The rushing waves splash upon woody serpentine mesh of stilt roots and spiny structures rising from the soft coastal mud, and come to rest. Swirling softly below the emerald green, bereft of its erosive force, the waves bring in fresh nutrients for the little nursery of marine fauna nestling within. Mangroves, a unique group of plants, create a labyrinth of roots that clutch on to the swishing silt to build an ecosystem of exemplary importance. Repetitive cycles stabilise sediments making the underlying soil surface higher, which in course of time, rises beyond the high water mark. Plants other than mangroves then occupy the newly created land. Mangroves reduce the risk of shoreline erosion – which brings waves closer to habitation – by dissipating surface wave energy. Early this year an alarming report was released on World Wetlands Day (2 February 2008), entitled The World’s Mangroves 1980-2005, published by UN Food and Agricultural Organisation (FAO). The Report states that world has lost around 3.6 million hectares of mangroves since 1980, equivalent to a 20 percent loss of total mangrove area. The Report however also reveals a silver lining despite the total mangrove area declining from 18.8 million hectares in 1980 to 15.2 million hectares in 2005. The good news is that there has been a slowdown in the rate of mangrove loss: from some 1,87,000 hectares destroyed annually in the 1980s to 1,02,000 hectares a year between 2000 and 2005, reflecting an increased awareness of mangrove ecosystems.
The Report continues, ‘Asia suffered the largest net loss of mangroves since 1980, with more than 1.9 million hectares destroyed, mainly due to changes in land use. At the country level, Indonesia, Mexico, Pakistan, Papua New Guinea and Panama recorded the largest losses of mangroves. A number of countries have increased mangrove cover, such as the Sundarban Reserved Forest in Bangladesh. It is well protected and no major changes in the extent of the area have occurred during the last few decades.’
In another declaration the FAO added that mangrove forests could have reduced damage resulting from the storm surge caused by cyclone Nargis in Myanmar. Parts of Myanmar’s coast have been cleared of mangrove forests to make way for agricultural land and fish ponds in recent decades and coastal communities are now more exposed to cyclone damage. Settlements have been established closer to sea and the combination of proximity to coastal hazards and lack of a protective forest buffer has increased the risks to human populations. The mangrove area in the Ayeyarwady Delta, severely hit by the cyclone, is now less than half the size it was in 1975.
Mangroves are important forested wetlands. It is an economically viable zone providing wood, food, fodder, medicine and honey and at the same time offering a habitat for crocodiles, snakes, tigers, deer, otters, dolphins and birds, besides a wide range of fish and shellfish. Indonesia, Australia, Brazil, Nigeria and Mexico together account for around 50 percent of the total global mangrove area. Mangroves are highly specialised ecosystems which have existed since the 3rd Century BC, as testified by the Greek philosopher Eratosthenes in his work Geographica. The Portuguese call these forests ‘mangue’. Nothing else is known about the origin of the word ‘mangrove’ which perhaps is a combination of the Portuguese word ‘mangue’ and the English word ‘grove’.
Mangroves fall under the ecological community named helophilous halophytes. Mangrove plants
have been subdivided into two groups: true mangroves and mangrove associates. True mangroves may be further sub divided into two groups. One group exhibits enhanced tolerance against salinity that may be higher than sea water while the other group is restricted to salinity less than sea water. Among these plants, red mangroves (comprising genera like Bruguiera, Ceriops, Kandelia and Rhizophora) are highly salt tolerant plant groups living close to sea water. Black mangroves (mainly Avicennia germinans) are moderately salt tolerant, deriving their name from the dark brown to nearly black wood that comprises their forests. The white mangrove plant groups (comprising genera like Laguncularia, Lumnitzera, etc.,) show lower endurance to salinity. Shrubby mangroves are grouped as buttonwood mangroves (such as Conocarpus erectus), as they sprout reddish brown button like fruits.
To overcome adverse salt dominant environmental conditions, mangroves have adapted themselves in complex and often spectacular ways. They possess two types of roots. The subterranean tap root system is provided with corky layers. In some cases it transforms into stilt roots to support the plant in tidal currents while some roots, pneumatophores, are profusely branched, rising above the soft mud to access the atmospheric oxygen. Exposed surfaces of pneumatophores are provided with numerous pores for gaseous exchange called pneumathodes. All aerial roots contain large air spaces or aerenchyma tissues which provide a reservoir of air during high tide when the aerial roots may be submerged. Stems or trunks of the arboreal species are stout, covered with thick periderm and usually develop succulence for water storage. Trunks and barks also possess a good amount of tannins to overcome adversity of the surrounding environment. Leaves are thick, leathery, usually succulent, small, glazed, with veritable salt glands capable of excreting salt that has penetrated its tissues. Fruits and seeds are light in weight to enable dispersion through flotation. Also most mangroves are viviparous. Since the embryo pierces the outer covering of the seed while it is still on the tree, their seeds germinate within fruits. By availing this adaptation, the propagules (propagules of Aegiceras fall off only at high tide), manage to quickly implant themselves in the silt along the coastline. Propagules can also survive long periods at sea. It floats vertically and sprouts its first leaf and roots in water so that when it touches soil, it can immediately transform itself into a viable plant, growing strong roots and sprouting a thatch of leaves. Young seedlings can even survive underwater until they are big enough to grow aerial roots, at about 1 to 2 years.
Mangrove in India
There are three coastal settings for Indian mangroves. These are deltaic (mouths of large rivers Ganges-Brahmaputra, Mahanadi, Krishna, Godavari and Cauvery), backwater and estuarine (tunnel shaped estuaries of Indus, Narmada, Tapti, etc.,) and insular locations (such as Andaman and Nicobar Islands).
India has about 4500 square km area under mangrove vegetation. About 59 per cent of this forest is found along the east coast (Bay of Bengal), 23 per cent occurs on the west coast (Arabian Sea) and 18 per cent is found on the Andaman and Nicobar Islands. Areas that support a fairly dense canopy include Sundarban in West Bengal, Kendrapara and Bhadrak in Orissa, Godavari in Andhra Pradesh, Pitchavaram in Tamil Nadu, Gulf of Kutch in Gujarat, coasts of Andaman and Nicobar Islands and Goa. An area wise distribution of mangrove vegetation is given in the Table 1.
Indian mangrove biota
Indian mangrove ecosystems constitute a large number of floral and faunal wealth. Over 1600 plants and 3700 animal species have been identified from these ecosystems. Out of a total 101 species of true mangrove plants recorded, 71 are reported to be present in India. Due to the rich presence of Heritiera fomes and H. minor in Sundarban, the entire deltaic region was named after the local name of Heritiera, i.e., ‘Sundari’. However, at present these two species are endangered.
Straight, round, stems and branches between 8 to 16 cm in basal diameter and between 5 to 7m long, are widely used as poles and rafters in constructing houses. Mangrove timber is used in making furniture, carts, boat hulls, masts and oars. The trunks of Avicennia marina are used for making small dug out canoes. People in coastal villages also use mangrove firewood. Tannin extracted from mangrove bark is mainly used by leather processing industries as tanning and dyeing agents. Mangroves have traditional medicinal uses also. The resin of Avicennia alba wood is used for birth control. Decoction of A. officinalis leaves is used to treat stomach and urinary disorders. Acanthus ilicifolius fruits are used for treating kidney stones. Leaves of Bruguiera cylindrica appear to have tumour inhibiting property. The astringent properties of Ceriops decandra and Xylocarpus granatum barks are commonly used in dermatology. Latex of Excoecaria agallocha is used to treat toothache. The leaf extracts of Rhizophora apiculata is administered against hernia. Avicennia foliage is a source of fodder for goats and cattle. Large leaves of Phoenix paludosa and Nypa fruticans are used for thatching houses as well as for making mats and basketry. Honey collection from the mangrove forest is a promising business opportunity in our country.
Mangrove vegetations are fast depleting world over. Anthropogenic activities including commercial exploitation of both wood and non-wood forest products, agriculture, fisheries and prawn culture are largely disrupting the integrity of these ecosystems. Increasing human population in the adjoining areas steadily destroy these ecosystems. Growing industrial areas along coastlines and discharges of domestic and industrial sewage are toxic to the trees, wiping out many species. Again siltation in the creeks and riverbeds causes stagnation of saline water which in turn disrupts certain physiological aspects of the entire mangrove biota, such as breeding of fishes.
In India, the need of mangrove conservation was perceived after Independence. The first management plan for mangroves was introduced in 1989 for the Sundarban Mangrove Forests. At present around 38 mangrove areas have been identified for intensive conservation and management.
Most countries have now banned the conversion of mangroves for aquaculture, assessing the impact on the environment before using mangrove areas for other purposes. But overall, the loss of these coastal forests remains alarming world over. The rate of mangrove loss is significantly higher than the loss of any other types of forests. If deforestation of mangroves continues, it can lead to severe losses of biodiversity and livelihoods, in addition to salt intrusion in coastal areas and siltation of coral reefs, ports and shipping lanes. In the future, sea level rise and increased frequency and intensity of storms arising from climate change are expected to put coastal areas at greater risk of damage. Lessons learned following the 2004 Indian Ocean tsunami and the Ayeyarwady Delta suggest that much can be done to improve the sustainability of coastal development in the region.
Discouraging further expansion of settlements close to the coast and maintaining healthy mangroves and other coastal forests will be important measures to protect coastal assets and populations. Early warning systems, evacuation plans, effective communication, transport infrastructure, and storm shelters should also be implemented as necessary measures to protect lives in the future from cyclones such as Nargis and tsunamis.