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Nature‐based solutions, through conservation or (re)creation of vegetated shorelines, are recognized to mitigate the impact of waves and erosion risks on shorelines. Wave attenuation is known to be dependent on plant traits, resulting in increasing wave attenuation rates with increasing shoot density, shoot thickness, height, and stiffness. However, following the allometric scaling theory, we hypothesize that increasing shoot density (a canopy‐scale trait) may be associated with decreasing shoot thickness and stiffness (a shoot‐scale trait), with potential opposing effects on overall wave attenuation. This study investigates (1) the presence of such allometric relations across intertidal shore plant species via existing literature and (2) the trade‐off effects on the overall wave attenuation capacity of shore vegetation through a flume experiment. Our results reveal for the first time the presence of allometric relationships between shoot‐scale and canopy‐scale plant properties in perennial intertidal plant species. Across different species, increasing shoot densities are indeed associated with decreasing shoot thickness and shoot stiffness. Next, we performed a wave flume experiment with plant mimics, showing that wave attenuation rate follows a logarithmic increase with increasing shoot density, even though the increasing shoot density was associated with thinner and more flexible individual shoots. Synthesis and applications. We conclude that wave attenuation is predominantly governed by canopy‐scale properties, but a trade‐off with shoot‐scale properties mediates the overall wave attenuation capacity of the vegetated shore. Our findings imply that nature‐based projects (re‐)creating vegetated shorelines should account for potential trade‐off effects of species‐specific plant traits at the canopy scale and individual shoot scale. [ABSTRACT FROM AUTHOR]

Aims: Tidal marsh vegetation along estuaries is exposed to strong environmental gradients that determine which species — enabled through specific traits — can establish and persist. With these ecosystems under anthropogenic pressure, in-depth knowledge on the conservation of remaining tidal wetlands and restoration potentials is needed. In this study we elaborate the habitat conditions in the natural vegetation of the Elbe estuary and analyze: (a) which abiotic factors drive species composition; and (b) which species traits are key to the plants’ strategies in this specific ecotone. Location: Three sites in the Elbe estuary (river kilometer 671–703). Methods: We collected data on soil nutrients, inundation period and wave height and sampled traits of the 17 most abundant plant species, which we analyzed by RLQ (three-table analysis, including environment variables [R], species abundance [L] and a species traits table [Q]). Results: We detected a strong ˈwave–disturbance–inundation gradientˈ, which separated sparsely vegetated low-lying plots receiving high wave impact and exposed to long inundation periods, from high-lying plots showing dense vegetation and aerated soils. Close to the shore, plants showed low organ density and high investment into rhizomes, with a correlation of 0.72 between mass fraction of rhizome and phosphorus content of rhizome tissue. On higher elevations, plants strongly expressed traits relating to competition, like high allocation to stem biomass. Further, we found species with high leaf chlorophyll content showing low specific leaf area values and a negative correlation with the nutrient gradient. Conclusion: The results of our study are particularly relevant for restoration measures in order to re-establish healthy marsh vegetation. However, changes in the environmental conditions, for instance stronger wave energies by a higher shipping frequency in the river channel and sea level rise may trigger changes in species composition through plant trait adaptations, for example by demanding a stem flexibility beyond what would be structurally feasible. [ABSTRACT FROM AUTHOR]

Many tidal marshes have been lost by past land use changes, but are nowadays increasingly restored and created to provide valuable ecosystem services such as nature‐based flood and erosion protection along estuarine shorelines. To be functional for flood and shoreline erosion protection, restored and created tidal marshes should develop erosion resistant sediment beds. Here, we investigated which factors drive the spatial variations in sediment strength and erosion resistance in a developing tidal marsh restoration site. Our results show that decreasing tidal inundation frequency, decreasing sedimentation rate, and better drainage led to stronger consolidation in deeper sediment layers. This consolidation resulted in greater sediment strength, quantified here by shear strength and penetration resistance. Generally, sediment strength was greater when sediment had higher bulk density, while a higher water and fine fraction (= clay and silt) content decreased sediment strength. Overall, all measurement locations were relatively erosion resistant, likely caused by the dense root network and cohesive sediment. To restore or create resilient tidal marshes for nature‐based flood and shoreline erosion protection, we should thus aim for sites with relatively low tidal inundation frequency, moderate sedimentation rates, and cohesive sediment mixtures of clay, silt, and sand, which are well drained and have potential for vegetation establishment. These conditions have a high likelihood of resulting in restored or created tidal marshes that contribute to nature‐based flood and shoreline erosion protection. [ABSTRACT FROM AUTHOR]

Tidal marsh conservation and restoration for nature-based mitigation of coastal flood and erosion risks is increasingly proposed as an additional, sustainable shoreline protection strategy to cope with increasing risks related to global change. Tidal marshes are known to provide shoreline protection as their aboveground biomass attenuates waves and their belowground biomass contributes to reducing erosion rates. The aim of this study was to quantify the impact of seasonal differences of both aboveground and belowground tidal marsh plant biomass on wave attenuation and erosion-sedimentation rates. Changes in hydrodynamics and sediment dynamics were measured during 17 months along three sea-to-land transects of 50 m length in the Elbe estuary (Germany). Simultaneously, changes in biomass of the monospecific pioneer vegetation (Bolboschoenus maritimus) were measured monthly. We found a positive correlation of wave and flow attenuation rate and the seasonal variation of aboveground biomass. In summer, above-ground biomass and associated wave and flow attenuation rates are highest; while above-ground biomass is washed away during the first storms in autumn or winter, resulting in low wave and attenuation rates. Contrastingly, maximum incoming wave heights and flow velocities occur during winter, indicating that wave and flow attenuation is most needed then. However, our results suggest that hibernating root biomass sustains low erosion rates in winter. Although wave attenuation by pioneer marshes is highly variable throughout seasons and pioneer marshes alone are not sufficiently effective, they might facilitate the survival of higher, more inland marshes, and as such contribute indirectly to nature-based shoreline protection. [ABSTRACT FROM AUTHOR]