Plant Species Responses to an Elevational Gradient in German North Sea Salt Marshes

Salt marsh vegetation often occurs in zones along an elevational gradient. We present a detailed analysis of the distribution of species in a data set of 2,691 plots from salt marshes along the German North Sea coast. For each 1 m(2) plot, we estimated species' percent cover and calculated elevation in relation to mean high tide. For the 19 most frequent species, distribution and dominance pattern along the elevational gradient (correlated with moisture and salinity) were quantified and species' responses to this gradient analyzed by Huisman-Olff-Fresco (HOF) modelling. Elevational ranges, distribution pattern, and response curves varied widely among the species, with most species' niches positioned in the middle part of the gradient. Non-parametric multiple comparisons (in total 171) did not reveal significant differences in species' ranges for 40 species pairings. Most HOF models resulted in unimodal skewed response curves, with the probability of occurrence decreasing towards the lower and more extreme end of the gradient. We conclude that elevation largely explains species distribution within the studied salt marshes and that species ranges follow a continuum along the gradient. Our data provide a baseline for future studies of possible changes in distribution pattern caused by conservation management or sea level rise.