Preface India is one of the 17 mega biodiversity countries in the world. However, taxonomic inventory of India biota is far from complete. The goal of this publication is to identify and describe the mangrove plant species of India. Towards this objectives, the taxonomical identity and distribution of mangrove species found in the country is revised and described. During field investigations, each species of mangrove and associated plants were identified and sampled. Description and high-resolution image material for each species is provided, which allows easy identification of individual species. This information is needed for coastal management planning and policy development, especially in relation to shoreline rehabilitation and expansion of appropriate shoreline livelihood projects in the face of climate change and sea level rise.

Mangroves Definition A mangrove is tree, shrub, palm or ground fern, generally exceeding one and a half meter in height that normally grows above mean sea level in the intertidal zone of marine coastal environment and estuarine regions of the tropical and subtropical coastlines (Duke 1992). The mangrove is also defined as assemblages of salt tolerant trees and shrubs. Mangroves are taxonomically heterogenous and more related to environment than terrestrial or aquatic plants. The term ‘mangroves’ is used to define both a group of plants and a community. To avoid confusion, Macnae (1968) proposed the term ‘mangal’ for the mangrove forest community, and the term ‘mangroves’ for the plants. Other terms for the mangrove forest community are coastal woodland, oceanic rainforest, intertidal forest, tidal forest, blue carbon forest, mangrove forest, and mangrove swamp. Generally, mangrove species are grouped into two types: Exclusive and Non-Exclusive. The exclusive species are limited to the mangrove habitat, and they are also referred to as strict mangroves, obligate mangroves or true mangroves. The non-exclusive species are mainly distributed in a terrestrial or aquatic habitat, but they also occur in the mangrove habitats and they are referred to as semi-mangroves, back mangroves or mangrove associates. Tansley and Fristch (1905) are the first to introduce the criteria to classify mangrove species into true mangroves and mangrove associates. Tomlinson (1986) used rigid criteria to distinguish true mangroves from mangrove associates. In his criteria, true mangroves possess all or most of the following features:

Origin and diversification of Mangroves Mangroves are quite old, possibly arising just after the first angiosperms (Duke 1992). Based on the fossil evidence and recent molecular studies, it is believed that mangroves originated along the shorelines of ancient Tethys Sea and there was a dispersal along the Tethys seaway westward into the Atlantic and further into the east Pacific (long before the closure of the Panama Isthmus) and eastward into South east Asia and Australia during the Late Cretaceous or early Eocene around 80 million years ago (Duke 2006). Nypa seems to be earliest fossil evidence. However, prior to closure of Tethys sea, around 40-55 million years ago, the mangrove genera - Nypa, Acrostichum, Rhizophora, Avicennia, Pelliciera, Sonneratia, Bruguiera, Ceriops, Heritiera, and Aegiceras - were present around shoreline (Duke 2017). The closure of Tethys Seaway by the mid-Tertiary (34-50 Million years ago) by northward movement of African landmass, created a physical barrier that completely terminated the Atlantic East Pacific (AEP) and Indo-West Pacific (IWP) exchange route, and resulted in subsequent diversifications within the AEP and IWP regions influenced by region-specific events (Ellison et al. 1999; Duke 2006). The plate tectonics and other vicariant events like Pleistocene sea level rise are attributed to the current distribution pattern and disjunct distributions of species within genera. Duke (2017) hypothesised two separate westward dispersal routes in AEP: (i) a southern path along the coasts of West Africa, South America and, (ii) a northern path along the North American east coast, whereas he hypothesised two isolated pathways in IWP towards eastward dispersal: (i) a northern path along the coast to the Middle East, Southeast Asia and, (ii) a southern path along the coasts of East Africa, India, and Australia. Further he noted that collision of India and Australia with Middle East and Southeast Asia respectively, resulted complication in dispersal route. Based on the available records it is presumed that diversification of mangrove plants has largely been driven by three key processes of speciation viz., Allopatric speciation, Peripatric speciation and Parapatric speciation (Duke 2017).

Distribution of Mangroves in India Mangrove forests in India are found along the coastline of 9 States and 4 Union Territories (Fig. 5). The mangrove habitats are broadly classified into three namely, Deltaic (Eastern Coast Mangroves), Estuarine & Backwater (Western Coast Mangroves) and Insular mangroves (A & N Islands) (Mandal and Naskar 2008). India has a total mangrove cover of 4975 km2; of which, 57% is present along the east coast (Bay of Bengal); 31% along the west coast (Arabian Sea) and the remaining 12% in the Andaman and Nicobar Islands (FSI 2019).

West Bengal In West Bengal, mangroves are present in Sundarbans, the large deltaic complex of three major rivers, the Ganges, Brahmaputra and Meghna, and 62% of total area is shared by Bangladesh and 38% by India. The Sundarban Biosphere Reserve (SBR), at the confluence of the Bay of Bengal covering an area of 9,630 km2, lies between 21°40′N and 22°40′N latitudes and 88°03′E and 89°07′E longitudes, spreading in districts of South and North 24-Parganas in West Bengal, India. The major portion of SBR is designated as Reserve Forest, covering nearly 4260 km2 and the area under mangrove vegetation is 2112 km2 (FSI 2019). Compared to 2017 assessment there has been a decrease of 2 km2 area of mangroves. The mangroves and their associates in the SBR exhibit great generic and species diversity and it is abode to rare and highly endangered flora and fauna, as well as, the only mangrove-tiger (Royal Bengal Tiger - Panthera Tigris tigris) kingdom in the world. Recognizing importance and uniqueness, the United Nations Educational, Scientific and Cultural Organization (UNESCO) declared the Indian part of Sundarbans as a ‘World Heritage Site’ in 1987, and also designated the ‘Sundarban Biosphere Reserve’ under the UNESCO Man and the Biosphere (MAB) program in 2001. Recently, Sundarbans has been declared as a Ramasar Site, considering ecological significance. Totally 33 true mangrove species belonging to 21 genera and 13 families have been identified in Indian Sundarbans. Among the mangrove species listed, Rhizophora apiculata, Bruguiera parviflora, Ceriops decandra, Kandelia candel, Aglaia cucullata, Excoecaria indica, Xylocarpus moluccensis, Xylocarpus granatum, Heritiera fomes, Scyphiphora hydrophyllacea, Brownlowia tersa, Nypa fruticans, Acanthus volubilis, Dolichandrone spathacea and Phoenix paludosa are under serious threat in Sundarbans (Kathiresan 2008; Gopal and Chauhan 2006). Particularly Heritiera fomes (locally called Sundari, from which Sundarbans derives its name), Nypa fruticans and Phoenix paludosa are declining rapidly due to the reduction of freshwater input (Gopal and Chauhan 2006).

Mangrove forests are ecologically significant and economically important (Kathiresan 2018). They provide ecosystem services worth at least US$ 1.6 billion each year and support coastal livelihoods worldwide (Costanza et al. 1997). They serve as the nursery, feeding and breeding grounds for crabs, prawns, molluscs, finfish, birds, reptiles and mammals. A large amount of global fish catches (up to 80%) is dependent on mangroves, thereby ensuring the food security of coastal people (Ellison 2008). The mangroves provide firewood, timber, cattle feed, honey, medicines and tourism development. They protect groundwater aquifers from seepage of seawater, thereby ensuring water security for coastal population. Mangrove forests remove coastal pollution particularly toxic heavy metals. They offer coastal protection against the fiery effects of natural calamities such as tsunami, storm surges, cyclone and floods (Kathiresan and Bingham 2001; Kathiresan and Qasim 2005; Kathiresan and Rajendran 2005). The potential of mangroves in carbon capture and sequestration is remarkable in mitigating the impacts of global warming and climate change (Kathiresan et al. 2013, 2014).

Threats There are sixteen threat factors have been identified from Indian mangroves (Kathiresan 2010). However, habitat conversion for agriculture and aquaculture activities, environmental pollution by upstream anthropogenic activities, Climate change processes (e.g. sea level rise; increasing frequency of cylones, etc.) are the major threat factors for mangroves. Mangroves are able to cope up with short-term oscillations and long-term fluctuations of climatic conditions, but the consequences of global climate change, such as increased extreme events, will have unprecedented effects on biota and threaten the resilience and recovery potential of the ecosystems (Harris et al. 2018; Sippo et al. 2018). Despite conservation measures, the mangroves still experience an annual loss of 0.2-0.7% during 2000-2012 and they remain the most threatened ecosystem of the world (Hamilton and Casey 2016).

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