The ecological status of grey alder (Alnus incana (L.) Moench) in the upper subalpine 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 finegrained 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.