Discovering Mangrove Swamps
Contributor: Leonie Twentyman Jones
Until fairly recently I have thought of mangrove swamps as being rather menacing, dark tangled places in the tropics, inhabited by alligators slithering over the mud, runaway fugitives or maybe a hideaway for pirates – no doubt fuelled by reading thrillers or adventure stories set in the Florida Everglades or Caribbean islands.
I had no idea that two of the most southern mangrove swamps in Africa are found in estuaries along our Transkei coast. Far from being menacing they are essential for a healthy marine ecology. Like the Knysna salt marshes they are part of an elaborate food chain and provide a safe nursery for young fish and other intriguing creatures, like the mudskipper. Mudskippers are amphibious fish, so when the tide goes out they skip over the mud-flats in search of food using their sturdy pectoral fins as legs. Mangrove swamps play an important role in protecting coastlines from storm damage as the trees and their roots decrease the effect of wind and wave action.
In the Umngazana River mouth we saw three different species of mangrove trees all growing densely packed together – the white, black and red mangroves. Each has evolved different ways of surviving in soils that are regularly waterlogged, airless and loaded with salt. High tides bring marine aquatic and estuarine conditions, while low tides expose the mud and roots to aridity, heat and desiccation. The White mangrove (Avicennia marina) is a pioneer tree, among the first to colonise open tidal habitats, later being restricted to the more inland parts of the swamp where conditions are at their most extreme. It has ‘pencil’ or breathing roots which stick up through the mud round the tree. These help to trap silt and stabilise the substratum. The trees provide shade so that other mangrove species can germinate. The White mangrove can exude excess salt through its leaves. The seeds germinate on the tree, so what is dropped and dispersed are actually seedlings which root quickly when they are washed ashore.
The Black mangrove (Bruguiera gymnorrhiza) grows behind the White mangrove on the inner side of the swamp where the soil is slightly drier and there is shade for its seedlings. It has elbow or knee-like roots protruding from the mud in order to obtain air. The tree gets rid of excess salt by depositing it in its leaves, which turn yellow, drop off and are immediately grabbed by the Red Mangrove crabs to drag them into their burrows to eat. Incidentally the crabs are a favourite food of the Mangrove Kingfisher! Fruit also germinates on the tree developing into a long cigar-shaped hypocotyl, so that what falls from the tree is a germinating seedling.
The Red mangrove tree (Rhizophora mucronata) grows in the shade often amongst the White mangrove and usually in or near water channels. The roots for taking in air and supporting the tree, branch off the trunk, as one writer describes ‘like the buttresses of medieval cathedrals’! Again germinating seedlings (propagules) fall from the tree and can survive at sea for several months before being washed ashore to establish roots.
Mangrove wood is well known for being water-proof, tough and resistant to borers so is used for building, fencing, fuel and charcoal – leading to human pressure on the swamps.