A mangrove is more than just a tree ...

For many people, mangroves look like muddy, swampy places filled with mosquitoes, snakes and spiders. But take a closer look....

Walking through a mangrove can be like going on a giant treasure hunt. Hidden within the twisted vines and branches are amazing reptiles, wild looking insects and plenty of crabs and other animals which call the mangroves their home.

Besides being a wonderful place to explore, mangroves are also important from an ecological standpoint. The Australian Institute of Marine Science has a number of fascinating research projects under way in an effort to better understand mangroves, for example:

Biologists estimate 75% of the commercially caught fish and prawns in Queensland spend at least some part of their life cycle living in the mangroves. For many species of fish, like the sea mullet and barramundi, the muddy waters of the mangroves are the nurseries where they raise their young. Because fish are so dependent on mangroves, protecting these forest communities is another way of protecting our fish populations. AIMS biologist Janet Ley is working to better understand what it is about mangroves that makes them so important to fish populations. She is studying what happens to fish populations when mangroves are altered by humans and natural actions.

AIMS biologist Dan Alongi has been tracking the cycle of nutrients in the mangroves in northeastern Australia and comparing them with mangroves in Malaysia and Vietnam. He hopes to find how different climates and conditions effect how mangroves grow.

Mangrove forests also provide safe nesting and feeding sites for herons, egrets and other birds. Biologists have recorded more than 230 species of birds flitting through Australian mangroves, and while only eight or nine species are restricted to mangroves in the Wet Tropics, the many other species visit and depend upon the mangroves for food, nesting or shelter.

Mangroves are also home to lots of snakes and spiders, flying foxes and a favourite spot for salt water crocodiles to tuck into for a rest and to look for food. All in all, biologists have found that mangrove forests are one of the most important habitats in the world.

Unfortunately, for a long time people didn't realise the important role mangroves played in the world and so they tore them up, filled them in and dug them under to create room for more houses, buildings and parking lots.

Today all mangrove forests and any part of a mangrove, including stumps, seeds and leaves, are protected in Queensland under the Queensland Fisheries Act, and any activity involving mangroves requires a permit from the Queensland Department of Primary Industry.

At the Australian Institute of Marine Science, biologist Barry Clough has been recording the changes in mangrove growth for the last ten years and the information he has gathered will help people better protect and manage important mangrove forests.

Awash in saltwater and up to their knees in mud, the plants in a mangrove forest have clever ways of coping with their environment.

Most plants have a very low tolerance for salt, but in the mangroves, twice a day, the high tide rushes in and covers many of the plants in saltwater. The trees, shrubs, palms, ferns, climbers, grasses and epiphytes which live in the mangrove forest must all be able to cope with salt. While these plants don't have to have salt to survive, studies have shown that mangroves do grow best in water that is 50% freshwater and 50% seawater. So how do mangrove plants defend themselves against the daily onslaught of salt?

Stopping the salt by filtering it out at the roots is the first line of defence for many of the plants. Some species of plants can exclude more than 90% of the salt in sea water this way. Once the salt has entered the plant's system, an other trick is to quickly excrete it through special salt glands in the leaves. Try licking a leaf, you should be able to taste the salt which the plant has excreted, or if you look closely at the leaf's surface you can see where tiny salt crystals have formed. Some plants cope with salt by concentrating it all in the bark or in older leaves which take the salt with them when they drop.

Conserving water is also important in the mangroves, and many of the plants have thick, waxy skins or dense hairs on their leaves to reduce the amount of water they lose. In addition, the leaves are often fat and succulent and store water in their fleshy internal tissue. Adjusting to life in the mangroves also means adapting to living in mud rather than soil.

Roots, for most land plants, provide stability and support to the plant as well pulling nutrients and oxygen out of the soil. In the mangroves, the unstable mud makes an extensive root system essential for holding the plant upright. This root system can be divided into three different types of roots with three different functions:

• Radiating cable roots with their tangle of anchor roots provide support.
• Little nutritive roots grow up out from the main cable root to feed on the rich soil just below the mud's surface.
• The third type of root collects oxygen for the plant.

Unlike soil, mud has very few air spaces for roots to gather oxygen for the plant, so many of the plants in the mangroves have developed some amazing methods of obtaining the oxygen they need to grow. The grey mangrove (Avicennia marina) grows a series of snorkels or pencil roots which poke out of the mud to get oxygen, while the orange mangrove (Bruguiera gymnorrhiza) has developed knee roots. These are cable roots which have grown-above the surface and then back down into the mud again, looking like small knees buried in mud. The red, stilt or spider mangrove (Rhizophora stylosa) has solved the problem of both stability and the need for oxygen by lifting itself out of the mud on root stilts.

The tangled growth of roots spread far and wide, providing anchors as well as a large surface area to absorb oxygen. Understanding what happens to mangrove forests when sea levels or the climate changes, as well as what happens when more mud gets deposited in the mangroves or eroded away is part of the work of AIMS biologist Joanna Ellison.

Humans can have an enormous influence on how well mangroves survive, and while mangrove plants are adapted to surviving in muddy, salty conditions, oil spills can spell disaster for them. Oil spills suffocate the trees' important oxygen obtaining pneumatophore roots, making it impossible for the plants to gain much needed oxygen. In addition, the effects of chemicals used to help clean up oil spills are still poorly understood. AIMS biologist Norm Duke is studying what happens to oil when it gets into a mangrove as well as helping to formulate some of the best methods for cleaning up a spill.

The research carried out by the scientists at the Australian Institute of Marine Science not only helps people better understand mangroves and the animals which are dependent on the mangroves during different phases of their life, but it also helps people around the world understand ways of protecting and managing mangroves for the benefit of all.

Further reading:
Field guide to the mangroves of Queensland
Mangroves index
Pollution

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Last updated - December 18, 2008

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