Sundarban is famous for its mangrove forests. Mangroves, which are also some times called as Mangals, are special group of plants of diverse families, which can survive in this tidal and saline swampland which are inundated and exposed routinely in every 6 hours by the coming and ebbing tides and have the various interesting adaptations like neumatophores (breathing roots), viviparous germination (germination of seeds when the fruits are attached with the parent plant to reduce the chances of washing off by the tide and to anchor in the soft mud as it falls), stilt and bow roots (to support the tree from falling down in the soft mud), salt glands (to excrete excess salt taken with in the body), Xerophytic leaves (leaves that can store and reduce loss of excess water) etc. Blasco (1975) defined mangroves as the name given to a type of coastal woody vegetation that fringes muddy saline shores and estuaries in tropical and subtropical regions and at high tide much of the ground is covered by the sea water.
Mangroves are important for many other reasons, e.g. these halophytic plants (plants which grow on salt rich soil in which osmotic pressure of soil sap is so high that it is very difficult for the plant to absorb water from that kind of soil, in fact that kind of soils are termed as physiologically dry) offer protection against storms, tidal waves, and erosion. Their root systems also provide haven for fish, crustaceans (a class of the invertebrate phylum arthropoda in which shrimps and crabs belong), and other marine organisms. The litter of mangroves (shedding of their leaves) contribute a great deal of organic matter to estuarine and other marine environments and provides food for tiny planktonic (organisms which can not swim and only drift in the water depending on the current), pelagic (organisms inhabiting the mass of water) and benthic (organisms which live in the bottom of water on or with in the substratum) organisms which, in turn, is being taken by larger organisms, and so on. As a result, marine life around mangrove communities is often greater than that found in other areas and this huge load of nutrient out welling support a highly productive fishery in our coastal area.
Mangroves swamps are formed in the coastlines world over where the sea is calm and shallow with gently sloping base and where seawater is properly diluted by the river or fresh water to form brackish water, in the estuaries or deltaic regions near the sea. In all those areas siltation and accretion is high to build muddy shores, which are regularly inundated and exposed, by incoming and ebbing tide. The ideal temperature is 20oC - 35oC, humidity 70-90%, and rainfall 100cm-300cm.
The mangrove regions of the world are divided in to two distinct groups by Tomlinson (1986), i.e., (1) Old World Mangrove and (2) New World Mangrove. 40 true mangrove species are distributed in the former in the Indian Ocean and West Pacific Islands and Australian region (36o North - 39o South and 30o East - 178o East) and 8 species of mangroves are found in the later in the Atlantics and East Pacific Ocean (36o North - 35o South and 5o East - 120o West).
Naskar (1983), have described 35 true mangroves, 28 mangroves associates and 7 species of obligatory mangroves from Sundarbans.
According to Chaudhuri and Chaudhury (1994) the Indian Sundarbans appears as a continuous green carpet in aerial survey, consisting of dense low trees ranging from 3-6 m in height, predominantly of Excoecaria agallocha (Genwa) and Ceriops decandra (Jath Garan). Excoecaria agallocha is particularly dominant close to the sea and Ceriops decandra is found largely as a degenerate form. Both species are increasing in number and are replacing other species.
Rhizophora apiculata (Garjan) and R. mucronata (Garjan) are found in the creeks, often in association with Nypa fruticans (Golpata). Four species of Bruguiera are present comprising B. cylindrica (Bakul Kankra), B. gymnorhiza (Kankra), B. parviflora (Bakul Kankra) and B. sexangula (Kankra). This genus has been over-exploited and regeneration is poor. Kandelia candel (Garia), a fourth genus of the family Rhizophoraceae, is uncommon.
Avicennia alba (Kalobani), Avicennia marina (Pairabani) and Avicennia officinalis (Sadabani) are members of the family Avicenniaceae found in the Indian Sundarbans. These species represent a pioneer genus in plant succession and occur in association with the sea-grass Porteresia coarctata (Dhani Ghas), in areas of accretion. Avicennia species also occur in gregarious forms along stretches of channels and creeks. A. alba has a degenerate form which occurs in areas where anthropogenic stress is high and the level of inundation has increased.
Sonneratia apetala (Keora) occurs in fringe areas in discontinuous strips, while S. caseolaris (Chak Keora) occurs only in the western Sundarbans. Xylocarpus granatum (Dundhal) and X. mekongensis (Passur), which were once common species in the Sundarbans, are found scattered and have become scarce because of selective felling and poor regeneration.
Stunted trees of Heritiera fomes (Sundari) may be found scattered all over the forests. In the eastern and southern blocks, Heritiera forms communities with Excoecaria agallocha. However, the size of H. fomes decreases gradually from east to west. Aglaia cucullata (Amur) is also found in association with Heritiera, in freshwater conditions. Aegialites rotundifolia (Tora), representing a monotypic genus, is found in low-lying areas with Acanthus ilicifolius (Haraguza).