Post-glacial dynamics of vegetation and tree-lines in the far north of Fennoscandia

Abstract

The transitional zone between the boreal forest and the northern coastal area of the Arctic Ocean in the far north of Fennoscandia is a globally important ecotone. The pollen stratigraphy of four lakes, Lakes Hopseidet and Ifjord in Norway and Lakes Skaidejavri and Rautuselkä in northern Finland, was studied to reveal the post-glacial dynamics of the vegetation in this ecotone. These lakes were selected to represent the different vegetation zones within the area and the altitudinal changes of these vegetation zones.

Pollen influx and concentration determinations were used to estimate the past locations of the tree-line, although this approach was partly hampered by the effects of sediment focusing and inwash. The results nevertheless show that between approx. 11 000-5 500 cal BP birch forests covered most of the Nordkinn Peninsula. However, pine never reached the two tundra lakes. The retreat of both the birch and pine tree-line started simultaneously and rapidly at approx. 5 500 cal BP leading to the disappearance of birch forest on the plateau of the Nordkinn Peninsula. Since then both species have retreated in a gradual manner.

Late-glacial sediment was found only in Lake Ifjord, where the basal pollen spectra belong to the Younger Dryas chronozone, being strongly dominated by Artemisia (max. > 70 %). The earliest Holocene stages were characterized by very sparse vegetation, suggesting extensive snow-beds and moist, open soils. Forest development started as extensive birch forests established between 11 200-10 000 cal BP. These forests resembled the present submaritime mountain birch forests, as they had a dense field layer dominated by ferns, lycopods, and tall herbs. A major change in vegetation took place when the gradual migration of pine started more than 10 000 cal BP. It replaced birch as the dominant tree species over large areas. During the later part of the Holocene, tundra has expanded along with the forest decline and, as shown by the results of detrended correspondence analysis, the zonation of vegetation has become clearer. The present tundra is very different to that of the early-Holocene, being mainly dominated by a pollen flora suggesting peaty, moist soils.

Pine migrated southwards from the coastal areas of the Arctic Ocean and it took more than 3 000 y to invade over northern Fennoscandia. The patterns of migration suggest that climate alone did not control the migration but that biological factors also played an important role as the migration of pine took place in an environment close to the limits of its ecological tolerance. The presence of a long time-lag in pine migration gives credence to the theory that early post-glacial tree migrations were controlled by a complex set of factors and that no detailed climatic interpretations should be made on the basis of the migration history of a single or few tree species.