Effects of shoot stiffness, shoot size and current velocity on scouring sediment from around seedlings and propagules

Abstract

Successful management and restoration of coastal vegetation requires a quantitative process-based understanding of thresholds hampering (re-)establishment of pioneer vegetation. We expected scouring to be important in explaining the disappearance of seedlings and/or small propagules of intertidal plant species, and therefore quantified the dependence of scouring on plant traits (flexibility, size) and physical forcing by current velocity. Flume studies with unidirectional flow revealed that scouring around seedlings increased exponentially with current velocity and according to a power relationship with plant size. Basal stem diameter rather than shoot length controlled scouring volume. Flexible shoots caused far less scouring than stiff shoots, provided that the bending occurred near the sediment surface as was the case for Zostera, and not on top of a solid tussock base as we observed for Puccinellia. Therefore, shoot stiffness is likely to strongly affect the chances for initial establishment in hydrodynamically exposed areas. Plant traits such as shoot stiffness are subject to a trade-off between advantages and disadvantages, the outcome of which depends on the physical settings.

Multiple Stable States and Catastrophic Shifts in Coastal Wetlands: Progress, Challenges, and Opportunities in Validating Theory Using Remote Sensing and Other Methods

“…Net sedimentation is also promoted by plant roots improving sediment cohesion [170][171][172][173][174]. Larger diameter and less flexible plant stems or clusters are associated with more scouring [145,175], but less flexible vegetation traps more total sediment if not submerged [176]. Within patches of pioneer plants (e.g., Spartina anglica tussocks), tidal currents are reduced and sediment accumulates, raising the plant in the tidal range and resulting in a positive feedback on plant growth.…”

Abiotic Factors Governing the Establishment and Expansion of Two Salt Marsh Plants in the Yangtze Estuary, China

“…But at the SP site the Scirpus seedlings also exhibit a (non-significant) tendency of a higher survival than the Spartina seedlings. It is possible that the growth form of Scirpus, with more flexible plants, makes the seedlings less vulnerable to erosion (Bouma et al 2009). …”

“…Spartina alterniflora is a perennial rhizomatous cord grass native to the coasts of North America (Teal 1985;Simenstad and Thom 1995). It is a strong ecosystem engineer (Crooks 2002), which similar to other Spartina species reduces erosion and enhances sediment accretion (van Hulzen et al 2007;Bouma et al 2009). This ability to accrete sediment is the main reason this species was introduced worldwide and has resulted in rapid expansion and invasion in many wetland ecosystems (Callaway and Josselyn 1992;Balletto et al 2005;).…”

Wave and tidally driven flows in eelgrass beds and their effect on sediment suspension

“…Reduction in velocities within seagrass beds is often accompanied by skimming flow, which increases velocities above the sea-grass canopy relative to ambient flow conditions (Fonseca et al 1982). This creates an inflection point of instability in the velocity profile, and shear layers are developed near the top of the canopy (Widdows et al 2008, Bouma et al 2009), which can locally enhance turbulent mixing into the canopy (Nepf & Vivoni 2000). Canopy friction exhibits a strong positive relationship to the percent of the water column occupied by the seagrass (Fonseca & Fisher 1986), and greater flow reductions are found inside canopies with increasing shoot density (Peterson et al 2004).…”

“…Most studies that have addressed the impacts of seagrass density and morphology on flow and sediment dynamics have been based on laboratory flume experiments or numerical modeling (Heller 1987, Chen et al 2007, Bouma et al 2009, Carr et al 2010). …”