Mangroves are a most common shrubs or trees especially near the coastal waterways in the tropics and sub-tropics, but mangrove species become less frequent in southern parts of the mainland and are not part of the Tasmanian floral makeup. As a group they have an interesting name, arise from a mixture for plant families and because of the habitats in which they grow have several unique features. Following are illustrated features of mangroves with the majority of images from the Kimberley coast, Western Australia (W.A.), supported with others from the Northern Territory (N.T.), Queensland (QLD.), New South Wales (N.S.W.) and South Australia (S.A.).
The word mangrove
The interesting point about the name mangrove is that it refers to both individual species that live in a particular habitat as well as the communities they form. In broad terms there are 41 different mangrove plant species found in Australia and their shrub and tree forms are all limited to tidally influenced habitats with most of these being in the tropical and sub-tropical bays, coastal lakes and river estuaries. Occasionally readers may come across the word mangal, this is a less commonly used alternative name for a mangrove community.
Botanical descriptions such a eucalypt, wattle or saltbush, refer to groups of species in Australia in three particular plant families ,respectively, Myrtaceae with about 700 eucalypt species, Mimosaceae with 980 wattles species and Chenopodiaceae with 60 saltbush species. Australian mangroves however belong in 19 diverse families that have one or more species that has adapted to grow within the mangrove habitat. The Rhizophoraceae family is the largest and includes three genera, Rhizophora (stilt mangroves), Ceriops (yellow mangroves) and Bruguiera (orange mangroves), each being represented by several species. At the other extreme Osbornia octodonta (myrtle mangrove) and Excoecaria agallocha (milky mangroves) are represented by just a single genus and single species and belong respectively in families Myrtaceae and Euphorbiaceae
The value of mangroves
Given that mangroves occupy tidally influenced waters and are associated with estuary river flats and sheltered bays they may become less pleasing to look at as a community at low tide than when flooded. Such sites were in the recent past often considered to be smelly (explained later) unattractive lowlands. Particularly in well populated Australian locations, such sites were often ’developed’ using landfill like sand and soil then used for ‘better’ waterside purposes such as parklands, housing estates with canals and factory building sites. In most cases planners and developers have seen the error their ways as the positive ecological values of the tidal flats and mangroves become widely known and accepted.
Ecologically, mangrove communities are extremely significant and a study in Darwin Harbour, cited by Norman Duke in Australia’s Mangroves, identified 9 species of bats, at least 11 other mammal species, 128 species of birds, and around 3000 species of invertebrates e.g. insects and spiders, all living as mangrove canopy dwellers. Aquatic fauna and organisms in the mud as sampled in the Darwin Harbour study found 36 species of crustaceans and 31 mollusc (shell fish) species associated with mangroves.
Crustaceans like crabs are most important since many they take large quantities of leaf material below the ground to be used as food. And as most recreational and commercial fishers are aware many fish species in their juvenile stage live within the mangroves.
Molluscs and fish as a food source, found withing mangroves, have been utilised by Indigenous people. Species of the Teredo worm that live in dead mangrove stems and trunks are also sought for food and medicinal purposes. The wood of some mangroves was used for making spears and boomerangs and naturally straight buoyant trunks of Camptostemon schultzii (kapok mangrove) was favoured in the Kimberley to make raft like watercraft.
Some characteristics of mangrove habitats.
Saturated soils Mangroves mostly live in soil that is regularly saturated. This means that all the pore space in the soil is filled with water and not the mixture of air and water as is the case for most other plant communities. With all the air replaced with water, anoxic (Oxygen free) conditions exist under-ground and mangroves have developed alternate appendages to overcome the problem of no soil Oxygen. Possibly the most obvious of these adaptations are the long, looping aerial roots as seen in stilt mangroves species in the genus Rhizophora. The stilt roots have many lenticels over their surface allowing for gaseous exchange -oxygen in and water and carbon dioxide out. Lenticels are small pores, occurring as raised sections formed by cells with lots of intercellular space.
(Above) Two examples of arching prop roots of long-style stilt mangrove (Rhizophora stylosa).
Pneumatophores are another adaptation of the root system They too are covered with lenticels and grow up above the soil level that is exposed with dropping tides. These grow up from cable roots that run out horizontally from the base of the plant as seen with grey mangrove (Avicennia marina). In this species pneumatophores are pencil shaped and generally less than 30 cm tall. With apple mangrove (Sonneratia alba) pneumatophores are more robust and cone shaped.
(Above) Two examples of the sharp triangular pneumatophores of apple mangrove (Sonneratia alba).
Other adaptations are knee roots where the plant forms tight loops in roots the rise above the ground surface, while other species have lenticel covered buttress shape roots formed at the base of their stem or trunk.
Break down of organic matter Bacteria are important organisms in the decomposition of soil organic matter such as chewed up fallen mangrove leaves that have been taken underground in mangrove systems by crabs. In the anoxic (oxygen depleted) environment of saturated soils, aerobic (oxygen requiring) bacteria cannot function and are replaced by species with an alternate form of cellular respiration.
In aerobic organisms (including ourselves) respiratory energy production during cellular processes relies on a food source like carbohydrates and Oxygen. Following a complicated set of enzyme-mediated chemical steps energy from the breakdown of carbohydrate is released along with Carbon Dioxide and water. Hydrogen is produced during the chemical breakdown and the cellular enzymes combine the Hydrogen with Oxygen to form the water.
In a summary: Carbohydrate + Oxygen → energy release + carbon dioxide + water
In anoxic conditions anaerobic bacteria take over the respiratory process to breakdown carbohydrates (from chewed up leaves) but they have enzymes that substitute Sulphur not Oxygen to combine with the Hydrogen. In this case, Hydrogen Sulphide, H2S or rotten egg gas, rather than water H2O, is produced.
In a summary: Carbohydrate + Sulphur → energy release + carbon dioxide + hydrogen sulphide
When soil is disturbed in mangrove habitats, we often get a whiff of the Hydrogen Sulphide! Alternatively, as the tide rises, water entering the crab holes or cracks in the soil. may push out the smelly gas H2S.
The influence of tides Most Australian mangroves have a habitat that is flooded at least once daily and many parts of the coast, with semi-diurnal tides, are flooded twice a day. Interestingly the range between low to high tide levels varies from day to day and so we have spring and neap tides.
Spring tides are a response to full moon and new moons while neaps fall between these two lunar phases. In summary during there are two spring tides and two neap tides each 29.5 days. During spring tides, as opposed to neap tides, the range between high and low tide increases. Shorter plants growing close to the water’s edge are therefore completely immersed in saline water by high tide on most days and it is the only the inner, landward margin of the mangal that is flooded during spring tides.
Whereas close to the open water plants may be flooded on more than 45 events per the 28 day lunar cycle, this reduces dramatically to less than 20 on the upland margin. Plants in the intermediate area will be flooded by normal high tides at a frequency between these two extremes.
In practical terms the soil surfaces in mid to lower tide are flushed with water during the ebb and flow of tides but soils above this on the landward upland margin of the mangal are only wet during the highest of the spring tide. Soils on these upland margins tend to become more saline and they accumulate salt following evaporation of the water once the tides have receded. In effect, the surface dries as the water is evaporated leaving the salt.
These differences in exposure to tidal flooding and salinity influence where particular mangrove species are able to grow so that plants of the same species are not randomly distributed but, in response to their habitat preferences, grow together. An aerial photo of a mangrove lined shore demonstrates this phenomenon with similar species forming bands of similar leaf texture, height and colour generally occurring parallel to the shoreline.
Salinity and position in the estuary There is also a gradient in salinity distribution along an estuary and this too means that different associations of mangrove species occur along the length of an estuary.
During the dry season of northern Australia for example there is reduced inflow of fresh water to estuaries from creeks and rivers. A stand of mangroves at the head of an estuary is a long way from the coast and at this point there will be reduced exchange of tidal water. These factors combined with evaporation from the estuary leads to more saline conditions. Closer to the estuary mouth however tidal exchange is more complete so salinity here remains much as it is in the open sea.
Adaptations to growing in saline water Some mangrove species have the ability to resist uptake of particular minerals (mainly sodium chloride) by their roots while others have developed glands that excrete salt crystals onto their leaf surfaces, some of which are likely to be the washed off during high tides and rain. Salts may also be removed during leaf fall and the shedding of bark.
There is the only one deciduous mangrove and it grows in the mid tidal position of river estuaries across northern Australia. This species the cedar mangrove (Xylocarpus moluccensis), changes leaf colour from green to yellow, red and orange and losses leaves during the dry season. There are suggestions that this function may be another adaptive mangrove strategy to remove accumulated salt.
Dispersal of species following seed setting. There are several mechanisms used by species to distribute offspring and one involves the production of a buoyant, woody round fruit that eventually break up to release the seeds after they have fallen -both Australian Xylocarpus species have these rounded buoyant fruits that split into four parts releasing seeds. Alternatively, many mangroves are viviparous where seeds germinate and partly develop while still attached to the parent plant. The seedling so produced is called a propagule and the stage of embryonic development reached before release depends on the species. The yellow mangroves Ceriops, produce seedlings with slender embryonic stems called the hypocotyl that is about 15 cm long in C. australis, 27 cm for C. decandra and is ribbed and up to 35 cm long in C. tagal. In the closely related Rhizophora species the hypocotyl is also buoyant but is 65cm long in R. stylosa (long style stilt mangrove) and around 80 cm in R. mucronata (upriver stilt mangrove).
These propagules are buoyant and rely on the tidal movement to disperse once they fall from the parent plant. After varying periods in the water they may be stranded and the partly developed roots on the tip of the hypocotyl may anchor the seedling in what may or may not be a suitable habitat for the development of an adult plant.
Distribution of Species in Australia. Whereas all mangrove species and most of their varieties grow in tropical waters, some like Rhizophora mucronate grow in the wet topic estuaries toward the tip of Cape York Peninsula. One like Avicennia integra, the stilt grey mangrove, only grows in estuaries of the Northern Territory with its definite wet and dry seasons. This area like the Kimberley and other parts of the Queensland coast have a monsoonal climate characterised by a wet season during the hotter time of the year and a dry season in the cool months.
In the arid tropics of the Western Australia such as the coastline of the Great Sandy Desert, there are up to nine species but further south the number of species is reduced with only one in the temperate south, Avicennia marina (grey and white mangroves). A similar pattern of distribution is found in eastern mainland Australia with a diminution of species southward. In some bays, inlets and gulfs of Victoria and in South Australia one of the varieties of A. marina is the only mangrove species represented.