Potential Impact of Climate Change on Mangroves in India

0894-B2

H.S. Singh[1]


Abstract

The earth’s climate changes on global and regional scales, especially in the northern hemisphere, have already affected hydrological systems and terrestrial and marine ecosystems, including mangroves. The extent and composition of mangroves in India (4 871 km2) may undergo major change, depending on the rate of climate change and anthropogenic activities, but the large extent of inter-tidal mudflats of 23 620 km2 in the country may provide a scope of adjustment and adaptation in some areas, mostly in the semi-arid region. It is expected that the diversity in mangroves may improve at higher latitudes like the Gulf of Kachchh; latitudinal range extension may occur at the expense of salt marsh communities; adaptation and survival chance of mangroves in deltaic region like Sundarbans will be higher than mangroves on Andaman and Nicobar Islands. It is also projected that change in climatic conditions may lead to an increase in species migration pole wards.

It is likely that some species in the tropical region, especially in Andaman and Nicobar Islands, may not be compete successfully and may lose out in favour of other species. On the other hand, new species may get the opportunity to regenerate under expected improved rainfall in the semi-arid region. Extensive hyper saline mudflats between mangroves and coast in subtropical regions like the Gulf of Kachchh may work as barriers against sea level rise and may provide opportunities for adaptation of mangroves with landward movement, depending on the rate of sea level rise. Strict protection, preparation of an action plan for each mangrove area in this context, afforestation in potential areas, plantation of species that fail to adapt to sea level rise, and introducing threatened species at higher latitudes are some of the mitigation and management options.


1. Introduction

Indian coasts fall within the bounds of the tropics, which measures about 7516.6 km. and are distributed among nine coastal states and four Union Territories (Anon, 2001). Of this, over 22.6% of the total length of the coasts of India is of islands (Andaman and Nicobar, Lakshadweep and Diu Islands). The climate along the Indian coast varies from one that of true tropical region in south to that of sub-tropical and arid environment in Kachchh in northwest. Rainfall varies from only 300 mm in the semi-arid region of Kachchh in western part of Gujarat State to average maximum of 3200 mm in Andaman-Nicobar Islands in south. On the eastern coast, there are gigantic deltas of the Ganges-Bramhaputra, Krishna-Godavari, Mahanadi, and Cauvery, which support large area of river estuaries and excellent growth of tidal forests. Most of the rivers in India flow to east-coast in the Bay of Bengal and carry tremendous amount of silt during monsoon. The coastal habitats, especially mangroves in India have strong linkages with coastal environment and are considered important areas for sustenance of the coastal communities.

Realising the importance of mangroves, the Government of India declared them as ecologically sensitive areas and provided legal protection under various legislations. The Ministry initiated plan on conservation and management of mangroves and coral reefs in the year 1986 and constituted a National Committee to advise the Government on relevant policies and programmes. On the recommendations of this committee, 31 mangrove areas in the country have been identified for intensive conservation. To provide protection to ecologically important areas, India initiated action through state governments to create network of the Marine Protected Areas (MPAs) under Wildlife (Protection) Act, 1972. Over half of the total mangroves in the country are already managed in over a dozen MPAs (Singh 2002). The Government of India has also notified three Biosphere Reserves- Sundarban in West Bengal, the Great Nicobar Island in southern most islands of Andaman and Nicobar archipelago and the Gulf of Mannar in Tamil Nadu, in marine environment which also support mangroves.

2. Materials and Methods

Reports published by the Government of India on coastal habitats, mangroves and corals reefs have been studied and relevant information has been extracted. ISRO (1998) has studied coastal wetlands of India using remote sensing data which has been used in this paper. Mangroves in the Gulf of Kachchh were extensively studied from 1998 to 2001 by the author and published scientific documents which have become relevant inputs to this paper. Most of the information used is based on the Government records but some of them, especially changes in the mangroves in the Gulf of Kachchh, have been collected from the field. Projected rate of climate change, as reported by the IPCC (2001) and national institutions, especially ISRO (1998), has been logically analysed with respect to mangroves in India.

3. Results and Discussion

3.1 Extent of mangroves

The most luxuriant growth of mangrove with the greatest diversity is in the Indo-Pacific region. The Sundarbans, a "World Heritage Site" in India and Bangladesh, account as the largest single mangrove unit globally. The area under Sundarbans was large in the past, but has been reduced to the present level due to degradation that took place during last two hundred years (WWF, 1992). Logging operation, aquaculture, reclamation of swamps, paddy cultivation on the east coast of India and salt production on the west coast are the main reasons for degradation, resulting into shrinking of tidal forests throughout the Indian coast. Series of conservation measures were initiated in late 1980s which has resulted into control of degradation and recovery in some states like Gujarat where the mangroves have expanded to more than double of its extent in the last two decades (Singh, 2000). This has been ascertained by a remote sensing study which has reported recovery of mangroves in India during last decade from 4,244 sq. km in 1991 to 4,871 in 1999 (FSI 1999).

Among various types of coastal wetlands, tidal mudflats (23,620 sq. km) and mangroves (4,871 sq. km) have major share. India harbours some of the best mangroves in the world which are located in the alluvial deltas of rivers such as the Ganga, the Mahanadi, the Godavari, the Krishna and the Cauvery as well as on the Andaman and Nicobar groups of Islands (FSI, 1999; Singh, 2000). Over a dozen Marine Protected Areas in India cover over half of the total mangroves of the country. Out of eight states and union territories which support mangroves, West Bengal (2,115 sq. km), Gujarat (1,031 sq. km), Andaman & Nicobar Islands (966 sq. km), Andhra Pradesh(397 sq. km), Orissa(215 sq. km) have maximum areas of mangroves (FSI 1999).

3.2 Climate and mangroves

The climate, salinity, tidal fluctuation, substrate or soil, and wind velocity are major factors which determine extent and type of tidal forest. Climatic factors like temperature fluctuation, humidity, precipitation, number of rainy days, regular wind flow, radiation and fresh water flow in the region act as the most significant factors for development and succession of mangroves.

The concentration of green house gases and aerosols has increased by nearly 31% during last millennium which may further increase in the current century at faster rate. Global temperature rise is expected between 1.4 to 5.8 0C by 2100 which is higher than increase during last century. Similarly continental precipitation increased by 5-10% over the 20th century in the Northern Hemisphere, although decrease is also recorded in some regions (IPCC 2001). Global mean sea level has also increased and projected a much higher rate in 20th century. Although accurate data is not available for Indian sub-continent, the trend may not be much different from average global scenario as seas level rise and erosion of coast have been observed in the western coast (IUCN 2002; Worldwatch Institute, 2002).

Normally, evapo-transpiration and precipitation govern the mangrove’s climate in an area. Based on the ratio of mean annual precipitation (P) and potential evapo-transpiration (Etp), the mangroves of the world can be classified in four categories (Anon, 1984; Singh 2000) i.e.-(i) mangroves of moist region (high rainfall, warm humid regions, P/Etp > 0.75, mild warm winter temperature, average temperature over 200C); (ii) mangroves of sub-humid region (rainfall from moderate to heavy, P/Etp between 0.5 and 0.75); (iii) mangroves of semi-arid region (very warm summer with low winter temperature and rainfall, P/Etp between 0.2 and 0.5); and (iv) mangroves of arid region (extremities of climate with scanty rainfall and wide variations in summer and winter temperature, P/Etp less than 0.2). The factors like precipitation (P) and potential evapo-transpiration (Etp) are expected to change in most of the regions, especially Northern Hemisphere, resulting alternation of mangrove’s setting. As discussed, climate and coastal processes, which are key factors for mangrove setting and their successions, will change significantly, resulting changes in extent and composition of the species of mangroves.

3.3 Impact on mangroves

Impact on highly diverse and productive ecosystems such as mangrove forests will depend upon the rate of sea level rise relative to growth rates and sediments supply, space for and obstacle to horizontal migration, changes in climate-ocean environment. Sea level rise will affect mangroves by eliminating or modifying their present habitats and creating new tidally inundated areas to which some mangrove species may shift (IPCC 2001). Extent of high tidal mudflats constitutes major share of the tidal mudflats, especially in Gujarat State. This will provide great potential to the mangroves of the region for adjustment and adaptation against sea level rise.

Climate unsuitability is another factor responsible for mangrove’s change and disappearance. Hardy species like Avicennia sp recolonised the area fast in the area like the Gulf of Kachchh, but this is not true in case of the species of genus Rhizophora, Ceriops, Sonneratia and Aegiceras in the Gulf of Kachchh and other regions and Heretia sp. in Sundarbans as they were unable to recover from the effect within short period of time. Global warming and sea level rise would bring changes in the parameter Etp in the most of the region, resulting in alternation in mangrove setting.

It was estimated that the average global sea level was 120-130m below the present levels as recently as 10,000 BC. Since then there has been relatively rapid rise to more or less the present level (Clough 1994). In deltaic mangroves, there is consistent seaward movement of the mangroves but this may take a reverse change due to rise in sea level. Topography of deltaic mangroves, especially in Sundarbans is such that landward migration of mangroves, coupled with accretion of sediment might more or less keeps pace with rising sea level.

Based on evidence and study of mangroves in other parts of the world, likely impact on mangroves at different rate of sea level rise has been projected below (Ellison and Stoddart 1991; Clough 1994).

(a) Low rate of sea level rise: Existing mangroves can keep pace with a relative sea level rise of 8-9cm/100 years. A few species will be highly vulnerable in deltaic mangroves but many species will be threatened on islands. Mangroves in high latitude may gain in some areas.

(b) Medium rate of sea level rise: Mangroves would be under stress, especially islands with a relative sea level rise of 9-12cm/100 years. Substantial number of species will be vulnerable and majority of the species on island may face high risk of extinction. Impact may be less in subtropical region.

(c) High rate of sea level rise: Mangroves cannot persist as expansive areas with a relative sea level rise of above 12cm/100 years. Loss of species would be very high in short period in tropical region, especially on islands. Total devastation may take place in the mangrove areas of the islands.

On the other hand, most predictions suggest that future rises in relative sea level will be of the order of 100-200cm/100 years. If this projection becomes reality, mangroves of the world may suffer serious loss and majority of the species may fail to adapt new environment. The extensive mangrove systems of the Sundarbans in the Bay of Bengal are examples of river-dominated systems where relative sea level may rise less owing to the influx of large amount of silt. There is little information on his aspect of mangrove ecology, but circumstances evidence suggests that mangroves can cope with accretion rates of the order of 10cm/100years, albeit with some change in community structure and species composition (Clough, 1994). Probably, moderate rate of sea level rise may keep the pace of setting of mangroves, but situation may favour selected colonisers. These mangroves may come under stress or cannot persist when rate of sea level rise is high as predicted. Scenario of overall impact would be different on islands where mangroves are already restricted in area by coastal topography and tidal amplitude. Mangroves in these areas, especially in Andaman and Nicobar Islands may come under stress or may not persist in moderate to high rate of sea level rise. As about 260 km of the coast of Andaman and Nicobar Islands are lined with mangroves and they have restricted scope of adjustment in response to sea level rise, the impact of climate change on extent and species composition of mangroves may be devastative when sea level rise exceed about 10cm/100 years.

An extensive hypersaline mudflat occurs between mangrove line and main coast in the Kachchh in Gijarat, which are on rise due to siltation and tectonic movement. In the background of this fact, sea level rise of 12 cm/100 years or even more may not have any negative impact. Increased land accretion may also contribute to rise on mudflats. Thus, this region may have least impact of sea level rise, but when relative rise is very high, the mangroves may shift towards the Rann of Kachchh, which was part of the sea in the past. Other mangrove areas do not have such an extensive barrier to allow this scale of adjustment against sea level rise.

3.4 Species diversity and vulnerability

Mangroves in tropical region extremely sensitive to global warming because strong temperature dependence of physiological rates places many tropical species near their optimum temperature. Increased species diversity at the community level will add to the competitive ability of mangrove communities as a whole. Outside the present latitudinal limits for mangroves, comparable saline coastal environments are generally occupied by salt marsh vegetation. It is likely, given the more herbaceous nature of the vegetation in these communities that mangroves will compete such species in the medium to long term and that a gradual replacement of salt marsh vegetation by scrubby mangroves, first of Avicennia and later of Rhizophora may be expected to occur (Pernetta, 1993).

It is also expected that average global rainfall will increase with marked regional variations (IPCC 2001). If this happens, climate change is likely to lead to an increase in species migration pole wards. This may result into better environment for mangroves in semi-arid region like Gulf of Kachchh. In absence of accurate prediction on extent and rate of climate change, it is not possible to develop a model for likely scenario of mangroves in India. Many species are sensitive to fast changes, especially to anthropogenic disturbance and sea level rise. If pace of sea level is high, these species may not be successful to compete and may loose in favour of hardy and great coloniser, especially Avicennia marina, A. alba, Acanthus ilicifolius and Suaeda sp. in semi-arid in Gujarat and, A. officinalis and other species in the moist region. It is expected that species diversity may suffer in some areas, especially in Andaman and Nicobar Islands.

Sensitivity and adaptability of the species in the critical ecosystem become important to assess the vulnerability of a species. Tidal forest of India support about three and half dozen species of core mangroves and majority of them may be sensitive to medium to high rate of changes and they be categorized as highly vulnerable species because they can survive for long period in only a relatively stable environment. Adaptive capacity of some of them can be improved by management intervention, especially by planting them in suitable areas after assessing the trend and rate of environmental changes. In case of climate change and sea level rise, loss of some species in one area, especially on islands, can be compensated by planting them in the area of higher latitudes, although over all loss can not be recovered.

4. Adaptation and Management Options

Inter-tidal mudflats, saline and less productive coastal lands provide opportunity to raise coastal forests as a multiple use ecosystems (sink for carbon; barrier against cycle, storm and salty winds, coastal land stability; sustainable agriculture behind shelter belt and basic needs of coastal community). Strict protection of existing mangroves against encroachment and cutting and its expansion by regenerating potential intertidal areas through plantation of suitable species, including vulnerable and threatened species appears to be necessary management options. Adaptability capability of the species, which may not adapt quickly to climate change, can be improved through management intervention, especially by facilitating their regeneration in new areas. The scientific studies and consistent monitoring the ecological changes and sea level rise should be done to provide continuous inputs for necessary management intervention. The response of tidal vegetation to climate change will vary from area to area and hence area specific plan based on inputs of continuous monitoring of changes should be prepared for implementation.

5. Conclusion

Present mangrove cover in India is only about 26 % of the area of treeless inter-tidal mudflats. At present hyper saline area of high tidal mudflats is not suitable for mangroves but scenario may change with sea level rise. Large such areas, especially in semi-arid region of Gujarat, are expected to provide adequate scope for adjustment and adaptation of mangroves against sea level rise. If rainfall improves in western part of India, as projected in the Northern Hemisphere, the large high tidal mudflats in the Gulf of Kachchh and Gulf of Khambhat can be used for regenerating suitable species to improve species diversity and also to compensate loss in other areas. Deltaic region of the Sundarbans also have scope for adjustment of mangroves against sea level rise but human habitation in the landward may prove barrier and limiting factor. In absence of accurate regional data on climate change, it is difficult to conclude likely impact on short and long term basis. Very high level of sea level rise may be devastating for most of the mangroves in Indian sub-continent. Mangroves of Andaman and Nicobar Islands are matter of concern for even low rate of sea level rise.

References

1. Anon., 2001. India 2001- A reference annual compiled and edited by Research, Reference and Training Division, Ministry of Information and Broad-Casting. Government of India: 873 p.

2. Clough, Barry F., 1994. Climate impacts on mangrove ecosystems. Conservation of mangrove forest genetic resources. M. S. Swaminathan Research Foundation, Chennai, India: 39-44:487 p.

3. Ellison, J. C. and D. R. Stoddard, 1991. Mangrove ecosystem collapse during prediction sea level rise. Holocene analogues and implication. Journal of Coastal Research; 7(1): 151-155.

4. FSI, 1999. State of Forest Report, 1999. Forest Survey of India, Dehradun, Ministry of Environment and Forest: 113p.

5. ISRO, 1998. Coastal Wetlands of India, National- wide wetland mapping project. Space Applications Centre), A’bad: 237p.

6. IPCC, 2001.Climate Change 2001. Synthesis Report. A contribution of the working group I, II and III to the Third

Assessment Report of the international Penal on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New Yark, NY-USA: 398 pp

7. IUCN, 2002. Climate Change Institute. IUCN-The World Conservation Union. Website 20/04/2002.

8. Pernetta, John C., 1993. Mangroves Forests, Climate Change and Sea Level Rise: Hydrological Influences on Community Structure and Survival, with examples from the Indo-West Pacific.

8. Singh, H.S. 2000. Mangroves in Gujarat-Current status and conservation strategy. Gujarat Ecological Education and Research (GEER) Foundation, Gandhinagar: 128 p.

9. Singh, H. S. 2002. Marine Protected Areas in India: Status of coastal wetlands conservation. GEER Foundation: 31p

10. Worldwatch Institute, 2002. State of the World 2002- Progress Towards a Sustainable Society.

11. WWF, 1992. India’s Wetlands-mangroves and coral reefs. World Wide foe Nature India, New Delhi: 61p.

Note: Revision of paper Paper submitted to the XII World Forestry Congress 2003, Quebec (Quebec) Canada after receiving comments from the peer reviewers of the WFC


[1] Conservator of Forests, Vadodara Circle, Kothi Building, Roapura, Vadodara-390001, India.
Tel: 091-265-2412338; 091-265-2340397; Fax: 091-265-2429748; Email: [email protected]