Contrasting elevational responses of regularly flooded marsh plants in navigable estuaries

Many studies have illustrated the elevational gradient of plant zonation in estuarine marshes. Nevertheless, little is known about the elevational patterns of plant zones and marsh edge existing along deepened navigation channels compared to their shallow anabranches. Our study analyzed the factors controlling these patterns. This knowledge is required when restoring tidal marshes to compensate for anthropogenic impacts. We determined the species-elevation-pattern in the navigable German Elbe and Weser estuaries. Using the key species Scirpus tabernaemontani, Scirpus maritimus, and Phragmites australis, we compared (1) marshes along navigation channels and anabranches and (2) their marsh edges. (3) Additionally, we tested how tidal range affects the lowest elevation of marsh edges. We analyzed data derived from digital vegetation maps and digital elevation models, and then normalized the elevation data using the mean tidal range. We determined the elevational distributions of plant zones on regularly flooded estuarine marshes and quantified the lowest elevation as an important niche boundary which reflects the resistance against physical stress. Our findings indicate that the patterns result not only from tidal range but also from hydrodynamics due to the dredging history, diversely shaped channels, bank morphologies, and vessel frequencies. The patterns found and the species-specific elevations need to be considered when choosing suitable elevations when restoring tidal marshes in dredged estuaries. The results might also inspire reconsidering the reference plants zonation while monitoring ecological potential of severely modified estuaries under Water Framework Directive. (C) 2018 European Regional Centre for Ecohydrology of the Polish Academy of Sciences. Published by Elsevier B.V. All rights reserved.