Fig. 1), and since the ampli- secondary growth-controlling factor originates from tude of variation in TRW usually changes over the the changing vitality of each individual tree, a factor lifetime of individual trees (Schweingruber, 1988), which is mainly related to biological age, and a third an index (It) of the climate signal is calculated by factor is the influence from changes in the physical division rather than by using subtraction. This ap- and ecological environments (Fritts, 1976; Schwein- proach simultaneously removes the age-related trend gruber, 1988). Using this basic concept, the observed and stabilizes the variance such that the mean and time series of annual wood tissue production in a variance no longer systematically change with time: tree may be defined as the cumulative result of three separate growth controlling factors: climate (C), tree Gt It = + ε t . (2) age (A), and non-climatic changes in the surrounding At environment (E). Hence, The age-related trend (A ) is commonly mod- = + + + , (1) t Gt Ct At Et ε t elled with a deterministic approach, i.e. by fitting a where Gt is the observed growth, measured as ring negative exponential function or linear trend to the width (TRW) or maximum latewood density (MXD) data, or by using more flexible functions such as a at calendar year t, and �εt results from random (error) smoothing spline (Fritts, 1976; Cook & Kairiukstis, growth processes not accounted for by these other 1990). These deterministic methods generally pre- processes (Cook, 1990). serve the high-frequency variation in the data while Tree-ring chronologies are constructed by averag- removing much of the slowly evolving changes such ing replicate data for each individual calendar year. as biological growth trends. However, they poten- However, when used for paleoclimate reconstruc- tially also remove a large part of any climatic trend. tion, non-climatic information (At and Et in Eq. 1) The problem is referred to as the “segment-length needs to be removed from the individual raw data curse” in dendrochronology (Cook et al., 1995). In series (Gt). This central concept in dendroclimatol- this analysis, therefore, three alternative approaches ogy is referred to as standardization (Fritts, 1976; are used. First, no standardization: the data are pre- Cook et al., 1990) and the aim is to preserve as much sented as raw data chronologies with no statistical of the climate-related information as possible while pre-treatment, thus preserving all information in the removing the un-wanted, non-climatic information. data. Secondly, using an empirical standardization Inevitably, however, a varying proportion of the method denoted Regional Curve Standardization low-frequency climatic information is also lost in (RCS) which has demonstrated a particular ability this process. The choice of standardization method to preserve long-timescale climate variability in long depends on the character of the tree-ring data but, in tree-ring chronologies while removing most of the general, more complex statistical methods are needed age related variance (Esper et al., 2003; D’Arrigo when the non-climatic environmental factors have a et al., 2006). The basic concept of the RCS method strong impact on tree growth (Cook et al., 1990). was used already by Stellan Erlandsson (1936) in his In this study, tree growth is assumed to be forced pioneer dendro-climatological studies at Torneträsk by two dominant factors: the summer climate and and the method was later employed and developed the age-related declining vitality of individual trees. by Keith Briffa and co-workers (1992) using a much This assumption is based on the strong connection larger set of tree-ring data from Torneträsk. Thirdly, to temperature for Torneträsk TRW and MXD data a deterministic approach to standardization is ap- (Briffa et al., 1990; Grudd et al., 2002). Non-cli- plied using smoothing splines, which removes all matic environmental factors will certainly have been low-frequency information in the data. important for individual trees at different times, but The RCS method has higher requirements on the these factors are regarded as haphazard events with data as compared to other standardization methods: random distribution over space and time and, hence, the cambial age of all tree rings must be known, i.e. they will be part of the random error variable (εt) samples should ideally be from a common height in in the conceptual linear aggregate equation (Eq. 1). the stem of all sampled trees and include the pith Theoretically, hence, the climate signal (Ct) can be (the centre of the tree). If the pith is absent, which extracted by simply subtracting the age-related trend often is the case in drilled core samples and subfossil (At) from the observed growth (Gt). However, since wood, then the pith offset (PO) has to be estimated.

4 Torneträsk tree-ring width and density AD 500 – 2004

Figure 3. Stacked plot of the 100 individual maximum latewood density (MXD) series (grey curves) aligned according to their cambial age, i.e. ring 1 = age 1; ring 2 = age 2; etc. Individual trees range in age between 37 and 620 years (25 trees are older than 344 years). The lower curve shows the number of replicate samples for each cambial age (sample depth). The replication is very low for trees older than about 400 years and the remaining trees show clear signs of climatic variation the oldest parts (c. 400-600 years). The Regional Curve (thick black line) was computed by linear regression using a restricted number of data (delimited by the vertical lines A-A), i.e. only data with cambial ages between 16 and 344 years were used in the regression. The thin black line represents a previously published Regional Curve for Torneträsk MXD (Briffa et al., 1992) where all data were included in the regression.

In the Torneträsk MXD data, the pith was present in the TRW data, a “Hugershoff” RCS-function is used 25 of the series and PO was approximated in the re- that was previously developed for the Torneträsk maining 75 series. The 620 TRW series were not ad- area (Grudd et al., 2002): justed for PO: here, the first ring in the series is sim- TRW 0.088 −0.017×T ply assumed to represent the ring of the first cambial AT = 0.642×T × e + 0.445 (4), year. When averaging all individual tree ring series similarly, TRW is the “regional” age-related TRW according to their cambial age (T) rather than their AT calendar year (t) and calculating the average growth, value at cambial age T. This function is essentially a it is presumed that the influence from climate forc- negative exponential curve. However, it is also capa- ing will be levelled out. Hence, when using the RCS ble of taking into account the positive growth trend method it is also very important that there is a wide in young trees. Hence, the RCS curves used in this distribution of the data over time, i.e. the data lined study have the form of a negative linear function for up for each cambial year should come from a wide the MXD data and a negative exponential function range of climatic scenarios. for the TRW data. The exponential shape for TRW Only data with sample depth >�25 was used to cal- can be justified by the fact that ring widths are meas- culate the RCS curve for MXD (Fig. 3). The first 15 ures of radial growth in the stem, i.e. new rings are years were also left out from the regression to avoid formed on the outside of an increasingly larger cyl- influence from the strong positive growth trends usu- inder and, thus, the volume of wood tissue must be ally found in juvenile trees. A linear age trend was distributed over a successively larger surface. Maxi- then fitted to the MXD data: mum tree-ring density has no such geometrical constraint and, hence, a linear trend is used. MXD -4 A T = 0.657 – 2.985 x 10 x T (3), Using Equation 2, and using cambial age T instead of calendar year t, each individual MXD and TRW MXD where AT is the “regional” age-related MXD series were then standardized with the corresponding value at cambial age T (Fig. 3). For standardizing RCS age function (Equations 3 and 4) to produce

5 Håkan Grudd

Figure 4. Chronologies from tree-ring width (TRW) and maximum latewood density (MXD): Figures a and b show the raw data chronologies, i.e. no de-trending of the data – chronologies are the arithmethic mean of the raw data. Figures c and d show the Regional Curve Standardised chronologies; e and f show the spline standardised chronologies. All chronologies are scaled to have the same mean and standard deviation over the period AD 500-2004. Grey curves show annual data. The annual data were smoothed with a 30-year spline filter (thin black curves) and a 100-year spline filter (thick black curves).

6 Torneträsk tree-ring width and density AD 500 – 2004

new sets of individual index series where most of the age related trends are removed. After RCS standardization, all individual index series were re-aligned according to their exact calendar year (t). An arithmetic mean was then calculated for each calendar year, which generated two RCS index -chronologies, one for MXD and one for TRW. In addition to RCS, the data were also standardized using a spline function fixed to 67% of the total length of the individual series. Hence, At in Equation 2 is a unique curve for each series, determined by the individual growth data. This is a more flexible method which preserves the high frequency variation Figure 5. Monthly average values of temperature and pre- in the data but removes a large part of the low-fre- cipitation for the period 1913-2004 at Abisko Research quency variation, including any climatic long-term Station, Royal Swedish Academy of Sciences. Annual trend. Figure 4 shows the variously produced chro- mean temperature is -0.6 C° and annual precipitation is nologies for MXD and TRW. 300 mm.

Climatic response function analysis nents (PC’s) of monthly temperature and precipi- In order to identify climatic signals in the MXD and tation data and regressing the PC’s on the annual TRW data and to investigate the optimum season tree-ring indices over a common period. In response for a climate reconstruction, the RCS-standardized functions, however, normal significance levels of chronologies (Fig. 4c-d) are used together with local coefficients may be misleading because error esti- climate data in a response function analysis (Fritts, mates are underestimated (Cropper, 1985). Hence, 1976). This involves calculating principal compo- the method used here involves the technique of boot-

Figure 6.Climatic forcing on tree growth at Torneträsk – tree ring width (TRW) and maximum latewood density (MXD) – is illustrated by bootstrapped response values for monthly temperature (Temp.) and precipitation (Prec.). The response function analysis starts with January in the year prior to growth and runs to December in the year concurrent with growth. Significant months (95% confidence) are marked with asterisks.

7 Håkan Grudd

strapped confidence intervals to estimate the signifi- Table 1. Calibration of the reconstruction equation. cance of both correlation and response function coefficients (Biondi & Waikul, 2004) which produces Calibration period 1869-1936 1937-2004 1869-2004 more accurate results. Monthly mean values of local Correlation 0.79 0.81 0.81 instrumental climate data for the period 1913-2004 Explained variance 0.62 0.65 0.65

(Fig. 5) were provided by the Scientific Research Sta- Observations 67 67 135 tion in Abisko, which is run by The Royal Swedish Regression weights Academy of Sciences and located in the Torneträsk TRW -0.359 0.150 0.057 area. t+1 TRW 1.152 1.203 0.900 The response function analysis gives an indication t of the direction and relative strength of the climatic MXD t 6.949 7.732 7.343 forcing and the results show that both MXD and Constant 1.859 0.349 1.328 TRW have a significant positive response to growing-season temperatures over the period 1913-2004 Veriﬁ cation period 1937-2004 1869-1936 – (Fig. 6a-b), while the variations in precipitation exert Explained variance 0.61 0.63 – no significant influence (Fig. 6c-d). Summer temperatures during the current year of growth have a strong Summary of the regression results for the two separate 68-year calibration/verification periods and for the full 136-year period effect on both MXD and TRW, but the response using overlapping meteorological and tree-ring data. time-window is broader for MXD including June, July and August temperatures. TRW forcing is dominated by July and to a less significant extent June TRW TRW temperatures of the current year of growth. Hence, index in year t; I t and I t+1 are the mean TRW the analysis shows that a reconstruction of mean indices in years t and t+1; a-d are the regression coef- June-August (JJA) summer temperature is valid. ficients. TRW also show significant response values to May and June temperatures of the previous year which re- flects the high auto-correlation usually found in ring Sensitivity of recent tree growth to tem- width chronologies (Cook, 1985) and indicates that perature the trees use excess energy from the previous year’s Possible changes in the sensitivity of recent tree growth to produce a new ring. Therefore, a lagged growth to summer temperature was examined by TRW-variable must be included in the linear regres- first testing the regression equation (Eq. 5) in a cali- sion model for summer temperature reconstruction: bration/verification exercise (Fritts, 1976; Cook & MXD TRW TRW Kairiukstis, 1990). In the analysis, an existing longer C = a I t + b I t + c I t+1 + d (5), t record of average summer (June – August) temper- where Ct is the reconstructed summer (JJA) tempera- ature from Abisko Research Station was used that MXD starts in AD 1869 (Holmgren & Tjus, 1996). This ture at Torneträsk in year t; is the mean MXD I t 136-year record was split in two independent 68-

Figure 7.The regression equation (Eq. 5) was verified in a cross calibration/verification exercise using RCS chronologies and local instrumental summer (June – August) temperature data. The data were split in two periods: 1869-1936 and 1937-2004 and the regression coefficients derived from the first period were used to reconstruct temperatures in the second period, and vice versa (Tab. 1), thus producing two sets of independent data. The thin curves are the instrumental data and the thick curves are the estimated data. Correlation is 0.78-0.79 between estimated and observed independent data.

8 Torneträsk tree-ring width and density AD 500 – 2004

Figure 8. Chronologies (black curves) from Regional Curve Standardized TRW (a) and MXD (c) plotted against summer (June �– August) mean temperature (grey curves). The time series have been scaled to have the same mean and standard deviation over the period 1869 �– 2004. Changes on 100-year timescales are emphasized using a smoothing spline filter. Figures b and d show the linear trends over the same period. year periods: 1869–1936 and 1937–2004. Equation of the variance in the independent instrumental data 5 was fitted over one period using multiple linear and that the regression weights are similarly high in regression, and with the derived coefficients applied both periods, with a large positive weight on MXD to the tree-ring data in the other period. The proc- (Tab. 1). Hence, the calibration/verification exercise ess was then repeated with the periods reversed. This shows no indication of a change in sensitivity of tree calibration/verification scheme produces independ- growth to temperature in the recent period (Fig. 7). ent estimates of temperature that can be compared to However, the strong weight from MXD in the re- the observed temperature data. The analysis shows construction equation means that minor changes that the tree-ring reconstruction recovers 61-63 % in the sensitivity of TRW may be overruled by the robust signal from MXD. Therefore, the chronologies of MXD and TRW were also examined sepa- rately in relation to summer temperature (Fig. 8). These results confirm that there is no evidence of a change in the sensitivity of MXD to temperature over the calibration period AD 1869 – 2004 (Fig. 8c- d). However, TRW shows a slightly diverging trend which indicates an increased sensitivity of TRW to temperature over this period (Fig. 8a-b). The starting Figure 9. Difference between the Regional Curve Stand- point of this apparent change in sensitivity of TRW ardized TRW and MXD chronologies (chronologies in is identified by comparing the longer-term trends in Fig. 4c-d). A linear function (thin line) was fitted to the the RCS standardized MXD and TRW chronologies anomaly over the period 1800 � 2004 and used to “correct” (Fig. 4c-d). Using linear regression, a new set of es- the data over this period (thin curve). timated TRW values were calculated by regressing

9 Håkan Grudd

Figure 10. Variously reconstructed summer (June-August) temperature for the period AD 500 to 2003 at Tor- neträsk based on maximum latewood density (MXD) and tree ring width (TRW). Details of the five reconstructions are given in Table 2. The annual data (in grey) has been filtered to emphasize climatic variability on 30-year and century time scales (black curves). All series are expressed as anomalies (in °C) from their 1951-1970 mean.

10 Torneträsk tree-ring width and density AD 500 – 2004

Table 2. Details of the various June – August temperature reconstructions.

Reconstruction I II III IV V Figure 10a 10b 10c 10d 10e Standardisation method a TRW A B B C C MXD A B B B C Correlation b 0.76 0.81 0.80 0.80 0.76 Explained variance c 58 65 65 64 58

Regression weights -0.627 0.057 0.127 0.115 0.296 TRW t+1 0.970 0.900 0.946 1.135 1.226 TRW t 12.897 7.343 7.511 7.716 6.965 MXD t Constant 1.926 1.328 1.391 0.922 1.134

a A – no standardisation, chronology is arithmetic mean of the raw data (Fig. 4a-b). B – regional curve standardisation (Fig. 4c-d). Reconstruction III uses the “corrected” TRW data (Fig. 7). C – standardisation using a spline of 67% length of individual series (Fig. 4e-f). b Correlation between instrumental and reconstructed summer (JJA) temperature over the calibration period 1869 – 2004. c Per cent (%) explained variance over the calibration period 1869 – 2004.

the TRW data on the MXD data. All data from AD 58% of the variation in the instrumental tempera- 500 to 2004 were used in the regression. The residu- ture data over the calibration period (AD 1869 –� als (observed - estimated) show an increasing trend 2004) and shows a great deal of low-frequency vari- from about AD 1800 (Fig. 9). Similarly diverging ation over the last 1,500 years. However, these data trends from about AD 1800 are also evident when are not standardized and are, therefore, biased by comparing the raw data MXD and TRW chronolo- intrinsic age-related growth trends in the constituent gies (Fig. 4a and 4b). series. In reconstruction II (Fig. 10b) the age-related growth trends were removed by RCS standardisation which significantly improved the correlation AD 500-2003 climate reconstruction with temperature (65% explained variance). This The reconstruction equation (Eq. 5) was finally re- reconstruction retains a large portion of the low-fre- calibrated using various combinations of the Tor- quency variation in the data while removing most neträsk MXD and TRW chronologies and the full of the age-related growth trends. However, it is bi- 136-year period of overlapping summer (June – Au- ased by the increased sensitivity of TRW to tempera- gust) temperature data (AD 1869 �– 2004). Five alter- ture over the last 200 years. To adjust for this bias, native reconstructions of summer temperature were a straight line was fitted to the “residual anomaly” made for the period AD 500 �– 2003 and expressed apparent from AD 1800 to 2004 in the TRW data as anomalies in °C from a common baseline period (Fig. 9) and a “corrected” RCS standardized TRW (Fig. 10). The choice of AD 1951 – 1970 as base pe- chronology was created by subtracting the trend-line riod enables the new results to be compared with data from the observed data. Reconstruction III uses previously published data from Torneträsk based on this “corrected” version of the TRW chronology this same period. Details of the five reconstructions together with the RCS standardized MXD chronol- (denoted I – V) are given in Table 2. ogy. The “correction” results in higher reconstructed Reconstruction I (Fig. 10a) uses the raw data temperatures prior to AD 1800 (Fig. 10c). It does chronologies (Fig. 4a-b) as predictor variables, i.e. not, however, improve the correlation statistics in data with no prior standardisation or other statisti- the calibration period (Tab. 2). Reconstruction IV cal treatment. This raw data reconstruction captures (Fig. 10d) uses a combination of the two used stand-

11 Håkan Grudd

Figure 11. The top panel (a) shows the new Torneträsk reconstructions of June – August temperature, smoothed with a 30-year filter. The black curve is the preferred Reconstruction IV. The grey band shows the range between Reconstruc- tions II and III (see Table 2 for details). In the lower panel (b), Reconstruction IV is compared with two previously published temperature reconstructions based on tree-ring data from Torneträsk: The thin curve is from Briffa et al. (1992) and based on TRW and MXD. The hatched curve is from Grudd et al. (2002) and based on TRW. All three reconstructions have been smoothed with a 100-year spline filter and have a common base period: AD 1951 – 1970.

ardization techniques: The MXD data was standard- “corrected” TRW chronology gives the highest tem- ized with the RCS technique (Fig. 4d) and the TRW peratures, while reconstruction II, which includes data was standardized with the spline technique (Fig. the “un-corrected” TRW data, consistently shows 4e). This reconstruction benefits from the fact that it the lowest temperatures. Reconstruction IV, which does not include any age-biased data or “corrected” is based on the RCS standardized MXD data and the TRW data, and that regression statistics remain high spline-standardized TRW data, gives parallel results (Tab. 2). Reconstruction V (Fig. 10e) is constructed mostly within the limits of the other two reconstruc- from the spline-standardized MXD and TRW chro- tions. Reconstruction IV is, therefore, chosen as the nologies (Fig. 4e-f) where most of the low-frequency preferred version and compared to previously pub- information is removed. lished reconstructions which were based on earlier The five reconstructions show annual variation versions of the MXD and TRW data from Torneträsk in summer temperature over the last 1,500 years. (Briffa et al., 1992; Grudd et al., 2002). The amplitude in annual temperature is about 5 °C Figure 11b shows that reconstruction IV has on with the coldest summer occurring in AD 1904 in all average 0.4 °C higher temperatures than Briffa et al. five reconstructions (Fig. 10). To emphasize climate (1992) and 1.2 °C higher temperatures than Grudd change on 30-year and 100-year timescales the data et al. (2002). The differences are even larger (+0.6 were also filtered using smoothing functions. Nota- °C and +1.3 °C respectively) in the warm 200-year bly cold periods occur around AD 650, 800, 1150, period around AD 1000. In the earlier reconstruc- 1600 and 1900. The warmest summer temperatures tions, a prominent cold period was centred on AD occur in a particularly long and warm period cen- 1600. This cold period is less prominent in the new tred on AD 1000. Other warm periods occur around reconstruction and matched by an equally cold pe- AD 750, 1175, 1400 and 1750. In none of these five riod around AD 1900. reconstructions does the 20th century stand out as a particularly warm period. Reconstructions II� – IV are based on the RCS Discussion standardized MXD chronology and various versions The transfer functions generally used for reconstruct- of the TRW chronologies (Fig. 11a). These three re- ing past climate have to be calibrated using instru- constructions show virtually identical results in the mental climate data. Hence, the quality of the data in calibration period (AD 1869 – 2004). However, in the overlapping period is fundamental, i.e. the tree- the earlier part of the record they show a range of ring chronologies must be sensitive to climate, and about 0.35 °C. Reconstruction III which includes the the instrumental data must reflect local conditions.

12 Torneträsk tree-ring width and density AD 500 – 2004

Furthermore, the overlapping period must be long structure was created (Fig. 2). As a result, the loss of enough to enable both a calibration and a verifica- sensitivity in recent MXD to temperature, apparent tion of the reconstruction equation using independ- in SEAL88, is now eliminated in the new Torneträsk ent data. The new Torneträsk tree-ring chronologies data (Fig. 1). meet with these requirements: they extend the tree- The RCS method is capable of preserving a large ring records for 24 years for MXD and 7 years for portion of the low-frequency climatic variation in TRW as compared to the previously existing data the data (Fig. 10). The Regional Curves used in this (Schweingruber et al., 1988; Grudd et al., 2002) and study have a linear form for MXD and a negative they significantly improve the quality of the data exponential form for TRW. The rationale for using used for analyses of growth response to climate. Us- a linear function for MXD is based on the fact that ing local instrumental climate data from Abisko, it is maximum density in the latewood (MXD) are point- established that Torneträsk tree growth is largely de- values which do not necessarily have the same geo- termined by temperatures in the summer period June metrical constraint as is the case with tree ring width − August (Fig. 6). This is a much shorter summer (TRW), which are measures of distance and thus relat- period than the five-month season (April – August) ed to the basal area increment for each year. Non-lin- defined by Briffa et al. (1990; 1992). However, the ear functions have been used in RCS standardization bootstrapped response-function analysis technique of MXD data, such as negative exponential curves used in this study (Biondi & Waikul, 2004) gives (Briffa et al., 2002a) and spline functions (Luckman more reliable estimates of the significance levels of & Wilson, 2005). However, pending more data from the response values as compared to the previous very old pine trees (>350 years), there is little support studies, where the significance levels probably were for using other than a linear Regional-Curve function overestimated. April is in fact a winter month in for the Torneträsk data. However, where data sets northern Sweden (Fig. 5) and it is not until May that have an age structure similar to the Torneträsk MXD snow starts to melt. It is, therefore, justified to use data, great care has to be taken when using linear the more restricted definition of a season for tem- or exponential Regional-Curve functions since they perature reconstruction. The previous reconstruc- can potentially introduce a new bias in the resulting tions of summer temperature at Torneträsk (Briffa MXD index chronologies which, also, would appear et al., 1990; Briffa et al., 1992; Grudd et al., 2002) as a loss in sensitivity to temperature in the 20th cen- have recovered, at best, 55% of the variance in the tury. This is illustrated in Figure 3 which shows how instrumental temperature data. The new Torneträsk the linear RCS function may be biased by including MXD and TRW data explain 65% of the variance data with low replication. In this study, hence, only in the instrumental June – August temperature data data where the sample replication is greater than 25 (Fig. 7 and Table 1). were included in the linear regression of the Region- Diverging trends between MXD and temperature al Curve, thus leaving out the oldest sections of the in the late 20th century is a common phenomenon 25 most elderly trees. Many of these are living trees across the high latitudes of the Northern Hemisphere that have survived the coldest phase of the Little Ice (Briffa et al., 1998b). This widespread apparent loss Age (Grove, 1988) which means that the variation in the sensitivity of density to temperature is, how- seen in the “tail” from about year 400 to 600 (cam- ever, not understood (Briffa et al., 2004). Several dif- bial age) is strongly influenced by the climate of the ferent explanations have been proposed, relating the 19th and 20th centuries. Hence, if these “old tail” phenomenon to changes in the atmosphere (Myneni data were to be included in the calculation of the et al., 1997; Briffa et al., 1998b; Briffa et al., 2004) linear Regional Curve, as was the case in Briffa et al. and to climate induced changes in the environment (Briffa et al., 1992), it would result in systematically (Vaganov et al., 1999; Barber et al., 2000; D’Arrigo depressed index values for data with cambial ages et al., 2004; Wilmking et al., 2004; Wilmking et al., greater than about 200 years (Fig. 3) and thus ap- 2005). However, as demonstrated here, using data pear as a loss in sensitivity of recent growth. Hence, with a disproportionately high cambial age in the the alternative hypothesis, that the widely reported most recent period can create a similar phenom- loss in sensitivity of MXD to temperature is related enon (Fig. 1). The inclusion of new data, including to the data and/or methods used, can not be rejected relatively young trees in the most recent period, has based on present results from Torneträsk. This calls significantly reduced the mean cambial age of MXD for further investigations of other MXD data series data in the 20th century and thus a more even age that show a similar phenomenon.

13 Håkan Grudd

The diverging trends between TRW and instru- tially biased trend in the last 200 years and the result mental temperature over the last 136-years indicate is mostly within the limits of the two reconstruc- an increased sensitivity of TRW to temperature (Fig. tions that makes use of the “corrected” (Rec. II) and 8b). Using the MXD data as a base, this increased “un-corrected” (Rec. III) RCS standardized TRW sensitivity apparently started around AD 1800 (Fig. chronology (Fig. 11a). Hence, reconstruction IV is 9). Diverging trends in MXD and TRW were regis- chosen as the preferred version of a new Torneträsk tered also by Briffa et al. (1992). However, they in- summer temperature reconstruction. terpreted this phenomenon as a loss in sensitivity of The local instrumental temperature record at Tor- MXD to temperature. The new and improved tree- neträsk shows an increase in summer temperature of ring and instrumental data used in this paper clearly about 1 °C over the last century (Fig. 8). This long- show, on the contrary, that the diverging trends are term trend is characteristic for a much larger area caused by an increased sensitivity of TRW to tem- in northern Sweden (Alexandersson, 2002). How- perature over the most recent 200-year period. Simi- ever, a large part of the trend is caused by unusu- lar phenomena have been reported also from other ally cold summers around AD 1900 and unusually parts of the high latitudes of the Northern Hemi- warm summers around AD 2000. The cold period sphere and attributed to a “fertilization” effect from around AD 1900 marks the culmination of the Lit- high CO2 concentrations in the atmosphere (Briffa tle Ice Age in northern Sweden, when many glaciers et al., 1998a), analogous to the “greening” effect ob- in this region advanced to their Holocene maximum served in other arctic and sub-arctic plants (Myneni position (Karlén, 1988), and warm summers in the et al., 1997). The starting point for this phenomenon late-20th century are commonly attributed to the en- in about AD 1800 in the Torneträsk data coincides hanced greenhouse effect (IPCC, 2001). Hence, the with the very initial stages of increased CO2 concen- century timescale temperature variation seen in the trations in the atmosphere, which indicates a causal instrumental records is most likely caused by a com- coupling to CO2. It seems, however, improbable that bination of natural and anthropogenic climate forc- TRW would have this immediate and strong reac- ing. The relative contribution from these two forcing tion on the moderately increased CO2 concentration mechanisms is, however, a matter of discussion (Es- around AD 1800. Furthermore, the apparent change per et al., 2005) and has a profound significance for in sensitivity around AD 1800 coincides with an im- predicting future climate scenarios. To realistically portant regeneration period in the Torneträsk area predict the natural component of climate variabil- which is expressed in the doubling of the sample ity, models should ideally be calibrated to the pre- depth at this time (Fig. 2b). Hence, pending further industrial (natural) climate, and since instrumental investigations of the Torneträsk data, the alternative or historical data from this period are scarce, proxy hypothesis which relates apparent changes in sensi- climate records have to be used. tivity to data and/or method remains valid also for The new Torneträsk reconstruction shows over- the TRW data. all higher temperatures as compared to the previ- The new reconstructions explain up to 65% of the ously published data (Briffa et al., 1992; Grudd et year-to-year variance in the instrumental tempera- al., 2002). The differences are especially significant ture data in the calibration period (Tab. 2). This is in the 200-year warm period centred on AD 1000 an outstanding result for a tree-ring based climate which coincides with the so called Medieval Warm reconstruction. On lower frequencies, the agree- Period (Hughes & Diaz, 1994), hence indicating that ment is even higher (Fig. 7 and 8) which indicates this period was much warmer that previously recog- that the decade-to-century timescale variations seen nized. The principal explanation for these large dif- in the 1,500-year reconstructions are real (Fig. 10). ferences in average temperature is the fact that earlier The strong weight from MXD in the reconstruc- reconstructions used tree-ring data that was biased tion equations (Eq. 5 and Tab. 2), however, means in the most recent c. 200 years, resulting in too low that most of this low-frequency variation emanates reconstructed temperatures prior to AD 1800. The from the density data. This is especially true for re- new and much improved Torneträsk summer-tem- construction IV which combines the (low-frequency) perature reconstruction shows that the late-20th cen- RCS-standardized MXD chronology with the (high- tury is not an exceptionally warm period in the last frequency) spline-standardized TRW chronology. 1,500 years (Fig. 11). In fact, summers around AD The use of a spline-standardized TRW chronology 750, 1000, 1400, and 1750, were all warmer than means that reconstruction IV is free from the poten- at present.

14 Torneträsk tree-ring width and density AD 500 – 2004

Conclusions Biondi, F. & Waikul, K. (2004) DENDROCLIM2002: A C++ program for statistical calibration of climate signals Torneträsk maximum latewood density (MXD) and in tree-ring chronologies. Computers & Geoscience, 30, tree-ring width (TRW) are updated to AD 2004. 303-311. These new data enable a much improved reconstruc- Briffa, K.R. (2000) Annual climate variability in the tion of summer (June –August) temperature for the Holocene: intepreting the message of ancient trees. Qua- last 1,500 years that recovers 64% of the variance ternary Science Reviews, 19, 87-105. in the instrumental temperature data (calibration pe- Briffa, K.R., Bartholin, T.S., Eckstein, D., Jones, P.D., Kar- riod: AD 1869 – 2004). The coldest period in the lén, W., Schweingruber, F.H., & Zetterberg, P. (1990) A record occurs around AD 1900, concurrent with geo- 1400-year tree-ring record of summer temperatures in logical evidence of maximum Little Ice Age glacier Fennoscandia. Nature, 346, 434-439. advance in the area, and the warmest period is cen- Briffa, K.R., Jones, P.D., Bartholin, T.S., Eckstein, D., Sch- tred on AD 1000. The 20th century temperature is weingruber, F.H., Karlén, W., Zetterberg, P., & Eronen, moderate in this record. M. (1992) Fennoscandian summer from AD 500: tem- TRW shows an apparent change in sensitivity to perature changes on short and long timescales. Climate Dynamics, 7, 111-119. temperature starting at about AD 1800. The reason for this change is not known and pending further Briffa, K.R. & Osborn, T.J. (2002) Blowing Hot and Cold. Science, 295, 2227-2228. investigations it can not be refuted that the phenomenon is an artefact of the data. Hence, the problem Briffa, K.R., Osborn, T.J., & Schweingruber, F.H. (2004) is circumvented by using spline standardized TRW Large-scale temperature inferences from tree rings: a review. Global and Planetary Change, 40, 11-26. data in the final reconstruction. Furthermore, MXD has a much stronger weight than TRW in the recon- Briffa, K.R., Osborn, T.J., Schweingruber, F.H., Jone, P.D., struction equation (quotient is about 7/1), which Shiyatov, S.G., & Vaganov, E.A. (2002a) Tree-ring width and density data around the Northern Hemisphere: Part means that most of the reconstructed temperature 1, local and regional climate signals. The Holocene, 12, variability emanates from MXD. 737-757. When MXD data are produced exclusively from th Briffa, K.R., Osborn, T.J., Schweingruber, F.H., Jone, P.D., old-aged trees in the 20 century, they show an ap- Shiyatov, S.G., & Vaganov, E.A. (2002b) Tree-ring width parent loss in the sensitivity of MXD to tempera- and density data around the Northern Hemisphere: Part ture in the most recent period. By including data 2, spatio-temporal variability and associated climate also from relatively young trees, this apparent loss in patterns. The Holocene, 12, 759-789. sensitivity is eliminated in the Torneträsk data. This Briffa, K.R., Schweingruber, F.H., Jones, P.D., Osborn, T.J., emphasizes the importance of data quality and calls Harris, I.C., Shiyatov, S.G., Vaganov, E.A., & Grudd, H. for further analyses of other MXD series around the (1998a) Trees tell of past climates: but are they speak- Northern Hemisphere where a similar phenomenon ing less clearly today? Philosophical Transactions of the Royal Society of London, B 353, 65-73. has been reported. Briffa, K.R., Schweingruber, F.H., Jones, P.D., Osborn, T.J., Shiyatov, S.G., & Vaganov, E.A. (1998b) Reduced sensitivity of recent tree-growth to temperature at high Acknowledgements northern latitudes. Nature, 391, 678-682. This work was financed by EC projects PINE (Con- Cook, E. (1990). A conceptual linear aggregate model for tract No EVK2-CT-2002-00136) and Millennium tree rings. In Methods of dendrochronology: applica- (Contract No 017008 GOCE). tions in the environmental science. E. Cook & L. Kairi- ukstis eds. Kluver Academic Publishers, Dordrecht. 394, pp. References Cook, E., Briffa, K.R., Shiyatov, S., & Mazepa, V. (1990). Alexandersson, H. (2002). Temperature and precipitation Tree-ring standardization and growth-trend estimation. in Sweden 1860-2001. SMHI Meteorologi,104. In Methods of Dendrochronology. Applications in the Environmental Sciences. E. Cook & L.A. Kariukstis eds. Barber, V.A., Juday, G.P., & Finney, B.P. (2000) Reduced Kluwer Academic Publishers, Dordrecht. 394, pp. growth of Alaskan white spruce in the twentieth century from temperature-induced drought stress. Nature, 405, Cook, E., Esper, J., & D’Arrigo, R. (2004) Extra-tropi- 668-673. cal Northern Hemisphere land temperature variability over the past 1000 years. Quaternary Science Review, Bartholin, T.S. (1987) Dendrochronology in Sweden. An- 23, 2063-2074. nales Academiae Scientiarum Fennicae, 145, 79-88.

15 Håkan Grudd

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16 Torneträsk tree-ring width and density AD 500 – 2004

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Paper III

GEOPHYSICAL RESEARCH LETTERS, VOL. 27, NO. 18, PAGES 2957-2960, SEPTEMBER 15, 2000

Swedish tree rings provide new evidence in support of a major, widespread environmental disruption in 1628 BC

H˚akan Grudd1 Climate Impacts Research Centre, Kiruna, Sweden Keith R. Briﬀa Climatic Research Unit, University of East Anglia, UK Bj¨orn E. Gunnarson and Hans W. Linderholm

Dept of Physical Geography, Stockholm University, Sweden

Abstract. Pine trees, recovered from a peat bog in south- the Aegean Sea, which at the time had been approximately 14 central Sweden, are used to develop a continuous, but “float- C-dated to the 17th century BC. Subsequently, evidence ing”, 200-year tree-ring chronology. By wiggle matching of a period of extremely low growth in Irish, English, and high-precision 14C determinations against the radiocarbon German oaks, starting in the year 1628 BC, was ascribed calibration curve, the chronology is positioned at 1695 - 1496 to the same event [Baillie and Munro, 1988; Baillie, 1990]. BC with an uncertainty of 65 years. One major event, More recently, Kuniholm et al. [1996], have identified a very denoted by four consecutive years with extremely narrow large growth anomaly in a tree-ring chronology from Ana- rings, indicative of highly unfavourable local tree-growth tolia (Turkey) that could also be dated to this time. Hence, conditions, occurred in the time window represented by the the current state of knowledge is of a major environmental chronology. This growth depression is dated to 1637 BC disruption, possibly a major volcanic eruption in 1628 BC, (65) and may be tentatively ascribed to the same phe- that had widespread climatic effects in the mid-latitude re- nomenon that caused frost damage in trees in California gions of the Northern Hemisphere. and a growth depression in European oak in 1628/27 BC, Evidence for the 1628/27 BC tree-ring event in higher lat- hence providing new evidence of a more northerly area of itudes has, until now, been lacking due to the few precisely influence of this widespread phenomenon. dated tree-ring chronologies across this period. We now report new evidence indicating a strong effect in south-central Introduction Sweden apparently at this same date. Tree rings are widely used as proxy records of environmental change. Different forms of growth stress can leave Materials and methods their imprint in the annual width of tree rings [Douglass, Hanvedsmossen is a peat bog situated 50 kilometres south 1920]. By combining sample measurements from several of Stockholm in south-central Sweden. Large-scale peat har- trees from within one site it is possible to construct a time vesting at this site has exposed hundreds of pine trees in the series of environmental change that has annual resolution contact zone between the peat and the substratum. Thus, and, where it is continuous to the present day, absolute dat- the trees pre-date peat inception at this site. This ancient ing accuracy [Fritts, 1976]. Such tree-ring chronologies can well-grown forest, radiocarbon dated to the 17th and 16th reach far into the past [Spurk et al., 1998] providing year- centuries BC, represents a 200-year period of relatively dry to-year information on local site conditions. conditions when pine was able to germinate on a shallow When, in 1984, LaMarche and Hirschboeck [1984] com- dried-out fen-moor surface underlayed by fine-grained min- piled a record of frost damaged tree-rings in bristlecone pine eral soils. The forest was killed by a change to more humid in the western USA, extending from the present back to 3435 conditions and was subsequently enveloped and preserved BC, they found good agreement in the timing of implied by peat [Gunnarson, 1999]. frost events and volcanic eruptions known from historical In this analysis we use 41 samples - full discs cut from sources. One very severe frost event in the prehistoric period pine trunks. Tree-rings were measured to the nearest 1/100 dated to 1627 BC. Since this was the only frost-ring event in mm in two radii in each disc. The two radii were cross- the second millennium BC it was tentatively attributed to checked for measurement error and averaged to produce a the cataclysmic Bronze Age eruption of Santorini (Thera) in mean tree-ring growth series for each tree. Mean tree-series were then crossdated following standard tree-ring methods [Schweingruber, 1988] and given relative dates using an ar- 1Also at Dept of Physical Geography, Stockholm University, Sweden bitrary but consistent time scale. The aim of the present study is to highlight extreme events, i.e. abrupt short-term Copyright 2000 by the American Geophysical Union. changes in the data. Thus, long-term influences, manifest as slowly evolving growth patterns, are of no interest, and Paper number 1999GL010852. can be filtered out leaving only the high frequency signal. 0094-8276/00/1999GL010852$05.00 This form of processing the data, known as “standardisa- 2957 2958 GRUDD ET AL.: SWEDISH TREE RINGS IN 1628 BC

Table 1. 14C-dates referred to in Figure 1 Cal. years BC -1750 -1700 -1650 -1600 -1550 -1500 -1450 -1400 Sample Lab. ID yr BP 1σ Ring Cal yr BC 3500 a St14158 3435 40 992 1770-1630 b St13577 3320 70 1007 1690-1500 c St13578 3200 60 1019 1530-1390 3400 d St14157 3300 30 1060 1605-1505 e St14159 3360 30 1113 1690-1530 f St14160 3270 40 1168 1600-1450 3300

C years BP C years

14 3200 tion” [Fritts, 1976; Cook and Kairiukstis, 1990], is routinely used in tree-ring chronology construction to highlight a spe- 3100 a b c de f cific environmental signal. Our choice of standardisation method involved two steps: (1) transformation of the data 990 1030 1070 1110 1150 1190 to logarithms in order to normalise the very large variabil- Relative ring number in chronology ity in the early juvenile phase of tree growth; (2) filtering of each individual tree-ring series by fitting a flexible smooth- Figure 1. Graphical representation of the wiggle match. See ing spline function with a 50% frequency cut-off of 100 years. also Table 1. This produces a set of indexed series where most of the low- frequency variation is filtered out. The indexed series were finally averaged to produce a mean chronology for the site. to our discussion because it bears directly on the extent to Extremes were identified and quantified by transforming which we can establish whether or not the 1628/27 BC event, the mean chronology to a series of z-scores, using the for- seen in California, central Europe and the British Isles, also mula: had an influence on Scandinavian trees. The wiggle match Zt =(It − m)/s, is by no means perfect, since we excluded one outlier in the calculation. However, the radiocarbon calibration curve in where It is the index value for each year t; m and s are Figure 2 is constructed from ten-year blocks of wood [Stuiver the mean and the standard deviation respectively. Z-scores and Becker, 1993] which means that it can be regarded as a outside 3 are considered as extreme events in the series. 10-year low pass smoothed record. Our 14C determinations Six samples from the chronology, with precisely known were made from 1-8 year blocks of wood and we should, separation (in years) and each representing a sample of wood therefore, expect our data to be more scattered than the with 1-8 annual rings were radiocarbon dated (Tab. 1). 14 ten-year smoothed calibration curve. Thus, we are confi- The C determinations were then wiggle matched [Pearson, dent that the 200-year chronology dates to 1695-1496 BC 1986], to the long radiocarbon calibration curve [Stuiver and with an uncertainty of 65 years. Becker, 1993] using the calibration software OxCal [Bronk The European Oak tree-ring width chronologies [Baillie, Ramsey, 1994]. This operation is essentially a least squares 1990] and the bristlecone pine frost-ring record [LaMarche calculation of the combined errors, which significantly in- 14 and Hirschboeck, 1984], are both absolutely dated, i.e. there creases the dating accuracy. One of the C-determinations is no uncertainty associated with the dates of individual (c in Tab. 1) was regarded as an outlier and was omitted rings. Therefore, we can be certain that oak trees in different from the calculation. parts of central Europe and the British Isles were affected by a period of stressful growth conditions starting in the year Results 1628 BC. We can also be certain that pine trees at the tim- The wiggle match (Fig. 1) provided a date to within berline in the White Mountains of California were affected 65 calendar years with 95% probability (23 years with by severe cold in the year 1627 BC. These lines of evidence p=68%). Thus, the 200-year chronology covers the time period from 1695 - 1496 BC (65). We can identify one major anomalous growth event in the Years BC (Wiggle matched) chronology: Four consecutive years, between relative rings -1650 -1600 -1550 -1500 z 1050-1053,have -scores below -3 (Fig. 2). The two most 3 extreme years have scores around -4, indicating a very extreme low-growth anomaly. The first year of this major 0 anomaly dates to 1637 BC 65 years (1625 BC 23 with Z-value p=68%). Minor low-growth anomalies can be identified at -3 relative rings 1034, 1086, 1124 and 1147 (Fig. 2). 1000 1050 1100 1150 R e la tive rin g n o . Discussion Figure 2. The mean chronology has been transformed to a time Our data from Hanvedsmossen represent an annually re- series of z-scores. The first year of the major growth reduction, solved and well-replicated record of environmental variation starting at relative ring 1050, was wiggle matched to 1625 BC. in a time window of 200 years in the second millennium BC. The shaded area represents the 1σ confidence interval for this The dating precision of this “floating” chronology is central date. GRUDD ET AL.: SWEDISH TREE RINGS IN 1628 BC 2959 point to a climate cooling in the mid-latitudes of the North- the tree-ring signal is generally a secondary, climatic effect: ern Hemisphere starting in the year 1628 BC. Supportive, Consequently, tree-ring derived attributions are only ever but less precise, tree-ring evidence of a climatic event at circumstantial. this time comes from a continuous but “floating” 1,503-year The 1628/27 BC event caused an extremely severe growth tree ring chronology from Anatolia in Turkey, which has depression in our data, lasting for about 4 years. Another also been dated using the radiocarbon wiggle-match tech- less severe depression, but one that lasted 7 years, occurred nique [Kuniholm et al., 1996]. Similarly, the new 200-year one hundred years later, and several other shorter but no- chronology from Hanvedsmossen now has a very unusual table growth reductions are also apparent (Fig. 2), indicat- growth reduction at 1637 BC 65 years (1625 BC 23). ing a high frequency of unfavourable tree growth periods. Our dating and the severe magnitude of this phenomenon This may be an indication of multiple volcanic eruptions suggest that it can be ascribed to the 1628/27 BC event, in this 200-year time frame. Indeed, several other eruptions hence providing new evidence of a wider, more northerly beside Santorini have been approximately dated to this time area of influence. [Vogel et al., 1990], and the ice cores have acid spikes that The tree-ring evidence, thus, points to a major environ- point to, as yet, unidentified volcanic activity. However, mental disruption in 1628/27 BC having effect over much based on present knowledge, Santorini was the largest ex- of the Northern Hemisphere. One likely candidate to have plosive volcanic eruption in this time. caused this effect is a major volcanic eruption. Large explosive volcanic eruptions are known to produce detectable Conclusions cooling over a period of 1-3 years, even on temperatures averaged across the Northern Hemisphere [Angell and Ko- A new, continuous but “floating”, 200-year tree-ring rshover, 1985]. Environmentally sensitive tree rings from a chronology from Hanvedsmossen in south-central Sweden is wide range of geographical areas have been shown to dis- radiocarbon dated, with the technique of wiggle matching, play this temperature effect, caused by increased amounts to the period 1695 - 1496 BC with an uncertainty of 65 of aerosols in the stratosphere which reduce solar radiation years. [Scuderi, 1990; Jones et al., 1995; Briffa et al., 1998]. Our In the time window represented by the chronology, one data from Hanvedsmossen show a very strong 4-year reduc- major environmental event, probably a severe cooling, oc- tion in growth, presumably because of a severe cooling, con- curred at 1637 BC 65 years. This event is therefore consistent with the hypothesis of a major volcanic eruption in temporaneous with other tree-ring evidence of a severe cli- 1628 BC. mate anomaly at around 1628 BC, previously identified in North America, western Europe and the Aegean, reinforcing Much of our present knowledge on the timing and poten- and extending the evidence to show an increased geograph- tial climate effectiveness of prehistoric volcanism comes from ical area of effect, now incorporating south-central Sweden. ice cores, through the measurements of acidity at different The evidence is consistent with the hypothesis of a major depths which represent volcanic aerosol deposition through Northern Hemisphere volcanic eruption in 1628 BC, which time [Hammer et al., 1980; Zielinski et al., 1994]. Ice-core may have been Santorini in the Aegean Sea. evidence of volcanic activity in the 17th century BC comes from three different deep cores from the Greenland Ice Sheet: Acknowledgments. 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letters to nature large single crystals; polycrystalline arrays may be brittle. Such ...... composites may be of use as high-performance damping materials, Climate variability 50,000 years ago since the ®gure of merit26 in the present results exceeds that of commonly used materials (E tand < 0:6 GPa) by a factor of more in mid-latitude Chile as than 20. For that purpose, sensitivity to temperature could be reduced via matching inclusion and matrix stiffness. Moreover, reconstructedfromtreerings pre-stressed or pre-buckled24 elements with minimal temperature sensitivity may be used as inclusions. Bounds on properties of Fidel A. Roig*, Carlos Le-Quesne²³, JoseÂ A. Boninsegna*, complex heterogeneous materials are generally derived assuming Keith R. Briffa§, Antonio Lara³,HaÊkan Gruddk, Philip D. Jones§ positive phase properties18. These bounds can be exceeded if & Carolina VillagraÂn¶ negative stiffness is allowed, permitting extreme properties not previously anticipated. As in thermoelastic and piezoelectric mate- * Laboratorio de DendrocronologõÂa, IANIGLA-CONICET, CC 330 (5500) rials, elasticity is coupled with temperature and electric ®eld Mendoza, Argentina respectively, these composites may ®nd use in high-performance ² Department BotaÂnica, Facultad de BiologõÂa, Universidad de Oviedo, sensors and actuators. These composites may also occur naturally CatedraÂtico Rodrigo UrõÂa s/n, 33006, Spain in rocks and in biological materials (in which anomalies were ³ Instituto de Silvicultura, Universidad Austral de Valdivia, Casilla 567, Valdivia, reported27); they may be considered in the context of deep-focus Chile earthquakes and attenuation of seismic waves. M § Climate Research Unit, School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK Received 25 July 2000; accepted 2 February 2001. k Department of Physical Geography, Stockholm University, S-10691 Stockholm, 1. Timoshenko, S. P. & Goodier, J. N. Theory of Elasticity 3rd edn (McGraw-Hill, New York, 1970). Sweden 2. Lakes, R. S. Foam structures with a negative Poisson's ratio. Science 235, 1038±1040 (1987). ¶ Department BiologõÂa, Facultad de Ciencias, Universidad de Chile, Casilla 653, 3. Lakes, R. S. Advances in negative Poisson's ratio materials. Adv. Mater. 5, 293±296 (1993). Santiago, Chile 4. Haeri, A. Y., Weidner, D. J. & Parise, J. B. Elasticity of a-cristobalite: a silicon dioxide with a negative Poisson's ratio. Science 257, 650±652 (1992)...... 5. Rothenburg, L., Berlin, A. A. & Bathurst, R. J. Microstructure of isotropic materials with negative High-resolution proxies of past climate are essential for a better Poisson's ratio. Nature 354, 470±472 (1991). 1 6. Baughman, R. H., Shacklette, J. M., Zakhidov, A. A. & Stafstrom, S. Negative Poisson's ratios as a understanding of the climate system . Tree rings are routinely common feature of cubic metals. Nature 392, 362±365 (1998). used to reconstruct Holocene climate variations at high temporal 7. Thompson, J. M. T. Stability prediction through a succession of folds. Phil. Trans. R. Soc. Lond. 292, resolution2, but only rarely have they offered insight into climate 1±23 (1979). variability during earlier periods3. Fitzroya cupressoidesÐa South 8. Thompson, J. M. T. `Paradoxical' mechanics under ¯uid ¯ow. Nature 296, 135±137 (1982). 9. Lakes, R. S. & Drugan, W. J. Stiff elastic composite materials with a negative stiffness phase. J. Mech. American conifer which attains ages up to 3,600 yearsÐhas been Phys. Solids (submitted). shown to record summer temperatures in northern Patagonia 10. Heckingbottom, R. & Linnett, J. W. Structure of vanadium dioxide. Nature 194, 678 (1962). during the past few millennia4. Here we report a ¯oating 1,229- 11. Paquet, D. & Leroux-Hagon, P. Electron correlations and electron-lattice interactions in the metal- year chronology developed from subfossil stumps of F. cupres- insulator ferroelastic transition in VO2: a thermodynamical study. Phys. Rev. B 22, 5284±5301 (1979). soides in southern Chile that dates back to approximately 50,000 12. Zhang, J. X., Yang, Z. H. & Fung, P. C. W. Dissipation function of the ®rst-order phase transformation 14C years before present. We use this chronology to calculate the in VO2 ceramics by internal friction measurements. Phys. Rev. B 52, 278±284 (1995). spectral characteristics of climate variability in this time, which 13. Handbook of Chemistry and Physics C-40 12±40 (CRC Press, Boca Raton, Florida, 1996). 14. Brodt, M., Cook, L. S. & Lakes, R. S. Apparatus for measuring viscoelastic properties over ten decades: was probably an interstadial (relatively warm) period. Growth re®nements. Rev. Sci. Instrum. 66, 5292±5297 (1995). oscillations at periods of 150±250, 87±94, 45.5, 24.1, 17.8, 9.3 and 15. Lakes, R. S. & Quackenbush, J. Viscoelastic behaviour in indium tin alloys over a wide range of 2.7±5.3 years are identi®ed in the annual subfossil record. A frequency and time. Phil. Mag. Lett. 74, 227±232 (1996). comparison with the power spectra of chronologies derived 16. Bramble, J. H. & Payne, L. E. On the uniqueness problem in the second boundary value problem in elasticity. Proc. 4th Natl Cong. Appl. Mech. 469±473 (American Society of Mechanical Engineers, from living F. cupressoides trees shows strong similarities with Berkeley, California, 1963). the 50,000-year-old chronology, indicating that similar growth 17. Knowles, J. K. & Sternberg, E. On the failure of ellipticity and the emergence of discontinuous forcing factors operated in this glacial interstadial phase as in the gradients in plane ®nite elastostatics. J. Elasticity 8, 329±379 (1978). 18. Hashin, Z. & Shtrikman, S. A variational approach to the theory of the elastic behavior of multiphase current interglacial conditions. materials. J. Mech. Phys. Solids 11, 127±140 (1963). With the exception of polar and tropical ice cores and some 19. Read, W. T. Stress analysis for compressible viscoelastic materials. J. Appl. Phys. 21, 671±674 (1950). ocean-sediment cores, palaeoclimate records with annual resolution 20. Hashin, Z. Viscoelastic behavior of heterogeneous media. J. Appl. Mech. Trans. ASME 32E, 630±636 based on studies of tree rings, coral, and varved sediments have been (1965). 5 21. Lakes, R. S. Extreme damping in composite materials with a negative stiffness phase. Phys. Rev. Lett. recovered only for the late Holocene . Here we report annually (in the press). resolved proxies of palaeoclimate data for southern South America 22. Rosakis, P., Ruina, A. & Lakes, R. S. Microbuckling instability in elastomeric cellular solids. J. Mater. for an interstadial period thought to correspond to marine oxygen Sci. 28, 4667±4672 (1993). isotope stage 3 (OIS 3; ref. 6). Our analysis is based on subfossil tree 23. Salje, E. Phase Transitions in Ferroelastic and Co-elastic Crystals 10, 72 (Cambridge Univ. Press, Cambridge, 1990). remnants found at Seno ReloncavõÂ in the southern Lake District of 24. Lakes, R. S. Extreme damping in compliant composites with a negative stiffness phase. Phil. Mag. Lett. Chile (408 009 to 428 309 S, 718 309 to 748 009 W). Here we use the 81, 95±100 (2001). term `subfossil' to mean wood preserved since the late Pleistocene 25. Ren, X. & Otsuka, K. Origin of rubber-like behaviour in metal alloys. Nature 389, 579±583 (1997). 26. Cremer, L., Heckl, M. A. & Ungar, E. E. Structure Borne Sound 2nd edn 243±247 (Springer, Berlin, epoch that still contains carbon material. Supported by glacial 1988). geology and palynology studies, the southern Lake District repre- 27. Pugh, J. W., Rose, R. M., Paul, I. L. & Radin, E. L. Mechanical resonance spectra in human cancellous sents an area where late Quaternary climate ¯uctuations have been bone. Science 181, 271±272 (1973). best established for the entire Andes7±10. Although the Lake District 28. Falk, F. Model free energy, mechanics, and thermodynamics of shape memory alloys. Acta Metall. 28, 1773±1780 (1980). was affected repeatedly by large ice lobes descending from the Andes 29. Duffy, W. Acoustic quality factor of aluminum alloys from 50 mK to 300 K. J. Appl. Phys. 68, 5601± at times . 14 kyr before present (BP), long-term full-glacial strati- 5609 (1990). graphic pollen records show that the forests did not completely disappear during the periods of ice advance and rapidly re- Acknowledgements expanded afterwards8,9. After ice recession, evergreen forest, We thank W. Drugan and R. Cooper for supportive comments and discussions. This work mainly dominated by broad-leaved and conifer elements, expanded was supported by the NSF. across the Lake District with a wide ecological amplitude, from Correspondence and requests for materials should be addressed to R.S.L. lowland ¯oodplains to near the modern, upper tree line. (e-mail: [email protected]). Annually resolved records in the region based on the characteristic

NATURE | VOL 410 | 29 MARCH 2001 | www.nature.com © 2001 Macmillan Magazines Ltd 567 letters to nature tree of the north Patagonian evergreen forest, F. cupressoides regarded as minimum ages for the subfossil wood. According to the (alerce)Ðwhich attains ages of 3,600 years11Ðwere available pre- chronological and stratigraphic glacial record proposed for the viously only for the past few millennia. Fitzroya, a monotypic area7, the wood samples pre-date the Last Glacial Maximum, and conifer restricted to wet environments, is a sensitive indicator of broadly correspond to the early portions of middle Llanquihue time climate; both the width and the density of the annual rings respond (correlated with the Middle Wisconsinan/Weichselian glacial stages primarily to variations of summer (December to March) tempera- in the Northern Hemisphere). Furthermore, the longer pollen tures11±13. In southern Chile, present seasonal climates are regulated records available for the region8,9 document the existence of an by the annual cycle of latitudinal migration of the South Paci®c arboreal taxa association with a signi®cant presence of Fitzroya- Subtropical High coupled with the cold off-shore Humboldt ocean related pollen types (Pilgerodendron type) in the early phases of 14 current and the orographic effects of the Andes14. The prevailing middle Llanquihue time (that is, about 47,000 CyrBP). This westerly storm tracks are related to equatorward shifts of the polar implies that tracts of interstadial forest immediately adapted to front. These low-pressure storms, which are most frequent during growth under the cool temperate, humid, climate regime in the winter, are responsible for the humid and equable climate that mid-latitudes of southern Chile at the early stages of OIS 3. The supports the temperate rain forests. simplest explanation, pending more precise dates, is that the In the forested landscape of southern Chile and Argentina, acidic subfossil Fitzroya stand became established during an interval of soils and climate are conducive to the preservation of tree remnants humid and cool climate, broadly corresponding to the onset of suitable for radiocarbon dating and dendrochronological OIS 3. research6,15. After the 1960 earthquake16 that affected the southern Subfossil wood samples from Pelluco were collected by different Lake District, an intertidal area alongside the north shore of Seno workers from Chile, Sweden and Argentina, and processed accord- ReloncavõÂ became subject to marine erosion. Well preserved sub- ing to standard dendrochronological techniques20. 51 polished fossil F. cupressoides stumps were exposed; the trees had been transverse cross-sectional surfaces from 30 trees were measured originally buried in situ by a lahar17 (a debris ¯ow composed chie¯y (with a precision of 6 0.01 mm), and the resulting individual ring- of volcaniclastic materials). Particularly at the Pelluco site (6 km east width series were subjected to extensive cross-dating comparisons of Puerto Montt, 41.58 S, 72.98 W), these stumps attain diameters of to identify missing rings and eliminate other possible errors at the up to 1±1.5 m. Few stumps recovered from Pelluco belong to measuring stage. After this procedure, 47 radii from 28 trees were Pilgerodendron uviferum. This is evocative of the present-day selected and used to construct a ¯oating mean tree-ring chronology Fitzroya±Pilgerodendron community on marshy, low-drainage 1,229 years long (the range of specimen length was 185± soils13. Because the outermost rings have been eroded from most 1,154 years). Tree-ring standardization20 was performed to remove of the cross-sections, it was impossible to ascertain if the trees were the inherent long-term growth trend associated with tree age and killed and buried simultaneously in sediment. However, the posi- forest dynamics. Individual series were detrended using a smooth- tion and orientation of the stumps suggest that this forest was ing spline with a ®xed 50% frequency response of 200 years, affected by a catastrophic single event. Tephra deposition, ¯ooding resulting in dimensionless indices, before averaging them into the and abrupt changes of the ground levelÐcaused, for example, by chronology. This standardization procedure preserves most of the earthquakesÐtoday affect the stability of F. cupressoides forests18; century-timescale tree growth variations21. The ®nal chronology, these factors must have operated similarly in the past. with the corresponding internal sample replication, is shown in The AMS (accelerator mass spectrometry) 14C ages of three Fig. 1. To our knowledge, this is the oldest known ¯oating (that is, wood samples containing the ®ve innermost rings are 49,780 6 not absolutely dated) tree-ring chronology developed to date. 14 14 14 2,040 CyrBP, 49,770 6 2,040 CyrBP, and 49,370 6 2,000 CyrBP To identify cycles contained in our Pelluco chronology, spectral (CAMS 46534, 46535, and 46536 respectively). Previous radio- analysis was performed using two different methods as de®ned in carbon dating showed similar results17,19. Because these dates are Fig. 2. Fourier analysis (using the Blackman-Tukey technique) close to or higher than the limit for 14C dating, they should be showed that long-term cyclic components exceeding the a priori

2.5

1.5

Ring width index 1

0.5

0 25

Number of cores 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 Number of rings Figure 1 The 1,229-year-long ¯oating tree-ring chronology from Pelluco, southern Chile. Missing rings were infrequent. The overall series mean ring-width is 0.42 mm and the A cross-dated set of 47 ring-width radii obtained from 28 subfossil trees make up the cross-correlation average between all series is 0.53, with a mean series length of ARSTAN (AutoRegressive STANdardization) chronology, whose best replicated section 430 years. The signal-to-noise ratio is relatively high at 5.25, and the variance explained (more than 5 series) exists between relative rings 278 and 1,139. The subfossil by the ®rst eigenvector is 32.27%. The ®rst-order autocorrelation is 0.85 and the mean chronology shows notable year-by-year changes in ring widths. Those periods with very sensitivity is 0.15. These values are similar to the statistics of living Fitzroya narrow rings (two cells wide) make measurement and cross-dating rather dif®cult. chronologies4,27.

95% signi®cance level occur at periods of 153±245 and 87±94 years, similar to those identi®ed in the subfossil series (that is, the peaks at which respectively account for 21.38% and 19.71% of the variance 82±95, 23.6 and 17.8 years in the living trees are close to peaks at in the power spectrum. Other signi®cant peaks in the spectrum, 87±94, 24.1 and 17.8 years detected in the subfossil chronology; Fig. which account for the 13% of total variance, fall within the higher 2a, b). In the low-frequency band of 80±254 years, the subfossil frequency band (periodicities in the range of 2±7 years; Fig. 2a chronology shows a high concentration of variance (41.09%) but inset). The same cycles are reproduced in the maximum entropy only 12.49% is reached in the modern chronology. Peaks at 47±51, power spectrum (not shown here). 35, 23.6, 20.1 and 17.8 years are statistically signi®cant in the The Blackman-Tukey and maximum-entropy spectral properties modern chronology but do not exceed the a priori 95% con®dence of the chronologies derived from living Fitzroya trees are broadly level in the Blackman-Tukey spectrum from the subfossil series. Association between the spectra of the living and subfossil records is also observed through a cross-spectra comparison in the frequency a 0.05 domain (Fig. 2c). The variances in the cross-amplitude spectra show 153-245 relatively strong correspondence over the resolved frequency range 87-94 Autocorrelations: 615 0.002 5.34 equivalent to periods centred at 136±153, 81±94, 47±53, 35, 24, 0.04 4.5 4.1 3.8-3.9 3.3 17.8 years and other peaks over the higher-frequency range between 0.001 3.1 2.86 2.78 6.6 and 2.14 years (Fig. 2c inset). The broad correspondence 0.03 0 between spectra suggest the possibility that similar processes have 0.15 0.20 0.25 0.30 0.35 Spectral estimates in¯uenced the radial growth of both subfossil and modern Fitzroya 45.5 Frequency (cycles yr–1) trees. 0.02 31.5 Focusing on longer timescales, it is evident from the spectral 24.1 density distribution that the ,150±250-year waveform is a char- Spectral estimates 0.01 17.8 acteristic of the Pelluco chronology, and may re¯ect low-frequency 9.3 variations of climate during the late Pleistocene. Oscillations of the order of ,200 years have been found in late Pleistocene±Holocene b 0.022 records from marine cores from the west side of the Antarctic 35 peninsula22. Solar modulation may be a possible forcing mechanism Autocorrelations: 615 14 47-51 0.004 6.54 at this frequency because there is evidence of a near 220-year C 4.96 0.017 0.003 2.8 23 4.0 3.2 2.9 cycle as a result of sunspots . Other cyclic ¯uctuations in the Pelluco 3.43 2.58 82-95 23.6 0.002 chronology on a decadal to centennial timescale may also be 20.1 0.001 attributed to varying solar activity. Atmospheric D14C oscillations 0.012 0 Spectral estimates 17.8 0.15 0.20 0.25 0.30 0.35 derived from tree-ring chronologies covering almost the entire Frequency (cycles yr–1) Holocene (a 9,600-yr record)23 show changes with periodicities

0.007 close to 22 and 90 years, cycles related respectively to Hale and Gleissberg global solar changes. In a similar way, the oscillatory Spectral estimates modes detected in the subfossil tree-ring chronology at the 87±94- 0.002 year and the 24.1-year periods may correspond to periodicities related to both Hale and Gleissberg solar cycles contained in the c 81-94 `modern' (Holocene) tree-ring records. 136-153 6.6 Determining the forcing mechanisms that produced the short- 5 4.58 0.006 5.1 term cycles in our record is less straightforward. The El NinÄo/ 47-53 0.004 4.3 3.7 Southern Oscillation (ENSO) appears to be the largest single source 3.2 2.77 4 35 0.002 of present-day global inter-annual climate variability. ENSO has 24

Cross-amplitude operated for most of the Holocene, and strong climate shifts 3 0 0.15 0.20 0.25 0.30 0.35 associated with El NinÄo events have been documented by, among 17.8 Frequency (cycles yr–1) other proxies, South American tree rings4,12,13. But for earlier times, 2 data from marine cores imply that during glacial sea-level low-

Cross-amplitude stands, the ocean/atmosphere conditions in the inter-tropical Paci- 1 11.8 ®c prevented or dramatically reduced the functioning of the ENSO system24,25. Furthermore, there are indications that the ENSO 0 system probably operated during middle±late Pleistocene intergla- 0 0.1 0.2 0.3 0.4 0.5 cials and warm interstadial periods, although overall climate and –1 Frequency (cycles yr ) boundary conditions at these stages may have been too different to Figure 2 The spectral and cross-spectral characteristics of the subfossil (a) and living (b) permit the characteristic ENSO climate variability in terms of F. cupressoides woods. We determined that 615 lags of the autocorrelation function frequency, intensity and recurrence26. Thus, some of the high- (,50% the series length) were adequate to resolve the frequency spectrum with high frequency oscillations at timescales of 2±7 years observed in our resolution and moderate stability. The 95% con®dence limits of the Blackman-Tukey28 subfossil chronology still remain to be assessed in relation to other (BT) spectrum were estimated from a ``red noise'' null continuum based on the lag-1 ancient high-resolution records. autocorrelation coef®cient. The periods are given in years for each individual peak. The F. cupressoides tree-rings are known to correlate well with summer (Hamming) bandwidth that we used was equal to 5. The maximum entropy29 (ME) spectrum temperatures4,11,13. This relationship permits reconstruction of Late (not shown) was estimated from a 123-term prediction error ®lter (10% of the series length) Holocene climate ¯uctuations in the northern Patagonian Andes, at using Burg's30 algorithm. For comparison, the same analyses were performed for a similar least during the past two millennia4. Uniformitarianism suggests time span (1,229-years) on a set of three millennium-long living Fitzroya chronologies that most of the tree-growth oscillations observed in our subfossil (Lenca11, Cisne27 and Costa31 chronologies, respectively) standardized in a similar way as tree-ring Pelluco chronology may be attributed to summer tem- the subfossil chronology. In this case we choose the amplitudes of the ®rst eigenvector to perature variability during Pleistocene interstadials at early stages of develop our spectral estimates. The spectra are shown over the frequency range 0±0.50. OIS 3. Hence, the oscillations of the order of 150±250 years detected c, The cross-spectral relationships between subfossil and modern chronologies. in this chronology may have the same forcing as century-scale Expanded details of each power spectrum are shown in the insets. oscillations observed in temperature reconstructions of the Late

Holocene derived from tree-ring data4. Acknowledgements Our study suggests that comparable cycles in tree growth We thank J. Betancourt, D. Stahle, V. Markgraf and J. Pilcher for comments on the occurred between interstadials of the last glaciation and today, manuscript. AMS dates were provided in part through funding by NSF (Earth System and hence that similar factors have affected the radial growth of History) and the Inter-American Institute (IAI). Fitzroya since the Late Pleistocene. Considering that the Earth's Correspondence and requests for materials should be addressed to F.A.R. atmosphereÐand the ocean/atmosphere systemÐwere signi®- (email: [email protected]). cantly disturbed during the glacial cycles of the Late Pleistocene, the notable similarity in the spectral properties of the Pelluco subfossil and modern F. cupressoides chronologies suggests that the forcing mechanisms of climate during the interstadials have not ...... changed dramatically. The potential for developing very long tree- ring chronologiesÐfrom the present day back several millenniaÐ Simulating the ampli®cation of is one of the most promising results in southern South America dendrochronology15,27. Our Pelluco tree-ring chronology represents orbital forcing by ocean feedbacks a high-resolution window that allows us the possibility of analysing ¯uctuations in tree growth and climate during warmer events of in the last glaciation the last glacial stage in southern Chile, where other very old palaeorecords (.50,000 yr) are rare. M M. Khodri, Y. Leclainche, G. Ramstein, P. Braconnot, O. Marti & E. Cortijo

Received 17 October 2000; accepted 18 January 2001. LSCE, CE-Saclay, 91191, Gif sur Yvette, France 1. Old®eld, F. (ed.) Global Change Report No. 45 (International Geosphere-Biosphere Programme ...... (IGBP), Stockholm, 1998). 2. Fritts, H. C. Reconstructing Large-Scale Climatic Patterns from Tree-Ring Data (Univ. Arizona Press, According to Milankovitch theory, the lower summer insolation Tucson, 1991). at high latitudes about 115,000 years ago allowed winter snow to 3. Pilcher, J. R., Baillie, M. G. L., Schmidt, B. & Becker, B. A 7,272-year tree-ring chronology for western persist throughout summer, leading to ice-sheet build-up and Europe. Nature 312, 150±152 (1984). 1 4. Villalba, R. et al.inClimate Fluctuations and Forcing Mechanisms of the Last 2,000 Years (eds Jones, glaciation . But attempts to simulate the last glaciation using P. D., Bradley, R. S. & Jouzel, J.) 161±189 (Springer, Berlin, 1996). global atmospheric models have failed to produce this outcome 5. Bradley, R. S. Quaternary Paleoclimatology (Chapman & Hall, London, 1992). when forced by insolation changes only2±5. These results point 6. Clapperton, C. Quaternary Geology and Geomorphology of South America (Elsevier, Amsterdam, 1993). towards the importance of feedback effectsÐfor example, 7. Denton, G. H. et al. Geomorphology, stratigraphy, and radiocarbon chronology of Llanquihue Drift in the area of the southern Lake District, Seno ReloncavõÂ, and Isla Grande de ChiloeÂ, Chile. Geogra®s. through changes in vegetation or the ocean circulationÐfor the 6±9 Ann. A 81, 167±229 (1999). ampli®cation of solar forcing . Here we present a fully coupled 8. Heusser, C. J., Heusser, L. E. & Lowell, T. V. Paleoecology of the southern Chilean Lake DistrictÐIsla ocean±atmosphere model of the last glaciation that produces a Grande de ChiloeÂ during middleÐlate Llanquihue glaciation and deglaciation. Geogra®s. Ann. A 81, build-up of perennial snow cover at known locations of ice sheets 231±284 (1999). 9. Heusser, C. J. et al. Pollen sequence from the Chilean Lake District during the Llanquihue glaciation in during this period. We show that ocean feedbacks lead to a cooling marine oxygen Isotope Stages 4-2. J. Quat. Sci. 15, 115±125 (2000). of the high northern latitudes, along with an increase in atmo- 10. Denton, G. H. et al. Interhemispheric linkage of paleoclimate during the last glaciation. Geogra®s. spheric moisture transport from the Equator to the poles. These Ann. A 81, 107±153 (1999). 10±15 11. Lara, A. & Villalba, R. A 3620-year temperature record from Fitzroya cupressoides tree rings in changes agree with available geological data and, together, southern South America. Science 260, 1104±1106 (1993). they lead to an increased delivery of snow to high northern 12. Villalba, R. Tree-ring and glacial evidence for the Medieval Warm Epoch and the Little Ice Age in latitudes. The mechanism we present explains the onset of southern South America. Clim. Change 26, 183±197 (1994). glaciationÐwhich would be ampli®ed by changes in vegeta- 13. Roig, F. A. Dendroklimatologische Untersuchungen an Fitzroya cupressoides im Gebiet der KuÈsten- kordillere un der SuÈdlichen Anden. Thesis, Basel Univ. (1996). tionÐin response to weak orbital forcing. 14. Miller, A. in Climates of Central and South America (ed. Schwerdtfeger, W.) 113±145 (Elsevier, Previous simulations using atmosphere general circulation Amsterdam, 1976). models have shown that the atmosphere alone is unable to produce 15. Roig, F. A., Roig, C., Rabassa, J. & Boninsegna, J. A. Fuegian ¯oating tree-ring chronologies from sub- perennial snow, which is required to start a glaciation2±7,16: several fossil Nothofagus woods. Holocene 6, 469±476 (1996). 3,16 16. Lomnitz, C. Major earthquakes and tsunamis in Chile during the period 1535 to 1965. Geol. Rundsch. studies related this failure to model resolution and snow para- 59, 938±960 (1970). metrization. However, a major drawback of these simulations was 17. Klohn, C. Beobachtungen uÈber die Reste eines spaÈteiszeitlichen Alerceswaldes. Z. Naturfreunde that the ocean surface temperature and vegetation cover were Wanderer 1975±1976, 75±78 (1976). 2±9 18. Lara, A. The dynamics and disturbance regimes of Fitzroya cupressoides forests in the south-central prescribed to their modern values . A previous study showed Andes of Chile. Thesis, Univ. Colorado (1991). that biosphere±atmosphere interactions help to create favourable 19. Porter, S. C. Pleistocene glaciation in the southern Lake District of Chile. Quat. Res. 16, 263±292 (1981). conditions for ice-sheet growth9, but did not succeed in simulating 20. Cook, E. R., Briffa, K. R., Shiyatov, S. G. & Mazepa, V. S. in Methods of Dendrochronology (eds Cook, perennial snow cover. For the period of the onset of the last E. R. & Kairiukstis, L. A.) 104±123 (Kluwer, Dordrecht, 1990). 21. Cook, E. R., Briffa, K. R., Meko, D. M., Graybill, D. A. & Funkhouser, G. The `segment length curse' in glaciation, studies of planktonic and benthic foraminifera from long tree-ring chronology development for palaeoclimatic studies. Holocene 5, 229±237 (1995). the North Atlantic Ocean led to the conclusion that the meridional 22. Leventer, A. et al. Productivity cycles of 200±300 years in the Antarctic Peninsula region: Under- gradient of sea surface temperature (SST) was enhanced before standing linkages among the sun, atmosphere, oceans, sea ice, and biota. Geol. Soc. Am. Bull. 108, glacial inceptionÐwith very cold SST at high latitudes10 (a cooling 1626±1644 (1996). 23. Stuiver, M. & Braziunas, T. F. Atmospheric 14C and century-scale solar oscillations. Nature 338, 405± of around 4 8C from present-day values) while low latitudes were 408 (1989). slightly warmer10,12 than at present. These studies10,14,17±19 assumed 24. Lammy, F., Hebbeln, D. & Wefer, G. Late Quaternary precessional cycles of terrigenous sediment input that the changes in SST pattern were probably due to a shift of deep- off the Norte Chico, Chile (27.5S) and palaeoclimatic implications. Palaeogeogr. Palaeoclimatol. Palaeoecol. 141, 233±251 (1998). water formation from the Norwegian Sea to the North Atlantic. 25. Salinger, M. J. Palaeoclimates north and south. Nature 291, 106±107 (1981). Therefore changes in ocean circulation must have had a large effect 26. Markgraf, V. et al. Evolution of late Pleistocene and Holocene climates in the circum-South Paci®c on the high-latitude heat budget. These hypotheses remain untested land areas. Clim. Dyn. 6, 193±211 (1992). by coupled ocean±atmosphere general circulation models. We tested 27. Boninsegna, J. A. in Climate Since A. D. 1500 (eds Jones, P. D. & Bradley, R. S.) 446±462 (Routledge, 20 London, 1992). this possible mechanism for glacial inception using the IPSL-CM2 28. Jenkins, G. M. & Watts, D. G. Spectral Analysis and its Applications (Holden-Day, San Francisco, 1968). ocean±atmosphere coupled model (see Methods section). 29. Marple, S. L. Jr. Digital Spectral Analysis with Applications (Prentice Hall, Englewood Cliffs, New We performed a 100-year-long sensitivity experiment, called here Jersey, 1987). the `115 kyr BP' run, which only differs from the 120-year control 30. Burg, J. P. in Modern Spectrum Analysis (ed. Childers, D. G.) 42±48 (IEEE Press, New York, 1978). 31. Lara, A. et al. in Dendrochronology in Latin America (ed. Roig, F. A.) 217±244 (EDIUNC, Mendoza, experiment in the Earth's orbital parameters, which are set for 21 2000). 115,000 years before present (BP) (Table 1). The concentration of

The Department of Physical Geography, Stockholm University Doctor of Philosophy Dissertations before 1994

Sigurdur Thorarinsson, 1944. Tefrokronologiska studier på Island. Þjórsárdálur och dess -förödelse. Fakultetsopponent: Prof. Lennart von Post.* Carl Mannerfelt, 1945. Några glacialmorfologiska formelement. Fakultetsopponent: Prof. Veinö - Tanner.* O. Harald Johnsson, 1946. Termisk-hydrologiska studier i sjön Klämmingen. Fakultetsopponent: Prof. B. Hellström.* Carl C. Wallén, 1949. Glacial meteorological investigations on the Kårsa Glacier in Swedish -Lappland 1942–1948. Fakultetsopponent: Prof. Harald Ulrik Sverdrup.* Carl-Gustav Holdar, 1957. Deglaciationsförloppet i Torneträsk-området. Fakultetsopponent: Doc. Carl C. Wallén. Valter Schytt, 1958. A. Snow studies at Maudheim. B Snow studies inland. C. The inner structure of the ice shelf at Maudheim as shown by core drilling. Fakultetsopponent: Doc. Carl C. Wallén. Gunnar Østrem, 1965. Studies of ice-cored moraines. Fakultetsopponent: Doc. Valter Schytt. P. J. Williams, 1969. Properties and behaviour of freezing soils. Fakultetsopponent: Prof. Gunnar - Beskow. Leif Wastenson, 1970. Flygbildstolkning av berghällar och blockmark. Metodstudier av tolkningsmöjligheten i olika flygbildsmaterial. Fakultetsopponent: Prof. Harald Svensson. Lars-Erik Åse, 1970. Shore-displacement in Eastern Svealand and Åland during the last 4,000 years. Fakultetsopponent: Prof. John Norrman. Stig Jonsson, 1970. Strukturstudier av subpolär glaciäris från Kebnekaiseområdet. Fakultetsopponent: Doc. Gunnar Østrem. Bo Strömberg, 1971. Isrecessionen i området kring Ålands hav. Fakultetsopponent: Doc. Erik Fromm. Erik Bergström, 1973. Den prerecenta lokalglaciationens utbredningshistoria inom Skanderna. Fakultetsopponent: Doc. Ragnar Dahl. Kurt Viking Abrahamsson, 1974. Äkäslompolo-områdets glacialmorfologi och deglaciation. Fakultetsopponent: Prof. Veikko Okko. Dietrich Soyez, 1974. Geomorfologiska inventeringar och deglaciationsstudier i Dalarna och Västerbotten. Fakultetsopponent: Prof. Paul Fogelberg. Weston Blake Jr., 1975. Studies of glacial history in the Queen Elizabeth Islands, Canadian arctic archipelago. Fakultetsopponent: Dr. Jan Mangerud. Wibjörn Karlén, 1976. Holocene climatic fluctuations indicated by glacier and tree-limit variations in northern Sweden. Fakultetsopponent: Doc. Jan Lundqvist. Bengt Lundén, 1977. Jordartskartering med flygbildsteknik. En metodundersökning i olika bildmaterial. Fakultetsopponent: Doc. Jan O. Mattsson. Margareta Ihse, 1979. Flygbildstolkning av vegetation. Metodstudier för översiktlig kartering. Fakultetsopponent: Prof. Måns Ryberg. Lill Lundgren, 1980. Soil erosion in Tanzanian mountain areas. Fakultetsopponent: Doc. Lennart Strömquist Kurt-Åke Zakrisson, 1980. Vårflödesprognoser med utgångspunkt från snötaxeringar i Malmagenområdet. Fakultetsopponent: Doc. Thorsten Stenborg. Olle Melander, 1980. Inlandsisens avsmältning i norvästra Lappland. Fakultetsopponent: 1. amanuensis Johan Ludvig Sollid. Ann-Cathrine Ulfstedt, 1980. Fjällen i Västerbotten och södra Norrbotten. Anmärkningar om geomorfologi och isrecession. Fakultetsopponent: Doc. Bo Strömberg. Wolter Arnberg, 1981. Multispectral reflectance measurements and signature analysis. Fakultetsopponent: Prof. Sipi Jaakkola. Carl Christiansson, 1981. Soil erosion and sedimentation in Semi-arid Tanzania. Fakultetsopponent: Doc. Lennart Strömquist. Johan Kleman, 1985. The spectral reflectance of coniferous tree stands and of barley influenced by stress. An analysis of field measured spectral data. Fakultetsopponent: Prof. Dag Gjessing. Lennart Vilborg, 1985. Cirque forms in northern and central Sweden. Studies 1959–1985. Fakultetsopponent: Prof. Anders Rapp. Per Holmlund, 1988. Studies of the drainage and the response to climatic change of Mikkaglaciären and Storglaciären. Fakultetsopponent: Dr. Helgi Björnsson. Henrik Österholm, 1989. The glacial history of Prins Oscars Land Nordaustlandet, Svalbard. Fakultetsopponent: Doc. Harald Agrell. Laine Boresjö, 1989. Multitemporal analysis of satellite data for vegetation mapping in Sweden. Fakultetsopponent: Prof. Friedrich Quiel. Ingmar Borgström, 1989. Terrängformerna och den glaciala utvecklingen i södra fjällen. Fakultetsopponent: Prof. Johan Ludvig Sollid. Christian Bronge, 1989. Climatic aspects of hydrology and lake sediments with examples from northern Scandinavia and Antarctica. Fakultetsopponent: Doc. Stig Jonsson. Kjell Wester, 1992. Spectral signature measurements and image processing for geological remote sensing. Fakultetsopponent: Prof. Friedrich Quiel. Gunhild Rosqvist, 1992. Late Quaternary equatorial glacier fluctuations. Fakultetsopponent: Prof. Chalmers Clapperton. Maj-Liz Nordberg, 1993. Integration of remote sensing and ancillary data for geographical analysis and risk assessment of forest regeneration failures. Fakultetsopponent: Prof. Stein Bie. * Dissertations in Physical Geography from the Department of Geography, Stockholm University College

The Department of Physical Geography, Stockholm University Dissertation Series

Peter Jansson, 1994. Studies of short-term variations in ice dynamics, Storglaciären, Sweden. Dissertation No. 1. Fakultetsopponent: Prof. Hans Röthlisberger. Elisabeth Isaksson, 1994. Climate records from shallow firn cores, Dronning Maud Land, Antarctica. Dissertation No. 2. Fakultetsopponent: Lektor. Claus Hammer. Karin Holmgren, 1995. Late Pleistocene climatic and environmental changes in Central Southern Africa. Dissertation No. 3. Opponent: Prof. Stephen Trudgill. Pius Zebhe Yanda, 1995. Temporal and spatial variations of soil degradation in Mwisanga Catchment, Kondoa, Tanzania. Dissertation No. 4. Fakultetsopponent: Doc. Jonas Åkerman. Anders Moberg, 1996. Temperature variations in Sweden since the 18th century. Dissertation No. 5. Fakultetsopponent: Dr. Phil D. Jones. Arjen P. Stroeven, 1996. Late Tertiary glaciations and climate dynamics in Antarctica: Evidence from the Sirius Group, Mount Fleming, Dry Valleys. Dissertation No. 6. Fakultetsopponent: Prof. David Sugden. Annika Dahlberg, 1996. Interpretations of environmental change and diversity: A study from North East District, Botswana. Dissertation No. 7. Fakultetsopponent: Prof. Andrew Warren. Helle Skånes, 1996. Landscape change and grassland dynamics. Retrospective studies based on aerial photographs and old cadastral maps during 200 years in South Sweden. Dissertation No. 8. Fakultetsopponent: Prof. Hubert Gulinck. Clas Hättestrand, 1997. Ribbed moraines and Fennoscandian palaeoglaciology. Dissertation No. 9. Fakultetsopponent: Dr. Chris Clark. Guðrún Gísladóttir, 1998. Environmental characterisation and change in South-western Iceland. Dissertation No. 10. Fakultetsopponent: Prof. Jesper Brandt. Jens-Ove Näslund, 1998. Ice sheet, climate, and landscape interactions in Dronning Maud Land, Antarctica. Dissertation No. 11. Fakultetsopponent: Prof. David Sugden. Mats G. Eriksson, 1998. Landscape and soil erosion history in Central Tanzania. A study based on lacustrine, colluvial and alluvial deposits. Dissertation No. 12. Fakultetsopponent: Prof. Michael Thomas. Nkobi M. Moleele, 1999. Bush encroachment and the role of browse in cattle production. An ecological perspective from a bush encroached grazing system, Olifants Drift, Kgatleng District, Southeast Botswana. Dissertation No. 13. Fakultetsopponent: Prof. Norman Owen-Smith. Lars-Ove Westerberg, 1999. Mass movement in East African Highlands: Processes, effects and scar recovery. Dissertation No. 14. Fakultetsopponent: Prof. Ana L. Coelho Netto. Malin M. Stenberg, 2000. Spatial variability and temporal changes in snow chemistry, Dronning Maud Land, Antartica. Dissertation No. 15. Fakultetsopponent: Prof. Jon-Ove Hagen. Ola Ahlqvist, 2000. Context sensitive transformation of geographic information. Dissertation No. 16. Fakultetsopponent: Prof. Peter Fisher.

The Department of Physical Geography and Quaternary Geology, Stockholm University. Thesis in Geography with emphasis on Physical Geography

Sara A. O. Cousins, 2001. Plant species diversity patterns in a Swedish rural landscape: Effects of the past and consequences for the future. Dissertation No. 17. Fakultetsopponent: Dr. Roy Haines-Young. Cecilia Richardson-Näslund, 2001. Spatial distribution of snow in Antarctica and other glacier studies using ground-penetrating radar. Dissertation No. 18. Fakultetsopponent: Prof. Robert W. Jacobel. Thomas Schneider, 2001. Hydrological processes in firn on Storglaciären, Sweden. Dissertation No. 19. Fakultetsopponent: Prof. Andrew Fountain. Hans W. Linderholm, 2001. Temporal and spatial couplings between tree-ring variability and climate in Scandinavia. Dissertation No. 20. Fakultetsopponent: Dr. Astrid Ogilvie. Marianne I. Lagerklint, 2001. Marine multi-proxy records of late Quaternary climate change from the Atlantic Ocean. Dissertation No. 21. Fakultetsopponent: Dr. Lloyd H. Burckle Richard Y. M. Kangalawe, 2001. Changing land-use patterns in the Irangi hills, central Tanzania. A study of soil degradation and adaptive farming strategies. Dissertation No. 22. Fakultetsopponent: Prof. William Adams. Anders Clarhäll, 2002. Glacial Erosion Zonation - Perspectives on topography, landforms, processes and time. Dissertation No. 23. Fakultetsopponent: Dr. Chris Clark. Krister N. Jansson, 2002. Glacial geomorphology of north-central Labrador-Ungava, Canada. Dissertation No. 24. Fakultetsopponent: Dr. Andrée Bolduc. Björne E. Gunnarsson, 2002. Holocene climate and environmental fluctuations from subfossil pines in central Sweden. Dissertation No. 25. Fakultetsopponent: Prof. Mike G. L. Baillie. Katarina Lövenhaft, 2002. Spatial and temporal perspectives on biodiversity for physical planning. Dissertation No. 26. Fakultetsopponent: Prof. Jan Bengtsson. Anna Allard, 2003. Vegetation changes in mountainous areas. A monitoring methodology based on aerial photographs, high-resolution satellite images, and field investigations. Dissertation No. 27. Fakultetsopponent: Doc. Timo Helle. Per Klingbjer, 2004. Glaciers and climate in northern Sweden during the 19th and 20th century. Dissertation No. 28. Fakultetsopponent: Dr. Georg Kaser. Ola Fredin, 2004. Mountain centered ice fields in Fennoscandian. Dissertation No. 29. - Fakultetsopponent: Prof. Jon Landvik. Johan M. Bonow, 2004. Palaeosurfaces and palaeovalleys on North Atlantic previously glaciated passive margins- reference forms for conclusions on uplift and erosion. Dissertation No. 30. Fakultetsopponent: Dr. Adrian Hall. Richard Pettersson, 2004. Dynamics of the cold surface layer of polythermal Storglaciären, Sweden. Dissertation No. 31 Fakultetsopponent: Prof. Helgi Björnsson. Katarina Lundblad, 2006. Studies on Tropical Palaeo-variation in Climate and Cosmic Ray Influx. Geochemical Data from Stalagmites Collected in Tanzania and Northern South Africa. Dissertation No. 32. Fakultetsopponent: Prof. Augusto Mangini. Lena Rubensdotter, 2006. Alpine lake sediment archives and catchment geomorphology; causal relationships and implications for paleoenvironmental reconstructions. Dissertation No. 33. Fakultetsopponent: Prof. Catherine Souch.

Dissertations from the Department of Physical Geography and Quaternary Geology

Håkan Grudd, 2006. Tree rings as sensitive proxies of past climate change. Dissertation No. 1. Fakultetsopponent: Prof. Brian Luckman.