Aspects of vegetation and settlement history in the Outer Hebrides, Scotland

Although the Outer Hebrides today are virtually treeless, many parts of the islands appear to have sustained woodland during the early Holocene. The reasons for the decline in trees and shrubs, which took place between the Mesolithic and Iron Age periods, may include natural factors (e.g. climate or soil change) and/or human impact. In order to gain an insight into the relationship between people and vegetation change, profiles from five sites were analysed for pollen, spores and microscopic charcoal content: Loch a' Chabhain and Loch Airigh na h-Achlais (South Uist), Fobost (a valley mire in South Uist); Loch Olabhat (North Uist), and the Neolithic archaeological site of Eilean Domhnuill (located in Loch Olabhat). Other than at the archaeological site, arboreal pollen values were high (>75%) at the beginning of the Holocene. There is no evidence for a clear Mesolithic presence at any of the sites. Inferred woodland decline started c. 7900 BP (8690 cal BP) at Frobost, probably due to an expansion of the mire, and c. 5300 BP (6080 cal BP) at Loch a' Chabhain, probably also due to natural factors. Both areas may have been used for grazing from the Neolithic onwards. At Loch Airigh na h-Achlais woodland reduction started in the Neolithic, accelerating during the Bronze Age, perhaps due to climatic deterioration and/or grazing pressures. The profile from Loch Olabhat has strong evidence of human impact during the early Neolithic: a decline in arboreal taxa, an increase in cultural indicators, and signs of erosion in the catchment area. Woodland removal and cultivation here may ultimately have led to rising loch levels and the inundation of Eilean Domhnuill. At Loch Airigh na h-Achlais and Loch Olabhat there may be evidence for heathland management by fire during prehistoric and historical times. Archaeological evidence points to a shift in settlement areas between the Iron Age and the Neolithic, from peat-covered inland areas to the machair along the west coast. A general expansion in heath and mire communities suggests that inland localities may have become increasingly infertile.