Coastal dune-systems provide opportunities for the study of plant successions and colonisation processes. The french atlantic dune-system is five hundred kilometres long from north to south. Grey dunes vegetation can be found on the fixed dune as a herb-grassland of xerotolerant species. An observatory of bryophytes and lichens on nine unforested dunes on the french atlantic coastal dune system has been set up to identify the patterns of the bryophytes and lichens in relation toenvironnement dynamic.The study of the patterns of the vegetation on the study area, along a transect from the beach to the forested dune, shows that bryophytes and lichens are distributed in five communities which their lifestrategies are identifiable by their biological traits and their significative relationships with some pedologic parameters. The distribution of each community in certains parts of the dune reflects modifications in the surface soil layer chemical composition of the substrate and shows trophic gradient, acidification and decalcification one from the beach to the forested dunes.Diachronic studies by the survey of the bryophytes and lichens communities and the relations between soil analysis and the species of some common response- traits of the species to disturbances suggest that the structure of the community and the mechanisms participating to the fixation of the dune are related to the abiotic environnemental conditions which varies with time, life-strategies of the species and the disturbances. The communities reflect three states of stability.Biotic interactions analysis in the community confirm that lichens take an important part in the maturation processes of the ecosystem. For their precise spatial zonation, their organizations, and their possible role in the ecosystem, the terricolous bryophytes and lichens communities can be considered as key-species in the functioning of the grey dunes.Terricolous brophytes and lichens communities allow precisions on the typology of the coastal dunesystemsand valuations of the state of dynamic. Results obtained can contribute to an ecological diagnosis of the sand dunes. They offer possibilities of decisions in the management and confirm that bryophytes and lichens are good indicators for the dune-manager.
