Carbon in mangroves – Donato et al 2011

When referencing the article regarding carbon sequestration in MAP News ISSUE #261, we want to emphasize certain salient points that reinforce MAP’s position since our founding in 1992:

We at MAP would like to urge our readers to review the referenced article on the importance of mangroves in sequestering and especially in below ground storage of carbon. The following excerpted points are especially relevant to MAP’s stance on mangroves since our founding in 1992:

“Tropical wetland forests (for example, peatlands) contain organic soils up to several metres deep and are among the largest organic C reserves in the terrestrial biosphere11–13. Peatlands’ disproportionate importance in the link between land use and climate change has received significant attention since 1997, when peat fires associated with land clearing in Indonesia increased atmospheric CO2 enrichment by 13–40% over global annual fossil fuel emissions11. This importance has prompted calls to specifically address tropical peatlands in international climate change mitigation strategies7,13.

Overlooked in this discussion are mangrove forests, which occur along the coasts of most major oceans in 118 countries, adding ∼30–35% to the global area of tropical wetland forest over peat swamps alone4,6,12. Renowned for an array of ecosystem services, including fisheries and fibre production, sediment regulation, and storm/tsunami protection2–4 , mangroves are nevertheles declining rapidly as a result of land clearing, aquaculture expansion, overharvesting, and development2–6. A 30–50% areal decline over the past half-century1,3 has prompted estimates that mangroves may functionally disappear in as little as 100 years (refs 1,2). Rapid twenty-first century sea-level rise has also been cited as a primary threat to mangroves14, which have responded to past sea-level changes by migrating landward or upward15 …”

“Carbon emissions from land-use change in mangroves are not well understood. Our data suggest a potential for large emissions owing to perturbation of large C stocks. The fate of below-ground pools is particularly understudied, but available evidence suggests that clearing, drainage, and/or conversion to aquaculture—aside from affecting vegetation biomass—also decreases mangrove soil C significantly16,22,26–28. In upland forests, the top 30 cm of soil are generally considered the most susceptible to land-use change9; however in wetland forests, drainage and oxidation of formerly suboxic soils may also influence deeper layers29. ..”

From this study reference is made to the following as a recent estimate of present global mangrove area:

“Coupled with published ranges of mangrove deforestation rate (1–2%; refs 1,4) and global area (13.7–15.2 million ha; refs 4,6),”

This must reflect that recent study that put present area of mangroves at 12% less than the previous accepted FAO estimates of 15 million ha. This study on Carbon footprint regarding various wetland and upland forests types provides excellent reference material for better reflection upon the truly significant role coastal wetlands, including mangroves and peatlands play in combating climate change, and how their rapid rates of loss pose grave threats to life on our planet.

A final point the study cites concerning the resilience of mangroves to adapt to rising sea levels is sobering, and this point was raised by MAP’s director at the “Mangroves As Fish Habitat” conference in Miami, Florida nearly a decade ago:

“In addition to direct losses of forest cover, land-use activities will also impact mangrove responses to sea-level rise14,15. Man- groves have been remarkably persistent through rapid sea-level rises (5–15 mm yr−1 ) during the late Quaternary Period (0–18,000 yr bp) because of (1) landward migration, and (2) autogenic changes in soil-surface elevation through below-ground organic matter production and/or sedimentation15. Under current climate trends, sea level is projected to rise 18–79cm from 1999–2099 (higher if ice-sheet melting continues accelerating)8,30, implying a period- averaged rate of ∼1.8–7.9 mm yr−1 , notwithstanding local variations and temporal nonlinearities. Although this rate is not unprecedented, it is unclear yet whether mangroves are currently keeping pace with sea levels14,15. Anthropogenic influences could constrain future resilience to sea-level rise through coastal developments that impede inland migration (for example, roads, infrastructure), upland land uses that alter sediment and water inputs (for example, dams, land clearing), and mangrove degradation that reduces below-ground productivity14. This synergy of land use and climate change impacts presents additional uncertainties for the fate and management of coastal C stores… “

“…Because land use in mangroves affects not only standing stocks but also ecosystem response to sea-level rise, maintaining these C stores will require both in situ mitigation (for example, reducing conversion rates) as well as facilitating adaptation to rising seas. The latter challenge is largely unique to management of coastal forests, calling for watershed-scale approaches, such as landscape buffers for accommodating inland migration where possible, maintenance of critical upstream sediment inputs, and addressing degradation of mangrove productivity from pollution and other exogenous impacts14,15.”