Featured image: Rhizophora mangroves. Photo: J. B. Friday/Flickr, CC BY 2.0
In the 2004 Sumatra-Andaman earthquake-tsunami, as land sank and the sea suddenly rose at Car Nicobar Island, mangroves facing the land were unable to survive. But the abrupt disturbance did not affect the sea-facing mangroves dominated by the Rhizophora spp., a study has said.
Seaward mangroves dominated by Rhizophora spp. could fight back the prolonged flooding and the pounding of the waves. But the landward mangroves comprising the Bruguiera spp., Lumnitzera spp., Sonneratia spp. could only take so much – they were unable to survive the sudden one-meter land subsidence at Kimios Bay in Car Nicobar Island, a part of the Nicobar Islands, during the major seismic event.
“The abrupt sea-level rise (SLR) in the Andaman and Nicobar Islands due to the sinking of the earth’s crust by 1.1 metre provided insights on species-level responses of mangroves to SLR,” said study author Nehru Prabakaran of Wildlife Institute of India. He suspects the resilience of Rhizophora spp. is probably due to the frequent geologic events in the Nicobar Islands and their adaptability to thrive in habitats that experience a long duration of flooding by seawater.
“There is a varying degree of resilience among species. For example, each mangrove species have different levels of resilience (high or low) to sea-level rise, mainly due to their morphological (structural) adaptations to thrive in different depths of tidal flooding,” said Prabakaran, stressing that the inter-specific resilience among mangrove species to SLR is a key to design conservation strategies for this economically important ecosystem that is among the most vulnerable to SLR.
The 2004 Sumatra-Andaman earthquake followed by the catastrophic tsunami gobbled up landmass. It stripped the coast off trees in the Nicobar Islands in the Indian Ocean – the landmass closest to the earthquake epicentre. Mangroves, which flourish where land and water meet, bore the brunt of the natural disaster. According to a 2018 study by Prabakaran, the event destroyed as much as 97% of mangrove cover on Nicobar Islands.
Among the sites with surviving mangroves, Kimios Bay in Car Nicobar Island was the only patch with more than 80 hectares of mangrove area that survived despite the 1.1 metre of land collapse. This 80-hectare patch is the unaffected Rhizophora vegetation. The affected spots dominated by Bruguiera gymnorhiza and Lumnitzera racemosa span roughly 100 ha.
Expanding on the ability of Rhizophora species to adjust to sea-level rise, Prabakaran said, across the world, these plants predominantly grow in the seaward side, mainly due to their pneumatophores/stilt roots that are specialised aerial breathing roots.
In Rhizophora spp, roots diverge from stems and branches and penetrate the soil some distance away from the main stem as in the banyan trees. “These roots (in Rhizophora spp.) strengthen the plants to withstand high wind speeds (frequent in the seaward zone) and longer hours of tidal water flooding. On the other hand, the Bruguiera spp. have knee roots, which means the roots go into the soil for a distance and then come out,” said Prabakaran.
Knee roots are horizontal roots growing just below the soil surface periodically grow vertically upwards then immediately loop downwards to resemble a bent knee. By repetition, a single horizontal root develops a series of knees at regular intervals. The aerial portions (knees) of these roots help in aeration of the whole root which because it spreads so widely, improves anchorage in the unstable mud, according to information about mangroves on a National University of Singapore webpage.
According to the India State of Forest Report (ISFR) 2019, about 40% of the world’s mangrove cover is found in southeast Asia and South Asia. India has about 3% of the total mangrove cover in south Asia. The current assessment shows that mangrove cover in the country is 4,975 sq km, which is 0.15% of the country’s total geographical area. Mangrove cover in the country has increased by 54 sq km (1.10%) compared to the previous assessment. West Bengal has 42.45% of India’s mangrove cover, followed by Gujarat with 23.66% and Andaman and Nicobar Islands with 12.39% cover.
Mangroves have a complex root system that efficiently dissipates seawave energy protecting the coastal areas from tsunamis, storm surge, and soil erosion. Their protective role has been widely recognised, especially, after the 2004 tsunami.
Keeping pace with sea-level rise
Mangroves don’t go down without a fight. They can keep pace with sea-level rise and avoid flooding by trapping sediments vertically, which allows them to maintain soil levels suitable for plant growth. “The faster root growth of Rhizophora spp. also allows them to quickly trap sediment and build soil to match up with the global sea-level rise,” said Prabakaran.
But a 2015 study on mangroves in the Indo-Pacific region suggests that mangrove forests at sites with low tidal range and low sediment supply could be submerged as early as 2070. In 69% of study sites, the rate at which the mangroves accrete sediment is not fast enough to match the current rate of sea-level rise, the paper said.
A recent Intergovernmental Panel on Climate Change special report cautioned that current ecosystem services from the ocean are expected to be reduced at 1.5 degrees Celsius of global warming, with losses being even greater at two degrees Celsius of global warming.
Agreeing with the findings at Kimios Bay, coastal systems research scientist R. Ramasubramanian at M. S. Swaminathan Research Foundation said a one-meter rise in sea level resulting in sudden submergence usually would not be a threat to Rhizophora plants. These plants grow 6-10 metres high in the Andaman and Nicobar Islands and have a large number of roots above the submergence level.
“The mangroves species such as Avicennia spp. and Bruguiera spp. can also withstand the normal sea-level rise of about a few millimeters a year (around 3 mm/year). This is because the root modifications (pneumatophores and knee roots) will also gradually grow along with the sea level and will be able to withstand the rise in sea level,” said Ramasubramanian.“But the 2004 event was a rare one where the SLR rise happened suddenly, and the submergence for a long period killed the mangroves that have shorter pneumatophores.”
The frontline plants that face the sea, such as Rhizophora spp., take the worst hit from tsunamis. “As the waves move towards the mangrove vegetation, their energy will be considerably reduced from sea to land. The seaward mangrove zone experiences the high wave energy, and the landward zone experiences the least. Therefore, the physical uprooting of trees is high in the frontline seaward mangrove zone. In contrast, the tree death due to subsidence/submergence related sea-level rise was high in the landward mangrove zone,” explained Prabakaran.
Legacy effects may have also shaped the resilience of Rhizophora assemblage. In the last two centuries, the Andaman and Nicobar Islands have experienced four strong earthquakes in 1847, 1881, 1941, and 2004 due to tectonic movements. The mangroves in these islands have perhaps undergone numerous sudden sea-level changes, the paper states.
“Records of mangrove and coastal vegetation being drowned due to subsidence are available across the Andaman and Nicobar Islands dating back to 1868 (Kurz 1868; Oldam 1884; Tipper 1911). Particularly noteworthy was the 1881 earthquake, which caused uplift and subsidence in Car Nicobar (Ortiz and Bilham 2003),” the study said.
The survival threshold of Rhizophora spp. appears to be between 1.1 m (as recorded in Car Nicobar) and 1.35 m of abrupt subsidence. However, further studies focusing on microcosm experiments to understand Rhizophora spp. resilience to rapid SLR at the study site is required to strengthen these observations, Prabakaran said.
“The Bruguiera spp. is coming back slowly. It is coming up in areas where earlier there were coconut plantations. These areas have become intertidal where the tidal water comes and goes,” said Prabakaran.
The United Nations Environmental Programme emphasises that mangrove forests are among the most powerful nature-based solutions to climate change. But with 67% of mangroves lost or degraded to date, and an additional 1.0% being lost each year, they are at risk of being eradicated. Without mangroves, 39% more people would be flooded annually and flood damage would increase by more than 16% and USD 82 billion. They protect shorelines from eroding and shield communities from floods, hurricanes, and storms, a more important service than ever as sea levels continue to rise. The UNEP recently came out with ‘Guidelines on Mangrove Ecosystem Restoration for the Western Indian Ocean Region’ to analyse risks and challenges to restoration projects and point to potential solutions.
Prabakaran emphasised that the choice of mangrove species matters for mangrove restoration projects. “We cannot confirm that Rhizophora species across the globe would show similar adaptability to a sudden increase in sea level, like in Car Nicobar. But a number of other researchers have confirmed that Rhizophora species across the globe are certainly performing better against the global sea-level rise (which is a gradual process unlike the sudden increase in Car Nicobar) compared to many other mangrove species,” he said.
“If the objective of any mangrove conservation and restoration projects is focused on combating sea level rise, then Rhizophora species would be comparatively the better species of choice for the frontline seaward mangrove zones, while other mangrove species should also be planted based on their habitat preferences (eg. seaward zone or landward zone),” Prabakaran added.
Most mangrove ecosystems have the species diversity to tolerate a wide range of salinity and submerge tolerance observed Ramasubramanian.
“Succession will take place naturally based on environmental conditions. Even in the Indian part of the Sunderbans, the freshwater-loving species Heritiera fomes are slowly disappearing, and other saltwater tolerant species are gradually occupying the area. The Sunderbans mangroves are resilient, and they will recover on their own. In Muthupet in Tamil Nadu, the recent cyclone Gaja had a huge impact. Large areas of mangroves were lost. Slowly it is recovering on its own,” added Ramasubramanian.