Pollen analysis and the vegetational history of Barra and South Uist in the Outer Hebrides, Scotland.

This study examines contemporary vegetation, pollen taphonomy and Holocene vegetation history across east-west trending environmental transects in the southern Outer Hebridean islands of South Uist and Barra. The pollen depositionalc haracteristicso f sixteenp lant communities are described and modern pollen deposition is found to reflect the distribution of major vegetation types on the islands. The history of Holocene vegetation is investigated at a number of sites which include exposed west coast inter-tidal peat deposits, wooded loch islands and pre-peat soils associated with archaeological features. These studies indicate that Holocene vegetation development followed a pattern typical for the region at similar latitudes. The early post-glacial vegetation mosaic of Empetrum heath, herb-rich grassland and dwarf shrub communities was progressivly altered by the subsequent establishment of predominantly deciduous woodlands. The pollen diagrams record an ordered sequence of tree migration to the islands in which Betula then Corylus were the first colonists followed later by Ulmus, Quercus, Pinus Alnus and Fraxinus. The scales of analysis employed in the study indicate that very local' pollen deposition is a characteristic of the islands' micro-fossil record - a feature which could be exploited in further archaeologically related studies. Woodland appears to have persisted in sheltered locations until c. 5,000 B. P. A gradual reduction of woodland from c. 5,000 B. P. was accompanied by an expansion of the herb-rich grassland and blanket bog communitiues which had been present on the islands from the early Holocene After c 4,000 B.P .w oodland decline accelerateda nd the grasslanda nd blanket peat communities increased to attain their present dominance in the islands vegetation. The vegetation changes recorded in the pollen diagrams are attributed to a combination of factors changing environmental conditions - such as the pedological effects of Holocene climatic conditions, sea level rise and human impacts. There is no archaeological evidence for mesolithic occupation of the islands, however at a point in the 'early' Holocene a brief episode of fluctuating woodland disturbance, charcoal and 'cereal type' pollen is noted at one east coast site, Loch Hellisdale. This data contributes to an increasing body of information which suggests some mesolithic presence along the eastern coast of South Uist.