- Grasses can boost mangrove restoration by strengthening erosion-riddled and nutrient-deficient patches in the Indian Sundarbans, scientists have demonstrated.
- They have identified chemical clues that can signal cryptic mangrove degradation and help screen mangrove species for restoration.
- Ecological restoration is successful depending on how sustainable the ecosystem functions are in the long term and if they can withstand disturbances without disrupting functional aspects such as nutrient recycling.
Baby mangroves with leathery leaves peep out through lush meadows of grass that greet the Bay of Bengal. Soon enough these densely clumped blades and tufts of salt-tolerant grasses, in a degraded patch in the Indian Sundarbans, will fix the erosion-riddled saltmarsh to aid mangroves to expand their turf.
“As they change the sea-soaked soil for the better by pumping in lost nutrients, the grasses support mangrove expansion in a variety of ways. They protect against erosion, shield from sea wave energy, trap mangrove seeds dispersed by tidal currents and offer structural support to the seedlings,” says Ranjan Pradhan, a mangrove restorer and resident of the Indian Sundarbans in West Bengal.
Pradhan tends to the grass meadows that are in transition to support mangroves at the degraded patch on the Durbachoti island in the western part of the Indian Sundarbans. As a field staff member, he works with botanist Krishna Ray on a Department of Biotechnology-funded project that has tried out a biorestoration technology to help mangroves spring back in eroded, sick patches of islands using four native grass species.
New research shows that Sundarbans mangroves, already at risk due to climate change, sea-level rise and land-use changes, are beginning to fast erode. A mix of hard engineered structures and soft structures like mangroves can act as a barrier for shoreline erosion. Suggested measures include first developing the eroded areas with grasses such as the deep-rooted Porteresia coarctata to stabilise the soil, and then raising the mangroves.
Ray’s team relies on Porteresia coarctata, Myriostachya wightiana, Paspalum vaginatum, Sporobolus virginicus for the biorestoration. “Every grass has special features. For example, tall, dense Myriostachya wightiana clumps can absorb a lot of the wave energy during storm surges and shield against erosion,” said Pradhan who attested the healthy grass cover to clumping together grasses next to each other, with little or no space in between, as was shown in a study conducted to restore degraded salt marshes in Florida and the Netherlands. It said getting closer to neighbours can spur positive interactions between the plants. “It took multiple trials for the grasses to grow properly, especially since the soil was getting eroded,” added Pradhan.
The first site that Ray’s team took a chance on was on the bank of the Mridangabhanga-Barchara river at Ramganga in Patharpratima block of the Indian Sundarbans in 2013 over a three-hectare area. They were able to “stabilise” the soil over 0.6 hectares of the area, with grasses, over several planting cycles. Once the soil was stabilised with grasses, it was followed by mangrove transplantation in different saline zones of the patch based on salt tolerance levels of different species.
“This erosion-affected site had only sparse stunted mangrove vegetation along the fringes of the mudflat but it had an expanse of herbs like Suaeda spp. and Sesuvium sp., etc. which are features of degraded mangrove landscapes. It lacked necessary soil nutrients and displayed high salinity,” recalls Ray, a scientist at West Bengal State University.
The team set up a mangrove nursery and started transplantation with 22-23 species with very low success with Heritiera fomes or Sundari trees, a dominant mangrove tree of the Sundarbans mangrove system, informed research scholar Chayan Giri. The site now represents a chaotic tangle of 33 mangrove plants and associate plant species including the trunkless Nypa palm (Nypa fruticans), a mangrove species that has become rarer in the Sundarbans due to over extraction. Mangrove growth has also paved the way for species such as crabs, snails and butterflies to thrive.
The site on Durbachoti, where grasses have just begun nursing the soil and mangrove saplings, is one among several identified for replicating and scaling up the restoration project, both in area and budget from 3.6 hectares and Rs. 67 lakh (Rs. 6.7 million) in 2013 to 60 hectares and 2.41 crore (Rs. 24.1 million) in 2020. The scale-up is supported by the forest department.
The restoration, which the scientists call a work in progress, and related published scientific investigations in the Indian Sundarbans also yielded a range of information that can help accurately screen mangrove species for transplantation and forecast cryptic mangrove degradation, an extremely slow process that goes unnoticed but shows up after certain time in the form adverse shift in mangrove species composition, or other unwelcome changes in the ecosystem.
“When we began our work we also recorded baseline information on soil profile, species composition and plant physiological characteristics in degraded and undisturbed mangrove patches that tell us how plants are responding to different levels of salinity and nutrients and how the restoration is working,” Ray said.
One such clue that can help screen mangrove species for restoration across different salinity-affected zones and signal the cryptic ecological degradation is the amount of specific osmolytes (the molecules that are used by cells of water-stressed organisms) the mangrove species start hoarding in their cells once they sense environmental disturbances, explains S.K. Basak at Sarat Centenary College, Hooghly, who is project co-lead and co-author on the papers. Other factors include levels of water storage tissue of the plants and thickness of leaves. They believe the predictive analysis will facilitate policymakers to be on guard and expedite ecorestoration programs.
At Ramganga, for example, Ranjan Pradhan says the team ran “lobon test” (salt test) to unravel the osmolyte accumulation levels to develop their mangrove plantation strategy from the river shore to the land. The lab tests showed that Rhizophora sp in degraded patches have high levels of specific osmolytes and therefore have gained a continuum of salt tolerance and are better able to adapt to the high salt stress in the frontline towards the river shore.
“It is assumed that this true mangrove species has its in-built, well-developed salt exclusion/excretion property in addition to strong morpho-adaptive features, that together helped them survive right in front of the shoreline against barreling waves,” observed Ray.
“We also transplanted Avicennia sp, Sonneratia sp and Bruguiera parviflora species on the frontline. Above that layer, a bit further in, in the intertidal zone are Ceriops spp., Bruguiera gymnorrhiza, Excoecaria agallocha among others. Towards the land we had Phoenix paludosa, Dalbergia spinosa, Caesalpinia crista and others,” she said.
Pradhan also notes that the Ramganga site (the flagship restoration site) where the mangrove cover was raised in front of a concrete river embankment was better protected compared to the adjoining patch of land where there was no mangrove cover in front of the embankment when cyclone Amphan pummelled the Sundarbans in 2020.
While further research is needed to investigate the disaster risk reduction capacity of the restored mangrove cover, ecologist S.K. Barik, who was on the Department of Biotechnology’s environmental biology task force and has inspected the site, says the demonstration of the grass-media landscape stabilisation shows that it works well in high erosion-prone areas such as the Sundarbans.
“One point that I have emphasised in my communication to the team is that when we carry out ecological restoration it doesn’t mean the restoration of the structural aspects of an ecosystem,” Barik, Director, CSIR-National Botanical Research Institute, Lucknow, told Mongabay-India.
“If only the structure is restored it may not be a sustainable solution but the functional aspects of the ecosystem, such as the establishment of nutrient cycling, sustainable energy flow and resilience, should be restored and monitored. We must ensure that after four or five years after the project implementation, these functional attributes of the ecosystem are in place and in the right direction. Then only you can say that the ecorestoration is sustainable or the ecosystem develops resilience,” Barik said, adding that the state government could take over the site following the project.
Read more: Remnant mangroves show faster recovery post tsunami in Nicobar Island
Banner image: Field staff Ranjan Pradhan planting grasses to stabilise the Sundarbans restoration site. Photo by the research team.