Tree-limit ecotonal response to Holocene climate change in the Scandes Mountains of west-central Sweden

University dissertation from Department of Geology, Lund University

Abstract: The aim of this thesis was to reconstruct the Holocene vegetational and climatic development in the Sylarna-Storulvån area, western Jämtland, in the central Scandes Mountains. Temporal trends and fluctuations in the elevation and vegetational character of the tree-limit ecotone were studied mainly by means of pollen and plant macrofossil analysis of two lake sediment sequences (Lakes Stentjärn and Spåime), located above the present-day tree-limit. The lake sediments were also subjected to high-resolution elemental and mineral magnetic measurements, which contributed useful complementary information on the local environmental development. Plant macrofossil data indicate the presence of a short-lived deglaciation flora, dominated by light-demanding herbs and dwarf-shrubs, followed by the establishment of birch-pine forest. The vegetational data obtained were compared with previously published records of radiocarbon-dated subfossil wood remains (megafossils), collected mainly in the study area. A general conformity was revealed between the stratigraphic plant macrofossil data and pollen accumulation rates, and the comparison between the non-stratigraphic megafossil data and the pollen influx/plant macrofossil records also revealed a high level of consistency of the inferred tree-limit variations for Pinus sylvestris, Betula pubescens, and Alnus incana. Records of climatic humidity inferred from peat humification data (DOH) were obtained from two separate profiles at a nearby peat deposit (Klocka Bog), situated below the forest-limit.

Evaluation of the DOH records exhibits millennial-scale trends, which are significantly correlated between profiles during the periods 6500-4000 cal yr BP and 2100-0 cal yr BP. Within these periods, the time between 5800 and 4800 cal yr BP, and 1800 cal yr BP until the present, are recognised as episodes of increasing climatic humidity. In general, the vegetational, geochemical and sedimentary records were shown to correlate with several Holocene climatic events and transitions, identified elsewhere in north-western Europe. The climatic forcing of some of these sub-Milankovitch scale perturbations is unclear, but a coupling to internal circulation dynamics of the North Atlantic Ocean is hypothesized.

Chronologies of the geological archives studied within the project were based on radiocarbon dating and tephrochronology. At Lake Stentjärn, three Holocene cryptotephra horizons were detected, one of which was geochemically correlated with the Icelandic Askja-1875 eruption. At Klocka Bog, at least seven cryptotephra horizons were recorded in the two peat profiles, and five of the horizons were geochemically correlated with the Askja-1875, Hekla-3, Kebister, Hekla-4, and Lairg A tephras, respectively.