Recent Retrogression of the Forest-Alpine Tundra Ecotone (Betula pubescens Ehrh. ssp. tortuosa (Ledeb.) Nyman) in the Scandes Mountains, Sweden

Abstract

Demography of mountain birch (Betula pubescens Ehrh. ssp. tortuosa (Ledeb.) Nyman) was studied in permanent plots (1972/73-85/86), in the forest- alpine tundra ecotone of the Swedish Scandes Mountains. The censused tree population, which to a large extent became established in response to the climatic warming c. 1915-50, declined in certain respects during the study period. A sympathetic cooling trend for the summers, and a few years with relatively severe and protracted snowpack influences were recorded for the study area. The number of tree-sized birches decreased by c. 16%, while sexual regeneration failed and many individuals declined in vigour. The major cause of mortality and vigour decline was mechanical stress, imposed by delayed snowmelt, possibly in combination with age-related mechanical weakening. A measure of tree-line inertia in relation to climatic variability was obtained by extrapolating the demographic trend of the study period indefinitely. Given the same degree of thermal change, extinction of the birch tree-line population needs much more time than establishing it.