Sonneratia ovata Backer (Lythraceae): status and
distribution of a Near Threatened mangrove species in tsunami impacted mangrove
habitats of Nicobar Islands, India
P. Nehru 1 & P. Balasubramanian 2
1,2 Division
of Landscape Ecology, Salim Ali Centre for
Ornithology and Natural History, Anaikatty (P.O.),
Coimbatore, Tamil Nadu 641108, India
Email: 1 nehrumcc@gmail.com
(corresponding author), 2 balusacon@yahoo.com
Date of publication (online): 26 December 2012
Date of publication (print): 26 December 2012
ISSN 0974-7907 (online) | 0974-7893 (print)
Editor: Marília Cunha Lignon
Manuscript details:
Ms #
o3009
Received 16 November 2011
Final received 20 August 2012
Finally accepted 25 October 2012
Citation: Nehru, P. & P. Balasubramanian (2012). Sonneratia ovata Backer (Lythraceae):
status and distribution of a Near Threatened mangrove species in tsunami
impacted mangrove habitats of Nicobar Islands, India. Journal
of Threatened Taxa 4(15): 3395–3400.
Copyright: © P. Nehru & P. Balasubramanian 2012. Creative Commons
Attribution 3.0 Unported License. JoTT allows unrestricted use of this article in any medium
for non-profit purposes, reproduction and distribution by providing adequate
credit to the authors and the source of publication.
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.
Abstract: The world’s most productive
ecosystem, the mangrove forest, is under immense pressure due to natural and
human induced disturbances. The Indian Ocean tsunami on 26 December 2004 had an
adverse effect on these habitats by breaking and uprooting the mangrove trees.
The mangrove vegetation and the coastal forest of Nicobar
Islands, India, were severally damaged by the force of the tsunami and the loss
of habitat due to the sudden rise in sea level. We studied the
re-colonization of mangroves species that began after the tsunami over 19
Islands and 25 locations present in the Nicobar group. Sonneratia ovata (Lythraceae), a Near Threatened
landward mangrove species, is reported for the first time from India. A total
of 43 individuals of S. ovata was recorded from two sites,
namely, Oh Hi Poh and Dhili Kadi, on Katchall Island. All the individuals of mangrove species
≥ 1cm girth at breast height were counted from both sites. The relative density for Bruguiera gymnorhiza (77%) and Sonneratia caseolaris (77%) are high at Oh Hi Poh and Dhili Kadi, respectively. Global
distribution, occurrence in India, threats and conservation of S. ovata are discussed in detail. The influence
of the tsunami in the dispersal of S. ovata from the nearest known sources Thailand, Malaysia or Indonesia
is not very clear. Hence, molecular based study is required for the
confirmation of the possible seed source location.
Keywords: Habitat
destruction, Indian Ocean tsunami, mangrove, recolonization,Sonneratia ovata.
For
figures, images -- click here
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 20th century (Valielaet 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, subductionof 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.
Materials and Methods
We surveyed the Nicobar
Islands from October 2010 to July 2011 for assessing the colonization of
mangrove species in the NFH. Excluding Batti Malv,
Isle of Man and Megapode Island, the entire coastal
line of all the other 19 islands, viz., Car Nicobar, Chowra,Bombuka, Teresa, Tillanchong,Kamorta, Trinket, Nancowrie,Katchall, Great Nicobar, Little Nicobar, Kondul, Kobra, Manchal, Pigeon, Pilomilo, Tris, Trak, and Meroe of the
Nicobar group were surveyed (Fig. 1).
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 ofS. 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 KatchallIsland (7051’–8001’N & 93017’–93028’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 Pohand 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 suborbicularcoriaceous 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.
Sonneratia ovata Backer, Bull. Jard. Bot. Buitenzorg Ser. 3, 2: 329. 1920; Qin & al. in Wu &
Raven, Fl. China
13: 287. 2007.
Trees up to 8m high; bark lenticellate; pneumatophores ca. 20cm high, thin,
pointed. Leaves opposite,
sub-orbicular to ovate-elliptic, 5.5–9 x 4–8 cm, attenuate to cuneate at base, entire to undulate at margins, obtuse at
apex, coriaceous; petioles 0.8–1.5 cm long. Flowers 1–3, terminal, ca. 3 cm
across, white. Bracts
orbicular, ca. 0.5 cm, cauducous. Pedicels
1.5–2 cm long. Sepals 6, coriaceous, verrucose outside,
smooth, pinkish at base inside. Petals 6, linear, 1.5–2 x 0.1–0.2 cm, white. Stamens numerous; filaments 2–3 cm long; anthers basifixed. Style 1.8–2.5 cm long; stigma capitate. Berry sub-globose,
2.5–3 x 3–4 cm, ca. 1/3 of the fruit enclosed within persistent
calyx; seeds numerous.
Flowering and fruiting: Throughout the year with peaks during February–March and
July–August (Images 2 a&b).
Specimen examined: 28.vi.2011, Andaman and Nicobar Islands, Nicobar District, Katchall Island, Dhili Kadi, P. Nehru (189). The voucher specimen is deposited in
SACON Herbarium.
Discussion and Conclusions
The genus Sonneratia has six species distributed in the tropical
mangrove forests from East Africa to Australia (Mabberley2005). 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. ovatawithin the Indian territory has not been reported
(Banerjee et al. 1989; Dagar et al. 1991; Banerjee et
al. 2002; Mandal & Naskar2008; 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 (Haininget 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) has noticed an individual of Sonneratia spp. at KatchallIsland 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 KatchallIsland. 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 1987). Hence, the scorching of littoral forests due to salt stress has resulted
in the formation of open area and the heliophilicnature 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 Sonneratiaspp. 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 KatchallIsland.
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