Effect of soil characteristics on plant distribution in coastal ecosystems of SW Iberian Peninsula sand spits

Some studies report particular soil conditions occurring on barrier islands and sand spits that contrast with those found on the mainland. The effects of soil characteristics on the distribution of plants established on a sand spit in the SW Iberian Peninsula were evaluated in order to elucidate the mechanisms that drive plant communities in this type of coastal ecosystem, furthering the knowledge regarding plant–soil interactions in this type of coastal ecosystem in comparison with the mainland. Effects of soil characteristics on plant species distribution, abundance, richness, and diversity were analyzed by multivariate analyses and correlation tests, and biotic interactions were also examined. Results revealed that soil pH, electrical conductivity, CaCO3 content, sand mobility (dunes), and flooding (marshes) played an important role in shaping plant communities, while biotic interactions became important only in advanced successional stages. Particularly, CaCO3 content determined general soil basicity in the spit, which favored the establishment of calcicolous species that were absent in the acidic dunes of the nearby mainland, and could reinforce positive biotic interactions in these dunes. In salt marshes, predominantly sandy soils favored the development of upper marsh communities, also favoring positive interactions in some areas, as well as the development of notable dune–marsh ecotones, compared to the muddy marshes that prevail on the mainland.