THE ECOLOGICAL STATUS OF GRAY ALDER (ALNUS-INCANA (L) MOENCH) IN THE UPPER SUB-ALPINE BIRCH FOREST OF THE CENTRAL SCANDES

Elevational distribution, site characteristics and regeneration patterns of grey alder [Alnus incana (L.) Moench] have been studied in subalpine forest of the southern Swedish Scandes, mainly as a basis for paleoecological inferences. Alder occurs very sparsely in the mountain birch forest and is strictly confined to sites with fresh fine-grained soils with an insignificant mor humus layer. It frequently grows in close association with moving water (inundation in the spring). The potential climatic tree-limit is inferred to appear about 30 m below that of mountain birch. Within a more than 200 m broad vertical zone below the tree-limit, sexual reproduction does not take place at the present. Conceivably, the alders here are old relicts from a warmer period in the past. In contrast to other tree species, the range-limit did not advance altitudinally in response to the 20th century warming (up to the 1930s/1940s) and a non-equilibrium relationship with climate is hypothesized to have developed after the maximum range extension in the early- or mid-Holocene. The virtual extinction of alder in the upper subalpine forest during the late Holocene is understandable in terms of pronounced thermophily of generative life-cycle stages and a narrow range of soil preference. Both these properties are acting regressively on reproductive success as a consequence of climatic cooling and long-term soil deterioration.