Tree-limit history during the Holocene in the Scandes mountains, Sweden, inferred from subfossil wood

Abstract

Certain aspects of the Holocene tree-limit history of the Swedish Scandes are inferred from 14C-dating of subfossil wood. The main objective was to test an earlier hypothesis that paucity of subfossil wood from between c. 7700 and 6200 yr B.P., above the current tree-limit of Pinus sylvestris, was due to a short erosional episode c. 6300 yr B.P., rather than a long-term general climatic cooling. Sampling was carried out in a restricted area where the absence of long, steep slopes diminished the potential for extensive and rapid mass movements. Some of the new datings fall within the above-mentioned gap and thus provide support for the earlier erosion hypothesis, although the exact nature and cause of the geomorphic processes is still hypothetical. Temporal clustering of dates around 6000 yr B.P. indicates a Holocene thermal optimum for pine at that time. It is deduced that the total range of summer temperatures between the above optimum and the long-term mean of the Little Ice Age was 2.3°C or slightly more. Starting around 6300 yr B.P. climatic seasonality appears to have decreased and snow depth and longevity increased, which preconditioned the emergence of a subalpine birch forest belt. It is inferred that the cooling (summers) of the Little Ice Age (c. 1300–1850 A.D.) amounted to approx. 1°C in relation to the 1931–1960 average. This triggered vegetational retrogression and destructive geomorphic processes. A synthesis of all available dates of subfossil pine from the region suggests that no neoglacial period occurred between c. 8600 and 3300 yr B.P.