Coping with waves: Plasticity in tidal marsh plants as self‐adapting coastal ecosystem engineers

Abstract

Tidal marsh vegetation is increasingly valued for its role in ecosystem‐based coastal protection due to its wave dissipating capacity. As the efficiency of wave dissipation is known to depend on specific vegetation properties, we quantified how these morphological, biochemical, and biomechanical properties of tidal marsh vegetation are, in turn, affected by wave exposure. This was achieved by field measurements at two locations, with contrasting wave exposure, in the brackish part of the Scheldt Estuary (SW Netherlands), where Scirpus maritimus is the dominant pioneer species. Our results show that shoots from more wave‐exposed conditions developed significantly shorter and thicker stems than the ones growing in more sheltered conditions. Furthermore, we show that the more exposed shoots are more flexible whereas the shoots growing in more sheltered conditions are stiffer. This may indicate plasticity in response to wave exposure following a stress‐avoidance strategy. Increasing stiffness was shown to be related to enhanced biogenic silica and lignin contents of the shoot tissue. These properties might affect the wave‐attenuating capacity of the marsh as stiff plants are known to mitigate waves more effectively than flexible ones. However, we also found higher shoot densities on the exposed site, which may partly explain why higher relative wave attenuation rates were found on the exposed site, despite the presence of more flexible individual shoots. This study highlights that the efficiency of wave attenuation by tidal marsh vegetation ultimately depends on mutual interactions between waves and plasticity in morphological, biochemical, and biomechanical plant properties.

How effective are tidal marshes as nature‐based shoreline protection throughout seasons?

“…The wave and flow attenuation rates that are reported in the literature, and that were typically measured during summer peak‐biomass conditions (Leonard and Croft ; Ysebaert et al ; Silinski et al ), are in the same range as our measurements for summer peak biomass. However in the present study, wave and flow attenuation rates were measured throughout the season.…”

“…Tidal pioneer marshes in the temperate climate regions may lose much of their aboveground biomass in winter which temporarily reduces the efficiency of the marsh for shoreline protection. Although the general processes of wave attenuation (Ysebaert et al ; Yang et al ; Möller et al ; Silinski et al ) and erosion reduction (Chen et al ; Francalanci et al ; Wang et al ) by tidal marshes are well studied, studies are mostly done under peak‐biomass (summer) conditions while studies under low biomass (winter) conditions are sparse (Coulombier et al ; Spencer et al ; Vuik et al ). In this study, we quantify seasonal changes in wave attenuation rates and sedimentation‐erosion rates in pioneer marsh vegetation.…”

“…Marsh vegetation exerts friction to the water motion and thereby attenuates currents and waves (Yang et al ; Möller et al ; Carus et al ; Vuik et al ). Even narrow fringes at the seaward edge of dikes have a stabilizing effect since the majority of incoming wave height, and thereby wave energy, is reduced in the first few meters of the pioneer marsh (80% over <50 m, Ysebaert et al ; 20–40% over 12 m, Silinski et al ). Although water depth on marshes is higher during storm surge conditions and the rate of wave attenuation is known to be reduced with greater water depth (Gedan et al ; Yang et al ), flume studies under simulated storm conditions show that up to 60% of wave height reduction can be attributed to marsh vegetation (Möller et al ).…”

Flexural Rigidity and Shoot Reconfiguration Determine Wake Length Behind Saltmarsh Vegetation Patches

“…The field data for flexural rigidity show an order of magnitude of variation in values. Previous studies of both freshwater and saltwater vegetation have also demonstrated large intraspecific variation in values for flexural rigidity between samples on different stems from the same site (Feagin et al, 2011;Miler et al, 2012;Paul et al, 2014;Rupprecht et al, 2015), and it is also likely that flexural rigidity varies depending on patch exposure to the flow and the local age of vegetation patches (Anderson & Smith, 2014;Silinski et al, 2018). The simplified model results (Figure 13c) demonstrate that the range of predicted flexural rigidities agrees well with the observed range.…”

Salt marshes for flood risk reduction: Quantifying long-term effectiveness and life-cycle costs

“…For high waves (significant wave heights over 1 m), relatively tall plant species will lose the majority of their aboveground biomass; a large-scale flume experiment showed 80% stem breakage of Elymus athericus (Rupprecht et al, 2017), and 50% reduction in stem density of Spartina alterniflora was observed after a Category 1 hurricane (Gittman et al, 2014). Plants at locations exposed to higher mean wave energy develop shorter and thicker stems, which makes them less vulnerable to stem breakage (Silinski et al, 2018). This implies that, similar to morphological stability, locations with low mean wave energy are most sensitive to stem breakage during severe episodic storm events (Fig.…”