By Jess Griffiths –
New research published in Ecosystems has found that areas of mangrove deforestation in Madagascar has lost 20% of carbon stored in the upper 1m of soil and 45% of total carbon accumulated in the past century [1], far more than previously thought. The CO2 emitted contributes to human-caused climate change, transforming mangroves and their soils from a vital carbon sink into a deadly carbon source.
Mangroves protect communities from coastal erosion [10] Image: onetreeplanted.org
Mangrove forests are among the most biologically and economically important ecosystems in the world [3]. They are buffers to coastal flooding and sea-level rise, host a diverse population of marine life (of which 210 million people reply on for their source of food and income [9]), support nutrient cycling, water filtering, and most importantly carbon sequestration [4].
Mangroves and carbon
Due to their high growth and low decomposition rates, mangroves have double the biomass and some of highest stocks of organic carbon of all forest ecosystems (5), and therefore, enormous impact on Earth’s climate. They are expert carbon storers, sequestering four times more than rainforests [8], although their extent is less than 3% of the Amazon alone.
Nonetheless, mangroves are being deforested at an alarming rate. Since 1990, over 20% of Madagascar’s mangroves (2% of global extent) were lost for charcoal and timber [1]. Previous studies have shown that clear-cutting causes soil subsidence, erosion, and loss of soil carbon [1]. Quantifying carbon emissions from deforestation has been an active area of research for decades but assessing the loss of soil carbon specifically is far more challenging and seen little success, until now.
What are the findings?
The study was carried out by Ariane Arias-Ortiz, a NOAA climate scientist at Berkely University. Arias-Ortiz and herteam sampled soils from five deforested plots in Tsimipaika Bay, Madagascar, and compared the carbon content with intact soils from surrounding areas. The soil cores were analysed at the University of Hawaii using an elemental analyser called an ‘isotope ratio mass spectrometer’.
Results showed a 20% depletion of carbon in the top 1m of soil due to lack of leaf litter, a 10-fold increase in soil mixing (caused by trampling and dragging logs during clear cutting [1]), and exposure to sunlight due to the removal of the protective canopy layer [6]. Overall, 45% of the carbon gradually accumulating over the last century has been lost since deforestation compared with intact soils [1].
What next?
Deforestation of mangroves is a significant source of emissions for many nations [1]. Since regrowing takes decades and with the window closing to prevent runaway climate change, restoring them as a carbon sink is becoming an unlikely solution.
Critical pressure should therefore be placed on policymakers for conservation of mangroves. If their protection for the sake of food supply, flood prevention, and rich biodiversity isn’t enough, then to reduce CO2 emissions for humanity’s survival should nudge them in the right direction.
References and further reading
[1] Arias-Ortiz, A., Masqué, P., Glass, L. et al. (2021) Losses of Soil Organic Carbon with Deforestation in Mangroves of Madagascar. Ecosystems 24, 1–19.
[2] Oceana (2020) Mangrove Forest.
https://oceana.org/marine-life/mangrove-forest/ [25-11-21]
[3] Earth Observatory (2010) Mapping Mangroves by Satellite
https://earthobservatory.nasa.gov/images/47427/mapping-mangroves-by-satellite [25-11-21]
[4] Barbier, E.B., Hacker, S.D., Kennedy, C., Koch, E.W., Stier, A.C. and Silliman, B.R., (2011) The value of estuarine and coastal ecosystem services. Ecological Monographs, 81, 169-193.
[5] Donato, D.C., Kauffman, J.B., Murdiyarso, D., Kurnianto, S., Stidham, M. and Kanninen, M., (2011) Mangroves among the most carbon-rich forests in the tropics. Nature Geoscience, 4, 293–297.
[6] Lovelock, C.E., Ruess, R.W. and Feller, I.C., (2011) CO2 efflux from cleared mangrove peat. Plos One, 6, 21279.
[7] Jones, T.G., Ratsimba, H.R., Carro, A., Ravaoarinorotsihoarana, L., Glass, L., Teoh, M., Benson, L., Cripps, G., Giri, C., Zafindrasilivonona, B. and Raherindray, R. (2016). The mangroves of Ambanja and Ambaro Bays, northwest Madagascar: Historical dynamics, current status and deforestation mitigation strategy. Estuaries: a lifeline of ecosystem services in the Western Indian Ocean 67-85.
[8] Sanderman, J., Hengl, T., Fiske, G., Solvik, K., Adame, M. F., Benson, L., & Duncan, C. (2018). A global map of mangrove forest soil carbon at 30 m spatial resolution. Environmental Research Letters.
[9] Wetlands International (2014) 210 million people benefit from mangrove associated fisherieshttps://www.wetlands.org/news/210-million-people-benefit-from-mangroves-associated-fisheries/ [27-11-21]
[10] Yudhishthra, N (2018) Forests by the sea https://kontinentalist.com/stories/forests-by-the-sea-mangroves-and-why-they-are-threatened [03-12-21]
