Some people don’t like mangroves, regarding them as muddy, mosquito and crocodile infested swamps. In the past their removal was seen as a sign of progress. So what is the point of preserving them? For a start, an estimated 75 percent of fish caught commercially spend some time in the mangroves or are dependent on food chains which can be traced back to these coastal forests. Mangroves also protect the coast by absorbing the energy of storm driven waves and wind. The only two yachts undamaged by Cyclone Tracey in Darwin in 1974 had sheltered in a mangrove creek. While providing a buffer for the land on one side, mangroves also interact with the sea on the other.
Sediments trapped by roots prevent silting of adjacent marine habitats where cloudy water might cause corals to die. In addition, mangrove plants and sediments have been shown to absorb pollution, including heavy metals. Worldwide, vast tracts of mangroves have been destroyed so we are lucky to have relatively large areas of Australia’s tallest and best-developed mangroves still existing on our doorstep. Now that their economic and ecological importance has been recognised we carry the responsibility to look after our mangroves.
A mangrove is a woody plant or plant community which lives between the sea and the land in areas which are inundated by tides. Thus a mangrove is a species as well as a community of plants. It can be a tree but (like a ‘rainforest plant’) it can also be a shrub or palm. All share the ability to live in salt water. As a general rule zones of dominant mangrove species run parallel to the shoreline or to the banks of tidal creek systems. The seaward side of the community is likely to be dominated by a fringe of grey mangroves Avicennia marina as it is best adapted to early colonisation and a wide range of soil conditions.
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Avicennia marina is a tough mangrove species – Australia’s most widespread due to its ability to tolerate low temperatures and a variety of other intertidal conditions. A pioneer, it is likely to be the first species to grow on newly-emerged mud banks, putting up its distinctive peg roots. Mangrove apple Sonneratia alba often grows in this zone too, but it is a more tropical mangrove. The red mangrove Rhyzophora, also known as the stilt or spider mangrove, is usually found behind this zone where its long prop roots give it a firm foothold against wind and waves. The next zone might be inundated only by periodic spring tides at the times of new and full moons. The soil will be firmer but more saline due to the evaporation of water leaving behind salt which will not be diluted until the next spring tide.
The more specialised yellow mangrove Ceriops species can be found in this zone, although conditions usually make it impossible for anything other than salt marshes or saline herb lands with succulent plants to thrive here. The resilient grey mangrove can appear again while less saline soils might be covered with a thick forest of the orange mangrove Bruguiera species. The greatest concentration of mangrove species is usually at the mouth of tidal creeks and rivers where salt and fresh water mix in ideal proportions and floodwaters deposit plenty of material to build up the banks. Red mangroves Rhizophora are frequently found here. While there are certain patterns to mangrove zone development, local conditions will always dictate which mangroves are found where. Mangroves don’t need salt.
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Some species have been kept in pots where they have grown and flowered regularly when given only fresh water. However, experiments have also shown that the best growth occurs where the plants live in sea water diluted half and half with fresh water. 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 intertidal conditions. In the optimum conditions of a tropical rainforest, diversity is great and competition fierce. On the edge of the sea (in Australia) about 38 species of mangroves have exclusive occupancy.
The richest mangrove communities occur in tropical and sub-tropical areas where the water temperature is greater than 24 oC in the warmest month, where the annual rainfall exceeds 1250 mm and mountain ranges greater than 700 m high are found close to the coast. (The proximity of mountains tends to ensure the rainfall. ) In addition, they need protection from high energy waves which can erode the shore and prevent seedlings from becoming established. In north Queensland the Great Barrier Reef performs this function while to the south a chain of sand islands provide shelter.
Shallow, gently-shelving shores allow mangrove seedlings to anchor, particularly in estuaries, rivers and bays. Mangroves exist in a constantly changing environment. Periodically the sea inundates the community with salty water while, at low tide, especially during periods of high rainfall, it may be exposed to floods of fresh water. Apart from suddenly altering the salinity levels, these fluctuations in water can alter temperatures as well.
Different mangrove species have different requirements. Some are more tolerant of salt than others. Other factors which affect their distribution include wave energy, soil oxygen levels, drainage and differing nutrient levels. Where one species finds its preferred conditions – or at least those which it is able to tolerate better than other plants – it tends to become dominant. This has led to quite clear zones among mangroves.
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Apart from coping with salt, mangroves also face common problems of water-logged, unstable and oxygen deficient soils. Despite belonging to many different families mangrove plants have come up with surprisingly similar solutions. Mangroves have long functioned as a storehouse of materials providing food, medicines, shelter and tools. Fish, crabs, shellfish, prawns and edible snakes and worms are found there. The fruit of certain species including the ny pa 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. Numerous medicines are derived from mangroves. Ashes or bark infusions of certain species can be applied to skin disorders and sores including leprosy. 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 Excoecaria agalloch a can indeed cause blindness but the powerful chemicals in it can be used on sores and to treat marine stings.
The leaves are also used for fishing; when 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, cannonball mangrove (relatives of the red cedar) and 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 ny pa palm are used for thatching and basket weaving. Various barks are used for tanning, pneumatophore’s (peg roots) make good fishing floats while the wood from yellow mangroves (Ceriops species) has a reputation for burning even when wet. Worldwide there are about 65 recognised species of mangrove plants belonging to 20 families.
Up to 35 mangrove species and three hybrids are known to occur in Queensland although figures can change as the definition of a mangrove is not clear and some plants such as cottonwood are regarded as mangrove by some and not by others. A study of Cairns mangroves found 24 mangrove tree and shrub species while a further 18 species of flowering plants were growing among the mangroves or on salt marshes. An additional 42 species of epiphytic plants and 25 species of fungi were identified growing on the mangroves.
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