Global Climate Change & Bangladesh....

Climate change & vulnerability of forest ecosystem

The Sundarbans is known as the single largest stretch of productive mangrove forest in the world. It is located in the northern limits of the Bay of Bengal and the estuaries of the Ganges-Brahmaputra-Meghna (GBM) river systems. It occupies an area of about one million hectares in south-west Bangladesh and south-eastern part of the State of West Bengal in IIidia (between 88°85 and 89°55 E and 21°30 and 22°40 N). About 62% of the forest covering an area of 577,000 hectare is situated within the territory of Bangladesh. The rest lies within India. About one third of the forest consists of water bodies in the forms of rivers, channels and tidal creeks. The forest land is highly influenced by tidal interactions because of the presence of these water bodies. The forest receives freshwater and sediment from a number of distributaries of the ganges. It hosts one of the richest natural genepool for forest flora and fauna species in the world including the most notable one: the Bengal Tiger. The forest has been endowed with a number of  commercially important mangrove species. The most common species are sundri (Heritierafomes) from which the forest gets its name and gewa(Excoecaria agallocha) The other important tree species include goran (Ceriops decandra), golpatta (Nypa fruticans) and keora (Sonneratia apetala).

In general, dicotyledonous tree species are represented by 22 families and 30 genus, while Rhizophoraceae is represented by all the 4 known genera and at least 6 species. The shrubs are represented by 12 species belonging to 11 genus under 7 families. Eleven different species of climbers belonging to 6 families have so far been identified in the Sundarbans. In addition to the rooted plants, there are epiphytic parasitic flora found in the forest. Orchids of 13 species and ferns of 7 species are identified in the Sundarbans.

However, the mangrove forest ecosystems deserve special mention not because they provide a number of valuable goods and services to the millions of people of Bangladesh, they are extremely rich in flora and fauna biodiversity. Since the fate of all such animal and plant species living in the forests would depend on the condition of the forests, it is of extreme importance to examine the vulnerability of the forests under climate change scenarios. For simplification the discussion will be limited only to the Sundarbans ecosystem.

Impacts on Mangrove Forests

Mangrove ecosystems of Bangladesh are located in the Sundarbans, the south-west parts of the country bordering India, and in offshore islands of Bhola, Moheshkhali and in Chokoria of Cox's Bazaar district . The former houses the largest patch of productive mangrove in the world, constituting more than 95% of the mangroves in the country. The other mangrove ecosystems are severely degraded due to continued encroachment, whereas the Chokoria Sundarbans (about 7500 hectares) has been completely denuded during the past three decades.

General Features of the Sundarbans

The Sundarbans is ideally located in the estuaries of the three mighty rivers: the Ganges, the Bramaputra and the Meghna. The landscape of the Sundarbans consists of a large number of fluvial and tidal geomorphological features created by the continual deposition of weathered materials carried by the GBM river systems. Mudflats are formed along the estuaries or riverbanks and are subjected to direct wave action, flow and turbulence of the water currents in the river. The lower parts of mudflat retruiins submerged during all high tides. Backswamps or basins are also found to occur as low lying saucer shaped depressions. These collect rainwater and sediments, the latter being washed away each year during the early monsoon season. Ridges or levees are found to exist due to sediment deposition on the edge of the riverbank. Some levees have inclined slopes on their outer edges, with steep gradients towards the channel side. There are creeks and streamlets that are influenced by tides and maintain inter-connection between rivers and cross channels.

The Sundarbans is influenced by tides that are semi-diurnal. There is a seasonal variation of tidal height ranging between 3.5 to 5.0 metres, while the mean tidal height is about 4.0 metres. Tidal height is also influenced by lunar periods. The mean water level of the rivers changes between wet and dry season due to the onset of monsoon winds and high discharge of freshwater. Thus the water level reaches its peak during the wet season (July- August) and its lowest level during the dry season (December-January).

All areas below the lowest water level are regularly flooded by almost all high tides. Elevations above this level are subjected to flooding during spring tide only and remain exposed during neap tide. Besides tidal variations, the occurrences of monsoon floods and cyclonic surges raise the water levels of the rivers, causing submergence of elevated lands that do not undergo regular inundation. River flow and tidal currents playa vital role in creating the environmental conditions of the estuary.

The soil of the Sundarbans is saline due to tidal interactions, although the salinity is low compared to soil salinity in other mangrove forests of the world (Karim, 1988). Soil salinity, however, is regulated by a number of other factors including surface runoff and groundwater seepage from adjacent areas, amount and seasonality of rainfall, evaporation, groundwater recharge and depth of impervious subsoil, soil type and topography etc. It is found that, conductivity of subsurface soil is much higher than that of surface soil (Chaffey et al., 1985). Using the salinity scale established by Walter (1971), the forest areas have been divided into three zones based on soil salinity (Karim, 1988).

Salinity zones in the Sundarbans Forest

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Oligohaline (ormiohaline) Zone
The zone is characterised by the soil containing less than 5 ppt of Nacl salt. The oligohaline zone occupies a small area of the north-eastern part of the forest.

Mesohaline Zone
The zone is characterized by Nacl content within the concentration range of 5 to 10 ppt in soil. This zone covers the north-central to south-central part of the forest.

Polyhaline Zone
The Nacl content of the soil in this zone is higher than 10 ppt. This zone covers the western portion of the forest.

Figure shows the three zones, based on data of soil salinity in dry season. The boundaries, however, of these zones are not static and their precision remain tentative owing to high variability of salinity conditions within the zones. The general feature of soil salinity is that it increases from east to west. The increasing trend of soil salinity from north to south is not uniform throughout the forest. Soil salinity influences the floral distribution of the forest.

A forest inventory completed by Chaffey et al., (1985) reported that the forest had ten forest types and it consisted of eight dominant plants namely sundri (H. fomes), gewa (E.agallocha), passur (X mekongensis), dhundul (X granatum), kankra (B. gymnorhiza), keora (S. apetala), baen (A. officinalis) and goran (C. decandra). It was also reported that H. fames - E. agallocha forest type covered the largest area (29.45%) followed by pure H. fomes (21%). There is a wide variation of vegetation in the Sundarbans. It varies from multi-storied forest forming closed canopies to scrubby bushes with widely dispersed stunted trees. Moderate to relatively freshwater areas (oligohaline zone) support the best developed forest. While in the sea front areas (polyhaline zone), the forest consists of poor growth trees and shrubs. The general vegetation types with the dominant species in respect to salinity zones are presented in Table-3.

The general vegetation types in respect to soil salinity zones

Source: Ahmed, 1998

Possible Impact on the Sundarbans Ecosystem

Examining the possible impacts of climate change it appears that there would be more rainfall in monsoon which would force high incidences of floods in terms of intensity and frequency. Rainfall runoff would provide increased freshwater discharge to all the major distributaries of the Ganges supplying freshwater to the Sundarbans. But increased sea level would cause backwater effect, thereby delaying the discharge process in the estuary. As a consequence, there would be relatively prolonged inundation in the Sundarbans areas in monsoon months (July-September) and also, increased rate of sedimentation/siltation in the backswamps and creeks inside the forest area.

A significantly different environmental condition might be expected in winter months. There would be significant lowering of freshwater discharge in the rivers coupled with high rate of evaporation. Just to offset increased evaporation, the rate of surface and ground water abstraction for irrigation in the upstream areas would be increased resulting into lesser amounts of freshwater discharge in rivers coming on to the Sundarbans. As a consequence, it is highly possible that the rate of salinity intrusion into the forest would increase.

The impact on salinity intrusion would be compounded with climate change induced increased incidences of cyclonic storm surges in the Bay. Storm surges would inundate high levees and backswamps that do not get submerged with saline water and thereby would be affected by salinity.

The biota of the Sundarbans forest are highly influenced by salinity regimes of surface and groundwater systems and more significantly, of soil. As it has been described above, salinity of a particular forest area defines which type of vegetation is expected to dominate. Natural regeneration of vegetation and forest succession also depends on salinity regime (Karim, 1994 and Siddiqi, 1994). Since soil salinity inside the forest would significantly change in respect to the present salinity, it is highly likely that increased salinity would have discernable adverse impacts on forest regeneration and succession. This would in turn affect the long-term sustainability of the ecosystem. Furthermore, the highly dense human settlements just outside the forest area would restrict the species migration to less saline areas. A combination of the two effects would therefore threaten the very existence of the ecosystem.

Even if the forest would manage to survive such threats, but due to increased salinity the present species composition will certainly change. It has been observed in the Indian side of the forest that, due to gradual increase in salinity the freshwater loving sundri (H. fomes) trees disappeared while the area has been covered by shrubs and saline tolerant grasses (Chaudhuri and Choudhury, 1994). It is also reported that the remaining sundri trees, those survived in the less saline areas, shrunk in size. Similar phenomenon would occur due to increased salinity in the Bangladesh's side of the forest under climate change conditions.

Ahmed (1998) reported that the existing oligohaline zone would be completely transformed into mesohaline zone. As a result areas with best quality standing timber predominated by long Avicennia officinalis and H. fomes would be replaced by inferior quality tree species, predominated by H. fomes - E. agallocha and Ceriops - Excoercaria forest types. In a similar fashion, a significant part of the existing mesohaline zone, especially the south-western parts of the Sibsa river up to 22°00 N would be transformed into polyhaline (saline areas with NaCl content >10ppt in soil) zone. As a result, grasses, thorny herbs/shrubs and trees of poor quality would dominate and gradually replace woody tree species. Under such conditions vegetation canopy would become sparse and plant height would be reduced significantly (Ahmed, 1998).

With a change in species composition the productivity of the forest would be severely constrained. Chaffey et al. (1985) demonstrated that, total merchantable wood volume per unit area of forest land decline with increasing salinity of soil and river water. Disappearance of oligohaline areas combined with decreasing mesohaline areas would result into over 50% loss of merchantable wood from the Sundarbans. Increase in salinity in the Indian side of the forest would have compounding effect to the existing poor productivity of the forest.

The salinity would not be a problem in monsoon. Because there would be enhanced flushing with freshwater and the surface water salinity would be pushed back towards the sea. But change in water level due to higher discharge and rainfall runoff combined with backwater effect due to sea level rise would cause drainage congestion in the estuaries. The latter would result into increased rate of sedimentation/siltation in the submerged areas. As a consequence the backswamps of the Sundarbans would gradually be filled up. This will also cause changes in regeneration capacity of the species as well as productivity in the long run. Moreover, the low-lying mudflats would be more frequently submerged which could slow down forest succession process.

Impacts of climate change will increase moisture stress in winter months, enhance and prolong floods in monsoon, produce higher salinity in the coastal areas and have possible higher incidences of tropical cyclones and storm surges affecting the coastal areas. Increased moisture stress in the Barind and Madhupur Tract areas would cause accelerated degradation of the forests. Prolonged floods would affect tree species that do not prefer water-logged conditions. Enhanced salinity intrusion would limit afforestation potential in the coastal villages. Cyclonic storm surges would cause wide-scale damage to most forest species in the affected areas as it has already been observed following the cyclonic storms of 1991,1994 and 1997.

The most significant impact, however, would be observed in the Sundarbans forest. A possible shift in salinity zones would force ecological transformation and the freshwater loving woody tree species would be gradually replaced by bushy shrubs/herbs, grasses and non-woody plants. Furthermore, the species migration to freshwater zones would be limited due to human pressure coupled with enhanced withdrawal of surface water for irrigation in the upstream. Such a transformation would, perhaps, lead to a ecological disaster. Many of the wild animals including the Bengal Tiger would find it difficult to adjust to such dramatic changes.

Source: Huq. S., karim, Z., Asaduzzaman, M., Mahtab, F., Vulnerability and Adaptation to Climate Change for Bangladesh, 1999, Kluwer Academic Publishers, The Netherlands.