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Grey mangrove

Grey mangrove

What are grey mangroves?

Mangroves are a group of trees that can grow along coastal areas in tidal salty water. Grey mangroves are the most common and widespread type of mangrove. They are found along the mainland coast of Australia. 

The scientific name of the grey mangrove is Avicennia marina.

What do grey mangroves look like?

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 eight centimetres long and five centimetres wide. They are oval shaped with a pointed tip. Arranged opposite one another along the stems, the tops of the leaves are glossy green whilst the undersides 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.

Where do grey mangroves grow?

Grey mangroves grow in intertidal zones of estuaries and river banks. An intertidal zone is the area between the high tide level and low tide level. Grey mangroves thrive in estuaries where there is a mix of salty and fresh water. They can grow in a range of soils from soft muds to sandy soils.

What eats grey mangroves?

The fallen leaves and seeds of mangroves support the food chain of the ecosystem. Mangroves produce huge amounts of leaf litter which is made up of  fallen mangrove leaves, twigs, bark, flowers and seeds. The leaf litter layer is broken down, or decomposed, by bacteria, fungi, algae and animals feeding on it. The decomposing material is eaten by detritivores. Detritivores are animals such as red-fingered marsh crabs, prawns and molluscs which feed on decaying and dead matter.

Structural adaptations

Growing in the mud can be challenging for plants as the soil doesn’t have enough oxygen for the roots to survive.  The pencil-like 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. Each pneumataphore has tiny pores along the outside, called lenticels, and spongy cells within the root to absorb oxygen and transport it to the roots.

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.

How do grey mangroves reproduce?

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 and have two folded leaves.

Hundreds of seeds are dropped by each tree. They float and are carried by the incoming tides. There is high mortality of the seeds due to being consumed by crabs and being carried to unsuitable locations.

Because the seeds partly germinate whilst still attached to the tree they can quickly establish themselves once they have settled into a favourable location. This adaptation of releasing partly germinated seeds is called vivipary.

Seedlings can stay as small seedlings with just a few leaves for many years waiting for an opening in the canopy to occur, such as from the fall of an old tree. 

Why are grey mangroves important?

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 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 fish. 

Preserving grey mangroves also benefits humans. 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 bees and beekeepers.

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