Avicennia marina, commonly known as grey mangrove or white mangrove, is a species of mangrove tree classified in the plant familyAcanthaceae. As with other mangroves, it occurs in the intertidal zones of estuarine areas.
In Australia, it extends much farther south than other mangroves, occurring in every mainland state. Its distribution is disjunct in Western Australia; the population of the Abrolhos Islands is further south than the nearest population of Shark Bay. Another mangrove system is found even further south at Bunbury. This colonisation of southerly climes may have occurred relatively recently, perhaps several thousand years ago, when they were transferred by the Leeuwin Current. The most inland occurrence of mangroves in Australia is a stand of grey mangroves in the Mandora Marsh, some 60 km from the coast. In South Australia along the Barker Inlet and Port River in Gulf St Vincent, as well as in sheltered bays in Spencer Gulf and the west coast of Eyre Peninsula, A. marina forests form hatcheries for much of the state's fish and shellfish commercial and recreational fisheries.
Description
Grey mangroves grow as a shrub or tree to a height of, or up to 14 m in tropical regions. The habit is a gnarled arrangement of multiple branches. It has smooth light-grey bark made up of thin, stiff, brittle flakes. This may be whitish, a characteristic described in the common name. The leaves are thick, long, a bright, glossy green on the upper surface, and silvery-white, or grey, with very small matted hairs on the surface below. As with other Avicennia species, it has aerial roots ; these grow to a height of about 20 cm, and a diameter of 1 cm. These allow the plant to absorb oxygen, which is deficient in its habitat. These roots also anchor the plant during the frequent inundation of seawater in the soft substrate of tidal systems. The flowers range from white to a golden yellow colour, are less than 1 cm across, and occur in clusters of three to five. The fruit contains large cotyledons that surround the new stem of a seedling. This produces a large, fleshy seed, often germinating on the tree and falling as a seedling. The grey mangrove can experience stunted growth in water conditions that are too saline, but thrive to their full height in waters where both salt and fresh water are present. The species can tolerate high salinity by excreting salts through its leaves. Grey mangrove is a highly variable tree, with a number of ecotypes, and in forms closely resembling other species. It has been reported to tolerate extreme weather conditions, high winds, and various pests and diseases. It is a pioneer in muddy soil conditions with a pH value of 6.5 to 8.0, but is intolerant of shade. A number of botanists have proposed division of the species, but currently three subspecies are recognised:
