The machair systems of the Outer Hebrides of Scotland are renowned for the diversity and conservation value of their plant communities. However, machair systems have a restricted distribution and are prone to disturbance from both anthropogenic and environmental sources. The effects of two major anthropogenic and environmental disturbances (agriculture and burial by wind-blown sand) on the Kildonan and Drimsdale machair systems on South Uist, and the dynamic responses of machair vegetation to these human and natural agencies were described and quantified. The investigated machair systems comprised a diversity of sand dune, grassland and marginal vegetation types which were spatially distributed along a primary environmental gradient of soil organic matter, soil moisture and soil pH. Vegetation types influenced by the effects of cultivation, notably potato-bed successions, abandoned field systems and fallow areas were also a conspicuous component of the Kildonan and Drimsdale machairs. Re-vegetation following cultivation occurred primarily through vegetative processes. Sexual reproduction via seed within different machair vegetation types was limited. The numbers of viable seeds recorded from machair seed banks ranged from 7 mˉ² soil in foredune vegetation to 58 mˉ² soil in newly-ploughed cereal land. Densities of viable seeds recorded from the machair seed rain ranged from 2 mˉ² soil in the dune slack to 24 mˉ² soil in a potato patch, fallow for two years. These values represent the first estimates of the size of machair seed budgets. Glasshouse-based burial experiments examined the effects of inundation by sand on four machair vegetation types. Foredune grassland, dune slack, three-year fallow grassland and unploughed grassland vegetation exhibited the capacity to survive and re-emerge from both intermittent and single depositions of sand. In terms of effects on plant frequency, intermittent burial by five depositions of 1cm of sand was more damaging than a single deposition of 5cm. However, all vegetation types responded to both intermittent and single burial events primarily through a change in the number of individual plants, rather than through a change in the number of plant species. Five different responses to burial by sand were identified for machair species. The suspension of photosynthetic activity and the maintenance of a low dark respiration rate were identified as important physiological responses to burial. Photosynthetic activity of machair vegetation was resumed on denudation, indicating the existence of an elastic physiological response to burial. Ecological processes in the dune slack habitat and the responses of slack vegetation to its dynamic environment were subtly different to those characterizing the other investigated sub-communities. These differences were attributed to the fact that machair slacks are characterized by an historical ecology distinct from that of other areas of the machair system. It is suggested that the formulation of effective management plans and conservation strategies for the machair systems of the Outer Hebrides ultimately lies in an understanding of their historical ecology, and in their past management and environmental histories.

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