Question: Is there a critical depth of burial by sand beyond which species and communities fail to recover, and does repeated incremental burial have a greater impact than a single large deposition?
Location: The machair on the calcareous sand dunes on South Uist, in the Outer Hebrides of Scotland, UK.
Methods: Eight turves were collected from each of four machair sub-community types. After acclimatization in an unheated polythene tunnel, they were buried with sterilized machair shell sand, either by one single burial to 5 cm or by five applications of 1 cm of sand at approximately seven-week intervals. Species response was recorded on five occasions.
Results: Within machair sub-communities, burial by sand reduced the abundance (local rooted frequency) of plants more than it reduced species richness. Intermittent burial was more damaging than a single burial event. Those species with the highest pre-burial frequencies tended to dominate recovery in the sub-community as a whole. Species occurring across all four sub-community types exhibited varying responses to community burial between the differing types. Samples from slack sub-communities had distinctly different response characteristics from those of foredunes and unploughed and three-year fallow dune grassland.
Conclusions: The perennial life-form of many machair species has been evolutionarily selected for and dominates throughout the machair habitat. Account needs to be taken of competitive interaction between species in relation to burial response. The results of the investigation show that the hypotheses of Gilbertson et al. and Kent et al. on ‘machair stratification’ require refinement in that frequency of shallow burial can be as important as overall burial depth.