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Mangroves - The Unsung Heroes of Bako

Stuart Green discusses the importance of mangrove
forests such as those found at Bako National Park

Introduction to the Mangrove Forest

When you think of Bako National Park, humid rainforests, sweaty vines, groups of monkeys, and so on spring to mind. Yet hidden in the background, and somewhat underestimated, are the mangrove forests, hard at work doing lots of magical little things to make the Bako experience even more holistic. Large stretches of mangroves are seen across Bako where they live in the no-man's-land where the sea meets dry land.

The mangrove is a remarkable piece of biology. It has evolved from land trees to adapt to its environment. Living by the sea and spending half of life flooded by sea water is not an easy existence. Strong currents, the occasional storm and numerous crabs, fish and shells living on, around and underneath makes the life of a mangrove tree exceedingly difficult. This is illustrated by the fact that there are less than sixty species of mangrove tree in the world. By contrast rainforest ecosystems have tens of thousands of species of trees, shrubs and plant-life, suggesting that adapting to life there is relatively easy compared to the mangroves.

Performing simple functions such as maintaining an inflow of carbon dioxide and oxygen and maintaining freshwater in your system whilst being covered in sea water for half the day is a hard stress to deal with, but the mangroves have 'adapted' to the environment. Prop roots, salt exuding pores and waxy leaves all help to overcome the hostile living conditions of the coastal areas where they live.

Stream and mangroves with tide out.
© 1998 Wayne Tarman
If you take a look at the ground surrounding a mangrove tree you should be able to see a large number of shoot-like structures popping out of the mud floor. These are the mangrove's Prop Roots (or pneumatophores). Their function is to provide stability for the trees themselves, but more importantly to provide air to the mangroves because their below-ground roots are mostly water-logged by sea water. The aerial roots also have a filter-like structure on the top which removes most of the salt in the water during transpiration - the process which sucks water from the roots up to the leaves.

Excessive salt isn't good, whoever you are, plant, animal or bacteria. It imbalances the biological system and therefore has to be removed as quickly and efficiently as possible. Mangroves have evolved salt pores to fulfill this task. These actively desalinate the water inside their vessels and push the salt out of the plant to maintain as low a salt ratio as possible inside the mangrove itself. Have a look at the leaves, run your finger along a leaf and taste the white powdery substance, plain old salt. Leaves release carbon dioxide and oxygen, but in most land trees water evaporates during the process. By having thick and waxy leaves less water evaporates in the hot sun of the day. This again helps the mangrove to conserve what fresh water it has and minimises the need to take up sea water through its roots.

During reproduction, as with most plants and trees, the mangroves first need to be pollinated. The many bats, bees and birds that live in the area do this rather successfully, carrying the pollen of the male flowers to the female flowers of the same species. Mangroves reproduce by dropping fruits ranging from round ones roughly the size of apples (e.g. Aviscennia sp.) to very long (up to 20 cm) pencil shaped fruits (e.g. Rhizophora sp.). At certain times of the year these drop and will either immediately plant themselves and begin rooting, or float away and get dispersed by the sea currents. When they next get carried up onto the shoreline, if the soil is right and the environment feels right, they drop their roots and begin to grow. Mangroves grow at different rates depending on their environment and species, but a rule of thumb would be in the order of about one foot (30 cm) per year.

Mangrove Zones

Mangrove forests can be divided into zones, each zone having different characteristics and different environmental regimes. This zoning makes it easier for us to analyse the biology of mangrove species. The various zones are mainly based on the frequency and strength of waves and currents, salinity and the soil types, the zones being a continuum from low tide level up to and beyond the high tide level. There are four main zones and the different species of mangrove prefer in a particular zone depending on their ability to survive the environmental conditions.

1. Seaward Zone - Here the daily tides water-log the area. The currents are strong, the water is at full salinity and the soil is almost always water-logged. Hardy mangrove species live here and they tend to have a lot of 'Pneumatophores' (aerial roots). Examples include those known as the true mangrove species as they spend most of their time under water, e.g. Aviscennia sp.

2. Mid Zone This area receives water every day except during neap (low) tides. The species found in this zone are less hardy. The soil is not so water-logged and the currents are slower. There is also less damage causing species, barnacles etc. coming in from the sea, e.g. Sonneratia sp.

3. Landward Zone - This area is only affected by Spring tides, so it is mostly dry area. It is a diverse area as there is less environmental stress from the sea. There is a lot of competition in the area with the rich soil supporting a large variety of species such as vines, epiphytes, etc.

4. Riverine Zone - This is a prime area for mangroves, with less saline water, slower currents, and less tidal damage, the soil can support a large variety of species ranging from banana trees to coconut trees and Nipah palms.

Enough Biology, What Do Mangroves Actually do?

For some people, mangroves are just a breeding place of mosquitoes and sand flies, smelly, muddy and one of those places that would be of more use logged and turned into a fishpond. Track-records of the Asian countries do rather well reflect these attitudes, a few facts to back this up; Thailand and the Philippines have chopped over 70% of their original mangroves for land reclamation and shrimp ponds with Malaysia having a better track record but still not that far behind. Good short term economics, but very bad long term economics when we consider the hidden 'services' that mangroves have given us since time immemorial. Presented below are details of the various 'services' that mangroves perform every day.

Juvenile and Spawning Grounds
Many delicious crustaceans fish and shellfish shelter amongst the mangrove roots at high tide. A significant number of species found at local fish markets such as crabs, shrimps and fish have a connection with the mangroves. What is your favorite seafood? It probably has a connection with the mangroves. Mangrove production figures are in the order of 400 kg of fish, 200 kg of crustaceans and 100 kg of molluscs (shellfish) per hectare of mangrove, per year.

Stabilization of the Coastline
Muds, sands and soils don't drift if they are bound up by mangroves. The mangrove roots bind soils in place and stop them from drifting away. On the other hand without mangroves, its good-bye soil, and hello coastal erosion. No mangroves means we can kiss good-bye to nearby beaches as the currents will get to them. No beaches, no tourists, no tourism revenues etc. To sustain the beaches and land would require coastal protection measures. Building big concrete coastal stabilisers or dredging regularly to replace lost sand is an expensive operation. It's much easier to just leave the mangroves in place and let them do the job.

De-Sedimentation and the Processing of Sewage
The mangroves clean the freshwater and sea water passing through them every day. They act as bio-filters which sieve out wastes, bind them up and begin the slow process of breaking them down. They also do this to our air, ensuring nice clean air to go with the clean sea water. They also prevent salt water from intruding into the water table and help to ensure that the fresh water from wells stays fresh.

A Home for Many Species of Fauna and Flora
Many species of animal, plant, bird spend part or all of their life in and around the mangroves. For example proboscis monkeys feed primarily on mangrove leaves; water monitor lizards and otters dwell nearby and rely on the mangrove areas for a steady supply of food; macaque monkeys search for crabs at low tide and birds find shelter and food in the mangroves.

Defensive Wall
Mangroves provide a protective barrier to the land and can reduce coastal damage from natural disasters such as heavy storms and tidal waves.

Commercial Uses
Charcoal, timber and furniture, tannins and leather, herbal medicines, fruits, honey and other foods; mangroves have many uses. In Thailand there are over 10 different herbal medicines that you can use from the mangrove areas. Different species having different leaves, fruits and woods and thus different uses. A 1990 study of the 8,728 hectare Sarawak Mangroves Forest Reserve found that keeping the mangroves intact prevents the loss of RM 70 million per year. This was due to a number of factors. Aside from providing housing materials, firewood and jobs (pole collection, fishing, etc.) for residents of 16 villages, the mangroves reserve supports the entire fishing industry of the Kuching Division which employs 3,000 people and produces an income of over RM 52 million per annum. It also protects the tourist beaches at Damai from mud deposits. Without the protection of the mangroves the white sand beaches at Damai would be deposited with mud from the nearby rivers. Furthermore the animals, birds and plants of mangroves were also important tourist attractions. The mangrove reserve was also harvested for a variety of forestry products and protects the coastline (and various villages, farms and even parts of Kuching) from erosion, salt intrusion, storm damage, flooding and siltation.

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