Mangroves among the most carbon-rich forests in the tropics

Mangrove forests occur along ocean coastlines throughout the tropics, and support numerous ecosystem services, including fisheries production and nutrient cycling. However, the areal extent of mangrove forests has declined by 30–50% over the past half century as a result of coastal development, aquaculture expansion and over-harvesting1–4. Carbon emissions resulting from mangrove loss are uncertain, owing in part to a lack of broad-scale data on the amount of carbon stored in these ecosystems, particularly below ground5. Here, we quantified whole-ecosystem carbon storage by measuring tree and dead wood biomass, soil carbon content, and soil depth in 25 mangrove forests across a broad area of the Indo-Pacific region—spanning 30 of latitude and 73 of longitude—where mangrove area and diversity are greatest4,6. These data indicate that mangroves are among the most carbon-rich forests in the tropics, containing on average 1,023 Mg carbon per hectare. Organic-rich soils ranged from 0.5 m to more than 3 m in depth and accounted for 49–98% of carbon storage in these systems. Combining our data with other published information, we estimate that mangrove deforestation generates emissions of 0.02–0.12 Pg carbon per year—as much as around 10% of emissions from deforestation globally, despite accounting for just 0.7% of tropical forest area6,7. Deforestation and land-use change currently account for 8–20% of global anthropogenic carbon dioxide (CO2) emissions, second only to fossil fuel combustion7,8. Recent international climate agreements highlight Reduced Emissions from Deforestation and Degradation (REDD+) as a key and relatively cost-effective option for mitigating climate change; the strategy aims to maintain terrestrial carbon (C) stores through financial incentives for forest conservation (for example, carbon credits). REDD+ and similar programs require rigorous monitoring of C pools and emissions8,9, underscoring the importance of robust C storage estimates for various forest types, particularly those with a combination of high C density and widespread land-use change10. 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.