Dendroclimatology in southern Norway: tree rings, demography and climate
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- Institutt for biologi 
Tree rings provide annually resolved palaeoclimate archives. In southern Norway, time series of tree-ring width and maximum latewood density of Scots pine growing at certain high-elevation locations are correlated to temperature and can be used to infer past summer temperature variability. This thesis focuses on a few questions fundamental to dendroclimatology: from which types of samples and from which locations can we expect trees to provide reliable climate information; how do the climate related conclusions we draw from different tree-growth proxies differ; at which pace do trees build their rings and is this representative of the growth year or is it influenced by the climate of previous years. To study memory effects in tree-ring data, which is of importance to the interpretation of climate reconstructions, the 14C content of sub-annual sections of Scots pine tree rings was measured over the radiocarbon bomb spike. Although a delay is observed in the measured radiocarbon content of cellulose compared to atmosphere, this does not conform to the hypothesis that memory effects in tree-ring data should be most clearly expressed in the earlywood. Historical construction timber in southern Norway was also analysed and deemed more suitable for the study of demographic changes and settlement history than for palaeotemperature reconstructions. The widespread building hiatus associated with the Black Death pandemic was dated to the late-13th and early-14th centuries in southern Norway. This onset is indicative of population decline several decades before the Plague, which might be linked to climatic deterioration in this period. From trees sampled in western Norway, summer temperature variability was reconstructed from tree-ring width and maximum latewood density back to the Little Ice Age. The reconstructed temperatures are most strongly correlated to instrumental temperatures in southern Norway and central Sweden, and with significant correlations to sea-surface temperature in the North Sea. Severely cold periods were reconstructed around AD 1480 and 1600. The reconstruction based on density data has the strongest correlation to instrumental temperature in the study area, which is expected from findings at other sites in Fennoscandia. The response to volcanic eruptions was investigated, along with links to large-scale anomaly patterns of sea-surface temperature and atmospheric circulation. The temperature reconstructions outlined in this project are among few based on tree rings in Norway, and among even fewer on the western side of the Scandinavian Mountains. They are therefore important for representing climate in southern Norway and southern Fennoscandia, which enables a better understanding of past temperature variability in northern Europe.