Tidal marshes are found along sediment-rich low-lands of coasts and estuaries and are highly dynamic and disturbance-driven environments. Their ecological value is of great importance as they provide precious ecosystem services such as carbon sequestration and water quality regulation as well as irreplaceable habitats for specialised organisms. In the face of climate change and sea-level rise, their capacity to substantially support coastal defence has been increasingly recognized and has led to the concept of ecosystem-based coastal defence. Nonetheless, areas in which marshes can survive naturally are increasingly threatened: storms, land-claim and sea-level rise have led to a worldwide loss of tidal marshes by 50 % since the 1980s. However, little is known about the critical thresholds of environmental variables for successful seaward expansion of marshes which is why further research is needed to better predict future marsh development. The aim of this thesis was to determine the critical conditions for horizontal seaward expansion of Scirpus maritimus-dominated marshes. The main focus is on the effects of wave exposure and wave period on colonization success of both the continuous pioneer marsh zone and individual seedlings and shoots. These aspects were investigated using (1) field experiments and monitoring campaigns at field sites in the Scheldt Estuary (SW Netherlands and N Belgium) with contrasting wave exposure (wave-sheltered and wave-exposed) and (2) by a wave flume experiment with contrasting wave periods (short waves representing wind waves and long waves as proxy for ship waves).
Our findings show that (1) the dominant and most successful colonization strategy of S. maritimus is clonal expansion from the marsh edge, with average rates of more than 1 m per year at the study sites. Wave exposure might influence the speed of this type of expansion as the expansion rate at the sheltered site was almost twice that of the exposed site. Moreover, the expansion rate was predictable at the sheltered site as it appeared to be linked to elevation at the marsh edge, and therefore related to inundation time. In contrast, at the more exposed site wave exposure appears to overrule any other limiting factors. (2) We detected that S. maritimus can follow both a stress-tolerance and a stress-avoidance strategy. The strategy depends on life stage (with flexible stress-avoiding seedlings and stiffer stress-tolerating shoots) and exposure (with flexible and shorter stress-avoiding shoots at more exposed sites and stiffer and taller stress-tolerating shoots at more sheltered sites). In our flume experiments, we found that these respective life-stage related strategies were successful for wind-generated waves while they failed under artificial long waves potentially caused by passing ships. (3) Our results further showed that important wave attenuation occurred over the first 12 m of the marsh and that relative wave attenuation was more important at the exposed site compared to the sheltered site. Although survival chances of transplants were generally better at the sheltered site, survival was equal within both marshes, emphasizing the relevance of wave sheltering capacities of marsh vegetation.
Overall our findings highlight that marshes create sheltered environments. Furthermore, within a range of wave conditions the S. maritimus marsh pioneer zone can adapt to the local conditions by following the appropriate survival strategy. This implies that in the face of climate change and sea-level rise as well as increasing shipping traffic, these marshes can be expected to adapt to increases in exposure and sustain themselves to a certain extent. While more precise thresholds of sea level and wave-induced disturbances for clonal marsh expansion still need to be determined, the contribution of marshes to future coastal protection is sure to be valuable, especially at exposed sites.
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