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Mangroves are a group of trees that can grow in salty water. Grey mangroves are the most common and widespread type of mangrove. They are found along the mainland coast of Australia. They are the only mangrove species that can grow in cooler climates, meaning they can grow as far south as Tasmania.
The scientific name of the grey mangrove is Avicennia marina.
Grey mangroves grow to 25 metres in height. They have a large trunk covered by light grey bark and have a spreading leafy crown.
The leaves grow up to 8 centimetres long and 5 centimetres wide. They are oval shaped with a pointed tip. The leaves are arranged opposite one another along the stems. The tops of the leaves are glossy green whilst the undersides (bottom) are pale grey and slightly hairy.
During the flowering season, grey mangroves produce clusters of small yellow flowers.
Grey mangroves have a distinctive root system featuring pneumatophores. Pneumatophores are spongy pencil-like roots which spread away from the base of the trunk and up through the soil.
Grey mangroves grow in intertidal zones of estuaries and river banks. This means they grow in the area between the high tide level and low tide level mark. They thrive best in estuaries where there is a mix of salty and fresh water. Grey mangroves can grow in a range of soils from soft muds to sandy soils.
The fallen leaves and seeds of mangroves support the food chain of the ecosystem. Mangroves produce huge amounts of leaf litter, which is all the fallen mangrove leaves, twigs, bark, flowers and seeds. The leaf litter layer is broken down, or decomposed, by bacteria, fungi and algae. The decomposed material is eaten by detritivores. Detritivores are animals, such as crabs, prawns and molluscs, which feed on decaying and dead matter.
Growing in the mud can be challenging for plants, as the soil doesn’t have enough oxygen for the roots to survive. The pneumatophores of grey mangroves act like snorkels, sticking up in the air from the roots beneath the soil to access oxygen directly from the atmosphere.
Like all plants, grey mangroves need fresh water to survive. However, grey mangroves grow in salty water. To ensure that only fresh water is used by the plant, grey mangroves have tiny pores on their roots that filter out salt as water enters the root system. Any excess salt that enters the plant is then excreted through special salt glands found on the surface of the leaves.
Grey mangroves produce flowers from mid to late summer. When pollinated, the flowers form flat, pale green, hairy fruits. Inside the fruits are seeds which have partially germinated (sprouted) and have two folded leaves. When the seeds are ready to settle into the soil, the fruits containing them drop into the water. Because the seeds germinate whilst still attached to the tree, they can quickly establish themselves once they have settled.
In the past, people viewed mangroves as wastelands and breeding grounds for mosquitoes. As a result, many Australian mangrove forests have been cleared or degraded.
It is important to preserve grey mangrove forests as their roots hold soil together and keep river banks and channels stable. The roots also trap fine sediments (soils) which helps keep the water clear. They act as a natural sea wall, protecting the shoreline from erosion and destruction caused by large waves, floods, storm surges, and winds.
Grey mangroves also provide habitat for waterbirds, shellfish, and juvenile (young) fish.
The habitat value of grey mangroves is used by humans to produce the food we eat. Grey mangroves are important habitat for many commercial fish species, and support oyster farms by collecting oyster spat (larvae). The flowers of grey mangroves are also a source of pollen for beekeepers.
ClimateWatch, n.d. Grey mangrove. [online] ClimateWatch. Available at: <https://www.climatewatch.org.au/species/plants/grey-mangrove>.
Lower Georges River Sustainability Initiative, 2012. Georges River mangroves. [online] Georges Riverkeeper. Available at: <https://georgesriver.org.au/learn-about-the-river/georges-river-mangroves>.
Queensland Government, 2018. Grey mangrove. [online] Department of Agriculture and Fisheries. Available at: <https://www.daf.qld.gov.au/business-priorities/fisheries/habitats/marine-plants-including-mangroves/common-mangroves/grey-mangrove>.
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