Tropical storms, over the past 21 years, have had an overall positive impact on the ability of mangroves in India to sequester carbon dioxide, a key greenhouse gas for global warming, according to a study.

Researchers at the Department of Geography at the University of Georgia in the United States examined how mangrove productivity or carbon uptake (indicated by gross primary productivity or GPP) had changed over 21 years and whether the frequency or l The intensity of tropical cyclones played a role in this trend. The main motivation was to explore the resilience of mangroves in India by examining the impact of storms on GPP in the past.

They examined GPP satellite data sets for seven mangrove sites along the east and west coasts of India, from January 2000 to July 2020, finding that there is a net increase in GPP for all sites in mangroves, and in particular for the mangroves on the east coast which are more frequently affected by cyclones compared to the west coast.

“It (the cyclone) creates conditions for growth and carbon uptake if there is adequate recovery time after a cyclonic event. For example, if dry conditions persist, a cyclone can temporarily relieve water stress, replenish soil moisture levels, provide nutrients, and improve rates of photosynthesis. Thus, tropical cyclones can increase the supply of fresh water and bring in more nutrients along the way, which can ultimately improve the growth of mangroves, ”study co-author and Ph.D. candidate Dina Nethisa Rasquinha, told Mongabay-India.

The seven mangrove sites from the east coast to the west coast explored in the study are: the Sundarbans in the Bay of Bengal; the mangroves of Bhitarkanika and the mangroves of the Mahanadi delta in Odisha which saw the super cyclone of 1999 and subsequent events such as cyclones Fani and Yaas; the Pichavaram mangroves in Tamil Nadu flanked by the Vellar and Coleroon estuaries of the Cauvery Delta, which mitigated the damaging effects of the 2004 tsunami on neighboring villages; the urban mangrove patches of Mumbai (Gorai) and the surrounding rural coastal belts of Maharashtra (Uran); and the bird paradise of Chorao in Goa.

While researchers hypothesize that “cyclones may have a net positive effect on mangrove GPP through intermittent and excessive nutrient input,” cyclones may have the opposite effect by negatively affecting the rate of carbon burial. and the decline in soil organic carbon (SOC). “We don’t have data to look at the SOC trend for the same time period, but that could be part of future research.”

The gaps must be filled by the lack of long-term measures to understand how the carbon balance in mangrove forests is likely to change with climate change.

“If the frequency and intensity of cyclones worsens with the progression of climate change, mangrove PPG may continue to increase as the article suggests or even level off or decrease, but we cannot say for sure because long-term measures are lacking and how mangrove forests may undergo changes in carbon balance is not well understood, ”said Rasquinha.

“Essentially, it’s possible that with the continued upward trend in GPP that we’re seeing, a tipping point could reach due to more intense or more frequent storms, and we don’t know how far that tipping point could be. occur that can decrease GPP, ”Rasquinha observed.

Filling the long-term data gap on mangroves in India

One way to fill these gaps, the authors suggest, is to invest in setting up a network of specialized measuring towers (flow towers) across the country to check how an ecosystem is doing at the local level. These towers measure how carbon dioxide and other gases are exchanged between the long-term soil-vegetation-atmosphere in what is known as the Eddy Covariance method, an atmospheric measurement technique.

Several of these towers have been installed in several natural ecosystems in India, including the mangroves of Pichavaram and Sundarbans. Climatologist Supriyo Chakraborty, who is not associated with the study, agrees that this estimate of GPP based on eddy covariance and specific surveys (soil nutrient levels and photosynthetic activity just before and after a cyclone) would help. to draw conclusions on the impact hypothesis. cyclones on carbon uptake in mangroves.

“Studying a one-to-one correlation, for example a hurricane event and a corresponding increase in primary productivity (shortly after that event) would give a clearer picture. This should be observed for several events, ”said Chakraborty, associated with the execution of the Indian government’s MetFlux India project to collect data on changes in carbon dioxide, energy and water vapor exchanges. between terrestrial vegetation and the atmosphere.

The authors also recommend developing mangrove forest research and education stations across the country. They recommend increasing collaborations with national and local laboratories and investing in laboratory capacity through field stations to conduct studies on mangrove vegetation in the field. They also support the training of local communities interested in conducting field and laboratory research through accredited certification programs and appropriate remuneration. They focus on expanding open source data sharing of mangrove research, deploying stand-alone instruments and tools, and building research networks to collect and analyze ethnoecological data on mangroves.

Rising sea levels, coastal developments and frequent tropical cyclones shape Indian mangroves which constitute three percent of the world’s mangrove forests. The last Intergovernmental Panel on Climate Change (IPCC) strongly emphasized the increasing likelihood of compound extreme climate events that are the combination of multiple factors and / or hazards that contribute to societal or environmental risk. One example is compound flooding (a storm surge combined with extreme rainfall and / or river flow), which destroys mangroves.

The east coast of India has a greater diversity of mangrove species (around 40 mangrove species) than the west coast (27 species). This variation in species diversity may play a role in the carbon uptake of mangroves.

“Some studies have shown that productivity can show an upward trend with greater diversity of species. Some species do well after a disturbance event due to their ability to regenerate quickly, while others are more susceptible to damage, ”explained Rasquinha.

Likewise, species zoning models (where a species is found along the coastal / estuarine belt) can also determine the tolerance or salinity / freshwater requirements of certain species and influence their carbon storage potential ”, she declared.

Because mangroves also pay a high price to guard against storm surges, for Odisha, the most cyclone-prone state in the country that witnessed the devastation of the 1999 supercyclone, researchers advise planting trees. mangrove species that have a greater capacity to assimilate carbon dioxide and can adapt to changing environments after cyclonic storms in the Bhitarkanika wildlife reserve and its southern part, the Mahanadi mangroves.

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“After cyclones, the salinity and nutrient profile of the substrate change and there is an increase in soil organic carbon. The biomass of species sensitive to these changes begins to deteriorate as others adapt better to these sequelae of cyclonic storms. We should opt for planting mangrove species that have a greater capacity to absorb carbon dioxide and adapt better to changes after cyclones to mitigate climate change, ”explained Kakoli Banerjee of the University. Odisha plant, Koraput, which worked on the carbon cycle in Bhitarkanika. and the mangroves from Mahanadi to Odisha.

Climate change is adding to the increasing pressures on the mangroves of Bhitarkanika and other Indian mangroves. While Bhitarkanika boasts of dense mangroves in a protected area and is an important crocodile nesting site, recently conservationists have warned of falling freshwater levels in Bhitarkanika due to the planned diversion of freshwater. to power a mega water project. The Mahanadi estuarine system with moderately dense mangroves faces constant pressures from human actions, including industrialization.

Avicennia marina, Avicennia officinalis, Excoecaria agallocha, Rhizophora mucronata and Xylocarpus granatum are the five main mangrove species in Bhitarkanika and Mahanadi. Among them, A. marina, Excoecaria agallocha, and Rhizophora mucronata are good options for post-cyclone plantings.

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On the west coast, the Government of Maharashtra’s Mangrove Cell is working on building its mangrove carbon sequestration database; They are working to understand to what extent the mangrove cover has mitigated the impacts of Cyclones Nisarga and Tauktae on the coastline in the districts of Raigad and Ratnagiri. The mangroves of Maharashtra cover 20 species over 300 square kilometers.

According to Manas Manjrekar, deputy director of research and capacity building at the Mangrove Foundation of the Mangrove Cell, “The carbon sequestration project will map the amount of carbon stored in the state’s mangroves. And the results of the study on the impacts of cyclones on the coastline bordered by mangroves will help us develop an effective coastal protection policy.

The State of the Forest of India 2019 report documented an increase in mangrove cover in the country of 54 km² compared to the previous assessment, but marked a decrease in mangrove cover in Tamil Nadu , West Bengal and the Andaman and Nicobar Islands.

(The story first appeared on Mongabay)