Grey Mangrove John Tann/Flickr

Grey Mangrove

Did You Know?

Trees found to be more than 700 years old, tells us about sea levels of ancient shorelines and the stability of the location

Large to small tree or shrub up to 25 m high (commonly around 5 m), branches, flower heads and lower surface of leaves greyish or silvery

Pneumatophores (erect, pencil-like aerial roots specialised for gaseous exchange) are numerous and project from shallow lateral roots.

Leaves

Leathery and measure up to 16 cm in length and 5 cm in width. They are ovate, pointed and arranged opposite one another on the stems. The leaves are glossy green above with a distinctive pale and slightly hairy, grey underside. Stomata (pores) and salt glands are scattered over the entire leaf surface but are more abundant on the underside. Leaves are often covered in crusted salt from the exuded secretions.

Flowers

Yellow-orange or golden. Flower clusters dense, arranged at leafy shoot tips; flower stalks 10 – 30 mm long. Flower 4 - 8 mm long with a corolla of four fused pointed petals 3 - 7 mm wide.

Fruits/Seeds

Fruit capsule soft, pubescent, pale green, flattened, 15 – 30 mm diameter; no seed stage with viviparous development; enclosed mostly solitary propagule has two fleshy cotyledons (seed leaves) with plumule and hairy young root between. Propagule matures and drops in advanced germination to allow for dispersal and quick establishment once the pod settles.

Behaviour

Phenological events are considered site specific, depending on temperature (linked to latitude) and moisture conditions.

Flowering

Generally occurs once a year in mid to late summer and progressively later in higher latitude sites. In far northern parts of Australia, flowering occurs around November and December. And in furthest southern parts of Australia, flowering generally occurs around May and June.

Propagules

Maturation (note, there are no seeds produced as development is viviparous) is more rapid in warmer climates, taking 3 - 4 months in low latitudes and 8 - 9 months in southern locations. Generally mature between January and April but in intermediate locations like the Brisbane River this can occur in August.

Propagules germinate and develop while attached to the parent tree (a process called vivipary) which facilitates quick establishment once the pod settles. As a pioneer species, grey mangroves are notably tolerant of a wide range of saline conditions. Plants deal with harmful salt taken up through their roots by actively expelling the salt from specialised secretion glands on their leaves, and by passing the salt to senescent leaves and bark. While grey mangroves can also withstand short periods of inundation by fresh water or hypersaline water, they are notably impacted when conditions change over more prolonged periods.

Field Guide

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What to Observe

Presence of trees/seedlings
Notable new leaf growth (smaller developing young leaves)
Many old yellowing leaves on the tree and dropping
Leaf galls and growths present on leaves and foliage
Flowers present on trees (first flowering, full flowering and end of flowering)
No flowering observed on trees
Mature or near mature propagules present on trees
Abundant propagule maturation on trees (record all days)
No propagules observed on trees
Both flowers and mature propagules present on trees
Neither flowers and mature propagules present on trees
Detached mature propagules observed on the ground, or floating
Visitor insects and/or birds on open flowers
Location of large, old trees with stem diameters < 1 m

Climate Adaptations

Mangroves are important trees, playing a significant role in providing food and resources for people and animals, nursery fishery habitat, protecting coasts, and storing huge amounts of carbon. Rising sea surface temperatures, heatwaves, droughts and sea level rise from climate change seriously threaten widespread mangrove species.

We expect grey mangroves to start shooting, flowering and fruiting earlier since they have been shown to be dependent on temperature and other climate factors. They may start appearing in new areas, as warmer conditions enable them to live in environments that were previously unsuitable or unreachable. They may also die from conditions exceeding their natural tolerances, like the mangrove forests of northern Australia did in late 2015.

When and Where

When To Look

Year round is useful
Germinating propagules may fall and establish on sand and mud banks at any time
Flowering mostly occurs in mid to late summer, but plants may flower and fruit at any time

Where To Look

Widespread along the mainland coast of Australia
Intertidal margins of estuaries, tidal river bank shorelines and brackish river areas
They occupy a range of substrates from soft muds to sandy soils, and rocky to coral shorelines
May grow with river, red and other mangrove species
Commonly colonise developing mud banks

What Else?

It is traditionally used to treat sting-ray and stonefish ‘stings’ (Milingimbi); ringworms, sores and boils (Yirrkala); scabies (general); 'cheeky' mangrove worm medicine for coughs (Tiwi).

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