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Papers by Björn E. Gunnarson

Journal of Archaeological Science, 2017

Dendroarchaeology almost exclusively uses ring-width (RW) data for dating historical structures a... more Dendroarchaeology almost exclusively uses ring-width (RW) data for dating historical structures and artefacts. Such data can be used to date tree-ring sequences when regional climate dominates RW variability. However, the signal in RW data can be obscured due to site specific ecological influences (natural and anthropogenic) that impact crossdating success. In this paper, using data from Scotland, we introduce a novel tree-ring parameter (Blue Intensity e BI) and explore its utility for facilitating dendro-historical dating of conifer samples. BI is similar to latewood density as they both reflect the combined hemicellulose, cellulose and lignin content in the latewood cell walls of conifer species and the amount of these compounds is strongly controlled, at least for trees growing in temperature limited locations, by late summer temperatures. BI not only expresses a strong climate signal, but is also less impacted by site specific ecological influences. It can be concurrently produced with RW data from images of finely sanded conifer samples but at a significantly reduced cost compared to traditional latewood density. Our study shows that the probability of successfully crossdating historical samples is greatly increased using BI compared to RW. Furthermore, due to the large spatial extent of the summer temperature signal expressed by such data, a sparse multi-species conifer network of long BI chronologies across Europe could be used to date and loosely provenance imported material.

Trees growing at their altitudinal or latitudinal distribution in Fennoscandia have been widely u... more Trees growing at their altitudinal or latitudinal distribution in Fennoscandia have been widely used to reconstruct warm season temperatures, and the region hosts some of the world's longest tree-ring chronologies. These multi-millennial long chronologies have mainly been built from tree remains found in lakes (subfossil wood from lake-shore trees). We used a unique dataset of Scots pine tree-ring data collected from wood remains found on a mountain slope in the central Scandinavian Mountains, yielding a chronology spanning over much of the last 1200 years. This data was compared with a local subfossil wood chronology with the aim to (1) describe growth variability in two environments during the Medieval Climate Anomaly (MCA) and the early Little Ice Age (LIA), and (2) investigate differences in growth characteristics during these contrasting periods. It was shown that the local tree-line during both the MCA and early LIA was almost 150 m higher that at present. Based on living pines from the two environments, tree-line pine growth was strongly associated with mid-summer temperatures, while the lake-shore trees showed an additional response to summer precipitation. During the MCA, regarded to be a period of favorable climate in the region, the tree-ring data from both environments showed strong coherency and moderate growth variability. In the early LIA, the two chronologies were less coherent, with the tree-line chronology showing more variability, suggesting different growth responses in the two environments during this period of less favorable growing conditions. Our results indicate that tree-ring width chronologies mainly based on lake-shore trees may need to be re-evaluated.

An improved and extended Pinus sylvestris L. (Scots Pine) tree-ring maximum density (MXD) chronol... more An improved and extended Pinus sylvestris L. (Scots Pine) tree-ring maximum density (MXD) chronology from the central Scandinavian Mountains was used to reconstruct warm-season (April-September) temperature back to 850 CE. Due to systematic bias from differences in elevation (or local environment) of the samples through time, the 5 data was "mean adjusted'. The new reconstruction, called C-Scan, was based on the RSFi standardisation method to preserve mid-and long-term climate variability. C-Scan, explaining more than 50 % of the warm-season temperature variance in a large area of Central Fennoscandia, agrees with the general profile of Northern Hemisphere temperature evolution during the last 12 centuries, supporting the occurrences 10

Arctic, Antarctic, and Alpine Research, 2001

Science advances, 2015

Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fen... more Climate model projections suggest widespread drying in the Mediterranean Basin and wetting in Fennoscandia in the coming decades largely as a consequence of greenhouse gas forcing of climate. To place these and other "Old World" climate projections into historical perspective based on more complete estimates of natural hydroclimatic variability, we have developed the "Old World Drought…

Climate of the Past Discussions, 2015

Climate of the Past, 2014

Here we explore two new tree-ring parameters, derived from measurements of wood density and blue ... more Here we explore two new tree-ring parameters, derived from measurements of wood density and blue intensity (BI). The new proxies show an increase in the interannual summer temperature signal compared to established proxies, and present the potential to improve long-term performance. At high latitudes, where tree growth is mainly limited by low temperatures, radiodensitometric measurements of wood density, specifically maximum latewood density (MXD), provides a temperature proxy that is superior to that of tree-ring widths. The high cost of developing MXD has led to experimentation with a less expensive method using optical flatbed scanners to produce a new proxy, herein referred to as maximum latewood blue absorption intensity (abbreviated MXBI). MXBI is shown to be very similar to MXD on annual timescales but less accurate on centennial timescales. This is due to the fact that extractives, such as resin, stain the wood differentially from tree to tree and from heartwood to sapwood. To overcome this problem, and to address similar potential problems in radiodensitometric measurements, the new parameters blue intensity ( BI) and density are designed by subtracting the ambient BI/density in the earlywood, as a background value, from the latewood measurements. As a case-study, based on Scots pine trees from Northern Sweden, we show that density can be used as a quality control of MXD values and that the reconstructive performance of warm-season mean temperatures is more focused towards the summer months (JJA -June, July, August), with an increase by roughly 20 % when also utilising the interannual information from the earlywood. How-ever, even though the new parameter BI experiences an improvement as well, there are still puzzling dissimilarities between density and BI on multicentennial timescales. As a consequence, temperature reconstructions based on BI will presently only be able to resolve information on decadalto-centennial timescales. The possibility of trying to calibrate BI into a measure of lignin content or density, similarly to how radiographic measurements are calibrated into density, could be a solution. If this works, only then can BI be used as a reliable proxy in multicentennial-scale climate reconstructions.

Abstract Despite the spatially homogenous summer temperature pattern in Fennoscandia, there are l... more Abstract Despite the spatially homogenous summer temperature
pattern in Fennoscandia, there are large spreads
among the many existing reconstructions, resulting in an
uncertainty in the timing and amplitude of past changes.
Also, there has been a general bias towards northernmost
Fennoscandia. In an attempt to provide a more spatially
coherent view of summer (June–August, JJA) temperature
variability within the last millennium, we utilized seven
density and three blue intensity Scots pine (Pinus sylvestris
L.) chronologies collected from the altitudinal (Scandinavian
Mountains) and latitudinal (northernmost part) treeline.
To attain a JJA temperature signal as strong as possible,
as well as preserving multicentury-scale variability,
we used a new tree-ring parameter, where the earlywood
information is removed from the maximum density and
blue intensity, and a modified signal-free standardization
method. Two skilful reconstructions for the period 1100–
2006 CE were made, one regional reconstruction based
on an average of the chronologies, and one field (gridded)
reconstruction. The new reconstructions were shown to
have much improved spatial representations compared to
those based on data from only northern sites, thus making
it more valid for the whole region. An examination of some of the forcings of JJA mean temperatures in the region
shows an association with sea-surface temperature over the
eastern North Atlantic, but also the subpolar and subtropical
gyres. Moreover, using Superposed Epoch Analysis, a
significant cooling in the year following a volcanic eruption
was noted, and for the largest explosive eruptions, the
effect could remain for up to 4 years. This new improved
reconstruction provides a mean to reinforce our understanding
of forcings on summer temperatures in the North
European sector.

Dendrochronologia, 2014

ABSTRACT Blue Intensity (BI) has the potential to provide information on past summer temperatures... more ABSTRACT Blue Intensity (BI) has the potential to provide information on past summer temperatures of a similar quality to maximum latewood density (MXD), but at a substantially reduced cost. This paper provides a methodological guide to the generation of BI data using a new and affordable BI measurement system; CooRecorder. Focussing on four sites in the Scottish Highlands from a wider network of 42 sites developed for the Scottish Pine Project, BI and MXD data from Scots pine (Pinus sylvestris L.) were used to facilitate a direct comparison between these parameters. A series of experiments aimed at identifying and addressing the limitations of BI suggest that while some potential limitations exist, these can be minimised by adhering to appropriate BI generation protocols. The comparison of BI data produced using different resin-extraction methods (acetone vs. ethanol) and measurement systems (CooRecorder vs. WinDendro) indicates that comparable results can be achieved. Using samples from the same trees, a comparison of both BI and MXD with instrumental climate data revealed that overall, BI performs as well as, if not better than, MXD in reconstructing past summer temperatures (BI r2 = 0.38 - 0.46; MXD r2 = 0.34 - 0.35). Although reconstructions developed using BI and MXD data appeared equally robust, BI chronologies were more sensitive to the choice of detrending method due to differences in the relative trends of non-detrended raw BI and MXD data. This observation suggests that the heartwood-sapwood colour difference is not entirely removed using either acetone or ethanol chemical treatment, which may ultimately pose a potential limitation for extracting centennial and longer timescale information when using BI data from tree species that exhibit a distinct heartwood-sapwood colour difference. Additional research is required in order to develop new methods to overcome this potential limitation. However, the ease with which BI data can be produced should help justify and recognise the role of this parameter as a potential alternative to MXD, particularly when MXD generation may be impractical or unfeasible for financial or other reasons.

Tree-Ring Research, 2011

Minimum blue intensity is a reflected light imaging technique that provides an inexpensive, robus... more Minimum blue intensity is a reflected light imaging technique that provides an inexpensive, robust and reliable surrogate for maximum latewood density. In this application it was found that temperature reconstructions from resin-extracted samples of Pinus sylvestris (L.) from Fennoscandia provide results equivalent to conventional x-ray densitometry. This paper describes the implementation of the blue intensity method using commercially available software and a flat-bed scanner. A calibration procedure is presented that permits results obtained by different laboratories, or using different scanners, to be compared. In addition, the use of carefully prepared and chemically treated 10-mm-diameter cores are explored; suggesting that it may not be necessary to produce thin laths with the rings aligned exactly perpendicular to the measurement surface.

The Holocene, 2008

The temporal variations in distribution pattern of Scots pines (Pinus sylvestris L.) have been us... more The temporal variations in distribution pattern of Scots pines (Pinus sylvestris L.) have been used as an annual resolution record of past lake-level changes. Logs, preserved for thousands of years in bog and lakes (subfossil wood), were sampled from small lakes of the Scandinavian Mountains in west-central Sweden to construct a tree-ring chronology from an area where pines are sensitive

Geografiska Annaler, Series A: Physical Geography, 1999

A 200-year tree-ring chronology of pine from a raised bog in Sweden: Implication for climate chan... more A 200-year tree-ring chronology of pine from a raised bog in Sweden: Implication for climate change? Geogr. Ann., 81 A (3): 421-430.

Geografiska Annaler, Series A: Physical Geography, 2002

ABSTRACT This is a detailed study of 50 wood cores sampled from 25 Canary Island pines (Pinus can... more ABSTRACT This is a detailed study of 50 wood cores sampled from 25 Canary Island pines (Pinus canariensis) growing in a subtropical climate close to the tree limit (at Lomo de Retamar, 2000–2100 m a.s.l.) on the southern, leeward side of Tenerife.The Canary Island pines form tree-rings and, similarly to temperate pines, earlywood is formed during spring. But many pines fail to form tree-rings every year and one purpose for this paper has been to find a way to overcome this problem. Another purpose has been to see what correlations, if any, exist between the ring widths and meteorological data. Meteorological data have been collected since 1916 at the nearby Izaña meteorological observatory. It was also important to find the limiting factor for growth of Canary Island pines close to the tree limit.Through cross-dating, missing rings were found to be typical for younger parts of older trees; in the same years younger trees instead had low growth. Some trees lacked more than 20 rings in a century. This study shows that it is possible to compensate for missing rings, at least when the number of missing rings is less than 10%.Correlation studies showed that the growth of the tree rings was mainly due to the precipitation factor, particularly the annual precipitation one year before the ring formation, but also due to temperature. Strangely enough, multiple linear regression gave the highest correlation coefficient (0.50) for a combination of annual precipitation one year earlier and temperature four years earlier.

Geografiska Annaler, Series A: Physical Geography, 2005

... along the Scandinavi-an Mountains is strongly influenced by summer temperatures, mainly in Ju... more ... along the Scandinavi-an Mountains is strongly influenced by summer temperatures, mainly in July (Aniol and Eckstein 1984; Briffa et al ... temperature, 0.870.95 July August mean temperature, 0.910.96 July tempera-tures) indicates a common summer temperature pat-tern in ...

Geochronometria, 2000

We present results from an analysis of tree ring width and stable carbon and oxygen isotopes in t... more We present results from an analysis of tree ring width and stable carbon and oxygen isotopes in tree ring cellulose of Siberian Spruce collected from remote forest islands in the northwestern Russian tundra. Ring width is often considered a proxy for summer temperatures. The aim of this pilot study was to test whether stable isotopes can provide additional information about climate during the growth of trees in this extreme environment. Comparison of δ 13 C and δ 18 O with observed meteorological data shows that there is a link between stable isotopes and winter precipitation. This may be explained by the strong influence that snow exerts on the isotopic composition of soil moisture during spring and early summer, when the new cellulose is formed. Our results show that winter precipitation in the study area was increasing from 1865-1900, and thereafter decreasing until ~1930. The 1960-1980 period was again rather humid, followed by a drying trend until 1990. The study highlights the potential of stable carbon and oxygen isotopes in tree rings as proxies for winter precipitation.

Climate Research, 2012

ABSTRACT In Scandinavia, dendrochronological reconstructions of past climate have mostly been bas... more ABSTRACT In Scandinavia, dendrochronological reconstructions of past climate have mostly been based on tree-ring data from forests in which there has been, supposedly, very little or no human impact. However, human land use in sub-alpine forests has a substantially longer history and more profound effects on the forest ecosystems than previously acknowledged. Therefore, to assess human influence on tree-ring patterns over the last 500 yr, we have analyzed tree-ring patterns using trees from 2 abandoned Sami settlements and a reference site with no human impact-all situated in the Tjeggelvas Nature Reserve in north-west Sweden. The hypothesis was that land use legacies have affected tree-ring patterns, and in turn, the resulting palaeoclimate inferences that have been made from these patterns. Our results show that climate signals are strongest at the reference site and weakest at one of the settlement sites. From the 1940s to the present, tree growth at this settlement site has been significantly lower than at the reference site. Lower tree growth at old settlements may have resulted from rapid changes in the traditional land use, or following the abrupt change when the settlements were abandoned. Without site-specific know ledge of past land use, there is a high risk of accidently sampling trees that have been affected by human-induced disturbances in the past. This may create bias in the climate signals inferred from such trees, and hence bias the outcome of climate reconstructions. We therefore recommend sampling several separate sites in study areas to improve the robustness of inferences.

Canadian Journal of Forest Research, 2010

In studies on natural dynamics, biodiversity and reference conditions legacies of preindustrial h... more In studies on natural dynamics, biodiversity and reference conditions legacies of preindustrial human land use are often neglected. In this study, using archaeology and dendrochronology combined with field surveys on present forest characteristics, we assessed the naturalness of a protected forest landscape and examined the role of indigenous peoples in shaping forest structure in the past. Our results show that the studied Scots pine (Pinus sylvestris L.) forest conforms to the generally accepted impression of pristine forests and that it has a long history of human utilization. Areas with human presence over long time periods, especially in and near settlements, show significant differences in current forest characteristics compared with the rest of the landscape: the forest is younger (mean age 140-190 years compared with >300 years), the volumes of deadwood lower (8-13 m 3 Áha -1 compared with >20 m 3 Áha -1 ), and the tree species composition is substantially different from the surrounding forest. We suggest that these disparities are strongly linked to past land use and that indigenous people can alter ecosystems substantially and that the legacies of their activity may last for centuries. Consequently, in ecological research and conservation strategies, forest characteristics should always be considered in the light of their historical context.

Boreas, 2003

Page 1. Holocene humidity fluctuations in Sweden inferred from dendrochronology and peat stratigr... more Page 1. Holocene humidity fluctuations in Sweden inferred from dendrochronology and peat stratigraphy BJO¨ RN ERIK GUNNARSON, ANDERS BORGMARK AND STEFAN WASTEGA˚ RD Gunnarson, BE, Borgmark, A. &amp; Wastegård ...

The ring-width (RW) chronology of Scots pine (Pinus sylvestris L.) from Forfjorddalen, Vesterålen... more The ring-width (RW) chronology of Scots pine (Pinus sylvestris L.) from Forfjorddalen, Vesterålen archipelago (68° 47.5&amp;amp;amp;#39;N, 15° 43.5&amp;amp;amp;#39;E) extends back to AD 812. July-August temperatures are reconstructed back to AD 1100 based on a subset of the RCS-detrended RW series (Radj2 = 42.0 %). This suggests warm summers persisted during: the Medieval period 1100-1250; from 1750-1850; and from 1915-1955. The