Origin and ecological significance of silty soils of Hatainville dune slacks (Manche, France)

Along the European coasts, dune massifs are characterized by numerous settlements with great ecological value in relation to phreatic water. Since about twenty years, pioneer and grassy species of those settlements are subjected to substantial decline. Dune massif of Hatainville is a perfect sample of this regressive trend. Protection of scarce grassy species requires a better knowledge of soils and their dynamics. Pedological investigation has been carried out concerning unexpected silty soils of massif dune slacks. These soils are original insofar as they own richest pioneer and grassy species, as Apium Repens, Littorella uniflora, Gentianella amarella, Pyrola rotundifolia). Origin and ecological value of the silty soils have been cleared by analytic data. Thoses silty soils are the result of the pedogenesis of the characeae deposits. The presence of characeae depends of the dune slack flood from rising of the local groundwater. However, pedologial development (primary and secondary carbonatation, crumb structure) and the topographic position of biogene silts rarely flooded show us that those silts come from ancient elements and result of higher level of groundwater surface. It seems that the decrease of groundwater is due to climatic change during the last thirty years. The rainfalls and potential evapotranspiration trends show a increase of the potential evapotranspiration more considerable than rainfalls. The rise of potential evapotraspiration, probably due to the global climatic change, could explain the decrease of the local groundwater. Therefore, the hydrological conditions for the development of characeae and for the forming of silty soils are disappeared. The preservation of the rare hygrophile vegetable communities, which have a fine link with silty soils, will depend of many means to allow the presence of temporary and permanently ponds