Straddling land and sea and swarming with life, mangroves are key to healthy coastal ecosystems. They are recognised for their role as storm barriers, protecting coastal areas from flooding and erosion by dissipating the energy of huge waves. They act as nurseries for fish, help filter river water of pollutants and trap excess sediment before it reaches the ocean. Increasingly, mangrove protection and restoration is being acknowledged as a viable option for mitigating the effects of growing greenhouse gas emissions worldwide.

Powerhouses for carbon storage

P Ragavan et al in a recent commentary in Current Science discuss how mangrove forests can serve as potential carbon sinks due to their high carbon storage capacity per unit area. Mangroves fare better than terrestrial forests in increasing soil carbon stocks and protecting carbon-rich soils, both crucial for achieving the Paris climate targets. Despite the fact that mangroves constitute a miniscule geographic area of the planet – just 0.1% of the earth’s continental surface - a study suggests that mangroves held around 6.4 billion metric tons of carbon between 2000 and 2015.

For some perspective – upto 122 million tons of this carbon was released due to mangrove forest loss – roughly equivalent to the annual emissions of Brazil.  

Lack of robust estimates for carbon stocks of global mangroves

The commentary highlights the lack of reliable estimates of mangroves and the discrepancy in the available data. Degradation rates of mangroves are increasing while conservation endeavours are failing. Mangroves have been lost to make room for various forms of aquaculture, and have often been destroyed due to construction of dams and other developments that curtail the flow of rivers. They also face the risk of mass submergence as global warming raises sea levels.

There is an imperative need for country-wise, site-specific precise estimates. An understanding of the spatial distribution of mangrove soil carbon stocks could help demonstrate the actual climate mitigation potential of mangroves, and could strengthen conservation efforts.  

Recently, efforts have been made to assess the carbon stocks of global mangroves. However, uncertainty still exists.

According to a recent estimate, carbon sequestration in mangroves amounts to 14.2 teragrams of carbon per year (Tg C/ yr), with an average sequestration rate of 171 ± 17.1 g C /m2 yr(grams of carbon per unit square metre per year) and average soil accretion rate of 5.8 mm per year. The estimated mean mangrove carbon stocks per unit area are higher than those of other forest ecosystems.

Unlike other tropical forests, for which the bulk of carbon storage is in the biomass, mangrove carbon is primarily stored in the soil. The estimated average soil carbon concentration of global mangroves is also highly variable.

Previous estimates of global mangrove soil organic carbon have relied solely on climate-based models or mean country-level statistics, and more recently, on remote sensing. The differences in carbon stock values among these studies are why there are such vast discrepancies in estimating the extent of global mangrove cover.

Recently, Rovai et al noted that mangrove soil organic carbon stocks have been underestimated by up to 44%  and overestimated by up to 86% in carbonate and deltaic settings respectively, in earlier studies.

Twilley et al have also estimated the global mangrove soil organic carbon budget and found that the mangrove soil organic carbon stocks vary markedly across different types of coastal environmental settings, increasing from river-dominated environs to tide or wave-dominated settings to carbonate coastlines. They noted that carbon storage in deltaic settings has been overestimated, while soil organic carbon stocks in carbonate settings have been underestimated by up to 50%.

Walcker et al reported that carbon sink capacity declines with ecosystem age and global projections of the above and below-ground reservoirs of carbon stock need to be accounted for in mangrove age structures, which result from historical changes in coastal morphology.  

This variation in the estimation of carbon stock of mangroves has been attributed to the severe limitations of country-wise data that exists on mangroves. Global mean values of earlier studies have thus been used to estimate carbon stocks of countries.

Based on the available literature, it is also evident that there can be significant variability in the soil carbon stocks across different mangrove forests and also within the same mangrove forest.  

For instance, Kuffman et al. have reported increasing soil organic carbon from the seaward to the landward side of mangroves, and Weiss et al have reported higher soil organic carbon stocks in natural marine mangroves as compared to estuarine mangroves. Similarly, mangroves in sub-humid or arid regions have much lower soil carbon stocks than mangroves in humid regions.

Furthermore, it has been reported that mixed mangrove stands have -

• 20% higher soil carbon stocks per unit area than monotypic mangrove stands,
• Carbon accumulation within the conserved environments is up to four times higher than that of the degraded or deforested environments, BUT
• Three-fold lower than those impacted by domestic or aquaculture effluents, and
• Two-fold lower than those impacted by storms and flooding.

The mean total carbon stocks of mangroves per unit area between regions also vary (highest in the Asia Pacific at 1094 megagram of carbonper ha) followed by Latin America (939 megagram of carbon per ha), West– Central Africa (799 megagram of carbon per ha) and the Arabian/Oman Gulf (217 megagram of carbon per ha).

Thus the carbon storage capacity of mangroves is site-dependent, and so global generalizations are of little value.  

It has also been reported that the controls on soil carbon stocks are diverse, involving both climatic and local edaphic factors that act in a complex manner. There is little information on methane emissions from mangroves across the world, too. Despite discrepancies and the lack of data, all the studies point to the fact that some soil carbon stocks per unit area of mangroves are higher than other forest ecosystems.

Despite three decades of conservation and rehabilitation efforts, land conversion for aquaculture and agriculture remains a major threat and mangroves experience an annual loss of 0.16–0.39%.

In addition, increasing anthropogenic activities and frequency of extreme natural calamities caused by global climate change also threaten the survival and productivity of mangroves.

The three most widely used mangrove management measures are:

• More private land acquisition (which includes mangroves);
• Legal protection;
• Expanded mangrove rehabilitation through mono-specific plantation.

In addition, Lewis et al recently proposed a fourth parallel management measure, namely early detection and pre-emptive rehabilitation, to prevent the complete loss of plant community structures and ecological function by monitoring changes in the hydrological and ecological status of mangroves in the long term.

However globally, current management and policy-making efforts have not been very successful in ensuring the conservation and sustainable use of mangrove resources. If the current rate of degradation continues, the world may lose all its mangroves by the next century. If all of the world’s mangrove forests are destroyed and assuming that 95% of all mangrove carbon is oxidized to carbon dioxide, the loss would be equivalent to 6.5 years of carbon emissions from global forest loss.

To sustain the ecosystem services of mangroves, country-wise, site-specific, long-term ecosystem-based conservation and management measures should be taken as an immediate step along with the existing conservation endeavours, suitably strengthened and amended site-wise if needed.

The commentary can be viewed here.