Holocene treeline dynamics and climate changes in the Scandes Mountains of mid-central Sweden as inferred from a multi-proxy lake sediment study
by
Dan Hammarlund
Dept of Quaternary Geology, Lund University, Sweden
Coauthors: Gaute Velle (Zoological Inst., University of Bergen, Norway), Thomas W.D. Edwards and Brent B. Wolfe (Dept of Earth Sciences, University of Waterloo, Canada), Barbara Wohlfarth and Sara Lamme (Dept of Quaternary Geology, Lund University, Sweden), Sofia Holmgren (Earth Sciences Centre, Göteborg University, Sweden)

Mountain ecosystems at high northern latitudes are known to be sensitive to climate change, and Scandinavian treeline vegetation is presently responding to the late twentieth century warming. As part of a project including stratigraphic studies of Holocene lake sediments and peat sequences along an altitudinal transect in the Scandes Mountains of mid-central Sweden, a 3.1 m sequence of lake sediments representing the last 10, 700 years, was studied by various biostratigraphic, geochemical, and mineral magnetic methods. Lake Spåime (informal name) is a small open-basin lake situated at 887 m a.s.l., c. 100 m above the present-day continuous treeline but close to the tree-limit of mountain birch.

At c. 3700 cal BP a major environmental change took place at the site, most likely representing a transition from forest to a treeless environment resembling present-day conditions within the lake catchment. This development is related to the long-term Holocene descent of the upper limit of several boreal tree species, particularly Scots pine, recorded by radiocarbon dating of subfossil wood remains from various altitudes in the region. As indicated by pollen data, mixed birch-pine forest, possibly with some contribution of broad-leaved trees, occurred in the area prior to 3700 cal BP. The presence of pine at the site at least from c. 6500 cal BP is confirmed by macrofossil and stomatal evidence. Increased aquatic productivity during this forest episode is suggested by elevated carbon and nitrogen contents of the sediments, relatively low C/N ratios, depletion of ¹³C in organic material, and increased deposition of bacterial magnetite as recorded by mineral magnetic data. The retreat of the boreal treeline to levels below the studied site at c. 3700 cal BP coincides with a period of cooling and increased precipitation which has been previously recorded elsewhere in northern Scandinavia. The pollen record indicates that a second phase of pronounced treeline descent took place at c. 1800 cal BP, possibly transforming adjacent valleys at c. 700 m a.s.l., presently occupied by birch-dominated forests, into subalpine tundra.

Boreal forests were probably established in the lake catchment already by c. 9500 cal BP. Slightly before 8000 cal BP, a distinct decrease in aquatic productivity was recorded, accompanied by an increase in the proportion of terrestrial organic material of the sediments. This perturbation likely reflects the 8200 cal BP cold-event, which may have led to decreased forest cover, or even to the disappearance of tree-sized vegetation at the site. Corresponding, although less well-developed, stratigraphic features were registered at about 7000, 6000, and 4900 cal BP respectively, possibly representing similar climatic fluctuations in the area.

In order to place these records of local environmental change in the perspective of possible large-scale rearrangements of hydrology and atmospheric circulation across the Scandes Mountains during the course of the Holocene, additional palaeoclimatic methods were applied to the sediment sequence. These include reconstruction of long-term changes in the oxygen-isotope composition of ambient lake water and precipitation by delta¹⁸O analysis of aquatic cellulose, and reconstruction of mean July temperature variations based on chironomid analysis.

Date received: March 29, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cagc-67.