Holocene tree-line dynamics and inferred climatic changes in the Abisko area, northern Sweden, based on macrofossil and pollen records

Lake-sediment records of pollen and macroscopic plant remains of terrestrial origin provide evidence of long-term changes in forest composition close to the altitudinal tree-line in the Abisko area, northernmost Sweden. A dominance of mountain birch (Betula pubescens Ehrh. ssp. tortuosa (Ledeb.) Nyman) with a field-layer of ferns, grasses and sedges suggests that an oceanic climate with relatively high precipitation existed during the early Holocene. Macrofossils of mountain birch were found 300-400 m above the present tree-line which corresponds to a 1.5-2 degrees C higher summer temperature than at present. Scots Pine (Pinus sylvestris) was present to some extent at altitudes above its present limit during the early Holocene. A pronounced expansion of pine took place above its present limit around 6300 cal. BP (5500 BP). This is interpreted as a result of successively drier and more continental climatic conditions. A decreased abundance of pine from c. 5200 cal. BP (4700 BP), followed by a gradual retraction of the upper limit of pine during the later part of the Holocene, can be attributed mainly to long-term changes in solar radiation. A decline in the upper limit of pine of c. 175 m since 4500 cal. BP (4000 BP) is believed to represent a decrease in growing-season temperature of c. 1.5 degrees C.