Vegetation associations along disturbance gradients on the sand dunes of Sable Island, Nova Scotia.

Sable Island, Nova Scotia, is a dynamic dune ecosystem that is composed of plant communities exposed to varying levels of disturbance. The island is exposed to extreme weather events throughout the year, and this plays an important role in dune succession; however, the vegetation dynamics of this ecosystem are poorly understood. I investigated plant community responses to natural disturbance gradients using field measurements of community composition, abiotic variables, and grazing (and/or browsing) pressure from the island’s population of feral horses. Sampling plots were distributed across the entire island using a stratified random sampling design to capture the maximum range of environmental gradients and vegetation types. I measured species composition at each site in combination with predictor environmental variables: slope, organic layer presence, distance from shore, and evidence of grazing. I identified three different vegetation assemblages via hierarchical cluster analysis and non-metric multidimensional scaling ordination, and examined their associations with different environmental conditions and plant traits. Multivariate analyses indicated a strong relationship between community composition and distance from shore. Slope was the most important variable affecting whether a plot had vegetation and instances of grazing. Species with traits better suited to withstand sand burial and salt spray were present in areas closer to shore. Areas with less disturbance contained more shrub and heath communities. Evidence of grazing was present in all vegetation types with no observed relationship to plant species composition. Dune succession on Sable Island was not linear and is better described as the vegetative response to dynamic environmental stress rather than the result of gradual soil development and competitive displacement