Remnant mangrove tree sites show faster recovery post 2004 tsunami in the Nicobar Islands

The 2004 Indian Ocean tsunami devastated mangroves across Southeast Asia. In the Nicobar archipelago, consisting of 22 islands situated close to the epicenter of the 2004 Sumatra Andaman earthquake, mangroves were hit hard by the fierce waves. According to a 2018 study, 97 percent of the mangroves in the Nicobar Islands were wiped out.

Recently, another study examining the recovery of mangroves in the region has found that mangrove colonisation in a site with remnant vegetation was much faster than in a site without it and that landward mangroves had higher recovery rates than those towards the sea.

The effects of subsidence — a process that lowers the elevation of a particular area — also contributed to the massive loss of mangrove forests in the Nicobar Islands. The 2004 Sumatra Andaman earthquake that preceded the tsunami lowered the elevation of land in the Nicobar Islands and this sudden drop in elevation (1 to 2.8m) in the coastal plains and mudflats resulted in seawater incursion.

“This study provided site-specific information on species zonation, site conditions and the vacant area available for future colonisation,” said Nehru Prabakaran, lead author of the study and DST-INSPIRE Faculty at the Wildlife Institute of India. “Such information can play a crucial role in planning and prioritising site-specific restoration strategies.”

The forest department has been restoring mangroves in the Nicobar Islands, Prabakaran says, noting that “their efforts have contributed enormously” to restoration in the region. “The study sites also have some plantations raised by the forest department recently,” he adds.

Mangroves are a group of trees and shrubs that grow along tropical and subtropical coastlines. They are adapted to survive in salty and low-oxygen environments. Mangrove forests provide a home to a diverse range of species and are also a source of livelihood to local communities.

In a previous study, Prabakaran’s team had identified 33 sites in the Nicobar Islands with mangrove colonisation that lacked surviving mangrove trees. Mangrove restoration programs, he says, should focus on these sites. Future plantation drives in the region can mimic the natural species zonation patterns—landward or seaward—observed in our study for better success, suggests Prabakaran.

What are the trends in mangrove recovery in the region? Does the presence of remnant vegetation affect recovery? To find out, the team surveyed mangroves during 2010- 2011 and 2019 in two severely impacted sites in the Nicobar Islands: Safed Balu creek situated in the north of Trinket Island without remnant mangrove vegetation and Kimios Bay located at the south of Car Nicobar Island, which had remnant trees. They also analysed satellite imagery from 2004 to 2019 looking at changes in vegetation cover at the two sites.

Faster recovery with surviving mangroves 

The team found that the site which had remnant mangrove vegetation recovered faster than the site without it. At Kimios Bay in Car Nicobar Island, the site with remnant vegetation, stem density increased by 640 percent between 2011 and 2019 while in Trinket Island that lacked vegetation, it increased by a mere 84 percent.

Remote sensing showed that vegetation cover at Trinket Island dropped from 29 percent immediately after the tsunami to 0 percent by 2006. Compared to pre-tsunami estimates, the mangrove cover increased marginally by 0.8 percent in 2019. On the other hand, mangrove cover in the Car Nicobar Island declined from 66 percent right after the tsunami to 35 percent in 2007. Compared to pre-tsunami estimates, it recovered to about 75 percent in 2019.

One obvious reason for the faster recovery of mangroves in Car Nicobar Island is that remnant trees can provide propagules or seeds for regeneration. There are also other factors involved. “The presence of mangrove vegetation is known to reduce wave action or water energy, which is highly important to facilitate the settling and establishment of seeds,” explains Prabakaran. “Without remnant vegetation, the seeds will be continuously disturbed by the water currents, which restricts the chances of seed settling and establishment.”

Krishna Ray, an assistant professor at West Bengal State University, who was not associated with the study, says that these findings are very similar to her observations at the sites of her restoration work on degraded mangroves in the Sundarbans. “Whenever there is at least some surviving vegetation, it becomes easier for restoration workers to do assisted ecological restoration, facilitating the already degraded site to get back its original ecosystem and communities,” Ray explains, adding that the process is very dynamic.

“We are never sure that the composition of the introduction or planting of species by us will remain as such for the long term. It is up to nature, which species will be selected for that site in the long run, and which ultimately dominate that particular site’s ecological communities.”

Landward versus seaward mangrove zones  

At both sites, the team observed a greater increase in stem density and species richness in the mangroves in the landward zone compared with the seaward zone. They also found that the species composition of mangroves in the landward zones changed drastically. Lumitzera racemosa, L. littorea, Bruguiera gymnorhiza and Sonneratia alba dominated the landward zones. Meanwhile, Rhizophora mucronata dominated the seaward zone of both sites.

The observed species composition may change in the years to come, says Ray, who expressed interest in visiting the mangroves in the Nicobar Islands after reading the paper.

“The seaward zone is typically the most challenging zone for the establishment of trees,” Prabakaran says. “Very few species could withstand the high salinity and the frequent tidal flooding, which are typical of the seaward zone. Whereas the landward zones provide the most optimal conditions for the growth and establishment of many species,” he says adding, “the landward zone is also the transition zone between mangrove and terrestrial vegetation and “the diversity of species is mostly higher in such transition zones.”

The team also noted a trend of landward shifting of the mangrove zones at both sites. Why does this happen? Prabakaran explains that when the sea level rises, the intertidal zones (under tidal water in high tide and exposed at low tide), where mangroves have evolved to grow, also shift towards the land.

“When the sea level increases or land drowning happens,” explains Prabakaran, “it will result in permanent inundation of seawater in the earlier intertidal zones and ultimately the areas become unsuitable for mangrove survival.” At the same time, he says, “it will also result in the creation of new intertidal areas in the terrestrial areas that existed just behind the mangroves.” It is these new intertidal areas that mangroves will take advantage of and colonise, says Prabakaran.

In the future, Prabakaran and his team would like to examine the role of various environmental and ecological factors in the natural colonisation of mangroves. He would also like to understand the impact of mangrove loss and restoration on the socio-economy of local communities in the Nicobar Islands.

“Such a holistic approach not only improves knowledge on the basic ecology of mangroves but also contributes to involvement of local stakeholders in the restoration and management of mangroves,” stresses Prabakaran.

Read more: Unpacking how cyclones impact mangroves’ ability to soak up carbon dioxide

Banner image: Mangrove re-establishment in the previous terrestrial zones like human habitation, coconut groves, and the terrestrial forest is commonly seen across the Nicobar Islands. The above image from the Trinket islands shows mangrove colonisation at the previous coconut grove. Photo by Nehru Prabakaran.