Increasing the success of salt marsh restoration: The relative importance of sediment stability and seedling growth rates on the required length of the Windows of Opportunity of the salt marsh pioneer species Spartina anglica

Abstract

Understanding the mechanisms underlying salt marsh establishment is of great importance due to the many ecosystem services salt marshes provide. Because salt marshes are worldwide degrading, and because they are gaining in importance because of their crucial role in coastal protection against sea level rise, new salt mars areas are created. However, these often fail because the circumstances under which salt marsh pioneer species can establish on new salt marsh sites are largely unknown. A framework that deals with early vegetation establishment in bio-geomorphic ecosystems as salt marshes is the Windows of Opportunity (WoO), which describes a disturbance free period, needed for seedlings to successfully establish on a bare tidal mudflat. In this research, more insight is gained in the required length of the WoO for successful seedling establishment. This is done via testing the effect of abiotic factors on seedling growth and sediment stability and in turn on seedling stability, i.e. the disturbance a seedling can cope with. A mesocosm study was performed in which seedlings of the salt marsh pioneer species Spartina anglica were grown under different salinities, inundations, drainages, and in different sediment types. After 20 days, the seedlings were harvested and seedling characteristics were measured. Also measured was the critical erosion depth, which describes the erosion the seedlings can handle before they topple over when exposed to wave energy. In this research, I showed that young seedlings are rather vital to abiotic stressors as salinity, inundation, and drainage, but are only resistant to a small erosion of the sediment they stand in, depending on root length, before they topple over. Therefore, I argue that the biggest constraint on seedling establishment is induced by the physical properties of the sediment as these determine the likelihood of sediment erosion in the field. Sediments with a higher erodibility ask for bigger seedlings to overcome this erosion and hence a longer disturbance free period (WoO). The results suggest that, salt marsh restoration is most successful on sites with stable sediments and/or low disturbance frequencies.