Wave attenuation by intertidal vegetation is mediated by trade-offs between shoot- and canopy-scale plant traits

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.