Botany

Mangroves



Tweet
Nilanjana Bose's image for:
"Mangroves"
Caption: 
Location: 
Image by: 
©  

*What are mangroves?

Mangroves are edgy forests straddling land and sea.  They exist in no-man zones between the oceans and rivers and hard lands, in isolated islands defined by the rushing and ebbing of tides.  They are trees that crowd together at the borderlines, of land and water, of salt and sweet, deep and shallow, mysterious inhabitants of a romantic world of diffused half-light and half-shadows. 

One of the definitions of mangroves is “any of the several evergreen trees or shrubs of the genus Rhizophora, having stilt-like roots and stems and forming dense thickets along tidal shores”.  But the term mangrove can be used in different senses.  The broadest is where mangrove is used to refer to a mangrove forest in its entirety, or mangal, as an ecosystem, and the narrowest where it defines just a specific tree, the mangrove tree, either red, white, black or buttonwood.  

There are about 70 different species which live and thrive in inter-tidal waters; the range of species includes palm and hibiscus, myrtle and holly, from low-lying shrubs to tall trees used as timber.   However, mangals can comprise of just one or two species also.  Compare this to a rain forest where the number of species can go into thousands.

Mangroves lie within a tropical band, 30 degrees north and south of the equator and are located in a thin fringe along the coasts of tropical and sub-tropical coastlines of Africa, Asia, Americas and Australia.  The total area of all the mangrove forests combined is around 150,000 sq km – that is an area less than that of Florida, USA. 

*Superb adapters

All mangrove species have one thing in common – they are superb adapters.  They survive and thrive in water salinities that would kill other plants.  Most of them  have perfected a complex ultra-filtration system to keep out the salt, some have specialised roots  called pneumatophores which stick out of the choking mud to take in air, yet others have prop roots to keep them upright in the shifting soils of inter tidal zones.   

The red mangrove, for instance, has a waxy substance on its roots that helps keep the salt out.  The salt that does penetrate this protective coating is sent to old leaves which are then shed by the plant.  The white mangrove tree, on the other hand, has specialised glands which secrete the salt out through the leaves, which then get speckled with it.   There are other adaptations that keep water in, such as the ability to close up pores in leaves, and turn them away from the sun to minimise transpiration or water loss from the leaf surface.

They have evolved special mechanisms to ensure the survival of their offspring as well.  The seeds of some of the mangrove species are buoyant and germinate while still on the parent tree.  Once it germinates, it grows within or out of the fruit in a propagule, which is a ready-to-go seedling capable of producing its own food through photosynthesis.  It can be dispersed by water over great distance surviving for more than a year. When it is ready to root, its density changes so that it now becomes vertical rather than horizontal.  If for some reason rooting does not occur, it can again change its density and float away to a more favourable spot.

*Complex ecosystems and buffers

Mangrove forests are one of the most diverse and complex ecosystems on earth.  They are home – hunting or nesting or breeding grounds - to a vast variety of life-forms, birds and mammals, insects and arachnids, reptiles and fish, all kinds of animals.  Some are the habitats of endangered species such as tigers in the mangrove forests of India/Bangladesh region, known as the Sunderbans, the last remaining saltwater haunt of this highly endangered animal.  Similarly, the mangroves of Florida in USA shelter hawks-bill turtles and bald eagles.

These forests provide buffers between the open oceans and threats their waters pose to the landmass, they stabilize shores and trap sediments.   Where mangrove forests are left intact, they form natural breakwaters in the event of a tsunami, highlighted starkly during the Indian Ocean tsunami of 2004.  The interlocking root systems of mangroves keep river-borne soil sediments from washing out to the sea helping in land conservation naturally.  Many years of research also shows that mangroves are effective carbon sinks, they take carbon dioxide out of circulation from the atmosphere and reduce the levels of greenhouse gases.

*Threats

Mangrove forests have huge significance for coastal economies, particularly in Asia and Africa.   Rural communities living on their fringes come into the forests to harvest its products - for timber and firewood, thatch grasses and leaves, fruits, nuts, medicinal plants, wild honey and seafood.  But despite their importance, mangroves worldwide are in danger.  Human encroachment and urbanisation have put mangroves under enormous stress.  Shrimp farming is the one of the direst threats.  Shrimp farmers uproot mangroves for creating the farm, and after a few crop cycles, they abandon the site and move to another fresh location to avoid risk of disease, and so destroy a new area of the mangrove forest in a vicious cycle.   Shrimp farming countries like Thailand, Ecuador, Brazil and Philippines have been uprooting and losing their mangrove forests for years together as demand for shrimps has sky-rocketed in the developing economies.

The other comes from rising sea levels, as is obvious, the mangroves will be the first to face the inundation that global warming is likely to bring.

There have been some initiatives to protect and conserve mangroves.  Examples include Bangladesh, which has re-planted mangroves in the Bay of Bengal and gained an area of 120,000 ha – a huge benefit to a land starved country.  In the aftermath of the 2004 tsunami, many Asian nations are following Bangladesh’s example and replanting their mangroves as a defence against future disasters.  In Eritrea, planting of mangroves in the Red Sea Coastal areas is helping the country to recover from a war and poverty by providing a fillip to both fisheries and animal husbandry. 

The Forestry and Agriculture Organization of the UN is also involved in projects around the world in conservation.  Despite the efforts mentioned above, a recent study by the UN estimated that mangroves are being lost at a rate three to four times higher than land based forests. Though there are 1200 protected sites safeguarding about a quarter of the world’s mangrove forests, any further decline due to shrimp farming and/or coastal development can result in significant ecological and economic imbalances.  In 2010, a joint project of UNEP and FAO compiled the World Mangrove Atlas involving more than 100 mangrove research groups and organizations across the globe.  According to the lead author of that publication, “Mangrove forests are the ultimate illustration why humans need nature.”  One fifth of the global mangroves have been lost in the last three decades, unless science and economics combine to drive policy shifts, they will remain under threat. 

Tweet
More about this author: Nilanjana Bose

From Around the Web




ARTICLE SOURCES AND CITATIONS