Different coastal marsh sites reflect similar topographic conditions for bare patches and vegetation recovery

Abstract. The presence of bare patches within otherwise vegetated coastal marshes is sometimes considered to be a symptom of marsh die-back and the subsequent loss of important ecosystem services. Here we studied the topographical conditions determining the presence and revegetation of bare patches in three marsh sites with contrasting tidal range, sediment supply and plant species: the Scheldt Estuary (the Netherlands), Venice Lagoon (Italy), and Blackwater Marshes (Maryland, USA). We analyzed topographic properties of bare patches, including elevation, size, distance and connectivity to channels based on GIS analyses of aerial and LIDAR imagery. Our results demonstrate that across the different marsh sites, bare patches connected to channels occur most frequently at the lowest elevations and farthest distance from the main channels. Bare patches disconnected from channels occur most frequently at intermediate elevations and distances from channels, and vegetated marshes dominate at highest elevations and shortest distances from channels. Revegetation in bare patches is observed in only one site with the highest tidal range and highest sediment availability, and preferentially occurs from the edges of small unconnected bare patches at intermediate elevations and intermediate distances from channels. Our results are discussed within the alternative stable state theory. We suggest the existence of two stable states, a high-elevated vegetated state close to channels that tends to remain high and vegetated, and a low-elevated state of bare connected patches far from channels that tends to remain bare, with an unstable state at intermediate channel distances where bare patches may form and rapidly become revegetated.