History of Picea abies in west central Sweden

Early Holocene Picea megafossils found in west central Sweden have challenged the prevailing theory based on pollen analysis that Picea migrated to Scandinavia during the second half of the Holocene. Pollen analyses is the most used method for the study of past vegetation dynamics; however, in the study of the postglacial immigration of Picea or for studies at the forest-tundra ecotone where the local pollen production is low, it is crucial to be able to separate locally produced tree pollen from pollen coming from distant forests. This thesis aims to explore methods to facilitate the separation of local pollen from background pollen and thereby enable the inference of the local establishment of Picea abies at a site, to evaluate whether the Picea megafossils can be confirmed by pollen analysis, and to investigate the pollen-vegetation relationship at the forest-tundra ecotone. Pollen analysis was carried out from mires situated near early to mid Holocene Picea megafossil sites in west central Sweden. The relationship between pollen from moss polsters and the surrounding vegetation was analysed with models for pollen dispersal and deposition. In a simulation study a landscape model for Holocene tree-line fluctuations based on palaeoecological data representing different time slices was evaluated by the comparison of numerous simulated pollen counts from different elevations with actual pollen diagrams. The results show that it is possible to confirm the megafossils with pollen analyses, provided that the pollen sampling site is within tens of meters from the megafossil site. The timing of local Picea establishment and expansion differed between nearby sites. Parameters to facilitate the understanding of pollen-vegetation relationships at the forest-tundra ecotone were established: pollen productivity estimates (PPE), relevant source area of pollen (RSAP), and background and local pollen. The PPE at the forest-tundra ecotone differed from estimates in southern Sweden, showing varied responses to climate on pollen production for assorted taxa. It was concluded that the use of multiple adjacent sites for pollen analyses, calibration of modern pollen and current vegetation, and the simulation approach are applications that facilitate the interpretation of pollen analysis in terms of local presence of trees