Orbital Forcing and Tree-Limit History: Hypothesis and Preliminary Interpretation of Evidence from Swedish Lappland

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Abstract
The Holocene tree-limit history for an area in Swedish Lappland is analysed from a new set of 67 14C-dated subfossil wood remains. The species involved are Scots pine ( Pinus sylvestris), mountain birch ( Betula pubescens ssp. tortuosa) and grey alder ( Alnus incana). The highest tree-limit was established about 8000 BP by pine. Up to the present it has retreated c. 200 m in altitude at a constant rate of c. 25 m per millennium. A subalpine belt of birch above the pine forest, initially with admixture of alder, developed rapidly after 6000 BP, when birch started to invade the former pine areas and even extended higher. Birch grew at these high elevations until c. 4000 BP, although not necessarily without interruption. After 4000 BP birch also seems to have declined in elevation, although the magnitude is unknown. The steady pine tree-limit recession and the replacement of pine by birch may reflect continuous cooling and change of thermal seasonality. The present results corroborate the general hypothesis that predictable changes in the earth-sun geometry basically force climatic development, and indirectly vegetation history, during an interglacial.