Saudi Green Initiative

Magic of mangroves: The plant that sequesters more CO2 than rainforests

As a plant, mangroves aren’t really much to look at. Clustered around the brackish waters of estuaries, a floating mop of messy green foliage is all you see from afar. It’s only when the tide goes out, and the tangle of knotted roots appears, that you really see the whole thing.

One of the first western records of this plant appeared in the works of Pliny the Elder, the Roman author and naturalist who lived between AD23 and AD79. He had never seen plants that could grow on water, and wrote of his discovery: “on the Red Sea, the trees are of a remarkable nature”.

Almost 2,000 years later, scientists are still discovering remarkable things about these plants, like the fact that they can sequester more CO2 than rainforests, says Professor Carlos Duarte, a leading marine ecologist at Saudi Arabia’s King Abdullah University of Science and Technology (Kaust), and they’re barely scratching the surface.

Indeed, there is still much we don’t know about mangroves, including precisely how many species of plants there are. Conservative estimates place the figure at around 50, but this can go up to more than 100 depending on who you consult and how the plants are classified. The core requirement is that it’s a tree or a shrub that’s able to live in the salt-rich and oxygen-poor waters of tropical and subtropical climates. Exactly how it does this varies, too.

Take for example the Rhizophora stylosa, an elongated plant found in Asia where it’s commonly known as the stilted mangrove. Its root is made up of three distinct layers of material that act as fine filters to remove as much as 90 per cent of the salts from the water before it enters the plant – a feature that could feed into the development of more effective desalination filters in future.

Meanwhile, if you look closely at the Avicennia marina, the grey mangrove, it’s often possible to spot salt crystals on the underside of its leaves. This species, the most prevalent variety around the world, has a similar salt filtration system in its roots but it’s not nearly as effective. Perhaps just 50 per cent of the salt is removed from the water before it enters the plant. The rest is stored in the leaves or bark, while any excesses are excreted through its leaves, just like sweat.

Another differentiator between mangroves and other plants is that most of their roots are above ground, some branch out from trunks like stilts while others erupt from the earth like stalagmites – or a spike pit if you’re being really imaginative – but always exposed during low tide rather than hidden below ground. This aerial root network not only helps to secure the plant in place against the bashing waves but it also increases the mangrove’s oxygen uptake from the water.

To complete their adaptation to their uniquely challenging habitat, all mangroves have leaves with waxy surfaces that lock the moisture in and automatically adjust their position to avoid the searing sun.

Defender of habitats

If mangroves’ unusual physiology isn’t enough to convince you of their remarkable nature then their myriad of other benefits surely will.

Thanks to their density and network of gnarly roots, mangroves are known for reducing beach erosion by holding the soil in place. Any sediment that washes in with the tide – and this includes, Professor Duarte says, the microplastics that are now so pervasive in the ocean – then gets trapped in this ever-expanding network of filters. Over time, as the clarity of the water improves, this sheltered environment becomes a nursery ground for fish and shellfish, which in turn help to support a rich ecosystem of marine life.

Above ground, mangroves are equally indispensable. They create environments suitable for other plants, including rare orchids, as well as insects, birds, lizards and even larger mammals like monkeys and tigers. This rich tapestry creates endless possibilities for scientific research.

In the Arabian Peninsula, for example, there are bees that harvest pollen exclusively from mangroves. Free from pesticides, these plants provide an uncontaminated source of food for the bees – so perhaps it’s no surprise that the bee population in Saudi Arabia is one of the few that’s not in decline. Scientists are now looking at sequencing the genes of these bees, says Professor Duarte, to better understand how colonies in other parts of the world can be protected.

Another field of study is the health benefits of mangroves; in particular, how it could be used in the treatment of breast cancer and other illnesses.

For populations that live along the shoreline, mangroves also act as a first line of defence against extreme weather phenomenon, such as cyclones and tsunamis, something that’s happening with increasing regularity as a result of climate change.

Red mangrove seedling grows in the waters of Saudi Arabia’s Red Sea

Professor Duarte was among a team of experts working with the UN to assess the damage to coastal areas in south-east Asia following the devastating Indian Ocean earthquake and tsunami in 2004, where nearly a quarter of a million people lost their lives. One surprising discovery they made was that those who lived in areas with mangroves suffered considerably less damage.

“Those villages where pockets of mangrove remain between the village and the ocean actually had almost no loss of human lives, and the losses of infrastructure were much less,” he explained. “So the mangroves are also the first line of defence for shorelines against cyclones, against tsunamis and against sea level rise.”

The value of mangroves are of course well known to communities that live near these protective forests. Professor Duarte cites the example of the Mekong Delta, where much of the mangrove forest was destroyed through the use of herbicides and napalm by US forces during the Vietnam War between 1955 and 1975. Recognising the importance of the resource, the Vietnamese people “took it upon themselves to replant the mangroves” after the war.

“They planted 1,700 sq km of mangroves in the 10 years after the war,” Professor Duarte adds, “which remains the most significant ecosystem restoration ever attempted by humans.”

Red mangrove flowers growing in the Red Sea off the coast of Saudi Arabia

Red mangroves growing in a tidal creek on the coast of Saudi Arabia

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