Figure 1.2. Pi chart showing mangrove cover in different regions.
In political perspective mangroves are found in 123 countries worldwide among which 12 countries comprise nearly two thirds of mangrove cover. Indonesia has maximum mangrove cover comprising 20.9 % of world total mangrove cover. Next to Indonesia, Brazil, Australia, Mexico, Nigeria, Malaysia, Myanmar, Bangladesh, Cuba, India, Papua New Guinea and Columbia have mangrove cover in descending order area wise (Table 1.1).
Table 1.1. Twelve countries with the largest mangrove area in the world, altogether comprising 68 percent of world’s total mangrove.
|Country||Mangrove Area (sq.km)||Proportion of Global Total|
|Papua New Guinea||2.8 %|
|Rest countries||22.5 %|
The minor elements of mangal are distinguished by their inability to form a conspicuous element of the vegetation. They occupy peripheral habitats and only rarely form pure communities. The Mangrove associates however, do not inhabit in habitat of strict mangrove communities, and may occur only in transitional vegetation and even exist as epiphytes. Based on above criteria Tomlinson has kept 34 species of 9 genus from 5 families under major elements mangrove, 20 species of 11 genus from 6 families in minor elements of mangroves.
Duke (1992) defines true mangrove more specifically as “a tree, shrub, palm, or ground fern generally exceeding 0.5 m in height and normally grows above mean sea level in the intertidal zone of tropical coastal or estuarine environments”. He has prepared an improve list of 69 mangrove species in 20 genera and 16 families in the world.
Kathiresan and Bingham (2003) have prepared a list of 65 species of 22 genera from 16 families which include Tomlinson's major and minor elements but not mangrove associates. They did not include three shrubby species; Acanthus illicifolius, Acanthus ebracteatus, Acanthus volubilis and two palm species; Nypa fruticans and Phoenix paludosa.
Most recently Spalding et al. (2010) in “World Atlas of Mangroves” have considered 73 species and hybrids as true mangroves. All these species have adopted to mangrove habitat. Out of 73 mangrove species, 38 species are considered as core species which typify mangroves and dominates in most mangrove ecosystems. The rest others are rarely abundant and more appropriately found on fringe of the mangrove habitats.
Polidoro et al. (2010) have considered 70 species as true mangrove based on Tomlinson's original list of major and minor mangroves supplemented by a few species added through the expanded definition provided by Duke (1992) and other new taxonomic additions by Sheue et al. (2003; 2009). This book follows Polidoro et al. (2010) list of mangroves. The list of true mangroves is given below (Table 1.2).
Pi Chart showing mangrove cover in percentage state and UT wise.
The maritime state Odisha has a coast of 480 km long coast. The beach of Odisha coast has been broken by various river mouths putting water into the Bay of Bengal. The major rivers of Odisha such as The Mahanadi, the Brahmani, the Baitarani, the Subarnarekha form estuary on their mouth region before falling into the sea. Mangrove exists on these river estuaries from the time immemorial guarding the hinterland against various vagaries of nature such as tropical cyclone, tsunami etc. The coastal districts Baleswar, Bhadrak, Kendrapara, Jagatsinghpur, and Puri has conspicuous mangrove on coastal tracts. Kendrapara district has the maximum 197 sq km of mangroves cover. The Bhitarkanika National Park and Bhitarkanika Wildlife Sanctuary and buffer zone of Gahirmatha Marine Sanctuary encompasses maximum mangrove areas of this district providing protection. North to Bhitarkanika the mangrove areas of Bhadrak district lies on the sea front and some offshore islands. In Baleswar district the mangroves are found on the estuary formed on the mouth region of the Subarnarekha river and known as Bichitrapur mangroves. On the estuary formed by the Devi river in Jagatsinghpur mangroves are present. In Puri very small patches of mangroves are found in southern bank of the Devi river mouth and in Chilika.
|Districts||Very Dense Mangrove in km2||Moderately Dense Mangrove in km2||Open Mangrove in km2||Total Mangrove Area in km2|
Although Odisha has less mangrove cover to West Bengal, Gujrat, Andhra Pradesh, Karnataka in Indian mainland, the mangrove diversity is highest. The Andaman and Nicobar Islands has a total of 38 true mangrove species belonging to 19 genera and 13 families whereas Odisha has 35 true mangrove species belonging to 20 genera and 14 families ( Ragavan et.al., 2016) highest in Indian main land. Chadha and Kar
At present the mangrove cover is only one third of the past mangrove cover of 1930s (Das, 2010). There rampart cutdown of mangrove trees for human settlement expansion as well as industrial development on the coast was the main reason behind this decrease of mangrove cover. The removal of mangrove from the coast has made the human settlements in the hinterlands prone to natural calamities coming from sea.
Generally, the two-storey system is observed in this mangrove forests since the ground flora is very poor. The dominant mangrove taxa constituting the top canopy are Rhizophora spp., Bruguiera spp., Sonneratia spp., Avicennia spp., Excoecaria agallocha, Heritiera spp., Cerbera odollam, etc. Soneratia apetala and Avicennia spp. are gregarious and luxuriant, colonising the banks of river and creeks. Xylocarpus granatum is sometimes observed in this habitat. Pongamia pinnata is also found in abundance, usually away from the water bodies. Phoenix paludosa, Thespesia populnea and Tamarix troupii in the peripheral regions where tidal inundation is irregular. In Bhitarkaika Forest Block, on elevated regions where tidal inundation is irregular there top canopy is formed by Diospyros malabarica. The second storey is composed of the shrubby elements and under-trees such as Brownlowia tersa, Kandelia candel, Lumnitzera racemosa, Ceriops decandra, Cynometra iripa, Clerodendrum inerme, Aegiceras corniculatum, Caesalpina crista and Hibiscus tiliaceus. These elements flourish where there is tidal influence as well as a high degree of salinity. B. tersa is plentiful along the fringes of water bodies and remains half-submerged in water. Like-wise, Rhizophora spp. trees with their interwoven knee roots are observed in the muddy flats. In the terrestrial and swampy areas (Flagellaria indica, Salvadora persica, Salacia prinoides, Azima tetracantha, Phoenix paludosa, etc. are quite common. Common climbers of this mangrove forest are Derris trifoliata, Derris scandens, Finlaysonia obovata, Sacrolobus globosus, Pentatropis capensis and Mucuna gigantea, etc. The ground flora which is very rare in Bhitarkanika mainly compose of Suaeda spp., Sesuvium portulacastrum and Salicornia brachiate. Mostly these species occur as indicator of degraded mangrove and accompany with dieback of top storeyed and mid storeyed mangrove tress. The elevated areas surrounded by the mangroves which are not inundated by tidal water regularly and have sandy soil have terrestrial plant such as Manikara hexandra, Ficus retusa, Ficus religiosa, Ficus virens, Toddalia asiatica, Ixora arborea, Zyziphus oenopila etc.
Among the creeks usually on the rear edge where salinity is low due to higher amount of freshwater inflow Acrosticum aureum and Acanthus ilicifolius are seen in close association. The spiny element Caesalpinia crista is also not uncommon in such localities. The ground flora is rather very poor. Usually in the muddy flats, Myriostachya wightiana is found in pure formation. Porteresia coarctata is also found in patches on creek banks. Other notable herbaceous elements are Suaeda maritima, S. nudiflora, Salicornia brachiata, etc. Tylophora tenuissima, Fimbristylis spp. etc. are found in more dry and elevated areas.
The offshore islands on the periphery of Bhitarkanika such as Kanika sands, Babubali, Mahakurdia and riverine island Kalibhanjadia has rich mangrove diversity.
The Subarnarekha estuary is endowed with a few species of mangroves such as A. alba, A. officinalis, A. marina, A. corniculatum, E. agallocha, R apiculata, R. mucronata, S. apetala and A. illicifolius etc.