RELATIONSHIPS BETWEEN VEGETATION AND ENVIRONMENTAL CHARACTERISTICS INA SALT-MARSH SYSTEM ON THE COAST OF NORTHWEST SPAIN

This study reports an investigation of relationships between environme ntal variables (electrical conductivity of groundwater, soil redox pot ential, water-table depth, and high-tide flooding depth) on vegetation zonation in a salt-marsh system on the coast of northwest Spain. Disc riminant analysis indicated that conductivity (a measure of salinity) and redox potential are correlated with vegetation type within the stu dy area. Conductivity declines with increasing altitude and distance f rom the sea, whereas redox potential does not vary along well-defined large-scale gradients. Soils with the most strongly oxidizing conditio ns (i.e. moderate salinity, with Eh greater than 200 mV and thus subto xic levels of Mn2+, Fe2+ and S2-) are occupied by the Halimione portul acoides community. Communities dominated by Juncus maritimus, and Phra gmites australis reedbeds, occur at more strongly reducing sites (Eh b etween 100 and 200 mV, with possibly toxic levels of Mn2+ but not of F e2+); the presence of these communities may thus be limited by Fe2+ to xicity. The most strongly reducing sites (with Eh low enough for the r eduction of Fe3+ to Fe2+) are occupied by Spartina maritima and Scirpu s maritimus communities. These communities appear to be tolerant of Fe 2+, and even of low concentrations of S2-.