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 in global warming, said a to 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) had changed over 21 years and whether the frequency or intensity tropical cyclones played a role in this trend. The main motivation was to explore the resilience of mangroves in India by examining how storms impacted gross primary productivity in the past.

They examined satellite gross primary productivity data sets for seven mangrove sites along the east and west coasts of India, from January 2000 to July 2020, and found that there was a net increase in productivity. raw primary for all mangrove sites, and in particular for eastern coastal mangroves which are more frequently hit by cyclones than the west coast.

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

Explored sites

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 reduced the damaging effects on neighboring villages of the 2004 tsunami, the urban mangroves 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 gross primary productivity of mangroves through intermittent and excessive input of nutrients”, cyclones may have the opposite effect by negatively affecting landfill rates. of carbon and the reduction of organic carbon in the soil. “We don’t have data to look at the trend in soil organic carbon 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 worsen with the progression of climate change, the gross primary productivity of mangroves may continue to increase as the document suggests or even stabilize or decline, but we cannot say that with certainty as long-term measures are lacking and how mangrove forests may experience changes in carbon balance is not well understood, ”said Rasquinha.

“Essentially, it’s possible that with the continuing upward trend in gross primary productivity 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 point is. tipping could occur which may decrease Gross Primary Productivity, ”Rasquinha observed.

Filling the data gaps

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 soil, vegetation and the atmosphere over the long term in what is called the eddy covariance method, an atmospheric measurement technique.

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

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

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 mangrove research data sharing, 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 3% of the world’s mangrove forests. The last Intergovernmental Panel on Climate Change 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.

Diversity of 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 species diversity. 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 patterns (where a species is found along the coastal / estuarine belt) may also determine the tolerance or salinity / freshwater requirements of certain species and influence their carbon storage potential. , she said.

Because mangroves also pay a high price to guard against storm surges, for Odisha, the country’s most cyclone-prone state 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.

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

Effect of climate change

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 a prominent crocodile nesting site, recently conservationists have warned of falling freshwater levels in Bhitarkanika due to the planned diversion of fresh water 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. Of them, A marina, Excoecaria agallocha and Rhizophora mucronata are good options for planting after a cyclone.

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 mitigated the impacts of Cyclones Nisarga and Tauktae on the coastline in Raigad and Ratnagiri districts. The mangroves of Maharashtra cover 20 species over 300 km².

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 India’s Forest Report 2019 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.

This article first appeared on Mongabay.