Melting glaciers in the swedish scandes provide new insights into palaeotreeline performance

Author: 
Leif Kullman

Climate warming and associated post-Little Ice Age recession of glacier ice and permanent snow in high mountains of the Swedish Scandes have opened a new view for alpine palaeoecology. Receding glaciers and snow/ice patches, high above current tree-lines of all boreal tree species, have exposed forefields with a plethora of megafossil tree remnants, representing discrete multi-species tree groves. Radiocarbon-dating of associated wood debris offers new insight into the early Holocene structure and species composition of the tree-line ecotone. The emerging pattern deviates substantially from the conventional view, deduced from the pollen stratigraphical approach, which needs to be questioned in general, both with respect to tree-line reconstruction and in the context of more general biogeographic evolution. Boreal tree species were present in the concerned settings from late-glacial, 16 800 cal. yr BP to the late Holocene, about 4000 cal. yr BP. The frequency of megafossil tree remains peaked 9600-9000 cal. yr BP, maximum 600-700 m above modern tree-lines. This implies (corrected for land uplift) that the summer temperatures may have been 3 °C warmer than current standards. The postglacial immigration history of Picea abies to the Scandes, a stumbling stock in Scandinavian palaeobiogeography, is at least 8000 years prior to traditional pollen-based text book narrative. Discrete tree groves, embracing most present-day boreal tree species, were strictly confined to empty glacier cirques and snow hollows. These populations may have played an important role as dispersal nodes for the postglacial afforestation of the mountain landscape. In addition, the results add to the increasingly recognized view that Larix sibirica, an eastern species, currently not native to Scandinavia, prevailed quite frequently at high elevations during the early Holocene. Further shortcomings of traditional methods of vegetation reconstruction are highlighted when viewed in perspective of megafossil analyses, which support a relatively firm framework on which the detailed vegetation history can be built.

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