Mangroves

Mangrove trees offer significant and unique habitat to birds, mammals, crustacea, and fish populations through a complex marine food chain, creation of breeding habitat, and establishment of restrictive areas that offer protection for maturing offspring. In addition, mangroves contribute to improved water quality by filtering and assimilating pollutants, stabilizing bottom sediments, and protecting shorelines from erosion in an already strained ecosystem.

Inadequacies in conventional red mangrove replenishment methods are primarily a result of their sensitivity to water depth, tidal action, and wave activity. A major problem in successful planting is the difficulty in finding suitable locations with adequate and appropriate environmental conditions favorable to the rooting and sustenance of the mangrove during its early stages of development. To have any potential of establishing thriving mangroves when using conventional methods, the seedlings must be planted only in areas adequately shielded from any substantial wave action or upland run-off. These conditions translate into restrictions not simply on the geographic location of a potential replenishment project, but also on the relative size and range of any replanting. Many areas that would be desirable for mangrove planting present formidable factors that prohibit the successful introduction of the tree.

Mangroves are plants which live between the sea and the land. A mangrove is not a species, but rather the name given to a community of unrelated plants living in areas which are inundated by tides. Thus a mangrove may be a tree but (like a 'rainforest plant') may also be a shrub, palm, fern, climber, grass or epiphyte - all of them sharing the ability to live in salt water.

MANGROVES

A teaspoon of mud from a North Queensland mangrove contains more than 10 billion bacteria. These densities are among the highest to be found in marine mud anywhere in the world and are an indication of the immensely high productivity of this coastal forest habitat.

Mangrove plants produce about one kilo of litter (mainly leaves, twigs, bark, fruit and flowers) per square metre per year. Some of this is consumed by crabs but most must be broken down before the nutrients become available to other animals. That is where the bacteria, along with fungi, come in. Dividing sometimes every few minutes, they feast on the litter, increasing its food value by reducing unusable carbohydrates and increasing the amount of protein - up to four times on a leaf which has been in seawater for a few months. Partly decomposed leaf particles, loaded with colonies of protein-rich micro-organisms, are then eaten by fish and prawns. They in turn produce waste which, along with the smallest mangrove debris, is munched up by molluscs and small crustaceans. Even dissolved substances are used by plankton or, if they land on the mud surface, are browsed by animals such as crabs and mud whelks.

This process is not confined to the mangroves. While some litter is recycled on the spot, this system is one of the few to export much of the - organic matter it produces. Every time the tide retreats it carries a cargo of food out to sea. Studies of the mangroves at the northern end of Hinchinbrook Island have shown that they export more than 12,500 tonnes of litter per year into the Great Barrier Reef lagoon. This material is deposited over an area of 260 square kilometres of seabed. Here bacteria densities are almost as high as those in the mangrove mud and they do much the same job, breaking down the litter to be consumed by botom-living fauna, by prawns and fish.

The seafood industry is the fifth largest primary industry in Queensland, with an annual commercial catch worth around $250 million. An estimated 75 per cent of commercially caught fish and prawns depend directly on mangroves at some time in their lives or feed on food chains leading back there. Since those species making up the remainder of the catch probably also owe much to nutrients exported from the mangroves, these coastal forests can be seen as one of our major assets.

MANGROVES - COPING WITH SALT

One particular advantage to growing in a salty environment is the lack of competition! Only a limited number of plants have invested evolutionary energy into adapting to such harsh conditions. In the optimum conditions of a tropical rainforest, diversity is great and competition fierce. On the edge of the sea (in Australia) about 30 species of mangroves have exclusive occupancy.

So how do mangroves manage to flourish in an environment which would kill most other plants'.' The first line of defence, for many mangroves is to stop much of the salt from entering at all by filtering it out at root level. Some species can exclude more than 90 per cent of salt in sea water. (Rhizophora, Ceriops, Bruguiera and Osbornia species are all 'salt-excluders'.)

Another trick, is to quickly excrete salt which has entered the system. The leaves of many mangroves have special salt glands which are among the most active salt-secreting systems known. It is quite possible to see and/or taste the salt on the leaf surfaces of species which choose this method. (Examples of 'salt-secreters include Avicennia, Sonneratia and Acanthus.)

A third method of coping with salt is to concentrate it in bark or in older leaves which carry it with them when they drop. (Lumnitzera, Avicennia, Ceriops and Sonneratia species all use this trick).

As can be seen from the examples given, some mangroves use only one of these methods but many use two or more. In addition, a number of features serve to conserve water. These include a thick waxy cuticle (skin on the leaf) or dense hairs to reduce transpiration - the loss of water. Most evaporation loss occurs through stomata - pores in the leaves - so these are often sunken below the leaf surface where they are protected from drying winds. Leaves are also commonly succulent, storing water in fleshy internal tissue.

MANGROVE USES

The mangroves have long functioned as a storehouse of materials providing food. medicines, shelter and tools. Fish, crabs, shellfish, prawns as well as edible snakes and worms are found there. The fruit of certain species, including the Nypa palm, can be eaten after preparation along with the nectar of some of the flowers. The best honey is considered to be that produced from mangroves, particularly the river mangrove (Aegiceras corniculatum).

Numerous medicines are derived from mangroves. Skin disorders and sores, including leprosy, may be treated with ashes or bark infusions of certain species. Headaches, rheumatism, snakebites, boils, ulcers, diarrhoea, haemorrhages...and many more conditions are traditionally treated with mangrove plants. The latex from the leaf of the blind-your-eye mangrove (Exoecaria agallocha) can indeed cause blindness, but the powerful chemicals in it can be used on sores and to treat marine stings. They are also used for fishing, when leaves are crushed and dropped in water fish are stupefied and float to the surface. This sap is currently being tested for its medical properties and may play a part in western medicine.

Certain tree species, notably the cedar mangrove and the cannonball mangrove (relatives of the red cedar) as well as the grey mangrove are prized for their hard wood and used for boat building and cabinet timber as well as for tools such as digging sticks, spears and boomerangs. The fronds of the Nypa palm are used for thatching and basket weaving. Various barks are used for tanning, pneumatophores (peg roots) make food fishing floats while the wood from yellow mangroves (Ceriops species) has a reputation for burning even when wet.

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