THE GENUS BETULA IN GREENLAND - HOLOCENE HISTORY, PRESENT DISTRIBUTION AND SYNECOLOGY

After its arrival in E Greenland c. 8 000 C-14 years B. P. Betula nana spread rapidly to become a dominant plant. A climatic change c. 5000 B. P. almost exterminated it at more oceanic sites, whereas inland it only lost little ground. Today it is common inland in many kinds of heath and other vegetation types. When it spread to Middle W Greenland around 6500 B.P. it only took two to three centuries to become dominating in the continental inland whereas the spreading to the coastal areas began later and was more gradual. After the Holocene climatic optimum it decreased here but kept its dominating position inland, where today it is a main plant in mossy heaths on N slopes and dry, grassy communities on S slopes. Betula glandulosa arrived in SW Greenland c. 5700 B.P., later on in S Greenland. It is a lowarctic, oceanic plant, missing only on the outer skerries, but being common in mossy heaths all through the fjords, where it also occurs in grassland vegetation types and, at the bead of the fjords, as undergrowth in the subarctic, usually 4-5 m high birch woodlands formed by B. pubescens, which did not arrive until c. 3500 B.P. The most common plant communities, in which the three Betula species occur in different parts of Greenland are summarized, partly by referring to other publications, partly by presenting six new tables of vegetation analyses, stored in the Botanical Museum or published in "grey literature". Where the species meet, hybridizing and introgression take place. Distribution maps of the three species and two hybrids are presented, and the taxonomy summarized.