Holocene climatic change and tree-line response in Leirdalen, central Jotunheimen, south central Norway

Abstract

The provisional results of pollen analysis of a small mire (Leirdalen 5) provide evidence for long-term changes in forest composition and extent at and around the present day altitudinal tree-line in Leirdalen valley, south central Norway. Following deglaciation c. 10,115cal.yrbp, dense pine forest migrated into the area c. 9785cal.yrbp, replacing vegetation dominated formerly by birch. The pollen evidence suggests that Pinus sylvestris then expanded significantly in response to a warmer climate with summer temperatures ca. 1.5–2.0°C higher than present. This climatic optimum was probably interrupted by climatic deterioration c. 8380cal.yrbp (the Finse event), that caused woodland around Leirdalen 5 to retreat to lower altitudes. A sudden shift from dense pine forest to birch woodland with Pinus sylvestris and Alnus in conjunction with the development of a sub-alpine Betula belt is postulated to have occurred c. 6870cal.yrbp Since c. 4850cal.yrbp a general decrease in woodland extent due to further climatic deterioration and increased human impact is evident.