Sonneratia ovata Backer ( Lythraceae ) : status and distribution of a Near Threatened mangrove species in tsunami impacted mangrove habitats of Nicobar Islands , India

Acknowledgements: The financial support received from the Forest Department of Andaman and Nicobar Islands is gratefully acknowledged. We thank Shri. Negi PCCF (WL), Shri. Ajai Saxena, CCF and Dinesh Kannan DFO of Forest Department of Andaman and Nicobar Islands for their interest in the project. Comments by Dr. Aris Dason, Assistant professor, Madras Christian College and Dr. H. N. Kumara, Scientist, SACON helped us to improve the manuscript. Mr. P. Rajan, SACON helped us in map preparation.

Mangrove forests are the most productive ecosystem adapted to thrive in the coastal margins of the tropics (Ellison & Fransworth 2001).The ecosystem service provided by mangroves includes breeding grounds for many marine organisms, a carbon sink and is also the main source of economy for coastal dwelling human communities (Kathiresan & Bingham 2001).Mangrove ecosystems play a major role in protecting coastal regions from natural calamities such as storms, hurricanes and tsunamis.Regardless of the services provided, mangrove forests are disappearing from the world at an alarming rate of 1% per year and almost 35% of the mangroves have disappeared in the last two decades of the 20 th century (Valiela et al. 2001).
The remaining mangrove forests are under immense anthropogenic pressure from felling, encroachment, shrimp cultivation, hydrological alterations, chemical spills, and land conversion (McKee 2005;Giri et al. 2008).Natural disturbances such as storms, hurricanes, tsunamis, land elevation or submergence due to tectonic movements, climate change and sea level rise are also threatening this biologically important ecosystem.
The mega earthquake of 9.15 magnitude and the following tsunami on 26 December 2004 had a serious impact on the human population and on the biodiversity of the coastal regions in the Asian countries such as India, Sri Lanka, Indonesia, Burma, Thailand, Singapore and Malaysia.The close vicinity of the Nicobar Islands to the epicenter of the earthquake resulted in a heavy impact on the mangroves and littoral forests.It was variously estimated that the tsunami wiped off 60-70 % of the mangrove forests in the Nicobar Islands (Ramachandran et al. 2005;Sridhar et al. 2006).More importantly, subduction of the Indian plate with the Burma plate during this mega earthquake caused the Nicobar Islands to sink into the sea by about 1m, this resulted in a rise in the sea level in the coastal areas where mangroves and littoral evergreen vegetation were prevalent (Sankaran 2005;Sivakumar 2007).Perennial submergence of pneumatophores resulted in the scorching of mangrove trees due to the hypoxic condition.This submergence also resulted in the formation of new inter tidal areas at the expense of flat coastal forests (littoral forest) and coconut plantations that existed adjacent to the coast (Nehru & Balasubramanian 2011) and mangroves started colonizing in these newly formed habitats (NFH).
The post tsunami research work in the Nicobar Islands on mangroves are almost all based only on remote sensing and GIS (Ramachandran et al. 2005;Sridhar et al. 2006;Porwal et al. 2012).A single field based study that described the mangrove plant species diversity in the NFH of Central Nicobar Group of Islands was by Nehru & Balasubramanian (2011).
The current study describes the population, distribution and microhabitat preference of the Near Threatened mangrove species Sonneratia ovata Backer (Lythraceae) from the NFH after the tsunami impact.
It is to be noted that the Megapode Island which was largely covered by mangrove vegetation prior to the tsunami was totally submerged into the sea after the tsunami (Sankaran 2005).Apart from a single scorched tree stump in the middle of the sea, no evidence was found during the present study confirming the existence of this island.
Qualitative data on inundation levels, substratum, freshwater influx, pre tsunami history of the habitat, adjacent forest type, tsunami destruction level, number of colonizing mangroves and mangrove associates were noted for each NFH by direct observation.Geographic coordinates were taken using the Garmin 76C SX at all the localities and the area was calculated using the GE-path software package (ver. 1.4.6).
All the individuals ≥1cm girth at breast height (GBH) of mangroves species present in the NFH were enumerated.The whole population of S. ovata was enumerated and each individual was tagged.Samples of all the mangrove species were collected for herbarium purpose.Relative density was calculated for mangrove species studied in Katchall Island.

Results
In total, 25 localities distributed over eight islands were observed to have NFH.S. ovata was observed only on Katchall Island (7 0 51'-8 0 01'N & 93 0 17'-93 0 28'E) at two localities namely Oh Hi Poh and Dhili Kadi where vast stretch of littoral forest existed prior to tsunami, but are presently colonized by mangroves (Fig. 1).The total area of NFH at these two locations was 23.9 and 4.64 ha, respectively.The littoral forest existing in these two locations experienced different modes of destruction during the tsunami leading to the transformation into mangrove forests.At Oh Hi Poh, the littoral forest that existed prior to the tsunami was uprooted and resulted in the deposition of copious amount of logs in the substratum (Image 1a).At Dhili Kadi, the entire stretch of littoral vegetation was scorched due to salt stress and hypoxia created by the rise of sea level leaving a vast stretch of snags (Image 1b).A total of 43 live individuals of S. ovata were recorded (13 at Oh Hi Poh and 30 at Dhili Kadi) along the edges of the NFH abutting the evergreen hill forest.The relative density was high for Bruguiera gymnorhiza (77%) and Sonneratia caseolaris (77%) at the NFH studied at Dhili Kadi (n=114) and Oh Hi Poh (n=237), respectively (Figs. 2&b).
S. ovata was always found intermixed with the associated species (viz.B. gymnorhiza, S. caseolaris, Barringtonia racemosa and Crataeva religiosa), whereas its conspecific S. caseolaris was mostly found as mono-dominat patches with out any associated species.The substratum in both these sites was muddy clay towards the sea and firm clay inland.S. ovata is found in the firm clay substratum towards the landward mangrove forest edge where constant seepage of freshwater from the hill forest was observed.
S. ovata is distinguished from other species of the genus Sonneratia by (i) having verrucose texture on the outer surface of the calyx, (ii) fruits are enclosed by persistent calyx, and the calyx are either reflexed or flat in the other species (iii) and suborbicular coriaceous leaves without a mucronate apex.S. ovata normally grows in the landward mangrove habitats where sea water enters only during full moon and spring tides.A detailed description and the nomenclature of the species are given below.
Flowering and fruiting: Throughout the year with peaks during February-March and July-August (Images 2 a&b).

Discussion and Conclusions
The genus Sonneratia has six species distributed in the tropical mangrove forests from East Africa to Australia (Mabberley 2005).In India, it is represented by four species namely S. alba, S. apetala, S. caseolaris and S. grifithii (Banerjee et al. 2002).Occurrence of S. ovata within the Indian territory has not been reported  (Banerjee et al. 1989;Dagar et al. 1991;Banerjee et al. 2002;Mandal & Naskar 2008;Yi-feng et al. 2011).Thus the present report is a new record for the flora of India.This species is distributed in East and Southeast Asian countries such as China, Malaysia, Singapore, Thailand, Indonesia, New Guinea and Vietnam (Haining et al. 2007) and categorized as Near-Threatened by IUCN due to severe habitat loss throughout its distributional range (Polidoro et al. 2010).
There are two probable reasons for the occurrence of S.ovata on Nicobar Islands.Dagar et al. (1991)    1987).Hence, the scorching of littoral forests due to salt stress has resulted in the formation of open area and the heliophilic nature of S. ovata has eventually ameliorated the species to colonize the NFH.The occurrence of S. ovata in the NFH abutting the evergreen hill forests confirms the landward movement of mangroves when subjected to sea level rise, as proposed by Ellison (2005).Landward mangroves are vulnerable to sea-level rise, since there may not be any habitat to re-establish because mangrove edges are highly influenced by human developmental activities (Polidoro et al. 2010).
It is strongly recommended to include landward mangrove species such as Sonneratia spp. in the mangrove restoration programs as many of the current programs give emphasis mainly to seaward mangroves such as Rhizophora spp., and Avicennia spp.(Ellison 2000).Further research based on molecular tools can provide more information on the occurrence of S.ovata on Katchall Island.
Image 1. a -A view of Oh Hi Poh NFH showing deposition of logs in the substratum by uprooted trees (23.xi.2010); b -A view of Dhili Kadi NFH showing vast stretch of snags (28.iv.2011) has noticed an individual of Sonneratia spp. at Katchall Island without any reproductive parts and speculated its identity as S. grifithii.If it was a misidentification of S. ovata, then possibly this species must have existed very rarely prior to the tsunami, and later became common in the NFH of Katchall Island.Or, the seeds of S. ovata from the nearest sources viz.Indonesia, Malaysia and Singapore must have been carried by the tsunami and established a new population on Katchall Island.Sonneratia spp.along with Avicennia are usually considered pioneers of mangrove swamps and seeds of Sonneratia spp.are intolerant to shade, germinating on bare or near bare mud banks (Duke & Jakes

Figure 2 .
Figure 2. Relative density of mangrove species at Dhili Kadi (a) and Oh Hi Poh (b) on Katchall Island