Altitudinal limits of tree Birch during the late Holocene in Northern Fennoscandia
by
Gina Hannon
Swedish University of Agricultural Sciences

Betula pubescens ssp. tortuosa (birch) is the principal tree at the forest-tundra ecotone of northern Fennoscandia. The late Holocene history of this ecotone has been studied using high-resolution plant macrofossil analyses of lake sediments from the Joatka region in northern Norway and Enontekio in northern Finland. The aim is to study the dynamics of an ecotone that is particularly sensitive to climate change.

Dated lake sediments from a site above present tree line at Joatka, Norway spanned c. 3000 years from 5000-2000 BP. This may well be the period of maximum lake and ecosystem productivity during the Holocene at this altitude when tree Birch grew around the site. Two layers of birch bark and leaf fragments together with Betula pubescens fruits gave firm evidence for tree growth around the site during the entire period. The falling organic content of the sediments and the strongly reduced sedimentation rate after c. 4200 BP, taken together with the subsequently increasing proportion of Betula nana fruits in the macrofossil record suggest that the forest cover began to break up. By 2200 BP the site still lay within the treeline, but treeline must have moved downslope soon after this time. A second site, below present treeline, is as yet undated but shows the same preliminary pattern of a gradual but distinct change from tree Birch to dwarf Birch (Betula nana). In addition, there has been a gradual decrease in sediment organic content with time, a pattern that is consistent between sites.

At a site above treeline at Enontekio, Finland, the early Holocene macrofossils included Betula pubescens, Juniperus and Salix, together with a rich herb assemblage. Pinus macrofossils occurred in the central part of the record just after the most significant charcoal presence in the sequence. Pinus pollen values are greatest during the period when Pinus bracts are recorded. These trends were less clear in the pollen diagram, which lacked the spatial resolution of the macrofossil record. The first continuous records of Picea pollen follow the peak value in the charcoal record, although no Picea macrofossils have been recovered from this site. Picea today only grows in the southern part of the National Park and has never grown around the site. Our data suggests that Picea first became abundant in the region at the same time as Pinus became locally established. First Pinus, then Betula pubescens macrofossils disappear from the record and Betula nana becomes of increased importance in the top 50 cm of the 2.1 m core. Betula pollen concentrations decline as B. pubescens gives way to B. nana, so the overall picture is detectable in the pollen record even though local detail of treeline composition and dynamics cannot be resolved using the pollen data alone.

Date received: March 24, 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-26.