Mangroves

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. 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 stylosa, 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 saltmarshes or saline herblands 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 stylosa 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. 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 24Ñ"C in the warmest month, where the annual rainfall exceeds 1250mm and mountain ranges greater than 700m 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.