[LUM#21] Forests Between Sea and Land
They cover just under 150,000square kilometers of our planet, at the boundary between land and sea, in tropical regions. Mangroves, these extraordinary forests, are home to a diverse and endangered ecosystem. Christophe Proisy, a researcher in space-based remote sensing at the Amap laboratory1, observes this unique ecosystem, whose secrets are still well-hidden in the intertwining roots of the mangroves.
When asked,“What are mangroves?”, Christophe Proisy replies without hesitation that they are“among the most fascinating forests in the world.” These forests are unlike any others; the researcher sometimes has to travel dozens of kilometers by boat to reach them, yet they can also extend up to 30 kilometers inland. And for good reason: mangroves are “forests that grow in the tidal zone of tropical and subtropical regions,” explains the researcher from the Amap laboratory.
A buffer zone between land and sea, yet one that seems rather inhospitable… Few trees are indeed capable of thriving in this muddy, loose soil, which is regularly submerged by brackish water. “The mangroves that form mangrove forests have adapted to this constantly changing environment. There are about fifty species worldwide,” explains Christophe Proisy , whose work involves documenting the diversity of mangrove forms and the dynamics of the ecosystem in relation to coastal dynamics.
Fascinating Forests
How do they survive with their roots in saltwater? “Some mangroves excrete salt crystals through their leaves, ” explains the researcher. These remarkable trees have an aerial root system that connects all the trees together, forming the characteristic intertwining network of these forests, which are impenetrable in places. “These aerial roots, called pneumatophores, are like the mangroves’ snorkels: they allow the trees to store oxygen at low tide to ensure the plant can breathe during high tide when the soil and roots are submerged, ” explains Christophe Proisy. Even the mangroves’ mode of reproduction is unique: their seeds can germinate on the tree and then detach from the parent tree to take root directly in the mud or drift further away with the currents, thereby expanding the mangrove’s range.
Mangroves, which can grow up to 45 meters tall in equatorial regions, are supported by a root system that is very shallow but spreads widely across the muddy substrate. “No sediment, no mangroves, ” sums up Christophe Proisy. “If a dam built upstream of the river traps the sediments, the mangrove will perish under the inexorable effect of erosion caused by waves or currents,” warns the specialist. Moreover, the artificial modification of the coastline, which disrupts hydrodynamics and prevents the natural movement of sediments, is one of the main causes of mangrove disappearance.
Marine nursery
A worrying decline: over the past twenty years, the area covered by mangroves has shrunk by at least 35%. “The conversion of mangrove lands into aquaculture ponds is the main factor driving the destruction of the ecosystem, ” explains the researcher. And with the mangroves, an entire ecosystem of biodiversity nestled among their roots is disappearing. “Fish fry and juvenile shrimp take refuge there to stay out of reach of their predators; it’s a bit like a marine nursery.”
The situation varies widely around the world; in French Guiana, for example, mangroves cover 80% of the coastline. “Mangroves find optimal conditions there; in just a few months, they can colonize hundreds of hectares of mudflats, growing up to 2 to 4 meters in height annually, ” explains Christophe Proisy, who has made these 300 km of coastline his research ground. This territory, where jaguars rub shoulders with crabs and shrimp, and where swells can uproot trees from their mud platforms, remains a mysterious and vulnerable ecosystem. And to better protect it, the researcher has been working for years to better understand it.
Project Magellan
To help achieve this, he will coordinate the Magellan project, funded by the National Program for Nature-Based Solutions, which brings together local stakeholders responsible for managing the French Guiana coastline. “One of the main goals of this project is to reconcile the preservation of this ecosystem with the development of a responsible economy,” explains Christophe Proisy. “With this in mind, the work will aim to use observations of the spatial dynamics of Guyana’s mangroves to create an early warning system for erosion, siltation, or the degradation of coastal biodiversity.” ” And the mangrove is also a means of subsistence for many residents who come here to fish or gather mangrove crabs, for example. Magellan also involves research in the humanities and social sciences to better understand how local communities perceive mangroves in the context of regional development.
The project, which began in early 2024, includes a significant educational component aimed at raising awareness about this environment and its rich yet little-known biodiversity—a source of many misconceptions and negative perceptions. “Mangrove mosquitoes, in particular, have been little studied, and we do not know whether or not they are disease vectors; this question is the subject of a thesis by a student from French Guiana co-supervised by the Pasteur Institute of French Guiana and the IRD, ” explains Christophe Proisy. For the researcher, a better overall understanding of the role of mangroves will also make it possible to better integrate them into sustainable coastal management plans and prevent the disappearance of these fascinating forests, which are essential for the adaptation of human populations along the coasts of the Global South.
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- Amap (UM, CIRAD, CNRS, INRAE, IRD)
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