The establishment of seedlings is a crucial and very sensitive phase during initial colonization and development of ecosystems. In dynamic ecosystems, such as tidal salt marshes, establishing seedlings are exposed to extreme soil conditions in combination with strong and recurring mechanical disturbances. We tested whether this environmental filter could already determine later community composition. A greenhouse experiment with three species characteristic for the three salt marsh zones in NW European salt marshes (i.e. Salicornia spp., Atriplex portulacoides and Elytrigia atherica) was conducted to investigate effects of waterlogged soil conditions on the stability of seedlings to withstand physical disturbances (i.e. erosion). Seedlings were grown under three sediment treatments (drained, partly waterlogged and fully waterlogged) and finally subjected to sediment erosion exposing the roots. The depth of erosion at which the seedlings toppled (Ecrit) was measured and related to different morphological properties of the seedlings.
Growth of seedlings from the upper salt marsh species Elytrigia was influenced strongest by the waterlogged sediment treatment: shoot height was reduced by approximately one third compared to the drained treatment, whereas root length was reduced by two thirds. Growth of the lower salt marsh and pioneer zone species, Atriplex and Salicornia respectively, was not influenced significantly by the treatments. However, resistance against erosion decreased in all three species from the drained to the waterlogged conditions. Although Elytrigia seedlings had the highest stability against erosion in the drained and partly waterlogged treatments, Ecrit for Elytrigia grown in waterlogged sediment was least of all species and treatments. A GLM indicates that the root length:shoot height ratio is the most important trait influencing Ecrit relative to seedling size. The experiment demonstrates that seedlings from the upper salt marsh species growing very fast during the first weeks of development are more stable against erosion, however, when grown under waterlogged soil conditions, seedlings from the lower salt marsh and pioneer zone show higher stability due to a larger root length:shoot height ratio. Our study offers new insights in the mechanisms determining species distribution along a combined stress and physical disturbance gradient.