Topographical, soil, and water determinants of the Vallevecchia coastal dune-marsh system

Due to their low elevation range, coastal environments are extremely sensitive to rising sea levels resulting from climate change and other direct human impacts. Increased knowledge about site and vegetation characteristics can contribute to improving their conservation and restoration. A survey of 51 sampling areas and seven transects in the sand dune, salt marsh system of Vallevecchia (Caorle, Italy) was conducted, while taking into account the vegetation and certain important topographical, soil, and hydrological characteristics. The most important factors differentiating the presence of sand dunes and salt marsh vegetation were found to be soil electric conductivity (EC: threshold EC = 0.4 mS cm−1) and site elevation (threshold + 0.70 cm a.s.l.). In psammophilous vegetation, the soil pH and EC decreased with elevation and the water table was composed of freshwater. A small amount of salt accumulation was found only at the white dune bottom. In salt marsh vegetation, the soils were flooded daily by high tides and were saline and alkaline at higher elevations, where less frequent tide flooding led to frequent soil drying and salt accumulation. The soil OM content of the salt marshes was highest due to the slow mineralization rate as a consequence of the high water content. The current low sediment transport from adjacent rivers was the cause of the regression of the central-eastern part of Vallevecchia, where the dunal system width and number of vegetation types decreased and the height and mean slope of the sea facing sides of the white dunes increased. The 60-cm local sea level rise predicted by 2100 will increase the salt marsh extent by 60%, which will be a disadvantage for the more rare non-saline retrodunal and psammophilous vegetation. Reactivating river sediment transport, reducing or eliminating the deep-rooted pine and Ammophila arenaria plantations and closing drainage ditches in the adjacent pinewoods can mitigate the negative impacts of sea level rise.