The Holocene history of Betula at Lake lilompolo,Inari Lapland,northeastern Finland

Betula pollen diameters were systematically measured in a series of Holocene sediment samples from a lake in the northern birch-forest region of eastern Finnish Lapland. The resulting size-frequency distributions were analysed statistically to infer the species identity of the birch populations and to reconstruct the local history of birch. Macrofossil analysis of birch fruits and catkin scales were used as supplementary evidence. The pollen record was also used to reconstruct the forest history. Pinus arrived in the area c. 7500-8000 BP. From 7000 to 4000 BP the surroundings of the lake were probably covered with pine forest where tree birch was also present. Between c. 3700 and 2500 BP the birch pollen size-frequency distribution diagram shows a distinct phase of larger grains with a size range larger than in the other samples. This period coincides with the retreat of pine and is interpreted as reflecting extensive hybridization amongst the birch species and the first appearance of mountain birch (Betula pubecens ssp. tortuosa) in an environment where climatic conditions were becoming more rigorous.