Mangroves: The Lifeline of Coastal Ecosystem

Coastal ecosystems are among the most prolific natural systems on the planet, and they support a diverse range of species. They provide vital ecological services such as storm protection and fish nursery grounds. Even they are among the world’s greatest carbon sinks, with CO₂ burial rates 20 times higher than any other terrestrial ecosystem, including boreal and tropical forests. Hence, Blue carbon, or organic carbon sequestered and retained over long periods by coastal vegetation ecosystems such as mangrove forests, seagrasses, and salt marshes, is rapidly gaining traction as a significant natural climate solution.

Mangrove forests, in particular, are often regarded as the most thoroughly studied blue carbon ecosystem, with increasing clarity on their worldwide extent, areal dynamics across time, and the regional distribution of biomass and soil carbon stocks. Mangrove forests are valuable for more than only reducing climate change; they also support human communities and biodiversity by regulating nutrients, sustaining fisheries, and protecting coastlines from storms. As a result, this article provides a thorough examination of mangroves, their various characteristics, importance, and how India is planning to launch blue carbon programmes to combat climate change.

Mangrove: A Halophyte like No Other

Mangroves are a halophytic assemblage of trees, shrubs, and other plants that grow in brackish to saltwater tidal waters along tropical and subtropical coasts. It can be a tree, shrub, palm, or ground fern that grows above the mean sea level in the intertidal zone of marine coastal habitats and estuary edges and can reach a height of more than half a metre. Mangroves are salt-tolerant plants that may be found mostly between the latitudes of 24°N and 38°S. They have evolved a wide range of morphological and physiological adaptations to cope with the limiting conditions of oxygen deficiency, excessive salinity, and daily tidal inundation. Mangroves have a wide range of adaptations, including succulent leaves, sunken stomata, pneumatophores, vivipary, stilt roots, buttresses, and more.

The world’s total mangrove cover is 15 million hectares, accounting for 1% of all tropical forests. Mangrove cover in India is 4975 km², or 0.15 percent of the country’s total geographical area, according to the latest assessment by the Forest Survey of India (FSI). Very thick mangroves cover 1476 km² (29.66%) of the country’s mangrove cover, moderately dense mangroves 1479 km² (29.73%), and open mangroves 2020 km² (40.61%). West Bengal alone accounts for 42.45% of India’s mangrove cover, with Gujarat (23.66%) and the Andaman and Nicobar Islands (12.39 %). According to the evaluation, Gujarat has exhibited a maximum growth of 37 km² in mangrove cover.

The importance of mangroves in preserving coastal biodiversity, livelihoods, and mitigating climate change has been extensively acknowledged in recent years. The dramatic increase in the number of peer-reviewed research articles on ecosystem services, carbon sequestration, and threats to mangroves in India during the last decade reflects the increased awareness regarding mangroves in India. The increased interest in mangroves also presents a chance to fill knowledge gaps using cutting-edge approaches and multi-institutional cooperation, advancing our knowledge and management of Indian mangroves.

Amphibious Defenders Need Protection

Mangroves are amphibian trees that are endangered by both marine and terrestrial pressures because they are very productive ecosystems. Storms, sea surges, and rising sea levels all affect them. They are also affected by the rise of profitable industries such as aquaculture and the construction of tourist infrastructure on land. These are highly adapted plants that can endure salinity and a lack of oxygen in the soil that other trees cannot. Their root systems act as fortresses, collecting silt and absorbing the energy of hurricane waves and sea surges. Mangroves, on the other hand, are vulnerable. A change in their hydrological conditions could put them to death very quickly. Hence, Mangroves’ declining health has an impact on their resilience and recovery capability in the face of climate change effects. Despite mangrove development in many locations, the ecological health of mangroves worldwide, including in India, remains poor, and implied species loss has been observed.

Mangroves, for example, helped to reduce human deaths in Kendarapara, one of Odisha’s worst-affected coastal districts, during the 1999 super cyclone. Villages with more mangroves between them and the coast had a lower death rate than those with fewer or no mangroves. Despite the fact that some researchers doubt mangroves’ ability to protect against tsunamis, there is mounting evidence, particularly after the deadly Indian Ocean tsunami of 2004, indicating mangroves mitigated tsunami impacts by reducing the destructive force of the water rushing inland. Tsunami height is reduced by 5 to 30 % when mangrove belts of several hundred metres wide are present. Hence, wider mangrove forests are more successful at reducing tsunami height, water speed, and flooded area. However, according to a new analysis on the losses inflicted by cyclone Amphan in the Sundarbans in 2020, the ecologically sensitive region has lost 24.55 % of its mangroves due to erosion over the last three decades. Reduced sediments owing to upstream dams (human-induced) and greater wave action due to storms (natural) are the two main drivers of irreversible land loss to water in the Sundarbans in the Bay of Bengal delta. This leads to a direct loss of mangrove forests. Increased erosion is caused by excessive wave action as a result of enhanced storminess (as in Amphan), sea-level rise (due to global climate change), and coastal squeeze (land cover change from construction, aquaculture ponds, etc.) that shrinks the width of the mangrove forest, leaving less space for mangrove forests to grow inwards. However, if the mangrove length has already deteriorated as a result of reduced sediment inflow (upstream dams), tree cutting, and pollution, an already eroding shoreline would experience further erosion cycles, resulting in an excessive loss.

To cope with sea-level rise, mangroves shift vertically and horizontally. Local abiotic (sediment inputs and geomorphic settings) and biotic (plant productivity, peat development, and the accumulation of refractory mangrove roots and benthic mat materials) variables, on the other hand, have a significant impact on these processes. Mangroves’ adaptive capability is influenced by their quality and quantity. Like, Avicennia plants predominate in the Sundarbans, but their root systems are not as dense as Rhizophora’s. Rhizophora has the ability to help combat larger storms. Avicennia has taken over the mangroves of Pichavaram in Tamil Nadu, and Rhizophora can only be found on the stream banks. The fundamental reason is that deltas are getting smaller. Freshwater availability is dwindling in the Sundarbans, but glacial melt is making up for it. However, in Pichavaram, freshwater is mostly supplied by the water-scarce Kaveri.

So, enhancing mangroves’ adaptive capabilities, such as increasing communication between surrounding ecosystems, preserving the hydrological connectivity of existing mangroves, and protecting natural corridors, is a pressing issue. A site-specific, long-term, and integrated ecosystem-based conservation, management, and rehabilitation approach with good scientific information and harsh legislative measures to limit/regulate coastal development activities is urgently needed. Aside from rigorously regulating development activities near mangroves, species-specific restoration for vulnerable species and long-term monitoring of mangrove ecological health are also required.

Mangroves are frequently governed by laws intended for forests in general. In many countries where mangroves exist, the term “mangrove” refers only to the ecosystem’s woody component (mangrove forests). But, mangroves, on the other hand, are strongly linked to nearby ecosystems in nature, both seaward and landward. To protect hydrological connectivity and neighbouring natural corridors, the statutory definition of mangroves should be amended to “mangroves as an integrated system.”

Carbon sequestration potential of Mangroves

The carbon density of mangroves is among the greatest of any tropical vegetation. These ‘blue carbon’ ecosystems can store enormous amounts of carbon over extended periods of time, and their preservation helps to mitigate climate change by reducing greenhouse gas emissions. Hence, to explicitly address how mangroves preserve the carbon, firstly, we need to dig into what is ‘blue carbon’.

Mangroves, tidal marshes, and seagrass meadows are coastal ecosystems with enormous carbon stores that have been accumulated overages by vegetation and natural processes. These ecosystems sequester and store more carbon – termed “blue carbon” – per unit area than terrestrial forests do. Because of their ability to capture carbon dioxide (CO₂) from the atmosphere and store it as biomass, these vegetative ecosystems are now being recognised for their importance in mitigating climate change. This process is known as carbon sequestration.

Mangrove forests are important players in the carbon cycle and play a vital role in maintaining tropical and subtropical coastal productivity, as well as sequestering enormous amounts of carbon underneath. Mangroves are one of the most carbon-dense forests in the tropics, with a carbon storage capacity up to 50 times that of tropical terrestrial forests. With a coastline of around 7516.6 km, including island territories, India had a mangrove cover of over 6749 km², accounting for roughly 89 percent of the total coastline. With a total predicted mangrove cover of 495,842 ha in 2020 (66 percent of India’s total coastline) and a carbon stock value of 386 tonne/ha, the mangroves’ total carbon sequestration potential is estimated to be 702.42 million tonnes of Carbon dioxide equivalent (CO₂ e). In 2030, carbon sequestration potential will reach 748.17 million tonnes CO₂ e. The increased sequestration capacity of 207.91 million tonnes of CO₂ e has been estimated if mangrove cover is conserved and protected. We examined the carbon sequestration capabilities of mangroves in detail in the preceding analysis, but it is critical to grasp the term carbon stock. Carbon stock is the total quantity of organic carbon (C_org) stored in a blue carbon ecosystem, usually expressed in million tonnes of organic carbon per hectare (million tonnes of C_org/ha) over a given soil depth. All relevant carbon pools within the researched area are added to determine these stocks.

Although methodological flaws make measuring primary production in mangrove forests difficult, but the best estimates imply that mangrove carbon generation is faster than that of other estuarine and marine primary producers. Depending on the methodology used, true rates of mangrove net primary production (NPP) range from 0.5 to 112.1 t dry weight (DW) ha-1 year-1. However, most approaches either considerably exaggerate (light attenuation method) or greatly underestimate (litterfall method) the true rates of production. At this time, the most logical way to quantify forest NPP is to measure aboveground biomass accumulation plus litterfall, and there are a variety of such metrics available for both mangroves and tropical terrestrial forests. The mean rate of aboveground NPP for mangroves is 11.1 t DW ha-1 year-1, with a median value of 8.1 t DW ha-1 year 1; the mean rate of aboveground NPP for tropical terrestrial forests is 11.9 t DW ha-1 year-1, with a median value of 11.4 t DW ha-1 year 1; and NPP declines with increasing latitude for both mangroves and terrestrial forests. Given the differences in biomass, height, age, and species within and between both forest types, the rates are fairly near, implying that NPP rates in mangroves and other forests are equivalent.

To precisely understand where carbon fixed by the trees is allocated. Hence, it is essential to our capacity to assess the function of mangroves in the coastal and global carbon cycle. Mangroves, like other woody plants, grow new leaf, reproductive organs, stem, branch, and root tissues, as well as store food and provide chemical defense. Mangroves return almost half of the CO₂ they absorb to the environment through above- and below-ground respiration. Because there is a dearth of factual data and monitoring root processes and woody part respiration is challenging, this is merely a rough estimate. Light intensity, species composition, nutrient and water availability, salinity, tides, waves, temperature, and climate influence the proportional allocation of fixed carbon within trees.

Mangroves collect sediment and associated particle materials, such as inorganic and organic carbon, because they are located at the land-sea interface. Surprisingly, their presence positively promotes material accumulation. Direct inputs of mangrove carbon into the soil pool and increased rates of mass sediment accumulation are two ways that carbon is collected in mangroves. Consumption by living organisms, particularly microorganisms, is one of the other flow channels for carbon produced by mangroves. Altogether, carbon is consumed, remineralised, and either CO₂ or dissolved inorganic carbon is released into the atmosphere. Tidal currents also transport dissolved and particulate organic carbon, which can be deposited, consumed, or mineralized offshore.

Mangroves thus account for about 3% of the carbon stored by the world’s tropical forests but account for only 1% of the total tropical forest area. However, if mangroves’ high per-hectare carbon stocks are disrupted, there is a risk of large Green House Gas (GHG) emissions. Clearing of mangroves, conversion to industrial estates/aquaculture, and changes in drainage patterns all result in dramatic changes in soil chemistry and, in most cases, rapid GHG emission rates, particularly CO₂. Although mangroves have a minor role in worldwide forest carbon sequestration, they play a much larger role in carbon burial in the global coastal ocean. Despite accounting for only 0.5 percent of the total coastal ocean area, mangroves provide an average of 14 percent to carbon sequestration in the world’s oceans, compared to other coastal ecosystems. As a result, when compared to any other ecosystem, terrestrial or marine, mangrove forests have the highest area rates of carbon sequestration and hence contribute disproportionately as a carbon sink.

A Potential Site for Carbon Finance Projects

Blue carbon, which is organic carbon trapped and retained over long periods by coastal vegetation ecosystems such as mangrove forests, seagrasses, and salt marshes, is rapidly gaining traction as a significant natural climate solution.

India agreed in its Nationally Determined Contribution (NDC) to the Paris Agreement in 2015 to build an extra carbon sink of 2.5-3 billion tonnes of carbon dioxide equivalent by 2030 through increased forest and tree cover. However, if the contribution of blue carbon in fulfilling the Paris Agreement’s targets is to be clearly achieved, severe data constraints must be resolved. The government must negotiate with the United Nations Framework Convention on Climate Change (UNFCCC) to recognise the carbon sequestered through coastal ecosystems (blue carbon) at the national level in achieving India’s NDC targets and mitigating climate change, along with that Indian mangroves can be considered a potential site for implementing carbon finance projects and trading carbon in the voluntary market. In addition, private sector finance is critical for a variety of mangrove conservation activities as well as achieving the NDC targets effectively and efficiently.

According to a study that analysed the global potential and constraints of mangrove blue carbon for climate change mitigation, India possesses 189 square kilometres of ‘profitable mangroves’ that qualify for blue carbon finance and are financially viable for the next 30 years. According to the report, up to 20% of global mangrove forests are eligible for blue carbon financing, and 10% of global mangrove forests are financially viable over the next 30 years; despite this limited global potential, mangrove blue carbon can be a financially viable means of meeting national-level climate goals. Beyond climate change mitigation, mangroves contribute to broader policy goals such as eco-disaster risk reduction, which coincides with numerous goals in the Sendai Framework for Disaster Risk Reduction, as well as some aims in the Sustainable Development Goals.

Carbon financing can save about one-fifth of all mangrove areas, with roughly half of this being cost-effective and commercially viable. The size of financially viable carbon finance gives ongoing and large-scale initiatives to protect mangrove forests across the tropics providing an economic incentive. Carbon finance might help to fund and implement these measures, but it could also be impacted by changes in deforestation patterns. As a result, having a more consistent and rigorous understanding of return-on-investment studies will assist us in closing the gap between the significant national and international policy interest in mangrove blue carbon and the small scale of existing carbon project implementation. However, despite its high carbon density, analysis shows that a variety of different conservation finance approaches are required to maintain the remaining 80% of mangroves from future threats. By defining the potential and limitations of mangrove blue carbon funding, more practical and realistic conservation plans and climate policies can be adopted, assuring the safeguard of high levels of carbon stocks and biodiversity in this unique ecosystem.

References

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