Response of grass covers for dikes to coastal stressors during establishment phase

Grass covers on dikes are subjected to a multitude of controlled and uncontrolled environmental stressors that can affect its vegetation development. Controlled stresses are applied during dike maintenance (e.g. mowing) to establish high coverage and a dense root system. Uncontrollable stresses during storm surges (e.g. flooding and wave loading) are likely to disturb this development. This species composition-based study analyses stress responses and their consequences for the ecological enhancement of grass covers in terms of dike safety. The responses of four different grass covers with increasing species numbers were investigated under three controlled experimental conditions, mimicking North Sea storm surge season in an outdoor wave-basin. We found significant differences in vegetation development indicating a shift of biomass from roots to shoots in response to flood stress. Resulting losses in the grass cover resistance were only minor for all grass covers. When additional wave stress was introduced no significant differences in root biomass were observed compared to the control group without flood or wave stress. The more herb and legume dominated grass covers seem to provide less erosion resistance for dike safety than the grass-dominated ones, as they develop the desired coverage slower and tend to have a lower shear strength. In conclusion, ecological enhancement by moderately increasing the number of species in the grass cover is possible without affecting dike safety. Evidence of disturbance in vegetation development highlight that thresholds and indicators for the early detection of stress responses need to be further investigated regarding the predicted intensification of uncontrolled environmental stressors. It is emphasised that long-term studies and adaptive monitoring are required to avoid unforeseen consequences by overloading the ability of grass covers to cope with and recover from coastal stressors and to find the optimal combination of ecological enhancement and dike safety.