Village-wise distribution of mangrove and its associated species.
Andaman and Nicobar Islands (ANI’s) being situated in the Tropical zone is the cradle of multi-disasters viz., cyclones, floods, droughts, land degradation, runoff, soil erosion, shallow landslides, epidemics, earthquakes, volcanism, tsunami and storm surges. Mangroves are one of the first visible reciprocators above land and sea surface to cyclonic storms, storm surges, and tsunamis among the coastal wetlands. The Indian Ocean 2004 tsunami was denoted as one of the most catastrophic ever recorded in humankind’s recent history. A mega-earthquake of Magnitude (9.3) near Indonesia ruptured the Andaman-Sunda plate triggered this tsunami. Physical fury, subsidence, upliftment, and prolonged water logging resulted in the massive loss of mangrove vegetation. A decade and half years after the 2004 tsunami, a study was initiated to assess the secondary ecological succession of mangrove in Tsunami Created Wetlands (TCWs) of south Andaman using Landsat satellite data products. Since natural ecological succession is a rather slow process and demands isotope techniques to establish a sequence of events succession. However, secondary ecological succession occurs in a short frame of time after any catastrophic event like a tsunami exemplifying nature\'s resilience. Band-5 (before tsunami, 2003) and Band-6 (after tsunami, 2018) of Landsat 7 and Landsat-8 satellite respectively were harnessed to delineate mangrove patches and TCWs in the focus area using ArcMap 10.5, Geographic Information Systems (GIS) software. From the study, it was understood that Fimbrisstylis littoralis is the pioneering key-stone plant followed by Acrostichum aureum and Acanthus ilicifolius facilitating Avicennia spp/Rhizophora spp for ecological succession in the TCWs.
- natural disasters
- Landsat (7 & 8)
- satellite image
- Short Wave Infra-Red
- GIS (Geographic Information Systems)
- fluvial influx
- mangrove biodiversity
A befitting example of the interaction of Sea, land, and air is the ‘coastal frontier’. This Coastal frontier comprises of fragile, sensitive, dynamic, and diverse ecosystems like forests, estuaries, coral reefs, tidal mudflats, salt marshes, seagrass, and mangroves [1, 2]. Mangroves are circum-tropical halophytes representing an ecotone between terrestrial and marine habitats which are adapted to wet and saline conditions having a vital ecological and economic relevance at global, regional, and local scales . These mangrove forests comprise of 65 true mangrove species and 6 hybrids , housed in one hundred and twenty-three countries between 32°N and 38°S covering an area of 1.5 million sq. Km . The highest concentration (60%) of global mangrove species (44) are reported from southeast Asia [5, 6]. The mangroves of Andaman and Nicobar Islands (ANI’s) represent the third-largest cover on the Indian subcontinent next to Gujarat and Sunderbans respectively . ANI’s comprise 38 true mangrove species belonging to 19 genera, and 13 families. Thus, ANI’s houses 50% of the global mangrove species [7, 8].
Globally mangrove forests are known as among one of the most productive and biologically important ecosystems because they deliver a variety of vital and distinctive ecosystem goods and services to humankind and other coastal marine ecosystems like the mudflats, coral reefs, seagrass, etc . Since time immemorial mangrove is been conventionally used for firewood, charcoal, alcohol, folk-lore therapeutics, roof thatching [10, 11]. They act as nursery and breeding ground for the juveniles of many commercial fish, crustaceans, including avifauna and reptiles [12, 13, 14, 15]. Also, they reduce coastal erosion, stabilize the shoreline, provide sediment and nutrient retention, improve water quality, and provide both flood and flow control as well as protection against storms, hurricanes, and tsunamis [16, 17, 18, 19, 20, 21]. Carbon sequestration is presently recognized as the most important service of the mangrove owing to the growing appreciation of the efficacy of these habitats in climate regulation through fixing carbon from the atmosphere [22, 23, 24].
The mangrove forests of the world are dwindling at a rate of 1–2% annually and if this trend continues the mangrove and its ecosystem shall be erased from the face of the earth by the 21st century [25, 26, 27]. The deterioration of mangrove is more alarming than any other ecosystem like the coral reef and marine forests. At this rate of destruction, the world would be deprived of mangrove and its ecosystem services by the end of this century . The loss of mangrove forests can be attributed to anthropogenic and natural factors. Anthropogenic factors such as dumping of wet and solid wastes generated by the urban population, deforestation, conversion for aquaculture, agriculture, industrial discharge, petroleum spills, the combustion of fossil fuels, automobile exhaust are responsible for the loss of mangrove forests [25, 27, 29, 30, 31, 32, 33]. Although the mangrove forest act as a bio-shield against natural disasters such as climate change, cyclones, hurricanes, typhoons, storm surges, and tsunamis [3, 16, 17, 18, 19, 20, 21]. On the contrary, these natural factors are also partly responsible for the loss of mangrove forests . However, Mangroves demonstrates the ability to be resilient to natural eventualities [18, 35, 36, 37, 38, 39, 40] by following the fluvial influx [39, 41].
The resilience of mangrove is naturally ensured by ecological succession. It is rather a slow process of development and adjustment of species compositions of the mangrove communities over time and space. Further, the ecological succession is dependent on the vital driving factors such as growth potential of the mangrove species, dispersal, settlement, competition, and external or biogenic changes in abiotic conditions . The fluvial influx in the landmass subsided zones due to the 2004 tsunami created a conducive environment for mangrove colonization (ecological succession). Hence, the present study aims at understanding the secondary ecological succession of mangrove in Tsunami Created Wetlands (TCWs) of South Andaman so that it would help in initiating anthropogenically induced massive restoration and rehabilitation of it in the future [6, 28, 43, 44, 45, 46, 47].
2. Study area
ANI’s is a union territory of India in the Bay of Bengal between peninsular India and Myanmar, trending in a north-south direction. Bounded by the coordinates (92° to 94° East and 6° to 14° North), it is an archipelago with > 500 islands/islets, stretching over 700 km . They are closer to the Indonesian landmass than to mainland India (1200 km), with the southernmost island only 150 km from Sumatra and the northernmost landfall, 190 km south of West Myanmar. ANI’s being the cradle of multi-disasters like cyclones, storm surges, earthquakes, and tsunami, the mangroves of this region are vulnerable to disaster. However, nature has its own plans for resilience after any disaster. The present study illustrates the ecological succession of mangrove in south Andaman after the 2004 devastating tsunami. Subsidence and Upliftment of landmass were observed in ANI’s due to the 2004 tsunami . Subsidence of landmass around the coastal frontiers rendered it to be permanently waterlogged thus creating wetlands that are very conducive for the mangroves to colonize [37, 38, 39]. The area under focus is bounded by the coordinates 11°27′00″and 11°45′00”N and 92°30′00″and 92°46′47″E (Figure 1) covering a land area of 333.18 km2 that encountered destruction from the 2004 tsunami and subsidence as well [48, 49].
2.1 Geology, soils, geomorphology, and drainage
The origin of the Andaman-Nicobar islands is approximately dated as late Pliocene to Pleistocene . The subsidence of landmass is defined by the rock type. Two types of rocks are encountered in the study area viz., (1) Sedimentary rock (Andaman flysch), and (2) Ophiolite suite of volcanic origin [51, 52]. Sedimentary rock comprises of greywacke, siltstone, chalk, limestone are soft and more susceptible to subsidence due to tectonic activity when compared to the Ophiolite suite (Figure 2a).
Geomorphogically the study area is dominated by the structural hill, valley trending N-S direction followed by pediments and coastal plains (Figure 2b). The coastal plains are dominated by alluvium and colluvium.
The soils of the study area have developed under the dominant influence of vegetation and climate and over diverse parent material. The soil is either present on the hill tops or deposited in the valleys or along the coast as escorted soil. Along the coast, the soil is sandy and contains shingles and old corals, etc. It is extremely porous. In the valley and in the lower slopes of hills, the soil is clayey loam. On the hills, it is rigid clay and dark red loam. There are three orders of soil Entisols, Inceptisols, and Alfisols  in six soil texture class viz., Clay, Clay loam, Loamy sand, Sandy, Sandy Clay, Sandy Clay loam. Clay loam is the dominant textural class of soil well distributed throughout the study area followed by clay. Sandy texture was seen along the coastal fringes (Figure 2c).
The drainage in the area under investigation exhibits dendritic and trellis patterns a typical structurally controlled drainage pattern of volcanic origin. In general, almost all the drainages are very young and terminate their first or second-order stream within a short distance. There are no landlocked watersheds hence all the streams empties into the adjacent sea (Figure 2d).
The study area is situated south of Tropic of Cancer and the region is surrounded by warm seas. The climate of this region is categorized as Warm and Humid. The recorded average temperature ranges from 25°C to 30.5°C. The prevalent temperature along with relatively high humidity gives rise to perceptible and sultry weather. However, this type of weather is moderated with pleasant sea breezes. The relative humidity is high throughout the year reaching > 90 % during the northeast monsoons. The maximum temperature recorded at Port Blair is 32°C. The average annual rainfall is around 3000 to 3500 mm. May to August is the rainiest months and April is the warmest month in this region. It is observed that the South-West monsoon brings in most of the rainfall. During May-June, the onset of the monsoon occurs and in September-October withdrawal of monsoon is observed. The North-East monsoons beginning in November and persists till the end of February. This transitional period is nonetheless disturbed by cyclonic storms which may be accompanied by thundershowers. Most of the storms experienced by the mainland and the area under investigation originate in the Bay of Bengal .
3. Conducive environment for mangrove ecosystem
The prevalent geology, soil, geomorphology, drainage system, and climatic conditions in the study area favour the tall and gregarious growth of mangrove flora. The rocks of sedimentary origin are more susceptible to weathering than volcanic rocks. Tropical rains weather the rock material and escort them to the coastal front through the natural drainage system along with abundant freshwater. The climate of any tropical intertidal zone acts as a vital and requisite factor for the natural growth, development, and succession of mangroves. Among these necessary climatic factors are (i) the temperature fluctuation-ranges between 20°C and 30°C [55, 56], (ii) the humidity is of a higher range , (iii) the total annual rainfall is above 1000 mm , (iv) there is regular wind flow, (v) the area is frost free , (vi) radiation and (vii) sedimentation along with upstream water supply plays a very dominant role for the growth and viability of mangrove in a holistic manner .
4. Materials and methodology
Landsat (7 & 8) satellite data products before (2003) and after (2018) tsunami respectively, for the study were downloaded from the website (www.earthexplorer.usgs.gov/). The study area is covered by the scene with path (134) and row (52). Mangrove patches and water bodies decipherably picked up very well by band-5 and band-6 by the short-wave infrared (SWIR) sensor of Landsat 7 and 8 satellites respectively from other features like the forest, human settlements, etc. Using ArcGIS Desktop 10.5 software mangrove patches and TCWs were demarcated.
Apart from the demarcation of TCWs, stream networks were delineated from the 1979 Survey of India (SOI) toposheet. An overlay analysis of stream network was comprehended over (1) satellite imageries, (2) geology map, (3) geomorphology map, (4) soil texture map, and (5) village administrative boundary map to understand the source of fluvial Influx dynamics and ecological succession.
Village-wise mangrove stand and TCWs (subsided landmass and permanent waterlogging thereafter) were inferred from before and after tsunami satellite image interpretation. A fishnet grid of 1 km2 covering mangroves and TCWs was generated with unique ID’s and the same was converted into Global Positioning System (GPS) compatible format (*.gpx). These grids were loaded in the handheld Garmin 62CSX, GPS for field investigation. Enumeration of mangrove species was carried out through a 150 m line transect technique  with a 50 m interval between each transect within the 1 km2 grid during the dry season (January-May, 2019 and March-April, 2020). These line transects were laid orthogonal to the coast either ways (land to sea and sea to land). A subplot of 4 m2 dimension was laid for enumerating individual plants . Mangrove phenology and habitat description were carried out as per Debnath 2004 .
5. Results and discussion
Through field survey, a total of twenty-eight mangrove species around existing mangrove and TCWs in forty-three village locations were enumerated and presented in Table 1. Also, village-wise pre-tsunami landuse with soil type and the maximum distance from the existing mangrove patch (km) were tabulated in Table 2.
|Sl no||Village name||Soil texture type||Pre-tsunami land use land cover||Max distance from the existing mangrove patch (km)|
|1*||Chidiyatapu||Clay loam||Agricultural Land & Settlement||1.2|
|2*||Manjeri||Clay loam||Agricultural Land & Settlement||0.12|
|3||Guptapara||Clay and loamy sand||—||—|
|5||Hashmatabad||Sandy clay loam||—||—|
|8*||Chouldari||Loamy sand||Agricultural Land & Settlement||0.55|
|9*||Portmout||Clay loam||Agricultural Land & Settlement||0.29|
|10*||Hobdipur||Clay||Agricultural Land & Settlement||0.35|
|11*||Balu Ghat||Clay loam and sandy clay loam||Open jungle||0.15|
|12*||Mohwa Dera||Sandy and sandy clay loam||Open jungle||1.5|
|13||Temple Myo||Clay loam||—||—|
|14*||Tirur||Clay loam, Loamy sand, clay, sandy clay loam||Agricultural Land & Settlement||0.25|
|15||Shore Point||Clay loam||—||—|
|16||Bamboo Flat||Clay loam and clay||—||—|
|17||Mathura||Clay and clay loam||—||—|
|18||Brindaban||Clay and clay loam||—||—|
|19*||Namunaghar||Clay and clay loam||Agricultural Land & Settlement||0.27|
|20*||Dundas Point||Clay loam||OpenJungle||0.65|
|21*||Mitha Khari||Clay||Plantation/Agricultural land||1.5|
|22*||Ograbraj||Clay and clay loam||Agricultural Land & Settlement||0.22|
|23||Badmasphar||Clay and clay loam||—||—|
|24||Craikabad||Sandy and clay loam||—||—|
|25||Dhanikhari||Clay and clay loam||—||—|
|26*||Sippighat||Clay and clay loam||Agricultural Land & Settlement||1.01|
|27*||Garacharma||Clay and clay loam||Agricultural Land & Settlement||0.45|
|28*||Dolligunj||Clay and clay loam||OpenJungle||1.05|
|29||Minne Bay||Clay and clay loam||—||—|
|30||Ward X||Clay loam||—||—|
|31||Ward VII||Sandy clay||—||—|
|32||Ward IV||Sandy clay||—||—|
|33*||Ward XVII||Clay||Agricultural Land||1.01|
|35||New Rangachang||Clay loam||—||—|
|36**||Nayasahar||Clay and clay loam||Agricultural Land & Settlement||2.5|
|37**||Bimblitian||Clay and clay loam||Agricultural Land & Settlement||1.2|
|38**||Taylerabad||Clay and clay loam||Agricultural Land & Settlement||0.87|
|39**||Muslim Basti||Clay and clay loam||Agricultural Land & Settlement||1.32|
|40**||Kanyapuram||Clay loam and clay||Agricultural Land & Settlement||0.95|
|41**||Govindapuram||Clay loam and clay||Agricultural Land & Settlement||1.47|
|42**||Stewardgunj||Clay loam and clay||Agricultural Land & Settlement||1.91|
|43**||Wimberlygunj||Clay loam and clay||Agricultural Land||2.02|
Tsunami is rather a rare disaster in the Indian Ocean . A mega-earthquake of magnitude 9.3 on the Richter scale struck near Indonesia On December 26th, 2004 at 07:58:53 local time [64, 65]. The epicenter was located 80km west of the coast of Northern Sumatra (at approximately 95°51' W and 3°25'N). The earthquake advanced thereafter approximately northward rupturing 1200 km to 1300 km (with an average rupture speed of 2.5 to 3 km/s) of the Andaman-Sunda plate in about 8 to 10 minutes [66, 67, 68] causing up to ∼6 m of bottom subsidence and ∼10 m of uplift parallel to the rupture and about100-150 km wide across the subduction area . Upliftment and subsidence of landmass  were generated as a consequence of earthquake elastic rebound, offshore of Banda Aceh, the northern tip of Sumatra . Rupture of the plate and coseismic activities spontaneously triggered a tsunami catastrophic devastation ever witnessed in the modern history of humankind [70, 71, 72, 73]. All the above sequential events just occurred in a short span of few hours resulting in unprecedented destruction and mangroves were one of the first visible responders of the tsunami [3, 74, 75, 76, 77].
Voluminous literature speaks about mangrove demonstrating resilience after a disaster like hurricane, cyclone, and tsunami [18, 35, 36, 37, 38, 39, 40, 78, 79]. However, very few studies were conducted to understand the dynamics of the ecological succession of mangroves after natural disasters like hurricanes and tsunamis . The mangroves of the study area faced the double impact of mortality due to 26th December 2004 tsunami viz., (1) physical fury, and (2) prolonged submergence due to subsidence of land mass [38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48]. Zones of subsided landmass were waterlogged permanently resulting in (TCWs). Nudation of mangrove (Figure 3) occurred due to a catastrophic event .
Overlay analysis of geology geomorphology and stream network of pre-post–tsunami satellite imageries suggest that subsidence of landmass (TCWs) has occurred in the regions of sedimentary rock and on the coastal plains. Sedimentary rocks (Andaman flysh) being soft are more susceptible to deformation due to tectonic activity when compared to volcanic rock (Ophiolite suite). Also, the streams once which were emptying itself in the shallow depths of the coastal frontiers started depositing in the TCWs (Figures 1,3, and 4). Mineral-rich fine sediments and abundant freshwater were deposited into TCWs through the conduits of natural streams network (Figure 4). Freshly deposited fine sediments are barren and are called as mud banks.
These mud banks in the TCWs were wet, saline, and poorly aerated proves unfavourable for higher plants  so, microbes and algae prepare the mud banks for the utilization of higher plants by aerating them [83, 84]. Also, sediments are counteracted by compaction and consolidation of both mud and peat . Coaction of non-woody key-stone species like
|Sl no||Species name||Phenology||Habitait|
|1||Mar-Jun||Jun-Aug||Common along tidal streams, inland borders of Mangrove swamps under the influence of salt or Brackish water|
|2||Apr-Jun||Jun-Aug||Gregarious in brachish swamps along the seashore and tidal streams|
|3||NA||NA||Landward side of mangrove, survives in TCWs completely cut off from sea|
|4||Throughout the year||Often found in inner mangroves along with |
|5||Apr-Jun||Jun-Aug||Often found in high intertidal and intermediate estuarine position also present in downstream and low intertidal areas. It is a dominant species in highly polluted areas|
|6||Jun-Aug||Aug-Oct||Often found in low and high intertidal position and also occur in mid and upper estuarine position along the banks of the creek.|
|7||Mar-Jun||Jun-Aug||Gregrious on stiff clay behind Avicennia, sometimes in association with Bruguiera gymnorrhiza|
|8||Throughout the year||Commonly occur in intertidal zone, along creeks, usually associated with Rhizophora apiculata and R. mucronata|
|9||Apr-Jul||Jul-Sep||Occur in intertidal zones of esturaine swamps in association with Bruguriera gymnorrhiza,Rhizophora apiculata and R. mucronata|
|10||Mar-Jul||Jul-Oct||Occur in intertidal banks of mangrove, also in areas nearer to and under esturaine influnce|
|11||Sep-Nov||Dec-Feb||Frequent to back mangrove in the Heritiera littoralis zones|
|12||May-Jun||Jun-Aug||Sporadic occurrence, found around the inner edge ofmangrove swamps in association with Sonneratia caseolaris and Heritiera littoralis|
|13||Mar-Jun||Jun-Aug||Occurs in muddy or sandy shores, commonly found in intertidal|
|14||Mar-Jun||Jun-Aug||Commonly found in intertial zone, frequently extending into muddy or sandy shores|
|15||Jan-Apr||Apr-Oct||Occurs in middle zone of mangrove forest, where soulble salts are more|
|16||Jan-Apr||Apr-Jul||Occurs in muddy or sandy elevated zones of esturaine and backwater|
|17||Feb-Jun||Jun-Sep||Sheltered intertidal creeks of mangrove swamps, preferably low saline regions|
|18||Aug-Dec||Dec-Apr||Occur along the edge of mangrove forests|
|19||Jan-May||May-Aug||Elevated muddy swamps, esturaine banks, can tolerate higher percentage of salt, even found on the sea coast too|
|20||Sep-Nov||Nov-Marp||Littoral shrub,often found in the tidal forests|
|21||May-Jul||Jul-Sep||Occur in intertidal regions of the creek in the in sheltered parts of mangrove|
|22||Jul-Sep||Sep-Nov||Occur in intertidal banks of creeks or in estuaries|
|23||Jul-Sep||Sep-Nov||Often found in mid to low intertidal and downstream tidal creeks; grows in a variety of habitats and disrupted mangrove areas. One distinctive niche is its ability to grow on edges of small coral islands, establishing on the coral substrate.|
|24||Mar-Aug||Mar-Aug||Along mangrove creeks|
|25||Mar-Jun||Jun-Sep||Occurs along the mouth of tidal creeks, grows on sandy or rocky soil|
|26||Mar-Jun||Jun-Sep||Often occurs on the landward edge of mangrove swamps in brackish water and muddy soil.|
|27||Throughout the year||Occur in the sheltered banks in association with Kandelia candel, Rhizophora sp and Sonneratia alba|
The mangrove seedlings were transported to the TCWs through the tidal influx from pre-existing mangrove (Tables 1 and 2) and thus had a stable environment. TCWs being situated in the shallow sheltered bays with low tidal amplitude favours the rooting of propagules [82, 89]. Mangroves follow the existing patterns of fluvial influx and their distribution is determined by the formation of banks, deltas, channels, levees, lagoons, and bays [55, 90, 91, 92]. Mangroves respond to geomorphic changes [93, 94] and attain a steady-state system in low energy tropical saline environments . Mangrove succession is a continuous process, where the species recruitment and replacement is systematic and anticipated . It has to be noted here that the likelihood of this phenomenon is of enormous benefit in assessing the evolution towards the climax species complex. The ecological succession from land towards the sea in TCWS in south Andaman is as follows:
From the present study, it is understood that secondary ecological succession has occurred in Andaman after the catastrophic 2004 tsunami. Key-stone species like
The author would like to acknowledge Dr. G. Narshimulu, guest faculty, Dept. Geography, JNRM, Port Blair, Andaman for the capturing field photos. Also, gratitude’s is due to Dr. Manoharan, PG- teacher, Rangat, Middle Andaman for inspiring to write this book chapter.
Field visits and other incidentals were spent from our own coffer and no external funding’s were received from either national or international agencies.
Conflict of interest