Magic Mangroves

Those living in the far southern regions of Australia who haven't been north of their state’s borders often harbour the belief that the seafront of states like Queensland are all long stretches of white sand and palm trees, brushed by tropical breezes and lapped by warm tropic seas. In some places that is a fairly accurate picture, except for much of the NQ coast where the more accurate picture is one of extensive mangrove swamps.

If you’ve ever tried to walk through a mangrove swamp — not a recommended activity — you might think that this is a messy, unpleasant, smelly hell hole rather than a sub-tropical haven. But it’s not quite as bad as it might seem, and mangroves are a vital and valuable component of the ecosystem, supporting the nearby white sands, palm trees, crystal clear water and flourishing marine life.

Of course, mangroves aren’t limited to the tropics; they extend right down the East Coast to near the Victorian border, as well as in smaller pockets around the Port Phillip and Western Port areas, the Spencer Gulf and Gulf St Vincent regions in South Australia, and from the Margaret River area of Western Australia all the way north and around the top of the continent.

What makes mangroves unique is their capacity to grow happily in heavily salted water. This generally means tidal inlets and rivers and creeks but can include sheltered coastal fringes. This doesn’t mean that mangrove trees need salt water, just that they tolerate it very well. Studies indicate that their optimum growing conditions involve a mix of fifty per cent saltwater and fifty per cent freshwater. Mangroves adapt to the salt by various tactics, filtering as much as 90 per cent of the water’s salt at the roots, and sometimes excreting salt through their leaves. Salt is also concentrated in the bark or older leaves so that it is lost from the plant as the leaves or bark fall.

Oh, and that smell you note when struggling through a mangrove swamp is hydrogen sulphide, a gas produced by bacteria that breaks down plant matter in the mud that settles around the roots of the trees.

Commonly there is a range of mangrove species growing in bands parallel with an inlet or shore line, ranging from those living in the softest mud that sees relatively little exposure, through to ground that gradually becomes harder and is subject to less and less inundation and greater degrees of drying. The richest zones are usually around the mouths of small creeks and rivers where there is a good mix of fresh and salt water and plenty of fresh sediment being imported and deposited.

Mangroves grow best where there is a warm climate, a minimum 1250mm of rainfall annually, hilly or mountainous land close to the coast (for enhanced run-off) and relatively still waters to enable juvenile plants to establish themselves. The Great Barrier Reef along the north-east coast and the chain of sand islands to the south serve to ensure the latter’s prosperity along the Queensland coast.

One of the secrets for success for mangroves is their roots systems. These are obviously important for all plant types, but much more so in mangroves, where there can be more of the plant below the substrate surface than above it. Depending on the environment, mangroves can have buttress roots, knee roots (long radiating roots with ‘knees’ where vertical roots descend into the substrate), stilt or prop roots (usually in higher wave energy environments) and smaller peg roots (vertical roots rising into the air to obtain air for metabolic processes). Some mangroves growing in soft, oxygen poor mud have special cells (lenticels) which ‘inhale’ air when exposed at lower water levels and then pass this along for respiration when covered with water.

Mangroves disperse their seeds, fruit and seedlings by flotation. Some will produce a seedling, growing out of the fruit for up to three years before it drops off into the water. Other species retain the live seedling within the fruit, which drops from the tree into the water. In high or very low salinity the seed coat remains attached, but in brackish water, especially in warm weather, the coat rapidly falls away so that the seedling can quickly lodge and begin to grow. One species, the cannonball mangrove, produces a fruit which explodes to scatter its seeds.

The dense and entangled environment among the mangrove roots provides an excellent refuge for juvenile marine creatures, ensuring their survival until they are able to better fend for themselves in the more open waters. They also provide a home for a rich ecology of other creatures, both vertebrates and invertebrates. This is the ideal home for mud crabs and all the way up to saltwater crocodiles. As much as 80 per cent of the global fish catch depends on mangroves, either directly or indirectly.

Mangroves also perform a valuable task in stabilising shorelines. They limit the impacts of storm surges and hurricanes and limit the damage from wave action. The dense root systems slow water flow and help trap pollutants and prevent their transportation downstream. Mangroves also sequester carbon at a rate of four to five times that of a tropical rainforest, so they are a valuable ally in diminishing the impacts of climate change.

The major threat to mangroves has been their removal by man to make way for coastal or near-shore development, as well as a simple belief that they are unsightly and unpleasant. However, in recent years, an increasing awareness of the role these trees play in ensuring the richness and diversity of the waters around them has led to a fresh approach of encouraging their growth where possible.