Mangroves

Some people see mangroves as muddy, mosquito and crocodile infested swamps, and in the past the removal of mangroves was seen as a sign of progress. So what is the point of preserving them?

For a start, an estimated 75 percent of fish caught commercially spend some time in mangroves or are dependent on food chains which can be traced back to these coastal forests.

Mangroves also protect the coast by absorbing the energy of storm driven waves and wind. The only two yachts undamaged by Cyclone Tracey in Darwin in 1974 were sheltered in a mangrove creek. While providing a buffer for the land on one side, mangroves also interact with the sea on the other. Sediments trapped by roots prevent silting of adjacent marine habitats where cloudy water might cause corals to die. In addition, mangrove plants and sediments have been shown to absorb pollution, including heavy metals.

Worldwide, vast tracts of mangroves have been destroyed so we are lucky to have relatively large areas of Australia’s tallest and best-developed mangroves still existing on our doorstep.

Now that their economic and ecological importance has been recognised we carry the responsibility to look after our mangroves.

What is a mangrove?

A mangrove is a woody plant or plant community that lives between the sea and the land in areas inundated by tides. Thus, a mangrove is a species as well as a community of plants. The species can be a tree, but it can also be a shrub or palm. All share the ability to live in salt water.

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 species because of its ability to tolerate low temperatures and a variety of other intertidal conditions. A pioneer, it is likely to be the first species to grow on newly-emerged mud banks, putting up its distinctive peg roots.

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 might be inundated only by periodic spring tides at the times of new and full moons. The soil will be firmer, but it contains more saline due to the evaporation of water leaving behind salt which 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 can appear again, while less saline soils might be covered with a thick forest of the orange mangrove (Bruguiera) species.

A number of factors may determine what happens to the landward side of this zone. In conditions of high rainfall — as occurs in north Queensland, particularly in the Daintree — regular flooding may lead to freshwater swamp areas dominated by the less salt-tolerant littoral margin (shore) species (such as cottonwood Hibiscus tiliaceus and Barringtonia acutangula).

Behind this may be a zone of paperbark swamps or wetlands and the beautiful flaky-barked red beech or golden guinea tree (Dillenia alata) as littoral (shore) vegetation merges into rainforest.

In areas of high seasonal rainfall, such as the Gladstone to Townsville region, the reverse may be the case, with evaporation and little fresh water input leading to an increase in salinity. This 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 corresponding roughly to decreasing salinity levels and ranges of other factors. The ever-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 plenty of material to build up the banks. Red mangroves (Rhizophora stylosa) are frequently found here. While there are certain patterns to mangrove zone development, local conditions will always dictate which mangroves are found and where.

Coping with salt

• The first line of defence for many mangroves is to prevent much of the salt from entering by filtering it out at root level. Some species can exclude more than 90 percent of salt in seawaters (Rhizophora, Ceriops, Bruguiera and Osbornia species are all ‘salt-excluders’).
• Another method is for the mangroves to quickly excrete salt that has entered the system. The leaves of many mangroves have special salt glands that are among the most active salt-secreting systems known. It is quite possible to see or taste the salt on the leaf surfaces of species that choose this method. Examples of ‘salt-secretors’ include Avicennia, Sonneratia and Acanthus.
• A third method of coping with salt is to concentrate it in bark or in older leaves which carry it with them when they drop. Lumnitzera, Avicennia, Ceriops and Sonneratia species all use this method.

As can be seen from the examples given, some mangroves use only one of these methods but many use two or more. In addition, a number of features serve to prevent water loss from the plant. These include a thick waxy cuticle (skin on the leaf) or dense hairs to reduce transpiration — the loss of water. 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?

It seems that the answer is ‘no’. As an experiment, some species have been kept in pots where they have grown and flowered regularly when given only fresh water. However, experiments have also shown that the best growth occurs where the plants live in sea water diluted half and half with fresh water.

One particular advantage to growing in a salty environment is the lack of competition! Only a limited number of plants have invested evolutionary energy into adapting to intertidal conditions. In the optimum conditions of a tropical rainforest, diversity is great and competition fierce. On the edge of the sea (in Australia) about 38 species of mangroves have exclusive occupancy.

The necessities of life

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

In addition, they need protection from high energy waves that can erode the shore and prevent seedlings from becoming established. In north Queensland, the Great Barrier Reef performs this function, while 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. Apart from suddenly altering the salinity levels, these fluctuations in water can alter temperatures as well.

Different mangrove species have different requirements. Some are more tolerant of salt than others. Other factors that affect their distribution include wave energy, soil oxygen levels, drainage and differing nutrient levels. Where one species finds its preferred conditions — or at least those which it is able to tolerate better than other plants — it tends to become dominant. This has led to quite clear zones among mangroves.

Water-logged!

Apart from coping with salt, mangroves also face common problems of water-logged, unstable and oxygen deficient soils. Despite belonging to many different families mangrove plants have come up with surprisingly similar solutions.

Roots

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 simply 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 metres — mangroves do not seem to grow deep tap roots, probably because of the poor oxygen supply below the surface.

Roots have different functions. Radiating cable roots, punctuated by descending anchor roots, provide support. From this framework sprout numerous little nutritive roots which feed on the rich soil just below the surface. The third type of roots collects the oxygen.

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

There is always a danger that the breathing roots of mangroves may become covered as sediments accumulate. Under normal conditions sediments build up at the rate of 1.5–2cm a year. To avoid being buried, the roots can grow vertically. Oil, in particular, can be fatal to plants. Once covered with it, lenticels can no longer draw in air and the plant can suffocate.

Different species have developed different ways of keeping their roots in the air.

Red, stilt or spider, mangrove (Rhizophora stylosa) is commonly found close to the seaward side of mangroves. It is, therefore, subjected to high wave energy and has developed a system of stilt or prop roots. These spread far and wide, providing numerous 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 percent air spaces in their tissues, but below this changes to 50 percent.

Grey mangrove Avicennia marina grows a series of snorkels or peg/pencil roots, known as pneumatophores. Experiments with a related Avicennia species have shown that those 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 and/or seed(ling)s of all mangrove plants can float, which is, of course, an excellent sea dispersal mechanism for plants that live along coastal waters.

Members of the Rhizophoraceae family (Rhizophora, Bruguiera and Ceriops species) have an intriguing viviparous method for successfully reproducing themselves. 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 called propagules. They may grow in place, attached to the parent tree, for one to three years, reaching lengths of up to one metre, before breaking off from the parent and falling into the water.

These seedlings then travel in an intriguing way. In buoyant sea water they lie horizontally and move quickly. On reaching fresher (brackish) water however, 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 (Rhizophora) 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.

Some other species (Avicennia, Aegialitis and Aegiceras) also produce live seedlings but these are still contained within the seed coat when it drops from the plant. The seed of Avicennia floats until this coat drops away. Interestingly, 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 (known as vivipary) is rare in plants other than mangroves. It occurs in a few seagrass species and a few succulents such as Agave species. It is possible that the well-developed seedling has a greater chance of surviving, once it has taken root, in a situation where it is likely to be battered by water-borne objects.

The presence of many mangrove species that do not produce viviparous seedlings shows this strategy is not strictly 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 means the seeds with a big storage capacity might survive longer.

The cannonball mangrove (Xylocarpus granatum) produces a large fruit that is 20cm in diameter and contains 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

Mangroves have long functioned as a storehouse of materials providing food, medicines, shelter and tools. Fish, crabs, shellfish, prawns and edible snakes and worms are found there. The fruit of certain species, including the nypapalm, 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).

Numerous medicines are derived from mangroves. Ashes or bark infusions of certain species can be 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; when crushed and dropped in water, fish are stupefied and float to the surface. This sap is currently being tested for its medical properties and may play a part in western medicine.

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, while the wood from yellow mangroves (Ceriops species) has a reputation for burning even when wet.

Facts and stats

How many species are there in the world?

Worldwide there are about 65 recognised species of mangrove plants belonging to 20 families. Up to 35 mangrove species and 3 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.

How many species are there in Cairns?

A study of Cairns mangroves found 24 mangrove tree and shrub species, while a further 18 species of flowering plants were growing among the mangroves or on salt marshes. An additional 42 species of epiphytic plants and 25 species of fungi were identified growing on the mangroves.

Australian endemics

Two mangrove species — Avicennia integra and Avicennia ovalis — are possibly found only in Australia. Many others occur widely throughout the Indo-West Pacific region. Some, such as the red mangrove (Rhizophora stylosa), are best developed in Australia.

Australian diversity

The north-east coast of Australia is home to the greatest diversity of mangroves and associated plants. This is because this region was close to the centre of origin and dispersal of mangroves, because 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 square kilometres in Australia, 4600 square kilometres of these in Queensland.

What’s in a name?

The origin of the name ‘mangrove’ is not certain. It could be a combination of the Portuguese ‘mangue’, meaning an individual mangrove tree, with the English ‘grove’, although early versions were ‘mangrove’ and ‘mangrave’. It may also be derived from the Malay ‘manggi-manggi’ or ‘mangin’.

The colours included in the common names of many mangrove trees often refer either to the bark — for example, grey mangrove — or to the blaze, the colour that shows when the bark is scraped — for example, the red mangrove.

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.

Food chain

Mangrove plants produce about 1kg 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 before the nutrients become available to other animals. That is where the bacteria and fungi come in. Dividing sometimes every few minutes, they 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. Partly decomposed leaf particles, loaded with colonies of protein-rich micro-organisms, are then eaten by fish and prawns. 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. Every time the tide retreats it carries a cargo of food out to sea. Studies of the mangroves at the northern end of Hinchinbrook Island have shown that they export more than 12,500 tonnes of litter a year into Great Barrier Reef waters. This material is deposited over 260 square kilometres 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 percent 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.

Last updated: 19 October 2007