Holocene Thermal Trend Inferred from Tree-Limit History in the Scandes Mountains

Abstract

The Holocene tree-limit history in the Scandes Mountains, Sweden is inferred from subfossil wood remains of Scots pine (Pinus sylvestris L.), mountain birch (Betula pubescens Ehrh. ssp. tortuosa (Ledeb.) Nyman) and grey alder (Alnus incana (L.) Moench). Corrected for glacio-isostatic land upheaval, the data indicate a linear 165 m elevational retreat of the pine tree-limit throughout the past 8000 years. The average rate of the process was 20 m per millennium. In contrast, the tree-limit of birch, and to some extent grey alder, seems to have advanced altitudinally during the earlyand mid-Holocene. These opposing performances of xeric pine and mesic broadleaved deciduous species are suggested to be consistent with the orbital forcing mechanism (Milankovitch theory) of climate change. Further support was obtained by converting the pine tree-limit retreat to thermal decline (lapse rate 0.6?C/100 m), which yields a cooling trend amounting to 0.12?C per millennium. This is the same value as predicted by Berger et al. (1990) for astronomically forced Northern Hemisphere cooling since the Holocene thermal maximum and during 55,000 years from the present.