Survival of the thickest? Impacts of extreme wave‐forcing on marsh seedlings are mediated by species morphology

Abstract

Although tidal marshes are known for their coastal defense function during storm surges, the impact of extreme wave forcing on tidal marsh development is poorly understood. Seedling survival in the first season after germination, which may involve exposure to extreme wave events, is crucial for the natural establishment and human restoration of marshes. We hypothesize that species-specific plant traits plays a significant role in seedlings survival and response to wave induced stress, i.e., through stem bending and uprooting. To test this hypothesis, seedlings of pioneer species (Bolboschoenus maritimus, Schoenoplectus tabernaemontani, Spartina anglica, and Puccinellia maritima) with contrasting biophysical traits were placed in the Large Wave Flume in Hannover (Germany) and exposed to storm wave conditions. Seedlings of P. maritima and S. anglica experienced a lower loss rate and bending angle after wave exposure compared to S. tabernaemontani and especially B. maritimus. The higher loss rates of B. maritimus and S. tabernaemontani result from deeper scouring around the stem base. Scouring depth was larger around stems of greater diameter and higher resistance to bending. Here, B. maritimus and S. tabernaemontani have both thicker and stiffer stems than S. anglica and P. maritima. Our results show that especially seedlings with thicker stems suffer from erosion and scouring, and have the highest risk of being lost during extreme wave events. This implies that for successful seedling establishment and eventually the establishment of a mature tidal marsh vegetation, the species composition and their capacity to cope with storm wave disturbances is crucial.

Stability of a Tidal Marsh Under Very High Flow Velocities and Implications for Nature-Based Flood Defense

“…For the latter, the basal 20 cm of the shoots were used to perform three-point bending tests with a universal testing device (Instron 5942, precision ± 0.5%). For more details on the methods to quantify the biomechanical properties we refer to Schoutens et al (2021). Belowground biomass was quantified from five replicate sediment cores of 0.10 m diameter sampled up to 0.40 m depth (i.e.…”

“…Only in the two highest flow conditions (after 10-12 h of cumulative flow exposure), uprooting of fine roots covering the entire sediment surface was observed (Figure 8). In addition, when roots get exposed at the sediment surface, they will cause local turbulence, which may result in scour features (Bouma et al, 2009;Schoutens et al, 2021). This might suggest that when high flow velocities would continue for a much longer period of time, the top layer of sediment may get damaged or removed, exposing the subsurface sediment layers with lower belowground biomass.…”

Adventures of salt marsh recruits

“…After emergence, dislodgement is recognized as the biggest challenge for seedling establishment and survival (Bywater-Reyes et al, 2015;Schoutens et al, 2021). During the early development stage, seedlings tend to be dislodged in form of slipping out of the substrate or cutting off roots when the drag force from hydraulic disturbance exceeds the anchoring force of seedlings (Fig.…”

“…For vegetated biogeomorphic systems such as salt marshes, completion of the life cycle through seed dispersal, seedling establishment, adult plant expansion, and subsequent reproduction is a key benchmark (Friess et al, 2012;Temmink et al, 2021). Variations in resilience, including resistance and recovery, at different life stages explain different evolutionary trajectories for this benchmark (De Battisti, 2021;Schoutens et al, 2021). Most resilience studies have focused on larger vegetation units (Bouma et al, 2009;Altieri et al, 2013;Leonardi et al, 2016), but few have tracked the resilience of salt marshes with a focus on individual seedlings.…”

“…Understanding seedling resilience implies unraveling the potential to either resist or to recover from different stress types (Leonardi et al, 2016;De Battisti, 2021). Dislodgement is regarded as the main fatal process for young seedlings and it can be triggered primarily through wave-induced bed shear stress and sediment scouring (Cao et al, 2018;Schoutens et al, 2021;Marion et al, 2021). Tidal currents are the main driver of long-distance dispersal following dislodgement (van der Stocken et al, 2019;Zhao et al, 2021b).…”