Mangroves

Distribution

Some wetland species (Avicennia integral, Avicennia marina var. australasica, Excoecaria agallocha var. agallocha, Excoecaria agallocha var. ovalis, Acanthus ebracteatus, Acanthus ebracteatus subsp. ebarbatus) are possibly found only in Australia while others occur widely throughout the Indo-West Pacific region.

The north-east coast of Australia is home to the greatest diversity of mangroves and associated plants. This region was close to the centre of origin and dispersal of mangroves. The climate is similar to that under which they first evolved, and the sheltered shallow waters of numerous estuaries are ideal for growth.

The distribution of mangroves has been mapped through the Queensland wetland mapping and more details can be found in the intertidal and subtidal mapping.

Zones in the mangroves

As a general rule, zones of dominant mangrove species run parallel to the shoreline or to the banks of tidal creek systems. The seaward side of the community is likely to be dominated by a fringe of grey mangroves (Avicennia marina) as it is best adapted to early colonisation and a wide range of soil conditions. Avicennia marina is a tough mangrove species; it is Australia’s most common mangrove because of its ability to tolerate low temperatures and intertidal conditions. A pioneer species, it is likely to be the first to grow on newly emerged mud banks.

Mangrove apple (Sonneratia alba) often grows in this zone as well, but it is a more tropical mangrove. The red mangrove (Rhizophora stylosa), also known as the stilt or spider mangrove, is usually found behind this zone where its long prop roots give it a firm foothold against wind and waves.

The next zone towards land is inundated only by periodic spring tides. The soil is firmer and is more saline because water evaporation leaves behind salt that will not be diluted until the next spring tide. The more specialised yellow mangrove (Ceriops) species can be found in this zone, although conditions usually make it impossible for anything other than saltmarshes or saline herblands with succulent plants to thrive here. The resilient grey mangrove might be found here and less saline soils might be covered with the orange mangrove (Bruguiera) species.

A number of factors determine what happens in the next zone towards the land. In high rainfall (as in north Queensland, particularly in the Daintree) regular flooding may lead to freshwater swamp areas being dominated by the less salt-tolerant littoral margin species (such as cottonwood Hibiscus tiliaceus and Barringtonia acutangula) that are not mangrove species.

In areas of high seasonal rainfall, such as the Gladstone to Townsville region, evaporation and little fresh water input might lead to increased salinity. The result could be a saltmarsh or salt flat zone where only the toughest yellow mangrove (Ceriops tagal), club mangrove (Aegialitis annulata) and grey mangrove (Avicennia marina) grow in patches bordering coastal saline herblands.

There is a similar change of species along rivers, where the zones relate to decreasing salinity levels and ranges of other factors. The adaptable grey mangrove (Avicennia marina), Excoecaria agallocha and Aegiceras corniculatum tend to be found throughout river systems, including the upper limit of tidal influence where fresh water is abundant.

Shrubs and herbs are rare in the mangroves, although very sparse Tecticornia spp. and Sporobolus virginicus may occur in the ground layer[2][1].

The greatest concentration of mangrove species is usually at the mouth of tidal creeks and rivers where salt and fresh water mix in ideal proportions and floodwaters deposit material to build up the banks. Red mangroves (Rhizophora stylosa) are frequently found here. Although there are overall patterns to mangrove zone development, local conditions will always dictate which mangroves are found where.

Coping with salt

Mangroves cope with salt in 3 ways:

• Salt is prevented from entering the plant by filtering it out at root level. Some species can exclude more than 90% of salt in saltwater. Rhizophora, Ceriops, Bruguiera and Osbornia species are all ‘salt excluders’.
• Salt is quickly excreted after it has entered the system. The leaves of many mangroves have special salt glands that are among the most active salt secreting systems known. You can see or taste the salt on the leaf surfaces of species that do this. Avicennia, Sonneratia and Acanthus are ‘salt secretors’.
• Salt concentrates in bark or in older leaves and is removed when the leaves fall e.g. Lumnitzera, Avicennia, Ceriops and Sonneratia.

Some mangroves use only one of these methods but many use two or more.

In addition, mangrove features prevent water loss. A thick waxy cuticle (skin on the leaf) or dense hairs reduce transpiration (water loss). Most evaporation loss occurs through stomata (pores in the leaves) so these are often sunken below the leaf surface where they are protected from drying winds. Leaves are also commonly succulent, storing water in fleshy internal tissue.

The necessities of life

The richest mangrove communities occur in tropical and subtropical areas where the water temperature is greater than 24 ºC in the warmest month, where the annual rainfall exceeds 1250mm and mountain ranges higher than 700m are found close to the coast.

Mangroves need protection from high energy waves that erode the shore and prevent seedlings from becoming established. In north Queensland, this protection comes from the Great Barrier Reef; to the south a chain of sand islands provide shelter. Shallow, gently-shelving shores allow mangrove seedlings to anchor, particularly in estuaries, rivers and bays.

Mangroves exist in a constantly changing environment. Periodically the sea inundates the community with salty water while, at low tide, especially during periods of high rainfall, it may be exposed to floods of fresh water. As well as suddenly altering the salinity levels, these fluctuations can alter growing medium temperatures as well.

Adaptation to conditions

As well as salt, other factors that affect mangrove distribution include wave energy, waterlogging, unstable and oxygen-deficient soils, drainage and nutrient levels. Where one species finds tolerable conditions, it tends to become dominant. This has led to the clear zonation among mangrove species. Mangroves have adapted to cope with these conditions.

Roots

Mangroves roots perform a number of functions for a plant, they support it and they obtain essential nutrients and oxygen.

In unstable, sometimes semi-fluid, soil an extensive root system is necessary to keep the trees upright. As a result, most mangroves have more living matter below the ground than above it. The main mass of roots, however, is generally within the top 2m—mangroves do not grow deep tap roots, probably because of the poor oxygen supply below the surface.

Roots have different functions and 3 different forms. Radiating cable roots, punctuated by descending anchor roots, provide support. From this framework sprout many little nutritive roots that feed on the rich soil just below the surface and collect oxygen.

Little oxygen is available in fine, often waterlogged, mud. Many mangroves adapt by raising part of their roots above the mud. These roots are covered with special breathing cells (lenticels) which draw in air. The lenticels are connected to spongy tissue within the roots. When the roots are submerged by water, the pressure within these tissues falls as the plant uses up the internal oxygen. The resulting negative pressure means that when the root is re-exposed when the tide drops, more air is drawn in through the lenticels.

The breathing roots of mangroves can become covered as sediments accumulate. Under normal conditions sediments build up at the rate of 1.5–2cm a year. To avoid being buried, species have developed different ways of keeping their roots in the air.

Red mangrove (Rhizophora stylosa) is commonly found close to the seaward side of communities. It is therefore subjected to high wave energy and has developed a system of stilt or prop roots. These spread far and wide, providing anchors for the tree as well as a large surface area for oxygen-absorbing lenticels. In common with other species, this mangrove also grows aerial roots (extra stilts) which arise from the branches or trunk. Studies have shown that these aerial roots alter dramatically in structure when they reach the mud: above it they have about 5% air spaces in their tissues, but 50% below.

Grey mangrove (Avicennia marina) grows a series of snorkels or peg or pencil roots, (pneumatophores). Experiments with related Avicennia species have shown that plants growing in coarse coral sand, with a good air supply to the roots, were able to survive after their pneumatophores were removed. However, those living in poorly aerated soil died when the pneumatophores were covered. In one situation, where they were covered with oil, the plants responded by growing aerial roots.

Orange mangrove (Bruguiera gymnorrhiza) develops knee roots. These are cable roots that have grown above the surface of the mud and then down into it again.

Looking glass mangrove (Heritiera littoralis) produces buttressed roots that are flattened, blade-like stilt roots.

Cannonball mangrove (Xylocarpus granatum) is buttressed, but the cable roots also appear above the ground in the fashion of knee roots.

Shoots

The fruits, seedlings and seeds of mangrove plants can float, an excellent dispersal mechanism for plants that live along coastal waters.

The Rhizophoraceae family (Rhizophora, Bruguiera and Ceriops species) successfully reproduce themselves viviparously. Fertilised seeds do not drop from the plants but begin to germinate, growing out from the base of the fruits to form long, spear-shaped stems and roots (propagules). They can grow in place, attached to the parent tree, for one to three years, reaching lengths of up to 1m, before breaking off from the parent and falling into the water.

These seedlings have evolved to travel in ways that change with water salinity. In buoyant salt water they lie horizontally and move quickly. On reaching fresher (brackish) water they turn vertically, roots down and lead buds up, making it easier for them to lodge in the mud at a suitable, less salty site. Some species of these floating seedlings can survive in a state of suspended animation for up to a year in the water. Once lodged in the mud they quickly produce additional roots and begin to grow.

Avicennia, Aegialitis and Aegiceras species also produce live seedlings but these are still contained within the seed coat when they drop from the plant. The seed of Avicennia floats until this coat drops away. The speed with which this happens depends on the temperature and salinity of the water. In water of high or low salinity the seed coat is slow to drop off, but in brackish water it is shed quickly allowing the seedling to lodge in the favoured habitat of this species. Higher temperatures also favour faster action. Avicennia seeds can stay alive in the water for only three to four days.

The production of live seedlings (vivipary) is rare in plants other than mangroves and many mangrove species do not produce viviparous seedlings so this strategy is not necessary for successful reproduction. However, all mangrove fruits and seeds are large, which suggests that bigger fruits and seedlings have a better chance of survival. It also suggests the seeds with a big storage capacity survive longer.

The cannonball mangrove (Xylocarpus granatum) produces a large fruit 20cm in diameter containing up to 18 tightly packed seeds. On ripening it explodes, scattering the seeds which float away on the tide. They often end up on mainland and island beaches.

The seed of the looking-glass mangrove (Heritiera littoralis) has a prominent ridge on one side. This can act as a sail when the seed is in the water.

Additional information

• The State of the World’s Mangroves 2022
• Global Mangrove Watch
• The Mangrove Restoration Tracker Tool
• Data collection protocol for mapping and monitoring mangrove communities in Queensland
• OzCoasts conceptual models
• Mangrove Watch
• Queensland Museum Mangrove Challenge
• The Nature Conservancy Policy Brief: Securing The Future Of Mangroves
• What's that smell? Seasonal mangrove odours
• Mangroves - City of Gold Coast
• Mangrove odours on Redlands Coast

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References

1. ^ Neldner, VJ, Wilson, BA, Thompson, EJ & Dillewaard, HA (2019), Methodology for survey and mapping of regional ecosystems and vegetation communities in Queensland. Version 5.0. [online], Queensland Herbarium, Queensland Department of Environment and Science, Brisbane. Available at: https://www.publications.qld.gov.au/dataset/redd/resource/6dee78ab-c12c-4692-9842-b7257c2511e4.
2. ^ Specht, RL (1970), Vegetation. In ‘The Australian Environment’.(Ed. GW Leeper.) pp. 44–67, CSIRO and Melbourne University Press: Melbourne.

Last updated: 14 November 2019