Summer air temperatures and tree line dynamics at Abisko

Abstract

Recorded and calculated summer air temperatures at Abisko (68?21'N, 18?49E, 388 m a.s.l.) from 1868 to 1994 are discussed in relation to changes of altitudinal tree line and distribution patterns of mountain birch near the beach of Lake Tornetr?sk. The broad features of the temperature variations are: there are only minor changes from the 1870's to c. 1900 when a rise of c. 1.5?C took place ending in c. 1940. Since then there has been a decline of c. 0.5?C in the long-term trend. The peak air temperatures measured in the 1930's appear to be associated with a 20-50 altitudinal rise of the species limit of mountain birch on the eastern slope of Mt Njulla whereas the potential altitudinal rise, calculated from the temperature increase of 1.0?C and typical mean summer lapse rates, is >200 m. Based on the continuous pine dendrochronologies from the Tornetr?sk region, it is suggested that the vertical changes of the tree line on the eastern slope of Mt Njulla are small since 500 AD. The reason is that 50 yr mean anomalies of summer temperatures may be too small to induce any significant altitudinal movements. The importance of edaphic and micrometeorological factors for the response to climate of the subalpine heaths below the altitudinal tree line are discussed.