Wave forces decrease seedling establishment in tidal marshes

Tidal marsh ecosystems, where water levels ebb and flow according to the tidal movement of the adjacent body of water, are an important conservation focus both due to their biodiversity and their ability to reduce the force of waves, even in extreme weather events. In recent decades, however, tidal marshes have degraded globally. Efforts to reestablish these areas depend on an understanding of seedling establishment requirements, including their ability to resist wave forces during this stage of growth.

As part of an Annals of Botany special issue on coastal flooding and storm risks, Haobing Cao and colleagues have set out to better understand how seedlings react to wave forces. The group developed a plant-shaking apparatus that mimics the wave-induced drag stress experienced by seedlings in order to assess the waves’ effect on plant survival, morphology, and biomechanical properties. Their ‘plant-shaking mesocosm,’ differs from previous such apparatus in that the plants move back and forth rather than the water, allowing a simpler build. The researchers tested seedlings of three different tidal marsh pioneer species, each with a different salinity preference, for a total of seven weeks.

The wave treatment led to decreased growth and survival rates compared to controls for all three species. Beyond causing greater seedling mortality, wave stress had the effect of increasing the proportion of biomass allocated to the roots, allowing for improved anchorage. The treated seedlings grew narrower stems that were more flexible, causing less frontal surface to be exposed to drag forces, but were less resistant to tensile stress. Overall, these changes make the seedlings less resistant to external stressors.

The study underscores the vulnerability of tidal marsh seedlings to wave forces during their establishment phase and suggests the need for wave-free periods to allow these ecosystems to reestablish. Creating these “windows of opportunity” could become increasingly difficult, however. “As ship-generated waves and extreme weather events may be expected to increase in the near future, wave-free windows of opportunity will become more rare,” the authors write. “Overall, the present study indicates that the likelihood of marsh establishment is reduced if wave energy increases.”