The role of seasonal vegetation properties in determining the wave attenuation capacity of coastal marshes: Implications for building natural defenses

Coastal marsh vegetation plays an important role in coastal protection by attenuating waves and mitigating shoreline erosion. However, the different vegetations show wide variability in biophysical properties on seasonal and spatial scales, and the effectiveness of coastal vegetation on wave attenuation remains uncertain. In the spring, summer, and autumn seasons of 2019, we investigated the spatiotemporal variability in vegetation properties and wave attenuating ability of two dominant marsh species, Scirpus mariqueter (short and flexible Cyperaceae) and Phragmites australis (tall and stiff Poaceae), in a coastal wetland in the Yangtze Estuary. The wave parameters and water depth were measured at the two marshes and ranged from 25 to 100 m landward of the low mudflat edge. The results indicated that the aboveground biomass and height of vegetation are crucial properties determining the ability of wave attenuation, and the pioneer vegetation belt has the highest wave dissipation efficiency. Phragmites, with greater biomass and stem structure, were more effective in reducing wave height than Scirpus. This study further revealed the importance of seasonal variability in vegetation growth in species-specific wave attenuation. Most Scirpus stems senesced and broke up from autumn onwards, losing their wave attenuation ability. Although Phragmites could not colonize the low-lying flats such as Scirpus, most Phragmites stems remained stiff and high biomass at the later growing stage, maintaining wave attenuation ability and wave decay values consistent with those observed in summer. Moreover, Phragmites has a great capacity to capture sediments and raise flat elevations, which might enhance the wave attenuation efficiency with vegetation-sediment interactions. Information from this study is helpful in evaluating the dynamic effectiveness of wave attenuation in coastal marshes. Therefore, we suggest that a hierarchical mixed community of Scirpus and Phragmites be established as an effective natural defense to offset their individual biological limitations due to seasonality and spatial niches.