Non-analogous tree flora in the Scandes Mountains, Sweden, during the early Holocene - macrofossil evidence of rapid geographic spread and response to palaeoclimate

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Abstract

Early-Holocene occurrences of broad-leaved temperate tree species at a site now within the subalpine belt of the Swedish Scandes are reported and analysed. Macrofossil remains (fruits and leaves) of Alnus glutinosa, Corylus avellana, Quercus robur and Ulmu.s glahra were recovered in a peat deposit far beyond and above their modern distributional limits. A previous nearby finding of Tilia cantata adds to this group of Hemiboreal lowland species, the regional presence of which in high-elevation northern Scandinavia has gone entirely unrecognized by pollen stratigraphical studies. Radiocarbon dates (AMS) range between 8500 and 8000 BP, indicating early and rapid spread, closely following the ice recession. Maximum tree species richness and distributional limits (latitude and altitude) were attained during the earliest part of the Holocene. In a regional perspective, the temperate trees coexisted with a dominant boreal (cold-climate) element, viz. Pinus sylvestris, Bclulu pubescens ssp. tortuosa, Betula pendula, Alnus incana, Picea ahies and Larix sibirica, which are all identified and dated from macrofossils. This climatically seemingly incompatible assemblage is without modern analogues. Presumably, this diverse tree flora owed its existence to a unique (Holocene perspective) climatic regime, ultimately forced by the Milankovitch cycles of insolation. The results strengthen previous palaeoclimatic inferences based on past tree-limit positions (Pinus sylvestris) and imply that Holocene summer warmth peaked fairly soon after the regional deglaciation, practically concurrently with the solar radiation maximum. Important questions are raised concerning several palynologically derived postulates within general palaeobiogeography. For example, rates of geographic tree spread are not necessarily individualistic, despite fundamentally different dispersal modes. The emergent efficiency of dispersal suggests that distribution patterns relate to climate and soil factors rather than time. Dispersal steps may have been fairly large, and the resultant geographic spread was epidemic, resulting in a sparse pattern of outlying stands, rather than sequentially following certain routes in the landscape. Rapid geographic expansion is likely to have been relatively unrestrained by physical barriers, e.g. mountain ranges, seas and large lakes. Even in Central Europe, close to the ice margin, there may have been more trees during the latest glaciation than previously realized.

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