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 specialized 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.
Fruit capsule soft, pubescent, pale green, flattened, 15–30 mm diam.; 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.
Phenological events in the Grey Mangrove 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.
Propagule 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-most locations. Propagules generally mature between January and April – but, in intermediate locations like the Brisbane River, this can occur in August. The 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 freshwater or hypersaline water, they are notably impacted when conditions change over more prolonged periods.
What to Observe
- Grey mangrove trees and/or seedlings present
- Notable new leaf growth (smaller developing young leaves) present
- Many old yellowing leaves on the tree and dropping
- Leaf galls and growths present on leaves and foliage
- Flowers present on trees (record all days; 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 greater than one metre
ClimateWatch Science Advisor
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 important and widespread mangrove species.
With climate change warming the earth, we expect grey mangrove plants to start shooting, flowering and fruiting earlier in the year since they have been shown to be dependent on temperature and other climate factors. They may also start appearing in new areas, as warmer conditions enable them to live in environments that were previously too cold, or un-reachable. They may also die from conditions exceeding their natural tolerances, like the mangrove forests of northern Australia did in late 2015 (see links to articles below).
Help scientists answer the question: "How are our animals, plants and ecosystems responding to climate change?"
When To Look
Grey mangroves occur commonly in 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. Grey mangroves are may grow with river, red and other mangrove species. As a key pioneer species, grey mangroves commonly colonise developing mud banks. Keep an eye out for seedlings too! These are our indicator of newly establishing stands.
We are also interested in recording the locations of large old grey mangrove trees. We have found trees more than 700 years old. Their presence tells us a lot about sea levels of ancient shorelines, as well as the stability of the location. So, if you find any trees with trunks greater than one metre in diameter, please take a GPS location record and photograph, and send these to us. We will use the diameter as a proxy to estimate the trees’ age. Your contribution will be acknowledged and the record added to the national database.
Note: ClimateWatch is looking for any changes outside of their known ranges so remember to keep a lookout beyond these regions too!
Where To Look
Year-round observation is useful. Germinating propagules may fall and become established on sand and mud banks at any time. And, while flowering mostly occurs in mid to late summer, there are important exceptions already, so plants may flower and fruit at any time. Let us know what the situation is in your area.
Note: ClimateWatch is looking for any changes in the timing of these events so remember to keep a lookout all year!
Duke, N.C., J.S. Bunt, and W.T. Williams 1984. Observations on the floral and vegetative phenologies of north-eastern Australian mangroves. Australian J. Botany 32: 87-99.
Duke, N.C. 1990. Phenological trends with latitude in the mangrove tree Avicennia marina, Journal of Ecology 78, 113-133.
Duke, N.C. 1992. Mangrove Floristics and Biogeography, pp. 63-100. In A.I. Robertson and D.M. Alongi, (eds.), Tropical Mangrove Ecosystems, Coastal and Estuarine Studies Series, American Geophysical Union, Washington, D.C., 329 pages.
Duke, N. C. 2001. Mangrove phenologies and the factors influencing them in the Australasian region, pp. 217-233. In L. D. de Lacerda (ed.), ‘Mangrove Ecosystems: Function and Management’. Berlin, Springer-Verlag. 292 pages.
Duke, N.C. 2006. ‘Australia’s Mangroves’. The authoritative guide to Australia’s mangrove plants. University of Queensland and Norman C Duke, Brisbane, 200 pages.
Duke, N.C. 2012. Mangroves of South-East Australia. In: ‘Estuary plants and what's happening to them in South-East Australia’. G. Sainty, J. Hosking, G. Carr and P. Adam. Potts Point, NSW, Sainty and Associates Pty Ltd: 134-155.
Duke, N.C. 2016. ‘Mangrove Click! Australia: expert ID for Australia's mangrove plants’. Currumbin, MangroveWatch Publication. ISBN: 978-0-9923659-2-9. https://itunes.apple.com/us/app/mangrove-au/id1157235522?mt=8
Duke, N.C. 2017. Climate calamity along Australia's gulf coast. Landscape Architecture Australia 153: 66-71.
Duke, N.C. 2017. Love at low tide: why mangroves deserve our admiration. Wildlife Australia, 54 (3). pp. 29-33.
Duke, N.C., and A.J. Watkinson. 2002. Chlorophyll-deficient propagules of Avicennia marina and apparent longer term deterioration of mangrove fitness in oil-polluted sediments. Marine Pollution Bulletin 44: 1269-1276.
Duke, N.C., J. Mackenzie, A. Wood and D. Burrows. 2015. New partnership network helps to protect Australian mangroves and saltmarsh. Wetlands Australia. National Wetlands Update 27: 48-50.
Duke, N.C. and K. Schmitt. 2016. Mangroves: Unusual Forests at the Seas Edge. ‘Tropical Forest Handbook’. L. Pancel and M. Köhl. Berlin Heidelberg, Springer-Verlag: 2: 1693-1724.
Duke, N. C., J. M. Kovacs, A. D. Griffiths, L. Preece, D. J. E. Hill, P. v. Oosterzee, J. Mackenzie, H. S. Morning and D. Burrows. 2017. Large-scale dieback of mangroves in Australia’s Gulf of Carpentaria: a severe ecosystem response, coincidental with an unusually extreme weather event. Marine and Freshwater Research, 68 (10): 1816-1829 http://dx.doi.org/10.1071/MF16322
Experts consulted: MangroveWatch; ClimateWatch Science Advisory Panel
Did You Know?
Avicennia marina is the most common and widespread mangrove found along the mainland coast of Australia.
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).
It exhibits latitudinal shifts in flowering and propagule maturation.