Mangroves are salt-tolerant forest ecosystems of tropical and subtropical intertidal regions of the world. The mangrove formation is the gift of the land and the sea. Mangrove depends on terrestrial and tidal waters for the nourishment, and slit deposits from upland erosion as substrate for support. The tides nourish of the forest, and mineral rich river-borne sediments enrich the swamp. Thus the mangroves derive their form and nurture from both marine and terrestrial influences.

It is one of the most productive ecosystems and a natural, renewable resource. However, on all the sides, the world's mangroves are beleaguered.

The depletion of mangroves is a cause of serious environmental and economic concern to many developing countries. At the same time, their primary production supports numerous forms of Wildlife and Avifauna as well as estuarine and near-shore fisheries. Consequently the continuing degradation and depletion of this vital resource will not only reduce terrestrial and aquatic production and wildlife habitats, but more importantly the environmental stability of coastal forests that afford protection to inland agricultural crops resulting in villages becoming seriously impaired.

According to the forest survey report, the mangrove cover in India is 4,872 Sq.meters (0.15% of the total land area). In India, this forest type is found in major deltaic regions like Gangetic, Sunderbans, Mahanadhi, Krishna, Cauvery and Andaman and Nicobar group of islands. Pichavaram and Muthupet are two potential places which have dense mangrove vegetation in Tamil Nadu.

Factors that determine area, diversity and growth of mangroves:

The health of the mangrove wetlands with reference to hydrological and soil conditions and the wealth of the mangrove wetlands in terms of area, species diversity, biomass and productivity are determined by degree of protection against high-energy waves, quantity and duration of freshwater flow and sediment supply, larger tidal amplitude and gentle sloping coastal topography. Though mangrove trees are capable of withstanding the forces of cyclones, storms and tsunami they grow only in coastal areas where wave energy is low or in places where mangrove wetlands are protected by sand barriers against high-energy waves. This protection is necessary for the seedlings of the mangrove plants to settle, establish and grow. The coastline of the Muthupet region of the then combined Thanjavur district of Tamil Nadu and that of Sunderbans in West Bengal are the best examples of low-energy wave coasts where mangroves grow luxuriantly along the seashore. In fact, in these areas, mangrove forest is slowly growing into the sea! In areas like the Godavari mangroves of Andhra Pradesh, a sand spit known as Hope Island and Kakinada Bay provide protection to mangroves against high waves. In the case of the Pichavaram mangroves of Tamil Nadu, a narrow sandy beach located between the sea and the mangroves prevents exposure of forest directly to the high-energy waves.

Thus, this essential condition of protection against high waves during the early stage of mangrove forest development rules out growing of mangroves all along the entire coast. Most of the mangrove plants require low salinity condition for their growth and reproduction. Hence, luxuriant mangrove forests can be seen only in the estuarine regions where large amount of freshwater is discharged for longer period of time in a year. For example, the Sunderbans mangrove forest of West Bengal, which receives freshwater from the River Ganges and the Brahmaputra throughout the year, harbours not only high number of mangrove plant species but also dense and tall mangrove forest. Whereas in the Pichavaram and Muthupet mangroves of Tamil Nadu, which receive only a low amount of freshwater and that too only for a few months in a year, both the number of plant species present as well as height of the tree is less. The area of the mangrove wetlands is determined by the tidal amplitude and slope of the coastline (tide is nothing but the temporary raise and fall of seawater due to gravitational pull of the moon and the sun and tidal amplitude is the difference between high tide and low tide). For example, tidal amplitude in the Sunderbans mangroves is about 4.8 m and the slope of the coast is also very gentle. As a result, seawater reaches up to 90 km inland and mangrove wetland is present up to this point. The total area of the Indian part of the Sunderbans mangrove wetland is about 4,26,000 ha (actual forest cover is about 2,12,500 ha). On the other hand, area of the Pichavaram mangrove wetland of Tamil Nadu, where the tidal amplitude varies from 0.40 to 0.65 m, is only about 1,400 ha (actual forest cover is only 700 ha).

Plants and animals in mangrove wetlands:

Plants The plant community in mangrove environment is classified into two types namely, true mangrove species and associate mangrove species. True mangrove species are found only in the mangrove wetlands whereas associated species are found both in the mangrove environment and in the nearby areas. Globally a total number of 69 plant species have been identified as true mangroves. All these species are able to grow in saline water but only a few of them have the ability to tolerate high salinity. For example, Avicennia marina can tolerate soil salinity as high as 90 grams per litre but many of the mangrove plants grow luxuriantly only in places where salinity is between 10 to 20 grams per litre.

Unique features of Mangroves:

Mangrove plants tolerate salinity of the soil and water by the following ways:
i) Salt excretion: Some mangrove plants take saline water as such through roots. But in the tissues, only water molecules and essential salts are retained. Excess salts are excreted through salt glands that are present in the leaves.
ii) Salt exclusion: In some of the mangrove plants the roots possess an ultra filtration mechanism called reverse osmosis by which water and salts in the seawater are separated in the root zone itself and only water is taken inside and the salts are rejected (reverse osmosis mechanism is widely used for producing drinking water from seawater!).
iii) Salt accumulation: In this type of mangrove, plants posses neither salt gland nor ultra-filtration system but these species have the capacity to accumulate a large amount of salts in their leaves.

True Mangroves and Mangrove associates:-

In general, plants of the mangrove wetlands are divided into two groups namely, a) true or exclusive mangrove and b) associate mangrove species. The following are, the characteristic features of true mangrove species:, a). True mangrove plants grow only in mangrove environment and do not extend, into terrestrial plant communities, b). They play a major role in determining structure of the plant community of the mangrove wetland and ability to form pure stands, c). They are morphologically adapted to live in waterlogged condition – e.g. aerial, roots associated with gas exchange, d). They are physiologically adapted to live in saline environment, e). They have viviparous reproduction, f). They are taxonomically isolated from terrestrial relatives, About 69 species in 27 genera, belonging to 20 families are considered as true, mangrove species.