Relationships between vegetation and environmental characteristics in a salt-marsh system on the coast of Northwest Spain

This study reports an investigation of relationships between environmental variables (electrical conductivity of groundwater, soil redox potential, water-table depth, and high-tide flooding depth) on vegetation zonation in a salt-marsh system on the coast of northwest Spain. Discriminant analysis indicated that conductivity (a measure of salinity) and redox potential are correlated with vegetation type within the study area. Conductivity declines with increasing altitude and distance from the sea, whereas redox potential does not vary along well-defined large-scale gradients. Soils with the most strongly oxidizing conditions (i.e. moderate salinity, with Eh greater than 200 mV and thus subtoxic levels of$\text{Mn}^{2+}$,$\text{Fe}^{2+}$and$\text{S}^{2-}$) are occupied by the Halimione portulacoides community. Communities dominated by Juncus maritimus, and Phragmites australis reedbeds, occur at more strongly reducing sites (Eh between 100 and 200 mV, with possibly toxic levels of$\text{Mn}^{2+}$but not of$\text{Fe}^{2+}$); the presence of these communities may thus be limited by$\text{Fe}^{2+}$toxicity. The most strongly reducing sites (with Eh low enough for the reduction of$\text{Fe}^{3+}$to$\text{Fe}^{2+}$) are occupied by Spartina maritima and Scirpus maritimus communities. These communities appear to be tolerant of$\text{Fe}^{2+}$, and even of low concentrations of$\text{S}^{2-}$.