REVIEW OF RELATED LITERATURE AND STUDIES
Mangroves play significant role in global carbon sequestration by acting as sinks of carbon within the tropical coastal zones. Despite occupying less than 1% of the coastal area, mangroves are estimated to contribute approximately 20% of all carbon emissions arising from deforestation (Caldeira, 2012). They are able to sequester atmospheric carbon both in their above and belowground biomass and also in sediments (Adame et al., 2013; Sawale & Thivakaran, 2013). Besides carbon dioxide, mangroves absorb carbon monoxide and sulphur dioxide, potentially reducing the impact of global warming and release significant amounts of oxygen to the atmosphere (Farley et al., 2009; Jennerjahn & Ittekkot, 2002; Moberg & Rönnbäck, 2003). The global benefits associated with carbon sequestration in the mangrove is the removal of the harmful greenhouse gases (including carbon dioxide) from the atmosphere and therefore helping in mitigating effects of climate change (Pendleton et al., 2012; Siikamäki et al., 2012; Alongi, 2009)
The term mangrove describes both the ecosystem and the plant families that have developed specialized adaptations to live in this tidal environment which are inundated twice daily by the tides 18. Mangroves possess a range of features which make them uniquely adaptable to their stressful environment, they are halophytic or salt tolerant, have aerial roots for gathering oxygen and seeds that geminate on the tree. Irrespective of the range of species and forest types, the manifold ecological role of mangrove ecosystems is, economically and socially, highly significant. The global carbon burials of mangroves are approximately 18.4 Terra grams of carbon per year (Tg C yr-1) and therefore they have the potential of providing an efficient sink of CO2. .( Patil V.et.,al).
Mangroves are among the most carbon-rich forests in the tropics and support numerous ecosystem services. These forests form an important part of the carbon stored in coastal and marine ecosystems, called “blue carbon.” They could play a significant role in reducing emissions, while also supporting biodiversity conservation, fisheries habitat protection and disaster risk reduction. Despite their large ecological impact, both have suffered huge declines, up to 50 percent in areas, in recent years ( www.cifor.org). Blue Carbon activities refer to a suite of sustainable policy, management and planning activities in coastal ecosystems to reduce emissions from conversion and degradation and to conserve and sustainable manage coastal carbon sinks” (Herr et al 2012).
Mangroves not only store carbon in their aboveground biomass (woody material and leaves) but also in their belowground biomass (roots and fine root structures) furthermore, approximately 25% of the leaf litter may also be trapped in the sediments (Jennerjahn and Ittekot 2002, in Breithaupt et al 2012). Mangroves also trap significant amounts of suspended sediments helping retain transported carbon in soils. While the overall percentages vary depending on the tidal flow, geographic location and underlying soil type, mangroves generally have been found to store greater amounts of carbon belowground than 10 aboveground. While there are generally low carbon turnover rates in intact mangrove forests, it can be released relatively quickly if converted to other uses (e.g., aquaculture).
The amount of organic carbon stored in any mangrove ecosystem depends on several factors such as sources of carbon which include, tidally suspended organic matter, amount of rainfall in 5 the hinterland or from local production by mangroves (Raza et al., 2011). Mangroves are able to sequester and store large quantities of carbon both in the plants biomass and also in the sediments below them (Murdiyarso, 2010; Pendleton et al., 2012; Adame et al., 2013).
Their high carbon content in the soil is as a result of high sedimentation rates and sustained anoxic conditions in the belowground which results into low decomposition rates of soil organic matter hence building up of soil carbon (Ray et al., 2011). Carbon sequestration in mangrove ecosystem is a continuous process which often results into large deposits of carbon which together with carbon stored in salt marshes and seagrass is collectively referred to as “Blue carbon” (Matsui et al., 2012; Pendleton et al., 2012). In mangrove ecosystems, this carbon is often more than a thousand years old, making these habitats among the most carbon-rich ecosystems on earth (Donato et al., 2011; Webb et al., 2013). Unlike the tangible benefits of provisioning despite mangrove ecosystems providing tremendous value and benefits to coastal communities and other associated species, they are currently being destroyed at alarming rates (Giri et al., 2011). Over the last 50 years, about one-third of the world’s mangrove forest cover has been lost (Giri et al., 2011; Caldeira, 2012) as a result of forest degradation and deforestation. Major threats facing mangroves include overexploitation of forest wood products, pollution and conversion of mangrove areas for other land uses (Murray, 2012; Farida-Hanum et al., 2012).
Climate change has been receiving increasing awareness relative to its potential impacts on the coastal zone, generally associated with sea level rise, increase in air and water temperature, increase in atmospheric CO2, alterations in the quantity and quality of the continental runoff and changes in the frequency and intensity of extreme meteorological events (Alongi 2008). Because of their location at the continent-ocean interface, mangroves are more likely to respond to these hazards resulting from global climate change. All of these will, to some extent, alter primary and secondary productivity and respiration of mangroves and their associated biocoenosis, and the transport of materials to adjacent terrestrial and marine ecosystems.
Until recently mangrove forests, particularly in developed nations, have been considered wastelands. Recognition by governments and the general public of the value of mangrove forests has been slow and is still limited. It is the ‘ecosystem products’ of mangrove forests which are most significant (Pernetta, 1995), but which are rarely recognized. These include: coastal protection from tidal erosion and storm surges, sediment trapping for land accretion (Pernetta, 1995) and use of mangrove habitats by juvenile fish and prawns (Robertson and Blabber, 1992).
On the other hand, information on mangroves is particularly relevant because they have the greatest potential to be incorporated into climate policy frameworks, especially in the near term.
Mangrove forests are among of the most productive and biologically important ecosystems of the world because they provide important and unique ecosystem goods and services to human society and coastal and marine systems. Mangroves allocate proportionally more carbon belowground, and have higher below- to above-ground carbon mass ratios than terrestrial trees. Most mangrove carbon is stored as large pools in soil and dead roots https://www.researchgate.net/publication/274116107_Carbon_sequestration_in_mangrove_forests accessed Aug 01 2018. The forests help stabilize shorelines and reduce the devastating impact of natural disasters such as tsunamis and hurricanes. They also provide breeding and nursing grounds for marine and pelagic species, and food, medicine, fuel and building materials for local communities. Mangroves, including associated soils, could sequester approximately 22.8 million metric tons of carbon each year.
According to Patil V. and Sawant B. A carbon sink is a natural or artificial reservoir that accumulates and stores some carbon containing chemical compound for an indefinite period. Whereas Carbon stock is the quantity of carbon contained in a pool, meaning a reservoir or system which has the capacity to accumulate or release carbon. Mangroves are important carbon sinks and sequester approximately 25.5 million tonnes of carbon every year. They also provide more than 10% of essential dissolved organic carbon that is supplied to the global ocean from land. Studies indicate that mangroves are able to sequester some 1.5 tonnes of carbon per hectare per year. This is approximately equivalent to the amount of carbon a motor vehicle releases to the atmosphere each year (assuming each car uses approximately 2,500 liters of petrol per year). In the context of CO2 sequestration, the relevant carbon sinks to be considered are: The burial of mangrove carbon in sediments – locally or in adjacent systems.
According to (GUSTAVO C.D. ESTRADA and MÁRIO L.G. SOARES 2016) that Global means of carbon stock and sequestration showed that mangroves are among the forest ecosystems with greater capacity of carbon storage in AGB per area.
A study done in the Philippines showed that, between 1990 and 2010, there was a decrease of 28,172 ha of mangrove forest, about 10% of the mangroves in the country, with the driving force behind this decrease in several regions being the expansion of aquaculture and mangrove wood extraction. However, the largest loss of mangroves occurred due a typhoon in 1990; however, his area showed signs of recuperation after the occurrence (Long et al. 2014). Human induced land-use change were also indicated as the major drivers of mangrove area decrease in Southeast Asia and Vietnam(Nguyen 2014) mainly due to clearing for areas for urban and rural expansion and for aquaculture, this is also true for other Asian countries like Myanmar and India (Rao et al. 2013, Rahu et al. 2012). Therefore, it is very difficult to monitor mangrove area changes in the Asian continent without the interference of direct human drivers.
Some recent reviews on the environmental pressures on mangroves due to climate change suggest that important threats to mangroves are mostly due to changes in salinity, wave regime, and the quantity and quality of the sediment loading (Giri et al. 2011), which may be further enhanced by increasing the frequency of extreme climatic events. For example, based on the IPCC maximum sea level rise scenario (Gilman et al. (2006, 2007) predicted an up to 13% loss for Pacific island mangroves by the year 2100. Similarly, Alongi (2008) arrives at a global loss rate of mangroves related to climate change of about 10-15%.
Some researchers however, have found that not all mangroves will respond negatively to a climate change scenario. Recent studies showed that in many locations mangrove vegetation are expanding their poleward limits. (Cavanaugh et al. (2014) Carbon stock and sequestration also vary according to physiographic types, indicating the importance of hydroperiod and edaphic parameters to the local variability of carbon stock. By demonstrating the contribution of local and regional-global factors to carbon stock, this study provides information to the forecast of the effects of future climate changes and local anthropogenic forcings on this ecosystem service. (http://www.scielo.br/ 2016)