Holocene climate change and peatland dynamics in southern Sweden based on tree-ring analysis of subfossil wood from peat deposits

Abstract: Dendrochronological analysis was applied to subfossil remains of Scots pine (Pinus sylvestris L.) buried in South Swedish peat deposits. By cross-dating Swedish bog-pine ring-width (RW) chronologies with corresponding material from North-west Germany, three Swedish RW chronologies were assigned absolute ages and yielded continuous, dated chronologies over the periods 5284-3728 BC and 2668-1108 BC. The obtained cross-match between RW chronologies from regions separated by 500-700 km is remarkably strong, which indicates that large-scale climate dynamics had a significant impact on bog-tree growth variability during the Holocene Thermal Maximum (HTM), a relatively warm and dry period generating favourable growth conditions on peatlands over large parts of North-west Europe. Nine additional RW chronologies, each about 200 years in length, were developed from pine, oak (Quercus robur L.) and alder (Alnus glutinosa L.). Most of these RW chronologies were dated by radiocarbon (14C). An approach combining bog-tree RW, replication and mean-age data, with peat stratigraphic records was explored to provide information about local hydrology, depositional history and peatland development. Registration of growth position of individual trees allowed assessment of the spatial dynamics of tree populations in response to hydrological changes and peatland development. Major bog-tree establishment and degeneration phases reflect changes in bog-surface wetness, often in response to climate-controlled groundwater fluctuations. Tree establishment phases coincide with stratigraphic layers of increased peat humification locally, and periods of increasing temperatures and widespread lake-level lowering regionally in southern Sweden. When conditions become more humid at the end of the HTM, increased lateral peatland expansion was recorded. This process could be studied in detail based on trees buried in the marginal zone of a peatland. An independent test of the hypothesis that bog-tree growth variability is controlled by effective moisture was performed by comparing RW data with tree-ring carbon (δ13C) and oxygen (δ18O) isotope records. Variations in the isotopic records confirm that growth depressions coincided with moister atmospheric conditions and reveal a lag of about three years in the growth response with respect to the isotopic signals, likely due to slow hydrologic response in the peatlands. This thesis demonstrates the usefulness of the South Swedish subfossil bog-tree material as a climate proxy with particular potential for decadal- to centennial-scale reconstructions of humidity fluctuations. It also demonstrates that subfossil bog-trees in combination with peat stratigraphy can be used for detailed reconstructions of peatland development and local groundwater variability, which are also highly relevant in a long-term regional palaeoclimatic context.