Tidal freshwater wetlands under salt intrusion

Coastal and estuarine wetlands have suffered extensive degradation due to centuries of human activity, but are now being restored as Nature-Based Solutions to combat climate change and support biodiversity. Among these, tidal freshwater wetlands—located in the freshwater zones of estuaries yet influenced by tides—are ecologically unique and provide vital services such as flood protection, water purification, and carbon storage. However, they are increasingly threatened by salt intrusion, driven by sea level rise, droughts, storm surges, and human interventions like dredging and groundwater extraction. This increase in salinity disrupts freshwater ecosystems, affecting vegetation, biodiversity, and wetland functions.
This thesis investigates how tidal freshwater wetlands, particularly floodplain forests, respond to salt intrusion. Mesocosm experiments (Chapters 2–3) revealed that black alder (Alnus glutinosa) is more salt-tolerant than white willow (Salix alba), regardless of the season or tidal conditions. Chapter 4 shows that natural marsh topography enhances drainage and reduce overall salt exposure compared to artificial landscapes. Chapter 5 explores the impact of a shift in vegetation towards salt-tolerant species. It shows that while this increases salinity within wetlands, it also promotes higher wetland elevation, potentially enhancing resilience to sea level rise. Overall, this thesis highlights how past and present human management, restoration, and creation of wetlands have often reduced their natural resilience to disturbances like salt intrusion. By studying natural wetland dynamics and species succession, it offers insights into how to better design adaptive and resilient systems capable of withstanding future climate challenges.