Tropical Topics A n i n t e r p r e t i v e n e w s l e t t e r f o r t h e t o u r i s m i n d u s t r y Mangroves I - the plants No. 19 April 1994 Mangroves - coping with salt Notes from the Mangroves are plants which live between the sea and the land. A mangrove is not a species, but rather the name given to a community of unrelated plants living in Editor areas which are inundated by tides. Thus a mangrove may be a tree but (like a Many people don’t like mangroves, ‘rainforest plant’) may also be a shrub, palm, fern, climber, grass or epiphyte - regarding them as muddy, mosquito- all of them sharing the ability to live in salt water. and crocodile-infested swamps. Their removal is seen as a sign of progress. Do mangroves need salt? It seems that The first line of defence, for many So what is the point of preserving the answer is no. Some species have mangroves, is to stop much of the salt them? For a start, it has been estimated been kept in pots where they have from entering at all by filtering it out at that up to 75 percent of fish caught grown happily and flowered regularly root level. Some species can exclude commercially either spend some time in when given only fresh water. more than 90 percent of salt in sea the mangroves or are dependent on However, experiments have also water. (Rhizophora, Ceriops, food chains which can be traced back shown that the best growth occurs Bruguiera and Osbornia species are to these coastal forests. where the plants live in sea water all ‘salt-excluders’.) diluted by about 50 per cent with fresh Mangroves also protect the coast by water. Another trick, is to quickly excrete salt absorbing the energy of storm-driven which has entered the system. The waves and wind. The only two yachts One particular advantage to growing leaves of many mangroves have undamaged by Cyclone Tracy in 1974 in a salty environment is the lack of special salt glands which are among had sheltered in a mangrove creek. competition! Only a limited number of the most active salt-secreting systems While providing a buffer for the land plants have invested evolutionary known. It is quite possible to see and/ on one side, mangroves also protect energy into adapting to such harsh or taste the salt on the leaf surfaces of the sea on the other. Sediments trapped conditions. In the optimum conditions species which choose this method. by roots prevent siltation of adjacent of a tropical rainforest, diversity is (‘Salt-secreters’ include Aegiceras, marine habitats where cloudy water great and competition fierce. On the Avicennia, Sonneratia and Acanthus.) may cause the death of corals. In edge of the sea (in Australia) about 30 addition, mangrove plants and species of mangroves have exclusive A third method of coping with salt is sediments have been shown to absorb occupancy. to concentrate it in bark or in older pollution, including heavy metals. leaves which carry it with them when So how do mangroves manage to they drop. (Lumnitzera, Avicennia, Worldwide, vast tracts of mangroves flourish in an environment which Ceriops and Sonneratia species all have been destroyed so we are lucky to would kill most other plants? use this trick.) have relatively large areas of Australia’s tallest and best-developed mangroves As can be seen from the examples still existing on our doorstep. A token given, some mangroves use only one preservation of small parts of these, of these methods but many use two or however, would be similar to preserving more. just a few reefs. Now that their economic and ecological importance has been In addition, a number of features serve recognised we carry the responsibility to conserve water. These include a to look after our mangroves. thick waxy cuticle (skin on the leaf) or dense hairs to reduce transpiration — the loss of water. Most evaporation Mangroves are too interesting to loss occurs through stomata (pores in pack into one Tropical Topics. This the leaves) so these are often sunken topic will therefore appear in two below the leaf surface where they are issues dealing first with plants and protected from drying winds. then with animals. Mangrove leaves are also frequently succulent, storing water in fleshy internal tissue. Where are the mangroves? The necessities of life The warmer and wetter the climate, the richer the mangrove community to be found there. The most diverse mangroves occur in gently-shelving shores allow mangrove tolerant of salt than others. Other factors tropical areas where the water seedlings to anchor, particularly in which affect their distribution include temperature is greater than 24deg. in estuaries, rivers and bays. wave energy, soil oxygen levels, drainage the warmest month, where the annual and differing nutrient levels. Where one rainfall exceeds 1250mm and mountain Mangroves exist in a constantly changing species finds its preferred conditions — ranges greater than 700m high are environment. Periodically the sea or at least those which it is able to tolerate found close to the coast. (The inundates the community with salty water better than other plants — it tends to proximity of mountains tends to while, at low tide, especially during become dominant. This has led to quite ensure the rainfall.) In addition, they periods of high rainfall, it may be exposed clear zones among mangroves. need protection from high waves to floods of fresh water. Apart from which can erode the shore and suddenly altering the salinity levels, these prevent seedlings from becoming fluctuations in water can alter established. In north Queensland the temperatures as well. Great Barrier Reef performs this function while to the south a chain of Different mangrove species have sand islands provide shelter. Shallow, different requirements. Some are more Orange mangrove Zones in the mangroves As a general rule bands of dominant mangrove (Ceriops species) may be flat zone where only mangrove species run parallel to the found in this zone, although the toughest shoreline or to the banks of tidal creek conditions may make it mangroves (for systems. The seaward side of the impossible for anything other example yellow community, where there is more than succulent plants to thrive mangrove (Ceriops Yellow salt water than fresh, is likely to here. The resilient grey mangrove tagal), club mangrove be dominated by a fringe of may appear again while less saline mangrove grey mangroves soils may be covered with a thick (Aegialitis (Avicennia marina). This forest of orange mangroves annulata) and tough species is Orange mangrove (Bruguiera species). grey Australia’s most mangrove widespread due to its A number of factors may (Avicennia ability to tolerate low determine what happens to the landward marina) grow. temperatures and a side of this zone. In conditions of high variety of conditions. rainfall, such as exist in north Queensland, There is a similar change of species along Grey A pioneer, it is likely particularly in the Daintree, regular rivers, the zones corresponding roughly mangrove to be the first flooding may lead to freshwater swamp to decreasing salinity levels and ranges of species to grow on areas dominated by the less salt-tolerant salinity (among other factors). The ever- newly-emerged mangrove species (such as adaptable grey mangrove tends to be mud banks, putting up its distinctive peg freshwater mangrove found throughout river systems, including roots. Mangrove apple (Barringtonia the upper limit of tidal influence where (Sonneratia alba) often acutangula) and fresh water is abundant. The greatest grows in this zone too. cottonwood concentration of mangrove species is (Hibiscus usually at the mouth of tidal creeks and The red (stilt or spider) tiliaceus). Behind rivers where salt and fresh water mix in mangrove (Rhizophora this may be a zone ideal proportions and floodwaters deposit stylosa) is usually of paperbarks and plenty of material to build up the banks. found behind this the beautiful flaky- Red mangroves are zone where its long barked red beech or frequently found prop roots give it a golden guinea tree here. firm foothold in wind (Dillenia alata), as and waves. littoral (shore) While there are Red vegetation merges certain patterns to mangrove The next zone may be inundated only by into rainforest. the development of periodic spring tides at the time of the new mangrove zones, it must be remembered and full moon. As a result the soil In areas of very that local conditions will always dictate will be firmer but more saline seasonal rainfall, such which mangroves are due to the evaporation of as those in the found where. water leaving behind salt Gladstone to Cottonwood which will not be Townsville region, the reverse diluted until the may be the case, evaporation and little next spring tide. fresh water input leading to an increase in Grey The tough yellow salinity. This may be a salt marsh or salt mangrove Yellow mangrove Mangrove uses Mangroves at sea The mangroves have long functioned as a storehouse of materials Mangroves occur not just on the coast of the mainland. They also pop up, where providing food, medicines, shelter and tools. conditions are suitable, on the fringes of continental islands and on wooded coral Fish, crabs, shellfish, prawns as well as islands of the reef. In these situations edible snakes and worms are found among rainfall and nutrient levels are likely to be the mangroves. The fruit of certain Blind-your-eye mangrove low so these communities generally lack species, including the Nypa palm, can be the diversity and luxuriance of those on eaten after preparation along with the the mainland.