Climate change in salt marshes - Effects of hydrodynamic forcing and higher temperatures on coastal vegetation
Final Report Abstract
Salt marshes are coastal ecosystems that are able to provide many valuable ecosystem services such as carbon sequestration or coastal protection. Despite their value, the global area of salt marshes is declining, mainly due to anthropogenic effects including global climate change. Climate change factors that might threatening the persistence of salt marshes in the future are sea level rise and higher temperatures as well as changes in the wind climate affecting local hydrodynamic conditions. Although salt marshes are formed in highly dynamic environments, their stability may be disturbed by more extreme events like a series of several storm surges that might lead to vegetation failure and a reduced marsh stability. So far, studies investigating wave impacts on marsh stability mainly focused on erosion while other plant-wave interactions such as wave-induced plant damage are poorly understood. This project aimed to address this knowledge gap by investigating the effect of an increased storminess and higher temperatures on marsh vegetation. In a true-to-scale flume study, single seedlings and fullydeveloped canopies of different tidal marsh species were exposed to storm surge conditions to quantify wave-induced plant damage and potentially discover determinants of plant resistance. Furthermore, plant response to increased temperatures were investigated in a whole ecosystem warming experiment in a salt marsh at the German North Sea coast. In both flume studies, we found a high robustness against hydrodynamic forcing for the studied species, which holds additional support for the high resilience of tidal marshes to storm impact. However, it should be noted that the conditions generated in the flume experiment reflect present storm surge conditions found in front of many NW European salt marshes. Nevertheless, the waveinduced plant damage we detected provides valuable insights into the hydrodynamic conditions the vegetation is able to withstand. Although uncertainties exists regarding future wave climate projections, findings of both flume studies suggest that an increase in wave intensities might increase the susceptibility of the vegetation to severe physical damage as well. Moreover, factors influencing or relating to the aboveground biomass of the vegetation (vegetation height, frontal area or density) as well as stem flexibility seem to be key elements for determining plant resistance. It might be therefore advisable to focus on these properties when evaluating marsh resilience to future environmental changes. However, there are plant properties that have not been investigated in this project but might be worth examining (e.g. stem density). Furthermore, the study conducted in the MERIT warming experiment, revealed an extension of the growing season under warming, which very likely led to an increase in aboveground biomass. A higher aboveground biomass in response to higher temperatures has the potential to enhance marsh stability as biomass increments has been shown to be positively correlated with vertical accretion rates. Further data analyses of warming-induced effects on aboveground biomass productivity, stem flexibility as well as current wave conditions in salt marshes in the German Wadden Sea (WP1) will complement the findings described here and certainly facilitate a better understanding of marsh resilience and coastal protection in the future.
Publications
- (2021): Survival of the thickest? Impacts of extreme wave-forcing on marsh seedlings are mediated by species morphology. Limnology and Oceanography n66, 2936-2951
Schoutens K, Reents S, Nolte S, Evans B, Paul M, Kudella M, Bouma TJ, Möller I, Temmerman S
(See online at https://doi.org/10.1002/lno.11850)
