Mangroves

Photo - Mangroves near Cairns Airport - Cathy Ellis

Mangroves are sometimes seen as muddy swamps infested with mosquitoes and crocodiles. Removing mangroves was once seen as a sign of progress. So, what is the point of preserving them?

For a start, an estimated 75 per cent of fish caught in Queensland spend some time in mangroves or depend on food chains that can be traced back to these coastal forests.

Marine plants are now recognised as a vital community asset supporting important commercial, recreational and indigenous fisheries. Mangroves are an integral part of this community resource. All marine plants are protected under the Fisheries Act 1994 and may not be removed, damaged or destroyed without a prior approval.

Mangroves protect the coast by absorbing the energy of storm-driven waves and wind. The only two yachts undamaged by Cyclone Tracy in Darwin in 1974 were sheltered in a mangrove creek. In 2006, mangroves protected vessels and the coastline during Cyclone Larry in far north Queensland: the damage bill would have been much higher if not for the existence of intact mangrove forests. As well as providing a buffer for the land, mangroves also interact with the sea. Sediment trapped by roots prevent silting of adjacent marine habitats where cloudy water might kill corals or smother seagrass meadows. In addition, mangrove plants and sediments have been shown to absorb pollution, including heavy metals. Mangroves are also very effective at storing carbon.

Worldwide, vast tracts of mangroves have been destroyed to make way for unsustainable development; Queensland still has relatively large areas of Australia’s tallest and best-developed mangroves.

Mangroves’ economic and ecological importance has been recognised.

What is a mangrove?

Mangroves are plants or plant communities between the sea and the land in areas inundated by tides, usually at the mean high water level. They can take the form of trees, shrubs or palms. All share the ability to live in salt water. OzCoasts: Mangrove community dynamics conceptual model

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 (Rhyzophora 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 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: the zones correspond roughly to decreasing salinity levels and ranges of other factors. The adaptable grey mangrove tends to be found throughout river systems, including the upper limit of tidal influence where fresh water is abundant.

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 three ways.

Salt is prevented from entering the plant by filtering it out at root level. Some species can exclude more than 90 per cent of salt in salt water. 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.

Do mangroves need salt?

Mangroves do not appear to need salt to thrive. Growing in a salty environment means the mangroves lack competition. Only a limited number of plants have adapted to intertidal conditions.

The necessities of life

The richest mangrove communities occur in tropical and sub-tropical areas where the water temperature is greater than 24 ºC in the warmest month, where the annual rainfall exceeds 1250 mm and mountain ranges higher than 700 m 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 2 m — mangroves do not grow deep tap roots, probably because of the poor oxygen supply below the surface.

Roots have different functions and three 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 1.5–2 cm 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 per cent air spaces in their tissues, but 50 per cent 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 1 m, 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 20 cm 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 (Heriteria littoralis) has a prominent ridge on one side. This can act as a sail when the seed is in the water.

Mangrove uses

Australian coastal Aborigines and Torres Strait Islanders and people of South East Asia have had a very close association with mangroves. Mangroves have long functioned as a storehouse of materials providing food, medicines, shelter and tools.

Current legislation in Queensland protects all marine plants including mangroves from any damage or disturbance. The following examples are historical uses of mangroves.

Mangrove communities provide food directly and indirectly. Fish, crabs, shellfish, prawns and edible snakes and worms are found there. The fruit of certain species, including the nypa palm, can be eaten after preparation along with the nectar of some of the flowers. The best honey is considered to be that produced from mangroves, particularly the river mangrove (Aegiceras corniculatum).

Medicines are derived from mangroves. Ashes or bark infusions of certain species were applied to skin disorders and sores, including leprosy. Headaches, rheumatism, snakebites, boils, ulcers, diarrhoea, haemorrhages and many more conditions are traditionally treated with mangrove plants. The latex from the leaf of the blind-your-eye mangrove (Excoecaria agallocha) can cause blindness, but the powerful chemicals in it can be used on sores and to treat marine stings. The leaves are also used for fishing: crushing the leaves and dropping them in water stupefies fish which float to the surface.

Certain tree species, notably the cedar mangrove, cannonball mangrove (relatives of the red cedar) and the grey mangrove, are prized for their hard wood and used for boat building and cabinet timber as well as for tools such as digging sticks, spears and boomerangs. The fronds of the nypa palm are used for thatching and basket weaving. Various barks are used for tanning, pneumatophores (peg roots) make good fishing floats.

The wood from yellow mangroves (Ceriops) has a reputation for burning even when wet.

Facts and statistics

Number of species

Worldwide there are about 65 recognised species of mangrove plants belonging to 20 families. Up to 39 mangrove species and hybrids are known to occur in Queensland, although figures can change as the definition of a mangrove is not clear. Some plants, such as cottonwood for example, are not universally regarded as mangrove.

Distribution

Avicennia integra, Avicennia marina var. australasica, Excoecaria agallocha var. agallocha, Excoecaria agallocha var. ovalis, Acanthus ebracteatus, Acanthus ebracteatus subsp. ebarbatus are possibly found only in Australia.

Many 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 because the sheltered shallow waters of numerous estuaries are ideal for growth.

Mangrove forests occupy approximately 11,600 km² in Australia, 4600 km² of these are in Queensland.

Productivity

A teaspoon of mud from a north Queensland mangrove forest contains more than 10 billion bacteria. These densities are among the highest to be found in marine mud anywhere in the world and are an indication of the immensely high productivity of this coastal forest habitat.

Mangrove plants produce about 1 kg of litter (mainly leaves, twigs, bark, fruit and flowers) a square metre a year. Some of this is eaten by crabs, but most must be broken down by bacteria and fungi before the nutrients become available to other animals. Dividing sometimes every few minutes, bacteria feast on the litter, increasing its food value by reducing unusable carbohydrates and increasing the amount of protein: up to four times on a leaf that has been in seawater for a few months. Fish and prawns eat the partly decomposed leaf particles. They in turn produce waste which, along with the smallest mangrove debris, is taken up by molluscs and small crustaceans. Even dissolved substances are used by plankton or, if they land on the mud surface, are browsed by animals such as crabs and mud whelks.

This process is not confined to the mangroves. While some litter is recycled on the spot, this system is one of the few to export much of the organic matter it produces. Studies of the mangroves at the northern end of Hinchinbrook Island have shown that they export more than 12,500 t of litter a year into Great Barrier Reef waters. This material is deposited over 260 km² of seabed. Here bacteria densities are almost as high as those in the mangrove mud and they do much the same job, breaking down the litter to be consumed by bottom-living fauna, by prawns and by fish.

The seafood industry is the fifth largest primary industry in Queensland, with an annual commercial catch worth several hundred million dollars. An estimated 75 per cent of commercially caught fish and prawns depend directly on mangroves at some time in their lives or feed on food chains leading back there. Since those species making up the remainder of the catch probably also owe much to nutrients exported from the mangroves, these coastal forests can be seen as one of our major assets.

Protection and management

All marine plants are protected under the Fisheries Act 1994. The destruction, damage or disturbance of marine plants without prior approval from Fisheries Queensland is prohibited.

Approvals for works that may impact on marine plants for lawful activities such as building a boat ramp or jetty may be applied for under the Sustainable Planning Act 2009. Self-assessable codes may also apply for some maintenance or minor impact new works. See the Fisheries Queensland or Department of Infrastructure and Planning websites for further information.