ZONATION OF HALOPHYTIC VEGETATION ALONG A TIDE EXPOSURE GRADIENT AND ASSOCIATED PROCESSES

Abstract

The zonation of vegetation in the salt-marsh of Mundaka-Gemika (Bay of Biscay, N. Spain) is studied along a small scale topographic gradient. Species distribution appears closely linked to abiotic factors deriving from topographical level, such as organic matter, pH, moisture and conductivity. The redox potential and the compactness of the soil vary in other ways, thus increasing the heterogeneity of the habitat. There is a critica1 elevation, close to MHW level, after which coexistence processes give way to dominance by the best equipped species. A model of the zonation is given. In this model the degree of overlap decreases towards the top area, where there is competitive exclusion and segregation of interior species to the most exposed area. It can not be concluded that the marked segregation of the species along the gradient is exclusively due to physiological requirements of the plants. Tidal dispersal probably plays an important role on the distributional pattem of the annual species Salicornia and Suaeda, but a minor role on the species with vegetative expansion, Spartina maritima, Arthrocnemum perenne, Halimioneportulacoides, and Arthrocnemumfr.uticosum. Plant zonation can only be explained bearing in mind associated processes such as physical disturbance and interspecific competition. As other authors found in higher latitudes, these processes will need to be considered in relation to edaphic factors in elucidating the underlying mechanisms of salt marsh plant zonation. Experimental work in field and laboratory is needed to determine the niche position of each species in the salt marshes in this part of the Atlantic Coast.