Mantel and partial Mantel tests suggest some factors that may control the local distribution of Aster laurentianus at Iles de la Madeleine, Quebec

Spatial heterogeneity is a significant aspect of ecosystem structure and function. Because ecological variables often are spatially autocorrelated, standard statistics frequently are not appropriate to analyze ecological data. In this paper, we use Mantel and partial Mantel tests to explore the spatial structure of and relationships between several variables - micro-topography, substrate salinity, wrack cover and the density of Aster laurentianus - at two sites at the periphery of a shallow lagoon at les de la Madeleine, Quebec. A. laurentianus is an annual halophyte endemic to the Gulf of St. Lawrence. It typically occurs in a narrow band (from ca. 50 to 200 cm wide) at the periphery of shallow lagoons. In this habitat, plants are exposed to frequent deposition of plant debris (wrack) and to the limiting effects of substrate salinity. Our analyses show that substrate salinity, wrack cover and density of A. laurentianus are significantly correlated with topography, and that wrack cover affects substrate salinity at both sites. At the first site, the abundance of A. laurentianus increases progressively with distance from the waterline, while at the second site, density increases initially, but then diminishes with distance from the waterline. At the first site, the most important structuring variable is wrack deposition, the effect of salinity on plant density being indirect. At the second site, which is somewhat sheltered, wrack deposition is far less important and substrate salinity predominates over wrack deposition as structuring variable. We suggest that A. laurentianus distribution along the shores of shallow lagoons is limited both by substrate salinity and wrack deposition, but that the relative importance of these structuring variables changes with the physical characteristics of the site.