Saltmarshes are among nature’s best defenses against rising sea levels, acting as natural barriers that protect coastlines. New research shows that dense vegetation on saltmarshes makes them more vulnerable to sea level rise because the vegetation hinders sediment transport. A surprising twist, as it was believed that dense vegetation played a crucial role in helping these ecosystems survive.
Using advanced computer modeling, the research team, led by Olivier Gourgue and Jean-Philippe Belliard from the University of Antwerp (Global Change Ecology Centre, department Ecosphere) and the Institute of Natural Sciences, examined how saltmarsh landscapes evolve over time. They discovered that while dense plants do trap sediment near tidal channels, they also block its movement toward the interior of the marsh. This means that instead of spreading evenly across the marshland, much of the sediment stays concentrated near the edges. Over time, this can lead to lower elevation levels in the marsh interior, making it harder for the ecosystem to keep up with rising sea levels.
These findings challenge the long-held assumption that denser vegetation always improves a marsh’s ability to survive environmental changes. By revealing how plant density affects sediment transport, the study, published in Limnology and Oceanography Letters, highlights the complexity of nature’s balancing act.
Saltmarshes and rising sea levels
A powerful numerical model called ‘Demeter’ was used to simulate 200 years of marsh development under different vegetation conditions. The model helped isolate the impact of plants on sediment movement, something difficult to observe in the field.
The computer predictions were later confirmed by real-world observations in China, where coastal areas with denser plant growth display the same uneven sediment patterns that increase vulnerability to rising seas. By combining their theoretical modelling with field observations published in Global Change Biology, Olivier Gourgue, Jean-Philippe Belliard and their collaborators provided a more complete picture of how marshes function over time.
Saltmarshes are critical for coastal protection, wildlife habitats and carbon storage. If they can’t maintain their elevation against rising seas, they may disappear. This would lead to increased coastal erosion, loss of biodiversity and reduced carbon sequestration.
Balancing sediment and vegetation
This research suggests that conservation efforts need to consider more than just planting more vegetation. It’s about understanding the entire sediment transport system and ensuring that marshes receive enough material to grow at a healthy pace. This carries important implications for policymakers and conservationists.
As Olivier Gourgue points out: “Coastal restoration projects often focus on planting more vegetation to stabilize wetlands. However, our findings suggest that encouraging a mix of plant densities or even allowing natural processes to determine vegetation growth may be more effective in the long run.”
Additionally, it’s crucial to ensure that sediment supply is maintained. Human activities such as damming rivers and dredging waterways can reduce the amount of sediment reaching saltmarshes, further endangering their survival and functioning as coastal protection. By understanding the delicate balance between vegetation and sediment movement, we can better protect these vital ecosystems and the benefits they provide for both nature and people.
The study that was published in Limnology and Oceanography Letters was the result of a collaboration between researchers affiliated to the ECOSPHERE Research Group of the University of Antwerp (Belgium), the Department of Earth and Environment of Boston University (USA), the Operational Directorate Natural Environment of the Institute of Natural Sciences (Belgium) and the Department of Physical Geography of Utrecht University (The Netherlands). For the Global Change Biology paper, the Belgian researchers collaborated with colleagues of the State Key Laboratory of Water Environment Simulation of Beijing Normal University, the Environmental Research Center of Duke Kunshan University and the State Key Laboratory of Estuarine and Coastal Research of East China Normal University (China).