Mangroves: the roots of the ocean
After discovering the marine flora and fauna, let’s have a closer look at the unique ecosystems forming the ocean. And what better than to start with what can be called “the roots of the ocean”?
With an appearance 75 million years ago, mangroves are not our usual pine trees occidental forests. They are located in the intertidal zone, the littoral area within the lowest tides and the highest tides limits. In other words, the area where the tide ascends and descends during the day. Their environment is then characterized by highly variable factors such as temperature, sedimentation and tidal currents.
Worldwide, 170 000 km2 of mangroves have been recorded in tropical and subtropical coastlines. The largest areas are found in Indonesia, Brazil and Australia. Mangroves only represent 0.1% of the world’s plant surface but are extremely important ecosystems ecologically, culturally and economically.
An extraordinary flora with a vital root system
There are 3 different mangroves type:
→ Forests associated with river deltas and estuaries
→ Forest associated with coastal embayments
→ Forests within reefs and lagunes
In any case, the trees found themselves submerged in saltwater at each tide. Why is it a problem? Salt in enormous quantities is toxic to plants.
But don’t worry, nature knows best! Mangrove trees have metabolic and physiological adaptations allowing them to exclude salt at the surface of their roots in order to extract freshwater mostly. Certain species even have salt glands to secrete it at their leaves' surface. So if one day you are walking by a mangrove forest and suddenly have a savory craving, try to taste some leaves, you may eat some salt crystals!
On top of having to adapt to salty water, the deep roots are constantly plunged in an anaerobic environment, that is to say without oxygen, on the contrary to terrestrial trees. The tree has to move the oxygen from the air down to the root system in order for it to continue its metabolic activity: absorb water and nutrients for plant growth. To help, the roots have aerenchyma cells specialized in oxygen transport that act like air bubbles.
Thanks to this optimal functioning, roots can bloom perfectly! In fact, the root system is the major component of mangroves ecosystem properties. Its role is fundamental in the process of accumulating material and increase soil volume. Indeed, in anaerobic conditions decomposition of organic material is much slower allowing a greater soil accumulation through time. And a well-supplied thick soil maintains the forest position on the coast. Therefore mangrove forests constitute coastal protection against storms and waves.
A diversified fauna
Between water and earth, mangroves are a biodiversity niche on the planet. They actually offer 3 different environments for the specialized fauna: the canopy, the roots surface and the space within the roots.
Their canopies proved shelter for diverse species such as the Bengal tiger in Bangladesh, flying foxes (the largest in the world), birds, macaques, serpents, insects and the list goes on.
In water, algal and sponge communities use the roots as a substrate for fixation. They slow down the velocity of tidal currents allowing suspended particles to deposit on the forest soil. I told you that roots were the crucial element of mangroves.
But there is also all the fauna navigating between the roots and around mangrove forests: fish, crabs, shrimps, dolphins, manatees, crocodiles and many more. The first 3 animals use mangroves as a nursery ground spending their juvenile phases there. Suspicion of a strong link between mangroves and fish abundance in offshore seas via migration is therefore high, but we still need rigorous proof on this linkage.
This richness is highly beneficial to local fishermen as well as to larger-scale fisheries: 30% of all commercial fish species are mangrove-dependent worldwide.
An unimaginable “blue carbon” pit
We keep hearing that the Amazon rainforest is the Earth’s lung and that it can stock an important quantity of carbon through photosynthesis. What if I told you that there was a lung even more powerful to sequestrate carbon?
Yes, even if they represent only 0.1% of the world’s plant surface, mangrove forests can stock up 50 times more carbon than other tropical forests! It is the ecosystem with the greatest carbon storage capacity worldwide.
I let you guess where all this carbon is located… Yes, in the roots! Hence the “blue carbon” name, because it is underwater. It is transported from the air and stocked for decades and millennia thanks to the soil thickness. And since the soil volume increases year after year, carbon storage as well.
Anyway, I will not repeat it enough: mangroves' roots are fascinating.
A resourceful forest, but at what cost?
This ecological complexity with such biodiversity was with no surprise noticed by humans. Nowadays mangrove forests sustain more than 70 human activities, whether they are cultural or economic.
Their exploitation began a long time ago by coastal populations. Their wood was used for construction, fire and charcoal. But more devastating methods with higher impact expanded over the years:
1. Wood industrialization: during colonial times for boat construction, which quickly led to an overexploitation
2. Agriculture: from 1900, mangroves suffered deforestation for rice, oil palm and coconut production. In China, rice farming is responsible for 48% of mangroves deforestation, which corresponds to 210 km2
3. Shrimp pond culture: the principal cause of mangroves deforestation worldwide. Ponds have to be relocated after 5 to 10 years towards a healthier mangrove forest due to eutrophication risk and toxins accumulation in water. Spoiler alert: the slaughtered mangrove that took so long to flourish will not recover within 10 years.
Intensive usage of mangroves resulted in their global loss between 1980 and 2000. 38% of mangrove forests were cut down in 20 years, being 30 000 km2, corresponding to a loss of 2% per year approximatively. And the worst is that those numbers don’t take into account Indonesian forests who would have lost 50 to 80% of their mangroves. This loss is not only dangerous to nature but also to humans (is it karma?). As mentioned before, mangroves protect littorals from heavy storms and waves. Yet, with their disappearance, coastal populations are more exposed to them. If the Indian mangrove forest was intact in 1999 when a cyclone hit the coast, more than 90% of deaths would have been avoided.
In addition to their usage, the construction of upstream walls and dams limits not only the mangroves expansion but also the arrival of freshwater and sediments, essential to their functioning.
A promising future?
All this is pretty depressing I agree. Luckily there are some positive elements! Indeed, the deforestation rate would be decreasing for 10 years now. But there is a new factor to take into consideration: sea-level rise. By 2100, oceans could rise by 2.5 meters. How will mangroves react? It will mostly depend on urban coastal development existing in their vicinity. If there are no dams, cities or walls, then no problem: mangroves could spread inland. If there is an obstacle, it is still uncertain. They will be prisoners, but it does not mean water will engulf them: their magnificent root system could cope with sea level rising by accumulating enough organic matter and thus maintain their position on the coast. Yet mangroves would need to be in a good state and urban constructions in their surroundings should be stopped.
To assure a future for mangrove forests, we need to plan their conservation at a local and global scale. Nowadays, 36% of mangroves are legally protected. But in my opinion, protected and efficiently managed are two different things. We should make sure in the future that regulations are respected in all protected mangroves, otherwise it is protection existing only on paper.
Reforestation of mangroves is the flagship and in vogue solution for a few years. Even if only half of the plants survive most of the time, I think it is a way to raise awareness among local communities on the economic benefit and ecological importance of a healthy ecosystem. In upcoming years, the best would be to improve reforestation techniques and plant more than one species of mangrove tree to assure good forest biodiversity.
How can I help?
We saw there are positive elements for mangroves maintenance, but what can we do from our home to help?
First of all, we can change our eating habits affecting mangroves. If you buy shrimps, be careful where they come from (by the way, the best would be to not eat them at all). The same goes for rice, palm oil and coconut. And of course, all Blutopia solutions will have a positive impact on our beautiful mangrove forests!
You can also directly help mangroves reforestation and conservation projects or NGOs by donating. I did a selection of a few initiatives for you:
· MCP a marine conservation center where I helped on coral reefs, but that also replant Philippines mangrove with the local community
· Mangrove Action Project that works with local communities and NGOs worldwide
· EcoViva in Mexico working with the local community
· ZSL that has a reforestation project of mangroves destroyed because of pond culture in the Philippines
· SeaCology that support projects all around the world
This article was written in collaboration with Blutopia, a french association which aim is to share ways on how to protect the ocean.
Sources
Friess, D. A. et al. The State of the World’s Mangrove Forests: Past, Present, and Future. Annu. Rev. Environ. Resour. 44, (2019).
Kathiresan, K. Importance of Mangrove Ecosystem. Int. J. Mar. Sci. (2012)
Malik, A., Fensholt, R. & Mertz, O. Mangrove exploitation effects on biodiversity and ecosystem services. Biodivers. Conserv. 24, 3543–3557 (2015).
Nagelkerken, I. et al. The habitat function of mangroves for terrestrial and marine fauna: A review. Aquat. Bot. 89, 155–185 (2008).
Trégarot, E. et al. Mangrove ecological services at the forefront of coastal change in the French overseas territories. Science of the Total Environment 763, (2021).
Valiela, I., Bowen, J. L. & York, J. K. Mangrove forests: One of the world’s threatened major tropical environments. BioScience 51, 807–815 (2001).
