Palaeoclimate and tree-line changes during the Holocene based on pollen and plant macrofossil records from six lakes at different altitudes in northern Sweden

Pollen and macrofossil samples were analysed from lake sediments where the lakes formed a transect across the altitudinal tree-line in the Tornetrask area, in the northern Scandinavian mountain range. The results show that the tree-line, consisting of mountain birch (Betula pubescens Ehrh. ssp. tortuosa (Ledeb.) Nyman), was 300-400 m above the present during the early and mid-Holocene. When the isostatic land up-lift is taken into account this reflects summer temperatures 1.5-2 degreesC higher than today. Expansion of mixed pine-birch forest took place 8300 cal BP in the eastern part of the Tornestrask area and about 700 years later in the western part at low altitudes. Optimal conditions for Scots pine (Pinus sylvestris L.) occurred during the mid-Holocene (6300-4500 cal BP) when mixed pine-birch forest reached 110 m and single pines reached 175 m above the present pine limit. The Holocene climate is interpreted as warm and moist during the early Holocene, warm and dry during the mid-Holocene, and finally cold and moist during the late Holocene. (C) 2001 Elsevier Science B.V. All rights reserved.