The coastal area of Bangladesh lies within the tropical zone between 21-23°N and 89-93°E. The mean annual rainfall varies from about 1500 mm in the west to over 3800 mm in the south-eastern region. The heaviest rainfall occurs during the monsoon period (July and August) while there is practically no rainfall during the dry winter months of December to February. Mean coastal temperature during January is 19.5°C while during July, the mean temperature is 27.2°C.
A. Shelter Belt:
Most of the 710 km coastline is dominated by the deltaic deposits of the Ganges- Brahmaputra-Meghna Rivers. Originating in the Himalayas, this river complex carries a huge amount of sediment. These sediments are transported and sorted by river flow and reworked by tidal and wind action, leading to extensive areas of accretion and erosion in the coastal area. With the fall in mean sea level after the monsoon, these newly formed deltaic deposits – locally known as ‘char’ lands – become increasingly exposed, often become dry completely during the winter months. Because of the nature of these ‘char’ lands, their partial submergence during subsequent monsoons, and their high interstitial salinity, these areas are not suitable for agriculture until salts have been leached out and soil ripening has occurred. Consequently, this new land remains bare until natural successional changes lead to a grass cover, usually dominated by halophytic uri-grass, Oryza coarctata. Protection of these lands from subsequent erosion, acceleration of their vertical accretion, to make these newly accreted land usable rapidly and to save the lives and resources of coastal people from cyclone and tidal surges are prime objectives of the coastal afforestation program. Experiencing the protective role of Sundarbans mangrove forests, Forest Department started planting mangroves in newly accreted char lands in early 1960s with Keora (Sonneratia apetlla) Bain (Avecinea officinalis).
B. Ecological Importance
Mangroves are woody plants that grow at the interface between land and sea in tropical and sub-tropical latitudes where they exist in conditions of high salinity, extreme tides, strong winds, high temperatures and muddy, anaerobic soils. There may be no other group of plants with such highly developed morphological and physiological adaptations to extreme conditions.
Mangroves create unique ecological environments that host rich assemblages of species. The muddy or sandy sediments of the mangal are home to a variety of epibenthic, infaunal, and meiofaunal invertebrates. Channels within the mangal support communities of phytoplankton, zooplankton and fish. The mangal may play a special role as nursery habitat for juveniles of fish whose adults occupy other habitats (e.g. coral reefs and sea grass beds).
Older mangrove plantation
Because they are surrounded by loose sediments, the submerged mangroves’ roots, trunks and branches are islands of habitat that may attract rich avifaunal communities including bacteria, fungi, macro algae and invertebrates. The aerial roots, trunks, leaves and branches host other groups of organisms. A number of crab species live among the roots, on the trunks or even forage in the canopy. Insects, reptiles, amphibians, birds and mammals thrive in the habitat and contribute to its unique character.
C. Economic Importance
Mangroves have enormous economic value. They protect and stabilize coastlines, enrich coastal waters, yield commercial forest products and support coastal fisheries. Extracts from mangroves and mangrove-dependent species have proven activity against human, animal and plant pathogens. Mangroves may be further developed as sources of high-value commercial products and fishery resources and as sites for a burgeoning ecotourism industry. Their unique features also make them ideal sites for experimental studies of biodiversity and ecosystem function.
Mangrove forests are among the world’s most productive ecosystems, producing organic carbon well in excess of the ecosystem requirements and contributing significantly to the global carbon cycle. Regarding mangrove plantation in the coastal areas several studies revealed that eco-system carbon stories (including bio-mass C and soil C) increases with the stand ages and greater than many other tropical forest of similar ages.
Government transferred 12,30,000 acres of newly accreted khas land in the Bay of Bengal, in the estuaries and influences of big rivers of Chittagong, Noakhali, Bhola and Patuakhali districts within 22°00´N and 21°30´N Latitude and 89°55´E and 92°30´E Longitude in 1976. Forest Department has established about 1,80,000 ha. mangrove plantation since 1965. It is estimated that a total of 1,20,000 ha. land has been reclaimed and stabilized. From this reclaimed and stabilized land Forest Department already handed over about 1,10,000 acre land to Ministry of Land for agriculture. These lands have now become raised to the level where they are no longer suitable for further mangrove plantation. Rest of the lands are under the process of reservation. On current estimates, there are some 50,000 ha. of suitable coastal lands presently available for further plantation. Hence, in future there will be an ample scope for coastal/mangrove plantation in Bangladesh.
The mangrove afforestation in Bangladesh is being carried out in one of the most dynamic natural environments on earth and as such, some problems associated with land stability must be expected and can never be completely eliminated. Four types of land stability problems have been recognized including: burial of mangrove seedlings when and where sediment accretion rates are particularly high; smothering of seedlings by sand in areas where wave action reworks large volumes of sand shorewards; the winnowing of fine sediment (clay and silt) from the plantation site during prolonged stormy periods, leaving a mobile sandy lag deposit; and the erosion of plantation margins through bank slumping by migrating tidal and river channels. All four types of coastal change have caused the loss of mangroves from existing plantations. However, the increasingly systematic identification of areas with land stability problems is being used to avoid planting sites where the risk of crop losses between planting and harvesting is high. In this way, mangrove losses due to adverse coastal changes should be progressively minimized.
Other problems in relation to mangrove plantations are: over and above the generally difficult communication and logistic conditions in the coastal areas, these problems include unauthorised cattle and buffalo grazing and the illegal occupation of newly raised land by the local villagers for cultivation of rice. On the other hand some influential and elites are interested to use the raised land for other non-agricultural purposes like shrimp/fish culture and they are trying to collect ownership document from the land department/ local administration for different purpose over the newly accreted land. Habitat destruction through human encroachment has been the primary cause of mangrove loss. Under this adverse condition it becomes difficult to keep control over the afforested mature land by the Forest Department.
A sizable amount of coastal lands are being encroached and its mangrove plantations are being degraded due to unplanned expansion of ship breaking industry. Some encroachers of the afforested coastal land are destroying coastal plantation and then making salt bed in that land. After several years these lands are suitable neither for use as salt bed nor crop production/afforestation.
Living at the interface between land and sea, mangroves are well adapted to deal with natural stressors (e.g. temperature, salinity, anoxia, UV). However, because they live close to their tolerance limits, they may be particularly sensitive to disturbances like those created by human activities. Because of their proximity to population centers, mangles have historically been favored sites for sewage disposal. Industrial effluents have contributed to heavy metal contamination in the sediments. Oil from spills and from petroleum production has flowed into many mangles. These insults have had significant negative effects on the mangroves.
Fresh water flows either from rain water or from upstream is critical to the development of mangrove forest. In the south-western region of Bangladesh, fresh water comes principally in the form of discharge from the Ganges and runoff from the catchment of various distributaries. Most of these distributaries have lost their connection with the Ganges due to geo-morphological process and human interventions.