Mangroves-World of their own?

by Matthew Norton

Mangrove trees are one of those rare groups of plants that can survive on the seashore. Plants struggle to survive here due to the periodic flooding with seawater, limited access to fresh (non-salty) water and the waterlogged sediment (soil or mud), which makes it harder to get oxygen and nutrients.

Mangroves thrive in this hostile environment with various coping mechanisms. To control salt levels in their body they have very fine filters in their roots, to limit how much salt gets in, and expel excess salt through the leaves. At the same time, they minimise water loss by producing waxed leaves and/or (depending on the species) moving them to avoid harsh sunlight and so avoid evaporation. Some species also produce thick leaves to store water.

Finally, to overcome the limited oxygen supply in the sediment they instead get oxygen from the air through specialised pores (lenticels) in their trunk. The insides of their roots are also ‘spongy’ with open channels that allow oxygen to flow quickly throughout the root system. Some species go even further with stilted roots to prop up the trees further above the water, so they spend more time in air, while others produce specialised ‘breathing tubes’ that stick out from the sediment, away from the trunk, and are packed with lenticels.

These adaptations are impressive, but let us not forget that mangrove forests include a wide diversity of life, including bacteria, sponges, crabs, insects, birds, mammals, seaweeds and mangrove seeds and seedlings. Mangrove trees support many of these organisms with their roots providing shelter from strong waves, hiding places from predators and a hard surface for animals to hold on to. By providing shelter from the waves, mangrove roots also encourage the settlement of fine sediment (i.e. mud), which provides habitat for burrowing species. Their seedlings, dead branches and dead leaves also provides food for many of the creatures on the forest floor.

In some cases these organisms support the mangrove trees in return, supplying them with the nutrients that can be in short supply. Inside the previously mentioned ‘breathing tubes’ of black mangroves there are cultures of cyanobacteria (photosynthetic bacteria) which extract nitrogen gas from the air and convert into nitrate, an essential plant nutrient. Also, some species of giant sponge grow on mangrove roots and produce tiny rootlets to help them to extract nutrients from the surrounding soil. Animals not attached to the trees can also be helpful, for example burrowing mix up the sediment and gets oxygen deeper down in the sediment where it reacts with iron to produce ferric oxide, another essential plant nutrient.

Sometimes, we don’t appreciate how stressful the seashore environment can be for living things, despite it being the border between two very different worlds. Evolution has responded with some of the hardiest plants and animals in the world, but this process acts on all aspects of the species, including its interactions with the environment and other species it shares that environment with. In other words no-one faces the world alone.

From a human perspective

Mangrove forests, like rainforests and coral reefs, are disappearing at an alarming rate with a 1-2% loss in forest area each year. The reasons for this decline include trees being cut down for firewood and cleared away for infrastructure and shrimp aquaculture. This is a devastating loss for humanity, as we benefit from mangrove forests in various ways.

For example they can reduce the impacts of climate change by extracting carbon dioxide from the atmosphere and then locking up that carbon as dead plant matter sinks into the sediment. Mangroves also provide important nursery habitats for many commercially valuable species of fish and shellfish (e.g. rainbow parrotfish) where they can develop and grow in a relatively safe and nutrient rich environment.

Another, more direct benefit afforded to humans by mangrove trees is that they protect us from the sea’s destructive side (flooding, coastal erosion, tsunamis) by reducing the power of the waves that pass through their roots. For example in the aftermath of the 2004 Indian Ocean earthquake, and the tsunamis they caused, surveys were conducted to assess the damage caused in different regions. In the Tamil Nadu coastal region man-made structures positioned behind thick mangrove cover suffered less damage from the tsunamis. Claims were also made of fewer human deaths in these areas.

However, while mangroves can protect us from the sea, other precautions must be taken. There are also doubts in some of the findings of the Indian Ocean tsunamis surveys due to criticisms in how the data was analysed. A more recent study (2018) between two sites in New Zealand showed just how much mangrove protection against smaller waves and coastal erosion can vary. One site in the Firth of Thames, where the mangrove trees were high and tightly packed together, was very effective at resisting the flow of both tides and storm surges. At a 2nd site in Tauranga harbour the smaller mangroves offered little protection and even that was compromised by drainage channels that diverted the water around the trees. Regardless, the possibility that mangroves can protect us from nature’s wrath is reason enough for us to protect them in return.

Fortunately, there have been attempts to replant lost areas of mangrove forest, but in some cases the results have not been promising. This is partly due to lack of planning with seedlings being planted in poor growing conditions (e.g. soil pH, topography), not ideal when some species are very fussy about this. In some cases seedlings have even been planted in places where there were no mangrove trees in the first place. Worse still, these replanting programmes often attempt to create plantations of a single mangrove species, rather than an integrated ecosystem of multiple mangrove species, and their associated animals and microorganisms.

In the past when we have thought of tropical environments it is usually rainforests and coral reefs that come to mind, with mangrove forests getting much less attention. This is a shame, but in more recent years their value has become more appreciated and I am optimistic that, with the right insight and action, they can recover and thrive as part of our world.

Sources

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Image sources

Veryn4ik89. 2015. [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)%5D. https://commons.wikimedia.org/wiki/File:Necklace_of_Mermaid.jpg

All other images are public domain and do not require attribution