A mangrove forest is a type of forest found in the intertropical zone that grows primarily on muddy soil. While mangroves are mainly found along coastlines, some grow on riverbanks, at river mouths, and sometimes even at the bottom of basins.
Regardless of their location, mangroves are rich and diverse ecosystems that grow at temperatures as low as 18 degrees Celsius and cannot tolerate excessive daily temperature fluctuations (the temperature difference between night and day must not exceed 10 degrees Celsius), and which thrive thanks to the tidal cycle (the alternation of high and low tides) and the presence of generally brackish water (although some mangroves grow in direct contact with the sea).
The iconic tree of the mangrove forest is the mangrove, whose location varies depending on the soil’s salt content. Thus, on the pioneer fronts (where the forest meets the sea), only red mangroves grow (Rhizophora mangle), which are well adapted to hyperhaline conditions (high salinity), whereas the farther one moves from the coastal zone, the less tolerant mangroves become of salt; the back mangrove area is even colonized by the golden fern (Acrostichum aureum) which, as its name suggests, is not a mangrove. There is therefore a linear gradient of salinity from the pioneer zone to the rear of the mangrove forest; accordingly, we find red, white, black, and gray mangroves, respectively (the last three genera of mangroves can sometimes be present simultaneously in the same location).
To fully appreciate how mangroves have adapted to their environment, it is worth noting that, when in contact with the sea, red mangroves—which are perfectly adapted to a saline environment—excrete salt (they release salt through small pores on their leaves), whereas other genera literally block the salt before it can penetrate the plant (this type of mangrove performs a sort of desalination of seawater).
While the presence of salt has forced mangroves to adapt, this is not their only adaptation; since the substrate is muddy, mangroves have developed two types of root systems: stilt roots or arching roots, which allow them to brace themselves in the mud and remain firmly anchored (as is the case with the red mangrove), and creeping roots, which pass beneath the mud and emerge at intervals as small, turgid protuberances called pneumatophores (as seen in the white mangrove). Many other adaptations exist …
While the flora has adapted to this very unique environment, the animals that thrive in the mangrove have adapted to it as well. To give just one example, the Periophthalmus, a fish that lives exclusively in mangroves, has developed a dual survival strategy. At high tide, it breathes (underwater) through its gills, while at low tide, it crawls across the mud and breathes out of the water using primitive lungs. The same is true of the Taxodes jaculator, a small fish that lives along mangrove fringes and hunts on tree branches. It may seem surprising, but this fish shoots a powerful jet of water (from its mouth) that dislodges insects from the branches of red mangroves, and once they’re in the water, all it has to do is catch them. Again, there are many other adaptations…
Ultimately, the mangrove ecosystem has very low plant diversity (there are only 60 genera of mangrove trees worldwide and about 15 in the Caribbean), but very high animal diversity.
In addition to these characteristics, it should be noted that mangroves provide real benefits to humans:
- Thanks to their tangled, nearly impenetrable root systems, they protect coastlines from erosion by dampening the energy of the waves; mangroves are therefore excellent barriers against tsunamis. Since the tsunami of December 26, 2004 (Southeast Asia), many countries have replanted vast areas of mangroves;
- are generally the only green spaces on coral islands (and as such, they contribute to the beauty of the landscape);
- These are true nurseries, as their intertwined roots protect the fry, which grow there in complete peace;
- Finally, many economic activities can be developed there, such as oyster farming, beekeeping, etc.
Given these circumstances, how can we explain why the mangroves are in such poor condition?
Worldwide, mangroves currently cover just under 100,000 km2 (which is roughly equivalent to the area of the island of Cuba—which is actually very small), and they continue to shrink because humans have turned them into unproductive spaces.
Since mangroves are areas where large volumes of organic matter decompose, they release methane, which sometimes gives them a foul odor. Through a simple association of ideas (bad smell = waste or dumps), people have turned mangroves into open-air dumps.
Whether in Saint Vincent, Martinique, Guadeloupe, Puerto Rico, Trinidad, or many other parts of the world, hundreds of km2 Mangrove forests have been turned into landfills or altered by human activity (construction of roads, ports, industrial zones, etc.). Yet the Amerindians, who were well aware of the benefits of mangroves, made them a cornerstone of their civilization. They used mangrove oysters for food; women adorned themselves with polished shells; the leaves were used for medicinal purposes; and the mud was used to protect against mosquitoes and to treat certain skin conditions, among other things.
Faced with such untapped—not to mention forgotten—wealth, isn’t it time to restore the mangrove to its rightful place at the center of our attention, since it is a habitat we need?.
In the context of climate change, some studies suggest that protecting mangroves—or even promoting their growth—is the best way to combat rising sea levels. What are we waiting for to take action?
Pascal SAFFACHE and Grégory ARIBO
