Left: A flow diagram showing the changes in vegetation types due to increased salinity. Right: A Dieback affected Sundari tree.

Most of the rivers of Bangladesh flow from north to south, silting up the mangroves delta and draining into the Bay of Bengal. The mangrove is a transitional territory between the freshwater rivers originating from the Ganges and the Bay of Bengal. The ecosystems of Sundarban react with the increase of salt concentrations. The environmental parameters with the direct influences on Sundarbans in terms of global climate change are sea-level rise, natural calamities like cyclones, rising temperature, salinity and drought. Salinity is more devastating than any other parameter in this territory. It is very difficult to manage salinity because of the lasting nature of its effects on terrestrial and aquatic ecosystems.
Most of the tree species grown in the Sundarbans cannot tolerate high level of salinity and are seriously affected when salts concentrate within the root zone. Trees are severely affected where groundwater is close enough to the surface to discharge or concentrate salts. The most significant off-site impact of salinity in the Sundarbans is the salinization of previous fresh water rivers and canals. The habitats of aquatic flora are also degraded by increased salinity. Predictions from Sundarbans territory show that salinity may be double over the next few decades posing risks for survival of flora in Sundarbans.
The greatest threat posed by increased salinity to the floral diversity of Sundarbans is the loss of both terrestrial and aquatic habitat. Salinity has severely affected riparian vegetations because they occupy the lowest parts of the landscape where they get submerged by saline water. Natural vegetations of such areas are being destructed causing major changes in landscapes and biodiversity. Destruction of remaining natural habitats in core areas, buffer zones and corridors are also occurring. Most of the coastal districts already face severe salinity problems, with saline water pushing up to 250 km inward during the dry season.
Increased salinity acts as a silent disaster in Shoronkhola, Morelganj, Rampal, Mongla, Dacope, Baithaghata and Sheyamnagar upazila. Brackish water from the Bay of Bengal is invading freshwater bodies percolating deep into the soil. But beyond the long-term peril, an immediate threat comes from increased salinity that jeopardizes the Sundarban ecosystems. The factors which contribute to the increase of salinity are, tidal flooding during monsoon, direct inundation by brackish water, and horizontal movement of brackish ground water during dry season. The whole eco-systems are sensitive to changes in salinity level and the plant communities are continuously struggling to adjust with the new conditions.
Silent killer of Sundari tree
The spread of Dieback of the pioneer species, Sundari, severe poses a large threat to the ecosystem. It is a condition where Sundari trees die or decline prematurely and often rapidly. Almost all Sundari trees are affected more or less. Dieback contributes to the loss of canopy coverage in the fresh water swamp forest eco-regions of Sundarbans. These eco-regions are located in between of Gangetic Lower Uplands' Moist Deciduous Forests and salt dominated Brackish Water Sundarban Mangroves bordering the Bay of Bengal. Water is only slightly brackish and becomes fresh during the monsoon. These areas have many luxuriant broad leafed tree species including Sundari.
Dieback-affected Sundari trees produce weak crowns, with sparse foliage and a good proportion of dead twigs and branches. In most cases shoots start to die from the top and root systems become poor. Sundari trees are found in Dangmari, Karamjal, Jongra, Mora Passur, Pashakhali, Nandobala, Harbouria, Choraputia, Buddhomari, Katakhali, Boroitola, Jeodhara, Amurbunia, Gulishakhali, Dhansagor, Kolomtezi, Nangli, Chandpai, Mrigamari, Andharmanik, Tamulbunia, Supoti, Bogi, Mora Bogi, Dumuria, Charkhali, Shapla, Chandeshwar, Shoronkhola, Panirghat, Bhola, Dashervarani and Kochikhali forest areas of Sundarbans. Increased salinity and prolonged inundation appear to contribute to Dieback of Sundari trees in all these areas. The writer apprehends that after 20 years Sundari trees will be extinct from the Sundarbans and its nomenclature will be changed as the name has been derived from the Sundari tree.
Declining trends of Nypa palm
Nypa palm is an indicator plant species in Sundarbans though it does not exploit truly littoral environment nor can it tolerate saline water for a long time. It occurs most commonly in areas where mixture of brackish water and fresh water flows. It can grow on low lands and depressions, at the base of eroding slopes and cliffs, or on sandy ridges or embankments. It is an undershrub in the riparian zones and the ecological climax of Nypa palm or its associates occurs in pure stands on islets in the main channels or on depressions of the interior river meanders having silt loam soil texture. These deposits are enriched frequently by floods or surface run-off from nearby rivers during monsoon. The abundance of Nypa palm depends on the availability of fresh water. Saline water tides are highly crucial for Nypa's seed dispersal and germination. Karamjal, Jongra, Mora Passur, Nandobala, Harbouria, Choraputia, Andharmanik, Tamulbunia and Supoti forest areas are considered as the paradise habitats for the Nypa palm. B declining trends in Nypa palm abundance has been observed in these areas over time.
Changes in forest types
The Sundarbans consists of fresh water swamp forest, salt water mixed forest, brackish swamp forest (true mangroves), mangrove scrub and littoral forest. The littoral forests are slightly different from the mangroves. They are found on high but sandy grounds with thick vegetation along the mainland coast offering habitation to birds. Mangrove scrub is the frontier mangrove forest dominated by small trees. Freshwater swamp forest is inundated with freshwater, either permanently or seasonally, while brackish swamp forest with saline water regularly. Salt water mixed forests are located in the transitional zone between freshwater supplied by rivers and saline water pushed by the Bay of Bengal.
Fresh water swamp is being converted into saline mixed forest while saline mixed forest into brakish swamp and brackish swamp into mangrove scrub. The littoral forests are becoming degraded forests due to higher mortality caused by increased salinity.
Changes in vegetation types
The species composition, natural regeneration, species richness, vertical and horizontal structure of Sundarbans is undergoing major changes due to increased salinity. Salinity causes a major threat to the successional mangrove ecosystem. The vegetations are responding by changing in productivity, canopy closure, tree coverage and species diversity, or by migrating. Salinity weakens the potentiality of natural regeneration by reducing the viability of seeds, seedling germination and seedling recruitment. The tree mortality rate is being accelerated due to increased salinity. The production of new leaves, leaf longevity and the leaf area, net photosynthesis rate, stomata conductance and transpiration rate of leaves are being reduced. Salinity is one of the most important factors of mangrove forest growth and distribution. A salt concentration of 20-40% is suitable for mangrove ecosystems, while 40-80% diminishes the number of species and their size. Only a few species can exist and grow in 90% salt concentration. Sundari, Bain, Kakra, Passur and Dhondul tree species are being quickly replaced by Gewa and Keora. Mixed forest stands are being converted into pure stands. The patchily distribution of mangrove date palm (Hanthol) is declining in the fresh swamp and mixed saline forests of Sundarbans.
The writer, a biodiversity specialist, is UNO, Mongla, Bagerhat (mizan_peroj @yahoo.com)