Those fascinating mangroves

Mudskippers and a frog amid stilt roots and pneumatophores

1. The other day my wife and I went for snacks at a riverside restaurant upon the bank of the Merbok River. It’s a blessing that mangroves still line the muddy river banks all the way to the tidal estuary and mudflats along the coasts.

2. I am very fascinated by mangroves. At high tide their dense, fresh green foliage seems to float on the water, but at low tide you can see their tangle of aerial roots on the surface of the mud, visibly the stilt roots or prop roots and the finger-like roots. Unlike other trees, many mangroves can breathe through their roots.

3. Mangroves do not have an easy life in tidal estuaries and muddy coastlines inundated daily with sea water. Their environment contains unfavourable conditions for plants. The water and soil within a mangrove ecosystem have a high salt content. Mangrove soils are also usually waterlogged and low in oxygen. In addition, the soft and muddy soils provide very little support for a plant to establish itself.

4. Mangroves have special adaptations that allow them to survive in their environment, namely, the ability to tolerate salty waters and the ability to survive in waterlogged and oxygen-poor soil. They survive in salty water by extracting freshwater from the seawater. Their roots are adapted to prevent the intake of too much salt found in seawater before it reaches the plant. Some species excrete salt through special glands in their leaves, while some species concentrate salt in older leaves or bark until the leaves drop or the bark sheds, thus ridding the tree of the stored salt.

5. Within the estuarine environment of the Merbok River, mangroves with the most notable adaptations include species of Rhizophora, Avicennia and Sonneratia. Rhizophora species produce stilt roots adapted to withstand daily submergence in salt water during high tide and the destructive action of tidal waves.

6. Stilt roots of a grown Rhizophora species can be 2 to 4 meters long and extend within a broad radius around the trunk. They develop from the base or lower part of the trunk. They grow laterally and arch towards the soil where many smaller roots develop to anchor themselves in the soil. Thus stilt roots anchor and stabilize the tree in the soft, loose soil and broaden the base of the tree, propping it up and ensuring its growing space. Apart from that, these roots play an important part in providing oxygen for respiration.

7. Both Avicennia and Sonneratia species have special finger-like roots and cone roots respectively. These are called “pneumatophores” which absorb oxygen from the air at low tide. These tube-like breathing structures, which stick up out of the mud, extend from underground lateral roots. They may be a foot or so high and spread out around the base of the tree.

8. Mangroves are extremely rich in biodiversity. They create habitats for many more plants and animals that make up the mangrove ecosystem. They support abundant life through a food chain that starts with the trees. Mangroves shed large numbers of nutrient-rich leaves. Decaying leaves, twigs, wood and other organic material break down into small particles (detritus), the food source for many species of molluscs (shellfish), crustaceans (crabs, shrimps and prawns) and fish.

These in turn are the food source for larger animals which include reptile, mammal, bird and amphibian.

9. Mangroves are excellent feeding, breeding and nursery grounds for fish, prawns and crabs. During the rising tide many fish enter mangrove forests in search of food. They remain in the forest during high tide and leave again as the tide goes out. The tangle of stilt roots protects them from larger predators that cannot move between the roots. Mangroves also provide nesting and nursery areas for the larger animals. Many water birds and migratory birds roost, feed and nest in mangrove areas.

10. In addition, mangroves perform crucial physical functions. They protect the coastline by acting as wave breaks. Their unique root system stabilise coastlines and promote coastal accretion. As a result, they preserve the coastline and prevent shoreline erosion. Mangroves also trap debris, sediments, excess nutrients and toxicants through their natural filtering processes. This improves the water quality of tidal rivers that drain through mangroves. Mangroves act as wind breaks as well. Dense mangrove forests reduce the force of winds that may destroy and damage property.

11. Taking all their benefits into account, I believe that it is wise to protect and conserve our mangrove forests. Development is one of the threats to this unique ecosystem. Without proper management and conservation there is a possibility that mangrove forests would gradually diminish from our coastlines, estuaries and river banks. If that happened, it would be a great loss to the natural environment.

12. By the way, mangroves give me inspiration for my children books. I have written a few storybooks which centre on the animals of the mangrove such as mudskipper, flying fox, otter, crab-eating macaque, and hermit crab. I believe this is one way I can show young children that mangroves are important natural resource for people, and that they are so important to biodiversity on the planet we live in.