Mangroves grow on sheltered shores around most of Australia. They are most plentiful in the north where the large trees form dense forests on shore lines, river banks and estuaries. Fewer species are present in more southern latitudes. They are important as a food source, forming the base of an important food chain based on detritus and as shelter for young fish and invertebrates.

• To be a vascular plant.
• To be able to use salt water as a source of water for transpiration.
• To exclude most of the salt from the water you use and to excrete or store excess salt.
• To grow with your roots in soil that contains no dissolved oxygen.
• To produce seeds which have germinated on the tree and can start growing rapidly after falling and that float.
• To have special structures (pneumatophores) which let oxygen enter the roots at low tide, but exclude water at high tide , to survive tidal flooding.

Mangroves come in all shapes and sizes of trees and shrubs. All except one, the Nypa palm, are dicotyledons. Most people recognise them because they grow in the soft mud between the half tide and the high tide mark, and by the extraordinary development of special root systems, which support and anchor them. Part of the root system (pneumatophores) are always above the ground level and are able to take in oxygen during low tide.

Mangroves are found on protected "low energy" (ie. little surf action ) coasts in the tropical and subtropical areas of the world. Plant communities dominated by mangroves are called "mangal" communities. Distribution of Mangroves in Australia. The areas occupied by mangroves on the shore line in relation to the tide varies depending on the latitude, as does the distribution and abundance of species. On the east coast of Australia, the number of species varies from about 28 in the far north (Daintree), 5 on the Queensland border, 2 in the Sydney region and one south of Merimbula. The most widely distributed species is Avicennia marina, which occurs in all areas where mangroves are found including Western Port Bay (38o 45Õ S) and in Spencers Gulf and Gulf St. Vincent in S.A. These mangroves are the closest to the pole of any mangroves in the world. Scattered mangroves occur on the west coast as far south as Fremantle. Tropical mangrove communities are made up of closed forests of large trees up to 30m high. This is especially true where rainfall is high. In sub-tropical regions the trees are not as large and areas of hyper-saline mud flats occur. In temperate areas, mangroves are small trees or shrubs and large areas of salt marsh appear, and in the south, only salt marsh occurs. There are no mangroves in Tasmania.

Mangroves are an important source of food for marine creatures both through leaf and litter fall and through predation on their roots by burrowing animals. As plants, mangroves have the same requirements as other land plants - light, a source of water and a supply of dissolved mineral nutrients. As trees, they are able to display their leaves to the sun, and the water around their roots provides a constant nutrient supply, the only problem being that it contains too much salt to be directly useable. Mangroves are able to "filter out" the salt from the sea water using the solar energy which drives evaporation from their leaves. This is the special physiological ability which separates mangroves from other plants. They obtain minerals from the soil and from the sea water and it is probably the availability of mineral nutrients which limits their growth.

Mangrove products (leaves, twigs and seeds) are eaten by detrital feeders. Mangrove productivity is high compared to most other communities, producing up to 1 kilogram of carbon per square metre per year. The main products are leaves and twigs (fairly low in nutrition) and seeds, which are high in nutrition. Many mangroves produce very large seed crops. Fungi and bacteria are probably important in making the food available to higher animals.

Mangroves are seed-producing plants and the flowers of mangroves are often visually attractive. The seeds of mangroves tend to be large when compared with other trees and most, if not all, undergo germination while still attached to the tree. When a mangrove seed drops from the tree it usually floats for a short period of time and then sinks to the bottom. This period of floating enables it to be dispersed into areas where it has a chance of growing, as it can only grow where it will have its roots in tidal water. The rate of growth depends on climate and the availability of mineral nutrients. Seeds which establish themselves in cleared areas and where old mangrove root systems have broken down (releasing nutrients?) are observed to grow very quickly, whereas most other seeds which germinate among established trees die within a couple of years.

The complexity of the mangrove community depends on how close it is to the equator. On the tropical north east coast of Australia (Daintree and Johnstone Rivers ) about 30 species occur. A wide variety of algae are found around the mangrove roots. Many animals make their home in the mangroves, the most famous in Australia being the salt water crocodile. In the Sundarabans of India, the mangroves form the most important remaining refuge of the Bengal tiger.

There appears to be no direct economic exploitation of mangroves in Australia although in the past they have been used as a specialised timber source for boat building and oyster racks, as a source of tanning bark and for the manufacture of charcoal. They are, of course, important as a fish habitat. In some parts of the world there are communities which are completely dependent on the exploitation of the mangrove habitat and the clearing of mangroves for farming, timber getting and the establishment of commercial mariculture, particularly prawns, is a serious social and environmental problem. In Bangladesh, the removal of mangroves have contributed to increasing damage caused by cyclones. The Sundarabans encompass the delta of the Ganges, the largest delta in the world and are probably the most complex mangrove community known (64 species). In the Indian portion a large area is set aside as a reserve primarily for the preservation of the Bengal tiger, together with salt water crocodile, the fishing cat and the Gangetic dolphin. Entry to the inner portions is restricted with controlled utilisation of the outer regions. About half of the original area of mangroves (9600 km2) has been converted to agriculture but 4260 km2 remain as reserved forest, of which 2585 km2 is within the tiger reserve. Parts of the forest are actively managed for timber and firewood. "Dire poverty urges the people of the Sundarbans to frequent the forests in search of livelihood. Trespassers in turn take undue advantage, pilfering forest products and poaching wild animals Some of these poor people and the trespassers entering the forest fall prey to the man-eating tiger each year. That the vulnerable mangrove eco-system is under stress due to such interference is manifested by the restlessness and abnormal behaviour of the animals at times. Occasional forays of the tigers into the neighbouring villages are nothing but the direct consequences of the evils of human interference in the mangrove forests." quoted from "Protect Sundarabans" Dept. of Forests, Govt. of West Bengal. 1994.

One of the most extraordinary features of the mangrove is its root system. All mangroves have specialised structures which facilitate the entry of oxygen into the root system (pneumatophores). The extent of these structures varies from the simple knob-like protuberances which arise from the roots of Aegiceras corniculatum to the spectacular looping root system of Rhizophora stylosa and the giant woody pneumatophores of Sonneratia alba. Mangrove root systems can be very extensive. As an example, at Towra Point in Botany Bay near Sydney, live Avicennia marina pneumatophores have been found up to 28 metres away from the base of the nearest tree of a stand, even though, none of the trees exceed 4 metres in height. The weight of root system below the ground can be greater that the weight of the tree above the ground!