Early- to mid-Holocene forest-line and climate dynamics in southern Scandes mountains inferred from contrasting megafossil and pollen data

Abstract

The result of 344 radiocarbon-dated megafossils is here presented and discussed. This study aims at elucidating early- to mid-Holocene forest-line and climate dynamics in the southern Scandes along a present gradient of decreasing forest-line elevations. Around 9.5 calibrated ka before present (BP), pine suddenly established vertical belts of at least 200 m. These represent the highest pine-forests during the Holocene, ca. 210–170 m higher than today when corrected for land uplift. By this, summer temperatures at least 1–1.3°C warmer than today are indicated for the early Holocene thermal maximum around 8.5–9.5 cal. ka BP. The most pronounced warming occurred in Jotunheimen, the highest mountain range in Scandinavia, because of an amplified ‘Massenerhebung’ effect. Megafossils show the establishment of birch-forests above pine-forests already from the early Holocene. Pine-forests started their decline in the early Holocene and became replaced by the less warmth-demanding birch-forests. Pine megafossil results and pollen studies from the same areas show that cooling around 8.5 cal. ka BP caused a significant decrease in pine pollen production whereas pine-forest-lines were more or less unaffected. In the following period of about 2000 years, the high-altitudinal pine-forests could hardly be detected in pollen diagrams. This shows how strongly past temperatures influenced on the pollen production of individuals and how this might obscure pollen-based reconstructions of past vegetation. To be able to correct for this error, there is a need for establishing exact present-day relationships between temperature and pollen production of prolific pollen producers.