Dendroclimatology on Scots Pine (Pinus Sylvestris L.) in Northern Norway
Total Page:16
File Type:pdf, Size:1020Kb
A DISSERTATION FOR THE DEGREE OF DOCTOR SCIENTIARUM Dendroclimatology on Scots pine (Pinus sylvestris L.) in northern Norway Andreas Joachim Kirchhefer December 1999 Department of Biology, Faculty of Science, University of Tromsø, Norway TABLE OF CONTENTS LIST OF PAPERS..........................................................................................................................iii ABSTRACT................................................................................................................................... iv ACKNOWLEDGEMENTS.............................................................................................................. v INTRODUCTION ........................................................................................................................... 1 Climate change and palaeoclimatology..................................................................................... 1 Tree rings and climate............................................................................................................... 2 Tree-ring research in Norway .................................................................................................... 5 Dendroclimatology in northern Fennoscandia ........................................................................... 9 THE AIMS OF THE STUDY ........................................................................................................ 13 METHODOLOGY......................................................................................................................... 14 THE MAIN RESULTS .................................................................................................................. 14 DISCUSSION............................................................................................................................... 15 Tree-ring chronologies............................................................................................................. 15 The general tree-ring signal..................................................................................................... 16 Regional variability................................................................................................................... 17 Slope aspect and June temperatures...................................................................................... 19 Twentieth century climate change ........................................................................................... 21 Climate-growth calibration ....................................................................................................... 22 Summer temperatures of northern Norway since AD 1358..................................................... 23 FUTURE STUDIES...................................................................................................................... 25 REFERENCES ............................................................................................................................ 27 Table of contents LIST OF PAPERS Paper 1: Reconstruction of summer temperature from tree rings of Scots pine, Pinus sylvestris L., in coastal northern Norway. The Holocene in press. 25 pp. Paper 2: Pine growth and climate AD 1800-1992 along a transect across the Scandes at 69°N. Submitted to Climatic Change. 25 pp. Paper 3: The influence of slope aspect on tree-ring growth of Pinus sylvestris L. in northern Norway and its implications for climate reconstruction. Submitted to Dendrochronologia. 20 pp. List of papers iii ABSTRACT A total of ten tree-ring chronologies of Scots pine, Pinus sylvestris L., was constructed between the Vesterålen archipelago and the Finnmarksvidda in order to investigate the regional variability of radial growth and climate response of pine. The longest tree-ring chronology, located in Forfjorddalen in Vesterålen, was highly significant back to AD 1354. The study area was divided into three dendroecological zones; the coast, the inner Scandes and the Finnmarksvidda. In all regions, July temperature was the most important growth-determining factor. At the coast, pine showed a significant positive response also to August temperatures. A partial study in the inner Scandes showed that the radial growth at north-facing slopes was enhanced by high June temperatures, most likely due to the influence of the midnight sun. Evidence of environmental stress due to global warming was seen in reduced growth during periods of warm-moist mid winters at the coast and, particularly in the warm 1930s, in the Scandes. Also, there were indications of drought stress in summer in the intra-alpine valleys of the Scandes and at the edaphically dry coastal site, Stonglandseidet. On the basis of the tree-ring chronologies, July temperatures were reconstructed back to AD 1800 for northern Norway 69°N and July-August temperatures along the coast back th to AD 1358. The 20 century since 1915 was a period of above-average temperatures and growth. In the present reconstruction, a comparable warm period occurred th previously only AD 1470-1540. In the 19 century, cool summers prevailed about AD 1810, in the 1830s and from the late 1860s to 1910. The 17th century, the coolest interval of the ‘Little Ice Age’, experienced three intervals of cool summers around AD 1605, 1640 and 1680. There was evidence of a lack of pine regeneration in the first half th of the 17 century. Major regional temperature differences were observed around AD 1760 with extraordinarily warm summers east of the Scandes, but average temperatures at the coast, and about AD 1800, when the coast was warm, but the inland cooling. An exploratory reconstruction of June temperatures from growth differences between north- and south-facing slopes demonstrated the potential of site-related growth responses for refined climate reconstructions. Abstract iv ACKNOWLEDGEMENTS I would like to thank my supervisor, professor Karl-Dag Vorren, for enabling the present thesis by his initiative to establish the tree-ring laboratory at the Department of Biology, University of Tromsø, and supporting my study and research plans in Tromsø and northern Norway. Prof. Dr. Dieter Eckstein, University of Hamburg, willingly became my external supervisor. The present project has been funded by the Norwegian Research Council under the programme ‘Climate and Ozone’, project no. 107733/720, during 1995-1998. Of all those who contributed to the present thesis, I would like to thank in particular: • Mauri Timonen and Håkan Grudd for generously loaning their chronologies; • Annette Stavseth Furset and Elisabeth Cooper for field assistance in Dividalen; • Marianne Iversen and Gunnar Kristiansen for inviting me to Imofossen; • Victoria Jonsson for precise and reliable measurement work; • all botany students and staff at the University of Tromsø for creating a pleasant working atmosphere. Sampling in the Nordreisa National Park was made possible by permission of the State County Governor’s Office of Troms, Environmental Department. Climate data were kindly provided by Eirik J. Førland, Inger Hanssen-Bauer, Stein Kristiansen and Per Øyvind Nordli at the Norwegian Meteorological Institute, Oslo. The collaboration in the FOREST-project (Forest Response to Environmental Stress at Timberlines), contract no. ENV4-CT95-0063, led to fruitful discussions and, through the dendrochronological workshop at the University of Marseille, strongly influenced the methods and results of the present thesis. Markus Lindholm, Mauri Timonen, Reinhard Mook and Elisabeth Cooper gave critical and encouraging comments on the three manuscripts. My warmest thanks to Sabine Cochrane for language consultation and all support during the writing process. Acknowledgements v INTRODUCTION CLIMATE CHANGE AND PALAEOCLIMATOLOGY The earth’s climate is a dynamic system. In pre-industrial times, climate variations were mainly caused by changes in solar irradiation and by aerosols from explosive volcanic eruptions (Sear et al., 1987; Rind and Overpeck, 1993; Sarachik et al., 1996). However today, by emitting large quantities of greenhouse gases and sulphate aerosols, also man plays an active role in the earth’s climate (Schimel et al., 1996; Tett et al., 1999). Since the late 19th century, the global mean temperature has increased by about 0.3°C to 0.6°C (Jones and Briffa, 1992; Parker et al., 1994; Nicholls et al., 1996). Major challenges in climate research are the attribution of causes for this temperature trend (Santer et al., 1996), the prediction of future climate change (Kattenberg et al., 1996), its spatial and temporal variability and its potential impact on the human environment. The instrumental climate record shows that the northern hemisphere warmed stepwise in the 1920s and since the 1970s, interrupted by a distinct cooling from the 1940s to the late 1960s (Jones and Briffa, 1992). Generally, the warming was strongest in the high latitudes and during winter. However, temperature changes in the regions bordering the North Atlantic Ocean were less representative of the hemisphere mean trends (Briffa and Jones, 1993). For instance, there was no significant warming trend during the 20th century in northern Europe. Spatially and seasonally varying trends also were observed within Norway (Hanssen-Bauer and Nordli, 1998). The months contributing most to the annual warming were June to August in the north and the autumn months, particularly October, in the south. A summer-autumn temperature maximum in the early 1920s was restricted to northern Norway.