Distribution of vegetation along environmental gradients on Sable Island, Nova Scotia.

Coastal sand dune ecosystems are known to be structured by disturbance along coast-to-inland gradients, but little is known about how such patterns might change on exposed islands where environmental gradients vector in multiple directions. We investigated responses in plant assemblages on Sable Island, a long (49 km) and narrow (1.25 km at the centre) mostly vegetated sand bar located 160 km off the east coast of Nova Scotia, Canada. We sampled vegetation composition across the island using a stratified random design to capture a range of environmental predictors potentially associated with substrate conditions and disturbance from coastal processes, as well as grazing by the island's feral horses. We identified 3 different vegetation assemblages using hierarchical cluster analysis and non-metric multidimensional scaling that were associated with predictor variables. Distance from shore (both north and south shore) and slope angle were strongly related to both vegetation distribution and community composition. Areas farther from shore (subject to less wind and wave disturbance) contained greater amounts of shrub and heath vegetation. However, all parts of the island contained non-vegetated areas or stress-tolerant plant communities. Patterns of vegetation succession inferred for Sable Island were not linear and are better described as responses to repeated environmental disturbance rather than to a gradual process of soil development and competitive displacement. In addition to highlighting the multi-directional environmental influences on community composition of island systems, our results establish baseline spatial information on vegetation communities necessary for the ecological monitoring of Sable Island as a new National Park Reserve.