Climate change in salt marshes - Effects of hydrodynamic forcing and higher temperatures on coastal vegetation

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Salt marshes and other coastal ecosystems provide important ecosystem services such as coastal protection, as the vegetation acts as an obstruction to hydrodynamic forces, thus leading to wave attenuation. While most studies so far have focused on the effects of vegetation on hydrodynamics, crucial knowledge on how hydrodynamics in turn affect and potentially damage salt marsh plants are missing. Additionally, effects of global warming on biophysical properties of marsh plants, which determine their wave attenuation capacity, are unknown. We will (i) quantify hydrodynamic forces using wave recording stations at the margin of Wadden Sea salt marshes and determine its spatial and temporal variation at sites differing in exposure. In parallel, vegetation structure and the eventual loss of biomass will be non-destructively determined and correlated to the recorded hydrodynamic conditions. Under controlled experimental conditions (ii) thresholds of hydrodynamic forces beyond which salt marsh plants suffer physical damage will be assessed in a true to scale flume study. We expect thresholds to be species specific, as plant species in salt marshes significantly differ in their biophysical properties such as plant flexibility. Furthermore, seasonal changes in these thresholds due to different plant development stages and differences in thresholds between single individuals of certain species and those associated in complex vegetation canopies will be determined. Finally, we will (iii) analyse effects of rising temperatures on biophysical properties and thus wave attenuation capacity of marsh plants in a world-unique whole ecosystem salt-marsh warming experiment. We expect that warming will increase above- and belowground biomass, prolong the growing season and increase the lignin content of plants. Overall, this might increase the coastal protection function of salt marshes in a warmer world. The results will be made available to be fed into both hydrodynamic models of wave impact on land-ward lying sea defences as well as into models of marsh evolution under different climate change and sea level rise scenarios. The findings will thus help to assess the coastal protection function of salt marshes adequately in the future and to better deal with impacts of ecological stochasticity and extreme events such as storm surges on coastal salt marshes.