MANDAL & NASKAR 131

Tropical Ecology 49(2): 131-146, 2008 ISSN 0564-3295 © International Society for Tropical Ecology www.tropecol.com

Diversity and classification of Indian mangroves: a review

R.N. MANDAL*1 & K.R. NASKAR2

1Central Institute of Freshwater Aquaculture P.O. Kausalyaganga, Bhubaneswar 751002, Orissa, India

2Central Inland Fisheries Research Institute, Regional Station, CGO Complex, Kolkata, India

Abstract: Indian mangrove vegetation covers about 6,749 km2 along the 7516.6 km long coast line, including Island territories. The entire mangrove habitats are situated in three zones: (1) East Coast, about 4700 km2, (2) West Coast, about 850 km2, and (3) Andaman & Nicobar Islands about 1190 km2, with Lakshadweep Atoll. These three zones have been further categorized into Deltaic, Coastal, and Island habitats following Thom’s classification of estuarine habitats. Estimates of the number of species considered mangrove in the world, range from 48 to 90, and in India from 50-60. We estimate 82 species of mangroves distributed in 52 genera and 36 families from all the 12 habitats in India. The relative mangrove diversity (RMD) of each of the 12 habitats is calculated as, RMD=100 × [(Fn+Gn+Sn)/N], where Fn, Gn and Sn are respectively, numbers of families, genera and species, and N = 170 (sum of numbers of families, genera and species in mangrove vegetation of all the 12 habitats in India). Sundarbans recorded the maximum RMD (90%) and Lakshadweep Atoll the minimum (9.4%). The inter tidal vegetation is classified into three categories: ‘Major mangroves,’ ‘Mangrove associates,’ and ‘Back mangal’, on the basis of their morpho-anatomical characters representing adaptation to halophytic condition.

Resumen: En la India la vegetación de manglar cubre cerca de 6,749 km2 a lo largo de

7,516.6 km de línea costera, incluyendo territorios isleños. Todos los hábitats de manglar están situados en tres zonas: (1) la Costa Oriental, con cerca de 4,700 km2, (2) la Costa Occidental, con cerca de 850 km2, y (3) las islas Andamán y Nicobar, con cerca de 1,190 km2, incluido el atolón Lakshadweep. Estas tres zonas fueron a su vez categorizadas en tres hábitats: deltaico, costero e insular, de acuerdo con la clasificación de Thom para los hábitats estuarinos. Las estimaciones del número de especies consideradas de manglar en el mundo varían entre 48 y 90, y en la India van de 50 a 60. Nosotros estimamos que hay 82 especies de manglar distribuidas en 52 géneros y 36 familias en los 12 hábitats en la India. La diversidad relativa del manglar (DRM) de cada uno de los 12 hábitats se calculó como DRM=100 × [(Fn+Gn+Sn)/N], donde Fn, Gn y Sn son, respectivamente, los números de familias, géneros y especies, y N = 170 (suma de los números de familias, géneros y especies en la vegetación de manglar de los 12 hábitats en la India). En Sundarbans se registró la máxima DRM (90%) y en el atolón Lakshadweep se registró la mínima (9.4%). La vegetación intermareal fue clasificada en tres categorías: ‘Principalmente manglar,’ ‘Asociada al manglar’ y ‘Manglar posterior’, con base en caracteres morfoanatómicos que representan adaptaciones a la condición halofítica.

Resumo: A vegetação dos mangais indianos cobrem cerca de 6,749 km2 ao longo da linha

de costa, incluindo os territórios insulares. O conjunto dos habitats do mangal encontram-se

* Corresponding Author; e-mail: rnmandal2003@yahoo.com

132 DIVERSITY & CLASSIFICATION OF MANGROVES

situados em três zonas: (1) Costa Oriental, cerca de 4700 km2, (2) Costa Ocidental, cerca de 850 km2 e (3) Ilhas de Andaman & Nicobar cerca de 1190 km2 com o Atoll de Lakshadweep. Estas três zonas foram posteriormente categorizadas nos habitats Estuarino, Costeiro e Insular de acordo com a classificação dos habitats estuarinos. Estimativas do número das espécies englobadas como de mangal, oscilam entre as 48 e as 90, e na Índia entre 50-60. De todos os 12 habitats na Índia estimou-se em 82 as espécies de mangal distribuídas por 52 géneros e 36 famílias. A diversidade relativa do mangal (RMD) de cada dos 12 habitats foi calculada como RMD=100 × [(Fn+Gn+Sn)/N], onde Fn, Gn e Sn são, respectivamente, números de famílias, genro e espécies, e N=170 (soma do número de famílias, géneros e espécies de vegetação de mangal de todos os 12 habitats na Índia). Sundarbans apresentou a maior RMD (90%) e o Atoll de Lakshadweep o mínimo (9,4%). A vegetação entre-marés, na base dos caracteres morfo-anatómicos representando as condições de adaptação halófita é classificada em três categorias: “Mangal principal”, “Associadas do mangal” e “Retaguarda do mangal”.

Key words: Classification, diversity, India, mangroves, mangrove habitats.

Introduction

Biodiversity is prevalent in the tropical estuarine system, particularly in the inter tidal forested vegetation known as Mangrove (Mooney et al. 1995), which covers about 240 x 103 km2 (Lugo et al. 1990; Twilley et al. 1992). This vegetation occurs in river deltas, lagoons and estuarine complexes (Thom 1982), and also colonizes shorelines and islands in sheltered coastal area, with locally variable topography and hydrology, leading to relatively specific structure and function (Lugo & Snedaker 1974).

‘Mangrove’ has been variously defined in literature. The oxford dictionary mentioned the words ‘mangrove’ and ‘mangrowe’ since 1613, indicating tropical trees or shrubs found in coastal swamps with tangled roots that grow above the ground, whereas the Americans, the Spanish, and the Portuguese used the term ‘Mangle’ and ‘Mangue’ indicating trees and shrubs of the genus Rhizophora (Mepham & Mepham 1984). Later, the term ‘mangrove’ was referred to the individual plant or tidal forest or both, as ‘Mangrove plants’ and ‘Mangrove ecosystem’ (MacNae 1968). Chapman (1976) used the term ‘mangrove’ for inter tidal plants, and considered plant communities of inter tidal forest as mangrove ecosystem called ‘mangal’. The term ‘mangal’ was also commonly used in French and in Portuguese to refer to both forest communities and to individual plants. Several workers have opined

that plants growing in between the highest and the lowest tidal limits may be considered ‘mangrove’ (Aubreville 1964; Blasco 1975, 1977; Clough 1982; Davis 1940; Grzimek et al. 1976; MacNae 1968; Naskar & GuhaBakshi 1987; Tomlinson 1986). The tidal limits of various habitats, however, can vary.

India with a long coastline of about 7516.6 km, including the island territories (Anonymous 1984), has a mangrove cover of about 6,749 km2, the fourth largest mangrove area in the world (Naskar & Mandal 1999). These mangrove habitats (69°E-89.5°E longitude and 7°N-23°N latitude) comprise three distinct zones: East coast habitats having a coast line of about 2700 km, facing Bay of Bengal, West coast habitats with a coast line of about 3000 km, facing Arabian sea, and Island Territories with about 1816.6 km coastline. The long coastlines and their mangrove vegetation have immense role in protecting coastal biodiversity. This paper highlights the diversity of mangrove habitats in relation to environmental factors, mangrove species diversity, along with their distribution, regeneration and growth, and classification of vegetation growing in and around mangrove habitats in India.

Habitat diversity

Based on Thom’s (1982) classification of coastal habitats, the Indian mangrove habitats are categorized as below:

MANDAL & NASKAR 133

(a) Deltaic mangrove habitat

It is characterized by high tidal range with associated strong bi-directional tidal currents. These currents are responsible for the dispersion of sediments brought to the coasts by rivers and the main river channels, which are funnel shaped and are fed by numerous tidal creeks. It includes the major estuaries of East coast and two gulfs of West coast, Gujarat.

(b) Coastal mangrove habitat

It is characterized by low tidal range. River discharge of fresh water and sediments lead to rapid deposition of terrigenous sands, silts and clays to form deltas. These deltas build seawards over flat offshore slopes composed of fine-grained pro-delta sediments. It includes the inter-tidal coastlines, minor river mouths, sheltered bays and backwater areas of the West coast.

(c) Island mangrove habitat

It is commonly found at the mouths where rivers are seen to border the open sea. Neither marine nor river deposition has been sufficient to fill up what is an open estuarine system. Sometimes the bedrock of onshore area has been drowned by a rising sea level. It includes shallow but protected inter tidal zones of ‘Bay Islands’ and Lakshadweep Atoll.

Considering geomorphological characters, the mangrove habitats of India can be viewed as follows:

(a) East coast mangrove habitats

Sundarban mangrove forest (SF), Ganga delta, West Bengal; Subarnarekha mangrove forest (SM), Bhitarkanika mangrove forest (BF), Mahanadi delta, Orissa; Godavari and Krishna delta (G&KD), Andhra Pradesh; Pichavaram Estuary

Fig. 1. Distribution of mangroves in 12 different habitats in India, along with that in Bangladesh. SF= Sundarbans mangrove forest, West Bengal; SM= Subarnarekha mangrove forest, Orissa; BF= Bhitarkanika mangrove forest, Orissa; G&KD= Godavari & Krishna delta, Andhra Pradesh; PE= Pichavaram mangrove estuary, Tamilnadu; CE= Cochin estuary, Kerala; C&MA= Coondapur/Malpe area, Karnataka; ZE= Zuary estuary, Goa; BM= Bombay mangrove creeks, Maharashtra; BE= Bhabnagar estuary, Gujarat; A&N= Andaman & Nicobar islands; LA= Lakshadweep Atoll; BDF= Bangladesh mangrove habitat..

134 DIVERSITY & CLASSIFICATION OF MANGROVES

(PE), Cauvery Estuary, Tamilnadu Fig. 1, are considered to fall under deltaic mangrove forest, situated at the mouth of respective major rivers carrying freshwater, facing Bay of Bengal by North to South and North-West to South directions (Jagtap et al. 1993; Mandal 1996; Sanyal et al. 1998; Untawale & Jagtap 1992). They altogether cover about 4700 km2 or 70% of the total mangrove area, and are rich in mangrove species. In East coast habitats, SF of Indian part alone covers about 4200 km2 (Govt. of India 1987), while the undivided Sundarbans (including Bangladesh part, BDF) is the largest single mangrove block in the world (Blasco 1977).

(b) West coast mangrove habitats

Cochin estuary (CE), Kerala; Coondapur/ Malpe area (C&MA), Karnataka; Zuary estuary (ZE), Goa; Bombay mangrove creeks (BM), Maharashtra; and Bhabnagar estuary (BE), Gujarat Fig. 1, are considered to fall under coastal mangrove habitats in the inter tidal zones along with mouths of minor rivers or minor estuaries and back waters, facing Arabian sea by North-East to West and East to West directions. They altogether occupy about 850 km2 (12% of the total Indian mangrove area), though Sidhu (1963) once mentioned the cover at about 1140 km2. The mangrove vegetation is sparse, less extended, and confined to patches due to scanty upstream freshwater supply, excessive amount of silt-clay deposition, low average rainfall and relatively low tidal fluctuation (Blasco & Aizpuru 1997; Naskar & GuhaBakshi 1987; Untawale 1984).

(c) Andaman & Nicobar and Lakshadweep

islands

Andaman & Nicobar Islands (A&N) and Lakshadweep Atoll (LA), comprising more than 200 and 37 islands, respectively, harbour island mangroves (Fig.1). This is the second largest mangrove coverage, estimated at about 1199.6 km2 or about 18% of the total Indian mangrove. Tall dense mangrove forest occurs in A&N Islands due to favourable climatic conditions such as short dry season and high tidal fluctuation, heavy amount of rainfall (Blasco & Aizpuru 1997; Dagar et al. 1991; Naskar & Mandal 1999). On the other hand, small and sparse mangroves with stunted growth are reported from LA (Newberri & Hill 1981; Spicer & Newberri 1979).

Habitat diversity based on tidal

fluctuation

Lear & Turner (1977) observed that mangrove vegetation is found between MSL (Mean sea level) and HATL (Highest Astronomical Tide Level). They recommended that mangrove might be subdivided into two groups: (i) Major mangroves zone - between the MSL and MHWSTL (Mean High Water Spring Tide Level), and (ii) Minor mangroves zone - between MHWSTL and HALT. Blasco (1975), following Aubreville’s view (1964), divided Indian mangrove habitats into two groups based on the tidal range: (i) Swampy mangroves - situated below the high tidal level and submerged by sea water twice a day, and (ii) Tidal mangroves - inundated only during spring tide and sometimes by unprecedented sea surges.

Factors responsible for mangrove

species regeneration and growth

Generally, tidal waters bring nutrients along with other essential minerals to the on-shore region where they become available to mangroves. This tidal water was earlier considered to be the only factor playing a major role in the regeneration and growth of mangroves. But, it has been observed that other factors also contribute; rainfall (200 cm - 300 cm), atmospheric humidity (60%-90%), and moderate temperatures (19°C - 35°C) have been considered ideal for mangroves’ growth (Blasco 1977; Naskar & Mandal 1999). In spite of having maximum tidal fluctuations, Bhabnagar estuary (BE), therefore, does not have high mangrove species diversity because of its low average rainfall (60 cm annum-1), and inadequate upstream freshwater supply (Blasco & Aizpuru 1997). Venkatesan (1966) has argued that the mangrove habitats would remain productive as long as they get inundated with tidal water, receive high rainfall annually, and are benefited with continuous upstream freshwater which usually carry silt, sediments and organic matter. Mandal (1996) supported the above views while investigating seed germination and seedling development of mangroves. The major mangroves initially require fresh water to continue their physiological process until they develop salt secretary organs, such as salt glands, corkwart, gall, and other related mechanisms (Naskar et al. 1997; Naskar & Mandal 1999).

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Mangrove species diversity

Till date, the usage of the term ‘mangrove’ and the number of mangrove species assigned on that basis vary remarkably among different workers. The species growing in the same region are designated differentially; some consider them as mangroves, but others do not. For instance, Jones (1971) reported 27 mangrove species from the Eastern Australia, whereas Walsh (1974) mentioned 55 mangrove species from the same region, and Bunt et al. (1982) selected 45 species among them as mangrove. World-wide, Chapman (1976) reported 90 species representing mangroves, Saenger et al. (1983) recorded 83 species, UNDP/UNESCO (1986) reported 65 species, and Tomlinson (1986) mentioned only 48 mangrove species, out of which 40 were considered true mangroves found in the Old world Tropics (Indo-West pacific region) and 8 true mangroves from the New world Tropics. In India also, the reported number of mangrove species varies among researchers: 50-60 species (Blasco et al. 1975); 33 species from West coast and 47 species from East coast, but about 55 species from all the habitats (Untawale 1987); 35 true mangrove species, 28 mangrove associates and 7 back mangals (Naskar & GuhaBakshi 1987); 59 species, out of which 34 species from East coast and 25 species from West coast (Banerjee et al. 1989); and 50 species (Jagtap et al. 1993).

Indian scenario

Usually, the coastal estuarine vegetation that occurs in inter-tidal zones i.e. between the highest and the lowest tidal limits is considered to be mangrove. A total of 82 mangrove species (Table 1), distributed in 52 genera and 36 families, has been recorded by different workers. Sundarban mangrove forest (SF), West Bengal, shows the highest taxa diversity: 69 species, 49 genera, 35 families, including two species, viz. Scyphiphora hydrophyllacea and Atalentia corea reported for the first time from Indian Sundarbans (Mandal et al. 1995). In addition, Acanthus volubilis, which was regarded as extinct, has been recorded again with its very limited population existing in confined location. This species has not been reported from any other mangrove habitat in India. Importantly, it has been found that both the mangrove palms: Nypa fruticans and Phoenix

paludosa are largely restricted to Sunderban mangrove area and to Andaman & Nicibar Islands; the latter species also occurs in Bhitarkanika mangrove forest. Other habitats have the following diversity (Appendix Table 1): Subarnarekha mangrove forest (SM), 18 species, 14 genera, 11 families; Bhitarkanika mangrove forest (BF), 57 species, 37 genera, 29 families, including one new species Heritiera kanikensis (Banerjee 1984) and with relatively higher number of true mangroves; Godavari & Krishna delta (G&KD), 36 species, 26 genera, 21 families; Pichavaram mangrove estuary (PE), 35 species, 26 genera, 20 families, including one new species Rhizophora annamalayana Kathir., the hybrid of R. apiculata x R. mucronata (Kathiresan 1995). Naskar (1993) has reported more than 60 mangrove species, including 25 major mangroves in East coast zones. In the West coast, Cochin estuary (CE) with 32 species, 24 genera, 19 families, despite negligible area coverage, has the second highest number of species after Zuary estuary, Goa, which has the highest numbers: 33 species, 25 genera, 19 families, among habitats in the West coast. Coondapur & Malpe area (C&MA) has 22 species, 16 genera, 12 families; Bombay mangroves (BM) has 29 species, 21 genera, 16 families, including Salvadora persica, a mangrove associate herb growing only in two habitats (BM and BE). Bhabnagar estuary (BE) has 26 species, 19 genera, 15 families. Andaman & Nicobar Islands (A&N) harbour 61 species distributed in 39 genera and 30 families, including two new species, R. lamarkii and R. stylosa (Singh et al. 1987), and has the maximum species of the genus Rhizophora (four species). Lakshadweep Atoll (AL) has 8 species distributed in 5 genera and 3 families.

Relative mangrove diversity of habitats

An index has been developed to represent the relative mangrove diversity (RMD) of different habitats. Relative mangrove diversity (RMD) = 100 x [(Fn+Gn+Sn)/N], where, Fn, Gn, and Sn are respectively, numbers of families, genera and species of a habitat, and N = 170 (sum of reported numbers of families, genera and species from all 12 mangroves habitats in India, i.e. 36+52+82 (Appendix Table 1). Among the 12 habitats, SF records 90% RMD, followed by A&N- 76.5%, BF- 72.3%, G&KD- 48.8%, PE- 47.6%, ZE- 45.3%, CE- 44.1%, BM-38.8%, BE- 35.3%, C&MA- 29.4%, SM-

136 DIVERSITY & CLASSIFICATION OF MANGROVES

25.3%, and LA- 9.4%. Thus SF shows the highest RMD and LA the lowest among all the habitats. Higher the RMD, greater is the resemblance of the habitat to overall taxa composition of mangrove vegetation of India.

Classification

Different opinions

Saenger et al. (1983) listed 60 mangrove species growing exclusively in the tidal zones, as ‘true mangroves’. Mepham & Mepham (1984) suggested that any arborescent species growing in the tidal zones might be referred to as “Potential mangrove” or ‘Frequent mangrove’. Tomlinson (1986) recommended that mangrove species were basically of two types, viz., (1) Major element of mangals or true mangroves – with complete fidelity to the mangrove environment, and (2) Minor element of mangals – not conspicuous in mangrove habitats, rather might prefer the peripheral habitats of mangrove regions. Later, Li & Lee (1997) used two terms, viz., ‘true mangrove’ and ‘semi mangrove’. Subsequently, several workers, e.g. Watson (1928), Tomlinson (1980), Chai (1982), Mepham & Mepham (1984), and Naskar (1993) coined the term ‘Mangrove associate’ for the flora representing non-arborescent, herbaceous, sub-woody and climber species, found growing mostly in regions bordering the tidal periphery of mangrove habitats.

Indian perspective

The present study enumerated the modified morphological and anatomical characters of leaves, stems, roots, and reproductive organs of 63 species and evaluated them in relation to halophytic adaptation. Leaves: lamina (1) fleshy, flattened, shiny, (2) coriaceous in dorsal surface, thus it reflects bright sunlight and checks transpiration rate, marked with symbols lm I, 2; anatomy – (1) thick cuticle & waxy epidermis, (2) dense hair, unicelled/multicelled glands, (3) sunken stomata, (4) hypodermal aqueous tissue, (5) central aqueous tissue, (6) sclereids/stone cells, (7) salt glands/cork wart, lenticels, each contributes to reduction of transpiration rate, storing of water, maintenance of succulence, provision of mechanical support, and excretion of excess salt, marked with symbols la 1, 2, 3, 4, 5, 6, 7. Stem: (1) swollen trunk base, (2)

gall, provide mechanical support and facilitate aeration, respectively, marked with symbols sm 1,

2; anatomy - (1) greater number of vessels mm-2

and multiseriate fibre, (2) secondary anomalous growth, perform functions like uplift of a huge amount of water and protection of conductive tissue, marked with symbols sa 1, 2. Roots: (1) all types of aerial roots, (2) pneumatophores, (3) cable roots, (4) pseudo taproots, provide mechanical support and facilitate breathing, marked with symbols rm 1, 2, 3, 4; anatomy – (1) lenticels, (2) number of air cavities, (3) sclereids, (4) circular frequent sclereid ring, aid in breathing and provision of mechanical support, marked with symbols ra 1, 2, 3, 4. Reproductive organ: (1) vivipary, (2) cryptovivipary, facilitate seed germination while attached with mother plant, marked with symbols rom 1, 2; anatomy – (1) presence of numerous air cavities, (2) conspicuous vacuum between seed and pericarp, support breathing and salt excretion, and buoyancy, respectively, marked with symbols roa 1, 2. In calculation, one modified part has been assigned one point and the sum of the total points has been put against each taxon (Appendix Table 2). Families such as Rhizophoraceae, Avicenniaceae, Sonneraceae, Combretaceae, Arecaceae, Sterculiaceae, Meliaceae, Euphorbiaceae, Rubiaceae, Agialitidaceae, Poaceae, and Acanthaceae score points between 7-14. They possess characters which belong to at least three modified organs out of four, viz., leaves, stems, roots, and reproductive organs. Members of all these families are considered ‘major mangrove’. Other families such as Acanthaceae, Tiliaceae, Asclepediaceae, Malvaceae, Chenopodiaceae, Loranthaceae, Aizoacae, Boraginaceae, Verbenaceae, Amaryllidaceae, Convolvulaceae, Rubiacae, and Pteridaceae score points betweens 2 - < 7. They have characters, which belong to at least one modified organ out of four. All such species have been considered ‘mangrove associate’. None of the members belonging to the families Fabaceae, Solanaceae, and Rutaceae scores any point because of absence of character modification essential for halophytic adaptation; and so, such species are considered ‘back mangals’. Under this assignment, 30 species fall under ‘major mangroves’, 21 species ‘mangrove associates’ and 12 species ‘back mangal’.

MANDAL & NASKAR 137

Loss of mangrove area

Inspite of their immense role in protecting human resource as well as biodiversity, these unique mangrove habitats of India have been facing tremendous threats due to indiscriminate exploitation of mangrove resources for multiple uses like fodder, fuel wood, timber for building material, alcohol, paper, charcoal and medicine (Upadhyay et al. 2002). Apart from those, conversion of forest area to aquaculture and agriculture, construction of port and harbour, extension of human inhabitation, over-grazing, urbanization, industrialization, and chemical pollution are major common occurrences that dwindle mangrove area (Blasco & Aizpuru 1997; Naskar 2004; Upadhyay et al. 2002). Owing to these threats > 33% of the Indian mangrove areas has been lost within the last 15 years. Of this, East coast area has lost about 28%; West coast area about 44%; and Andaman & Nicobar Islands about 32% (Jagtap et al. 1993; Naskar 2004).

Conclusions

The proposed Relative Mangrove Diversity (RMD) of the Indian habitats can help in assessing the diversity of Indian mangrove species in different habitats: gain or loss in course of time. Study of morpho-anatomical characters in relation to adaptation to halophytic conditions can help in formulating strategic plans to afforest mangroves; the classification system based on these characters may also be a guideline to select the appropriate zone for respective flora.

Acknowledgements

We are grateful to the Director, CIFRI, Barrackpore for providing us laboratory facilities for carrying out some investigation of mangroves. We record our sincere thanks to those who helped us a lot during field study. We duly acknowledge the help extended by Dr. K. K. Sharma, Dr. N. K. Maity, Dr. M. Samanta and B. K. Sarker for their valuable suggestions for preparing this manuscript.

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