Hidden champions

The influence of salt marshes on sediment dynamics is of increasing interest in the light of climate change. Increased hydrodynamic forcing can lead to a destabilisation of sediments, which may be aggravated by rising sea levels. Salt marshes can counteract both processes by stabilising existing sediment and inducing vertical accretion which can reduce relative sea level rise. This thesis addresses the question how sediment dynamics are affected by biomechanical and morphological plant traits with the aim to promote salt marsh establishment and incorporate the sediment stabilising functions in coastal protection. Following an introduction to relevant plant traits, systematic studies with surrogates are presented that elucidate the effect of individual plant traits on drag forces under waves and currents. The study under waves showed that stiffness and the dynamic frontal area (i.e., frontal area resulting from bending which depends on stiffness and hydrodynamic forcing) determines drag forces. However, the results showed that forces do not increase linearly with increasing material volume and it is proposed that the interaction of neighbouring surrogates results in these additional drag forces. The study under unidirectional currents highlighted the relevance of along-plant variation of biomechanical and morphological plant traits for plant posture and resulting drag forces. The results show that using stem-averaged properties may result in errors in predicted drag force of up to 26% and highlight the need to consider the reconfiguration of variable rigidity stems. The results obtained from the studies with surrogates were verified during a laboratory experiment using seedlings of four marsh species with different plant traits. Drag forces experienced by seedlings tended to increase with horizontal orbital velocity, and with stem length and diameter. The interplay of both plant traits was complex and experienced drag forces appeared to be affected by species-specific traits such as rigidity and leaf growth. To estimate potential changes in the coastal protection capacity of salt marshes in the future, the effect of climate change on biomechanical plant traits was assessed for the common salt marsh species Spartina anglica and Elymus athericus. Specimens of both species were exposed to future climate conditions and afterwards the effect on biomechanical plant traits was assessed. Despite some differences between the future climate scenario and present conditions, all values lie within the natural trait ranges for the two species, suggesting that the effect of salt marsh vegetation on hydrodynamics and the resulting drag will not change in the future. Winter stages of the same species were then used to quantify their effect on sediment erosion under breaking waves. Results yielded no effect of the above ground biomass on mean erosion, even though the species responded differently with Elymus athericus buckling and Spartina anglica withstanding the hydrodynamic forcing. Equally, no significant differences were observed between the species which is attributed to the comparable dry root biomass and soil bulk density of the used vegetated patches. The data, thus, suggest that a species independent erosion protection efficiency persists throughout the year. Finally, the insights gained in the effect of plant traits on drag forces are discussed in the context of sedimentation and erosion. The results provide improved understanding of the role of biomechanical and morphological traits of individual plants and based on these recommendations are developed to guide salt marsh establishment as part of coastal protection strategies. However, the interaction of multiple plants within a meadow could not be elucidated and the quantification of density thresholds which define the transition from erosive to depositional conditions within and around salt marsh vegetation remains open for investigation.