Low Atmospheric Nitrogen Loads Lead to Grass Encroachment in Coastal Dunes, but Only on Acid Soils

Abstract

The impact of atmospheric N-deposition on succession from open sand to dry, lichen-rich, short grassland, and tall grass vegetation dominated by Carex arenaria was surveyed in 19 coastal dune sites along the Baltic Sea. Coastal dunes with acid or slightly calcareous sand reacted differently to atmospheric wet deposition of 5–8 kg N ha−1 y−1. Accelerated acidification, as well as increased growth of Carex and accumulation of organic matter, was observed only at acid sites with pHNaCl of the parent material below 6.0. At sites with slightly calcareous parent material, increased N-deposition had no effect. A trigger for grass encroachment seems to be high acidification in early successional stages to below pHNaCl 4.0. Metals like Al or Fe become freely available and may hamper intolerant species. At acid sites, N-mineralization increases with elevated N-deposition, which may further stimulate Carex arenaria. Due to high growth plasticity, efficient resource allocation and tolerance of high metal concentrations, C. arenaria is a superior competitor under these conditions and can start to dominate the dune system. Carex-dominated vegetation is species-poor. Even at the moderate N-loads in this study, foliose lichens, forbs and grasses were reduced in short grass vegetation at acid sites. Species indicating these first effects of atmospheric deposition on dry, lichen-rich, short grasslands are identified and recommendations for restoration of grass-encroached sites given.