Abstract
Tidal marshes often present cyclic dynamics, with alternating phases of expansion and lateral retreat. Marsh seedling establishment is a primary condition for marsh colonization and expansion. Successful establishment is often episodic due to bio-physical thresholds, as expressed in the ‘Windows of Opportunity’ concept. Identifying critical factors and understanding how they
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enable or hamper seedling establishment, is highly relevant for coastal management and restoration schemes.
This study quantified the effects of critical factors in tidal marsh establishment processes. Firstly, by exposing marsh seedlings to different disturbances in terms of sedimentation regime and wave exposure, this study found that these disturbances affect seedling establishment both directly and via bio-morphological adjustments (Chapter 2 and 3). Secondly, channel related drainage relief was found to facilitate seedling survival, especially in the early phase of marsh establishment in muddy systems (Chapter 4). Thirdly, by using marsh seedlings with different disturbance-free periods in both laboratory and field experiments, this study confirmed that a longer disturbance-free period can strongly enhance seedling survival as well as their resistance to later extreme disturbances (Chapter 2 and 4).
The erosion of tidal marshes often leads to the development of a cliff at the boundary between tidal flat and marsh. The formation of a cliff shifts marsh development from a phase of lateral expansion to a retreating phase, as a result of the interdependent feedbacks between marsh vegetation and external forcing. This thesis investigated the tipping point conditions of cliff formation at marsh edges with different marsh species. The clonal growth of vegetation at marsh edges was found to respond to different sediment types and to affect the cliff formation processes. Species-specific clonal growth strategies in step length have important effects on initiating cliffs, lateral retreat and thus landscape dynamics at marsh edges (Chapter 5).
In summary, this study provides mechanistic insight into the primary expansion (i.e., seedling establishment) of tidal marshes under a variety of critical forcing factors, as well as insight in the factors contributing to cliff formation and the onset of lateral retreat. It highlights the variability in relevant species-specific responses, as well as the importance of location-dependent factors. These results add quantitative support to models predicting the critical transitions in marsh dynamics, both marsh expansion and marsh retreat. The findings of this study are instructive for marsh restoration and nature based coastal defense schemes. While they contribute to understanding the overall stability of marsh ecosystems in the face of global change, they also highlight the need for tailor-made solutions for the application of nature-based solutions at particular locations.
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