Zonation of vegetation along a burial gradient on the leeward slopes of Lake Huron sand dunes

The zonation of coastal dune plant communities from the beach to their inland margin is recognized worldwide; however, the cause of this pattern remains controversial because of the covariance of several environmental factors, such as sand burial, salt spray, and microclimate, along a gradient perpendicular to the shoreline. To minimize the confounding influence of this complex shore-inland gradient and determine the direct effects of burial on plant community composition, we examined stands along a burial gradient that extended parallel to the Lake Huron coastline, produced by variable blowout activity amongst a series of parabolic dunes comprising the second ridge inland from the coast. We used the point-quarter method and 1 m x 1 m plots to quantify overstorey and understorey plant communities in each parabolic dune stand and determined species importance, here defined as the sum of density, frequency, and dominance for the overstorey and the sum of frequency and dominance only for the understorey. Correspondence analyses of the species importance - dune stand matrices elucidated a pattern of plant community composition on the primary ordination axis that was strongly related to an index of burial activity (r2 = 0.40 and 0.87 for the overstorey and understorey, respectively). Burial was associated with changes in species richness and diversity, shifts in dominant species, and species replacement based on burial tolerance across the gradient. These data support the hypothesis that burial in sand dunes is a major causative factor of zonation, which can extend beyond the foredunes and include communities of woody species.