THE LATE WEICHSELIAN AND EARLY HOLOCENE HISTORY OF TREE BIRCH IN SOUTH NORWAY AND THE BOLLING BETULA TIME-LAG IN NORTHWEST EUROPE

This study compares late-glacial pollen data from northwest Europe to elucidate the northward migration of tree birch during the Boiling warming about 13,000 BP and the subsequent birch-forest development. The late-glacial tree-birch history in south Norway is emphasized. The Balling warming initiated a rapid northward migration of tree birch, that extended across most of northwest Europe within c. 300 yr. Comparison of pollen influx studies shows that values of about 200 tree-birch pollen grains cm(-2)a(-1) indicate local tree birch. The time that elapsed between the Belling warming and the local birch-forest development, the so-called Boiling Betula time-lag, lasted up to 1000 yr, though in southern and/or protected sites this lag was about 500 yr or even less. In all areas of northwest Europe, strong winds are assumed to have been an important factor that delayed the birch-forest development in the first half of the late-glacial. Other important factors were drought effects in southern areas (e.g. south Britain) and sparse snow cover and frost in northern areas. The Arctic treeline lay in north Rogaland, southwest Norway, about 12,500 BP. During the Allerod Chronozone, tree birch migrated northward along the ice-free coastal strip of south Norway to More and Romsdal. However, the development of Allerod birch forest in Norway was restricted to Rogaland. The Younger Dryas cooling (11,000-10,000 BP) caused the near-extinction of tree birch in Norway, and only in sheltered sites in south Rogaland scattered individuals were able to survive. Hence, vegetational boundaries crossed Rogaland, thus representing an ecotonal area, through major parts of the late-glacial. A rapid development of closed birch forests all along the coast of south Norway occurred after the Holocene warming (10,000 BP).