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

Abstract

Spatial heterogeneity is a significant aspect of ecosystem structure andfunction. 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 spatialstructure of and relationships between several variables –micro-topography, substrate salinity, wrack cover and the density ofAster laurentianus – at two sites at the peripheryofa shallow lagoon at Îles de la Madeleine, Québec.A. laurentianus is an annual halophyte endemic tothe Gulf of St. Lawrence. It typically occurs in a narrow band (from ca. 50 to200 cm wide) at the periphery of shallow lagoons. In this habitat,plants are exposed to frequent deposition of plant debris (wrack) and to thelimiting effects of substrate salinity.Our analyses show that substrate salinity, wrack cover and density ofA. laurentianus are significantly correlated withtopography, and that wrack cover affects substrate salinity at both sites. Atthe first site, the abundance of A. laurentianus increasesprogressively with distance from the waterline, while at the second site,density increases initially, but then diminishes with distance from thewaterline. At the first site, the most important structuring variable is wrackdeposition, the effect of salinity on plant density being indirect. At thesecond site, which is somewhat sheltered, wrack deposition is far lessimportantand substrate salinity predominates over wrack deposition as structuringvariable. We suggest that A. laurentianus distributionalong the shores of shallow lagoons is limited both by substrate salinity andwrack deposition, but that the relative importance of these structuringvariables changes with the physical characteristics of the site.