Immigration of Picea abies into North-Central Sweden. New evidence of regional expansion and tree-limit evolution

Analyses of subfossil tree remains in peats and raw humus soils account for the immigration and spread of Picea abies (Norway spruce) into Sweden and the evolution of the alpine tree-limit ecotone. Picea abies is recorded for the first time about 11 000 BP, on an early emerging nunatak in the southern Swedish Scandes. Prior to c. 8 000 BP, Picea was strictly bound to high elevations in the west. Farther to the east in North-Central Sweden, Picea emerged in the subfossil record mainly after c. 6 000 BP. Later on, growth of local founder populations and landscape-scale expansion may have been forced chiefly by a successively less seasonal climate (the Milankovitch model of orbital forcing), promoting increased net soil moisture and possibly a deeper and more persistent snow cover. Already at the Weichselian/Holocene transition, an elevational tree-limit ecotone was established and arborescent Picea grew at least 400 in higher than the modem tree-limit. Until about 8 000 BP, the species-limit descended, whereafter it stabilized up to the present day. Lack of significant species-limit retraction after c. 8 000 BP may seem paradoxical in perspective of independently inferred climate cooling in response to reduced insulation and land uplift. This could be a consequence of substantial phenotypic plasticity and increased snow accumulation, mitigating the long-term cooling. Thus, the elevational species-limit (krummholz) of Picea abies is out of equilibrium with the modern thermal climate. The striking incongruence between the results exposed here and earlier palynological interpretations of the same biogeographical process will have implications for the use of pollen data for range-limit reconstructions within historical biogeography, and urges for re-evaluation of certain aspects of the Fennoscandian forest history.