Plant species richness and its determinants on a coastal dune system at Îles de la Madeleine, Québec (Canada)
2008
Houle, Gilles
On coastal dunes, significant changes in several environmental factors that affect plant species richness, such as salt spray, substrate salinity, and sand movement, occur as distance inland from the beach increases. In the present study, I test a model (confirmatory path analysis) of the environmental factors controlling plant species richness on the ridges and in the slacks of a coastal dune system at Îles de la Madeleine, Québec, Canada. Elevation, substrate organic matter, pH and salinity, and plant species richness were determined in 50-m² quadrats on 9 ridges and 9 slacks along each of 12 transects perpendicular to the coastline. Each variable was analyzed for differences between ridges and slacks and among distances from the high drift line. Path analysis allowed me to determine the effects of the abiotic variables on species richness, separately for ridges and for slacks. Elevation, substrate organic matter, pH and salinity, and richness all varied substantially with distance from the high drift line. Only elevation and richness differed between ridges and slacks; however, differences were not constant with distance from the beach. The path model for ridges explained 68% and that for slacks explained 50% of the variance of species richness. Substrate organic matter content, an indirect measure of substrate fertility, was the variable having the strongest direct effect (positive) on richness, both on ridges and in slacks. Because primary production typically increases with an increase in substrate fertility, the positive association reported here between richness and substrate organic matter may correspond to the increasing portion of the hump-shaped richness/production relationship proposed by Grime (1973; 1979) and often reported in the literature.
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