2024-2025 SALTGARDEN project kick-off & first results

- Authored by SALTGARDEN -

The SALTGARDEN project is a transdisciplinary project that explores new restoration concepts that aim to maintain the ecological integrity of salt marsh systems and strengthen their associated ecosystem services. The past year the project started to investigate how vegetation, sediment, hydrodynamics, and human values interact to shape the resilience of dynamic coastal salt marshes under climate and environmental stress.

The SALTGARDEN project comprises a substantial field investigation focusing on the comparison of ecosystem services between historically established and recently restored marshes. The first campaign took place in Wierum, the Netherlands, where measurements of sediment erodibility, carbon content, wave characteristics, and biodiversity were taken along a transect from the high marsh toward the mudflat, providing a detailed picture of how ecosystem services vary across zones. Additional campaigns in the Netherlands and Germany will follow through 2025 and 2026 to capture regional variation across restoration types.

Extensive field sampling has also taken place across Denmark and Germany (Schleswig-Holstein and Lower Saxony). Sediment cores were collected and vegetation surveys conducted to establish links between plant communities, geomorphology, and sediment characteristics. These datasets will feed into comparative analyses to better understand how natural processes and human interventions jointly shape salt marsh development.

Mesocosm experiments were set-up to analyse the growth responses of Salicornia species, a key pioneer species, under different climate treatments. Next, more mesocosm experiments will be set-up to test responses of different plant species present in the low marsh, as well as the monodominant Elysmus species found in the high marsh. These salt marsh species will all be exposed to various climate and nutrient changes. After one growing season, the plants will be transferred to a wave basin to observe how stressed vegetation (of both dynamic and well-established static salt marshes) interacts with sediments and hydrodynamics. These experiments will provide insights into how different salt marsh plant communities can influence sediment stability, wave attenuation, and coastal protection under climate change.

Mesocosm experiment at Alfred Wegener Institute (AWI) – Sylt, Germany. Left: Mesocosm experiment installations; Right: Salicornia species growth under climate treatment.

Numerical modelling work has also started this year, initially focussing on the plant-scale interactions between salt marsh vegetation and intertidal hydrodynamics. Computational fluid dynamics simulations are being developed to get a better understanding how vegetation structure influences hydrodynamic processes. Starting from single-plant scale models, the research aims to bridge to larger, landscape-scale analyses that connect plant morphology and biodiversity with sediment dynamics and marsh stability.

At the landscape scale, satellite and remote sensing analyses are being used to trace long-term changes in the Wadden Sea salt marshes. Using data from LANDSAT, PlanetScope, LiDAR, and sea level and subsidence records, researchers are mapping both vertical and lateral dynamics across the entire trilateral region. These analyses will reveal how salt marshes have responded to past sea level rise, how they might adapt to future changes, and how resilient they are to disturbances. By combining remote sensing with TMAP vegetation monitoring data, the team aims to evaluate how different management regimes, and resulting vegetation differences, affect the adaptive capacity of salt marshes across the Wadden Sea.

Stakeholder interaction through field visits and qualitative interviews has been a central focus during the first year of the project. Around fifty so-called walking interviews were carried out in coastal communities across the Netherlands, Germany (Schleswig-Holstein and Lower Saxony) and Denmark. Applying the NCP approach (Nature`s Contributions to People), we explored how residents perceive and value their nearby salt marshes and what roles these landscapes play in their daily life and identity. All interviews were voice-recorded, transcribed, and are now being analysed. Alongside this, a “living” document was initiated to map the social network of various institutions involved in salt marsh management.

Early Findings

Since all SALTGARDEN researchers have only started in the last year, most datasets are still to be collected or under analysis. Yet, we can already present some preliminary findings. The mesocosm experiments are beginning to show visible differences in how vegetation zones respond to climate treatments, suggesting that biodiversity and functional diversity may indeed influence resilience. 
Fieldwork at Wierum revealed a strong variability in carbon content, biodiversity, and wave dynamics along the marsh gradient, illustrating the complexity of these environments. This variability will be compared to patterns in restored marshes to assess how restoration techniques influence the ecological functioning of salt marshes.
From the remote sensing work, accretion rates for salt marshes in the Netherlands and Denmark have been calculated from LiDAR data, though validation with field measurements is still required. Inundation frequency changes have also been mapped across the entire Wadden Sea, revealing patterns of increasing tidal influence in some areas. These analyses will contribute to understanding how different salt marshes are coping with sea level rise and whether they can continue to keep pace with future changes.

Interviews with the various local communities across the Wadden Sea revealed a strong attachment of the people to their local salt marshes. When people have grown up at the study sites, our analysis show a strong identity-forming function of salt marshes. Most frequently mentioned were nonmaterial NCPs (i.e. psychological, cultural, or social benefits derived from salt marshes) such as recreation, stress relief, nature observation, beauty, environmental education, etc.. However, regulating services (e.g. carbon sequestration) has been only rarely mentioned by the people. All interviewees expressed their interest in further collaboration with the SALTGARDEN team, highlighting learning and co-production of knowledge in regards to the social-ecological system of salt marshes (e.g. restoration) through workshops and guided field visits.

Outlook

In the coming year, further mesocosm experiments with species of the low marsh and high marsh will provide the basis for the wave basin experiments, testing the resilience and ecosystem services of future salt marsh plant communities. Field teams will continue their campaigns at additional marsh sites to complete the trilateral dataset covering salt marshes in Denmark, Germany and the Netherlands. Modelling work will move from small-scale validation to large-scale simulations linking vegetation structure and sediment dynamics of salt marshes, now and in the future. Stakeholder mapping and qualitative analyses of the interviews with the local communities will also continue through autumn and winter. In the coming year, we are planning workshops for our associated partners and local stakeholder in each of the Wadden Sea countries to establish new collaborations and to strengthen our knowledge of the socio-economic relevance and resilience of the salt marshes.

Through this combined interdisciplinary approach, the SALTGARDEN team intents to integrate the natural and social sciences. Our understanding of how ecological dynamics shape physical interactions and ecosystem services goes hand-in-hand with our understanding of the value, perceptions, and management practices that influence how these landscapes are cared for. The intended results of the SALTGARDEN project will provide quantitative insights in the dynamics and resilience of managed and static salt marshes in comparison to the functioning of biodiverse and dynamic salt marshes. Together with local communities and stakeholders, we intend to co-develop ‘Nature-based Gardening’ strategies that will enable such biodiverse and dynamic salt marshes, creating landscapes that deliver the highest ecological value and ecosystem services, whilst being adaptive and resilient in the long term. We believe that these Nature-based Gardening strategies can strengthen the ecological status and ecosystem services of the Wadden Sea salt marshes for future generations.

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