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 th e tree-line, consisting of mountain birch (Betula pubescens Ehrh. ssp. tort uosa (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 re flects summer temperatures 1.5-2 degreesC higher than today. Expansion of m ixed pine-birch forest took place 8300 cal BP in the eastern part of the To rnestrask area and about 700 years later in the western part at low altitud es. Optimal conditions for Scots pine (Pinus sylvestris L.) occurred during the mid-Holocene (6300-4500 cal BP) when mixed pine-birch forest reached 1 10 m and single pines reached 175 m above the present pine limit. The Holoc ene climate is interpreted as warm and moist during the early Holocene, war m and dry during the mid-Holocene, and finally cold and moist during the la te Holocene. (C) 2001 Elsevier Science B.V. All rights reserved.