Forests between sea and land

They cover just under 150,000 ^km2 of our planet, on the border between land and sea, in tropical zones. Mangroves, these extravagant forests, harbor a diverse and threatened biodiversity. Christophe Proisy, a researcher in spatial remote sensing at the Amap laboratory¹laboratory, observes this unique ecosystem, whose secrets are still well-guarded in the labyrinth of mangrove roots.

To the question " What are mangroves ?", Christophe Proisy answers without hesitation that they are " among the most fascinating forests in the world ". Forests unlike any others, which the researcher sometimes only reaches after travelling dozens of kilometers by boat, but which can also extend up to 30 kilometers inland. And with good reason: mangroves are " forests that develop in the tidal range of tropical and intertropical regions ", explains the Amap laboratory researcher.

It's a buffer zone between land and sea, but one that doesn't seem very welcoming... Few trees are able to thrive on this loose, muddy soil, regularly submerged by brackish water. "The mangrove trees that form the mangrove forests have adapted to this constantly changing environment. There are around fifty species in the world," explains Christophe Proisy, whose work focuses on the diversity of mangrove forms and the dynamics of ecosystem functioning in relation to coastal dynamics.

Fascinating forests

How do they live with their feet in salt water? " Some mangroves excrete salt crystals through their leaves," explains the researcher. These astonishing trees have an aerial root system that connects all the trees together, forming interlacing patterns characteristic of these impenetrable forests. " These aerial roots, known as pneumatophores, are like the tubas of mangroves: they store oxygen at low tide, enabling the plant to breathe during high tide when the soil and roots are submerged", says Christophe Proisy. Even the mangroves' mode of reproduction is original: their seeds can germinate on the tree and then detach from the mother tree to be planted directly in the mud, or they can establish themselves further away at the mercy of the currents, thus extending the mangrove domain.

Mangroves, which can reach heights of up to 45 meters in equatorial regions, are supported by a shallow root system that spreads out over the muddy substrate. " No sediment, no mangrove", sums up Christophe Proisy. " If a dam built upstream of the river holds back the sediment, the mangrove will perish under the inexorable effect of erosion by swells or currents", warns the specialist. Moreover, the artificialization of the coastline, which disrupts hydrodynamics and prevents the natural movement of sediments, is one of the main causes of the mangrove's disappearance.

Marine nursery

A worrying disappearance: over the last twenty years, the mangrove area has shrunk by at least 35%. " The conversion of mangrove land into aquaculture ponds is the main factor in the destruction of the ecosystem", explains the researcher. And it's a whole biodiversity nestled between the mangrove roots that's disappearing with them. "Fish fry and juvenile shrimp take refuge there to get out of the reach of their predators; it's a bit like a marine nursery.

In French Guiana, for example, mangroves cover 80% of the coastline. " Mangroves find optimal conditions here, and in just a few months they can colonize hundreds of hectares of mud banks, with rapid growth in annual height reaching 2 to 4 meters," explains Christophe Proisy, who has made these 300 km of coastline his research ground. This territory, where jaguars live side by side with crabs and shrimps, where swell can tear 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.

Magellan Project

To help achieve this, he will be coordinating the Magellan project, financed by the National Program for Nature-based Solutions, which brings together local players in charge of managing Guiana's coastline. One of the aims 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 the observation of the spatial dynamics of Guiana's mangroves as an early warning system for erosion, silting or the degradation of coastal biodiversity." And mangroves are also a means of subsistence for many locals, who come here to fish or collect mangrove crabs, for example. Magellan also involves human and social science surveys to find out more about how people perceive mangroves in the development of the region.

The project, which began at the dawn of 2024, includes an important educational dimension to raise awareness of this environment and its rich but little-known biodiversity, the source of many misconceptions and depreciation. "Mangrove mosquitoes in particular are little studied, and we don't know whether or not they are vectors of disease. This question is the subject of a thesis by a Guyanese student co-supervised by the Institut Pasteur de Guyane and the IRD," explains Christophe Proisy. For the researcher, a better overall understanding of the role of mangroves will also enable them to be better integrated into sustainable coastal development plans and prevent the disappearance of these fascinating forests, which are essential for the adaptation of human populations on the coasts of the South.

• Watch Christophe Proisy's lecture on how mangroves work

1. Amap (UM, CIRAD, CNRS, INRAE, IRD)
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