Sonneratia mangroves colonise mudflats in the Straits of Malacca.
WHERE tropical riverine freshwater and salty water meet, mangroves appear less colourful than coral reefs and thus attract much less attention. Fringing the sea, they are massive energy soaks of the destructive forces of storms, cyclones, even tsunamis, yet provide a unique habitat to specialised flora and fauna.
The world’s largest mangrove ecosystem, covering 20,000 sq km, is where the River Ganges enters the Bay of Bengal. It is this, here in the Sundarbans, a Unesco World Heritage Site in Bangladesh, where 26 species of the 50 species of mangrove trees worldwide may be found in a national park covering 10,000 square miles. That area, not unlike the Caribbean islands which were recently hit by two hurricanes within a few days, has seen the tearing out of mangrove trees left uprooted and abandoned by the ebb tide, along kilometres of beaches and tidal flats.
In Southeast Asia, many areas of mangrove have been cleared for land reclamation for farming, urbanisation, and tourism developments. Vietnam’s mangrove swamps, once damaged by war, are recovering as millions of mangrove trees have been subsequently planted to reduce storm-effects and to protect the villages just inland. Both Vietnam and Malaysia have taken the lead in replanting mangroves through youth schemes, so that the next generation can value their environment. These youthful, sterling efforts in Sarawak and Sabah have been regularly reported in The Borneo Post.
Inter-tidal forests
Located mainly in muddy waters, in estuaries or wherever rivers deposit their silt upon entering the sea, mangroves are lone species of plants in an otherwise hostile environment. In Southeast Asia, there are more than 100 species of mangrove plant per ha, whereas there are only eight species on the Atlantic/Caribbean shorelines. Their distribution is obviously related to sea-warmth of 20 degrees Celsius and above, where they manage to survive in oxygen-starved, waterlogged, muddy soils as well as tolerating salty or brackish waters.
Vertical asparagus-like rootlets of the Sonneratia species emerge for air.
Inhospitable conditions
To combat the anaerobic conditions of waterlogged mud, mangroves have developed a rooting system, either growing upwards to or downwards from their trunks. Technically, they possess ‘flying buttresses’ of roots that provide anchorage. They need oxygen to survive and this is achieved by passing air from the roots above the mud to those roots lying beneath. The oxygen is taken in by pores, called lenticels, in the root system, which is then diffused through sponge-like tissues along the roots. Relying on atmospheric pressure differences between the rise and fall of tides, these lenticels close whilst submerged and open as the tide ebbs.
Mangroves are classified as haloseres (salt-tolerant plants) and have developed several ways of coping with their salty environment. They restrict how much salt gets into their root systems by adapting to high salt levels in their tissues. They exude excess salt by depositing it on their roots and leaves. In fact, they have an inbuilt desalinisation system, as the sap of their trunks is less salty than the seawater surrounding their roots.
Mangrove zones
In taking a boat to Bako to the national park headquarters at Telok Asam, it appears that the coast is smothered in mangrove vegetation. If you look carefully you will see three zones of different species of mangrove plants. The seaward zone is dominated by two species of mangrove, Sonneratia and Avicennia.
These species, through their roots, stabilise the mud by binding the fine, river deposited, silt particles together as well as trapping tidal debris. To combat their twice daily covering of mud and saltwater and in order to breathe, they send upwards hundreds of slender vertical rootlets to absorb air. These rootlets look like asparagus-tips.
Beyond this zone, and landwards, is the middle zone, characterised by Rhizophora mucronata found colonising the higher mud banks in areas only submerged by high tides. This species produces brown, globular, knob-like seeds, with up to 60 per tree, which hang downwards from the branches. Small roots have already evolved on these seed pods and thus they may be considered embryo plants which drop off the tree, spear-like, to enter the mud below.
A vertical shoot almost immediately appears at the top of the knob and a new tree begins to grow. The Rhizophora species is characterised by stilt roots radiating from the main trunk and arching outwards before vertically entering the mud. These above water root-arches contain the lenticels to supply the plant with oxygen.
Yet further landwards, the third zone, near the high tide mark, is dominated by the buttress roots of the species Bruguiera, as well as by Nipah palms. These roots loop up and below the trunk and only in a severe storm or during a ‘king tide’ do they face total submergence. Interestingly, the famous naturalist, Alfred Russel Wallace, makes scant mention of mangrove swamp vegetation at Santubong in his 1879 publication, whereas, in 1916, Robert Shelford’s book, ‘A Naturalist in Borneo’, does refer to the fauna of the mangroves at both Santubong and Buntal.
Sonneratia mangroves at Santubong beach.
Rich fauna of mangroves
Mangroves provide ‘rich pickings’ for animals, birds and fish and no doubt insects. As I have only visited mangrove swamplands in Sabah, Sarawak and Johor, I’ll only stick to the flavour of what I have seen! The proboscis monkey (Nasalis larvatus) comes readily to mind from experiences along the Kinabatangan River in Sabah and at Bako National Park. With their distinctive red noses, bulging bellies and orangey brown coats with whitish tails, they specialise in stripping off the leaves of Sonneratia trees and busily devouring them for their daily salt intake.
Numerous ‘gangs’ of macaques (Macacus irus) also colonise the mangroves but today tend towards living near kampungs or tourist areas where their scavenging is easier. Expert swimmers and divers, the thickness of their skins prevents saltwater from entering their bodies. Saltwater or estuarine crocodiles also make their presence felt as we have read about in Sarawak and Northern Australia.
On the edge of the mangroves near Batu Pahat, Johor, I have seen Brahminy kites (Haliastur indus) perching and nesting on tree branches and even white-bellied sea eagles take off from the mangroves before swooping down on an unsuspecting fish in inshore waters.
Tidal life
The roots of mangrove trees are often encrusted with barnacles, oysters, mussels, sponges, sea anemones and annelid worms. Barnacles can choke a mangrove tree by covering their lenticels and thus preventing gas exchanges to the plant. Sponges, however, protect the plants from wood-boring animals, shellfish and ship worms.
Graspid and Fiddler crabs abound at low tide. The former feed on the exposed roots of the trees and especially on seedlings in the mud. The latter crab-species, characterised by the male’s brightly coloured, enlarged claw, sift out and forage through the sediment. Their predators are kingfishers, herons, snakes, fish and larger crab species; thus they are constantly on the move.
Mudskippers literally skip over the mudflats and can even climb mangrove trees, using their pectoral fins as climbing aids. In Borneo, two species are most common, the blue-spotted and the orange-spotted versions. They carry a mixture of oxygen and water in their enlarged gill chambers, which serve as aqualungs when on land. With a flex of their tail, they can produce a powerful leap of a metre or so and their bulging, periscopic eyes can spot an insect at three metres.
To us, mangrove swamps usually appear inhospitable environments but these areas have evolved a most incredible ecosystem. With climate change and man’s increasing land-reclamation and intrusion, the mangrove areas of our world are highly fragile and should not be overlooked.
Rhizophora plants with stilt roots near Batu Pahat, Johor.