Mangroves among the most carbon-rich forests in the tropics

Study Number:

1

Author:

D. C. Donato, J. B. Kauffman, D. Murdiyarso, S. Kurnianto, M. Stidham & M. Kanninen

Abstract:

Mangrove forests occur along ocean coastlines throughout the tropics, and support numerous ecosystem services, including fisheries production and nutrient cycling. However, the aerial extent of mangrove forests has declined by 30–50% over the past half century as a result of coastal development, aquaculture expansion and over-harvesting (deforestation) (Duke et al., 2007; Polidoro, B. A. et al., 2010; Alongi, 2002; Food and Agriculture Organization of the United Nations, 2007). 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 ground. 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 30º of latitude and 73º of longitude—where mangrove area and diversity are greatest (Food and Agriculture Organization of the United Nations, 2007; Giri, C. et al., 2011). These data indicate that mangroves are among the most carbon-rich forests in the tropics, containing on average 1,023Mg carbon per hectare. Organic-rich soils ranged from 0.5m to more than 3m 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 area (Giri, C. et al., 2011; van der Werf et al., 2009).

Main Results and Conclusions:

• A significant part of the world’s wetland forest is composed of mangroves: “mangrove forests…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 alone (Food and Agriculture Organization of the United Nations; Giri, C. et al. 2011; Page, Rieley, J. O. & Banks, C. 2011)” (293).
• Two mangrove settings were studied: Estuarine/river-delta and oceanic/fringe
• Mangrove soils are good examples of carbon sinks:
• “Mangrove soils consist of a variably thick, tidally submerged suboxic layer (variously called `peat’ or `muck’) supporting anaerobic decomposition pathways and having moderate to high C concentration (Kristensen et al., 2008; Alongi, D. M. et al., 2004; Chmura et al., 2003)” (293).
•  “These data indicate that high productivity and C flux rate in mangroves (Kristensen et al., 2008; Eong 1993) are indeed accompanied by high C storage, especially below ground” (294).
• “High per-hectare C storage coupled with a pan-tropical distribution (total area 14 million ha; refs 4,6) suggests mangroves are a globally important surface C reserve” (294).
• “We found that mangroves are among the most C-dense forests in the tropics (sample-wide mean: 1,023MgC ha-1 88 s,e.m.), and exceptionally high compared to mean C storage of the world’s major forest domains (Fig. 2)”(294).
• Clearing mangrove habitat can release large amounts of carbon: “To provide some constraints on estimated emissions, we used a similar uncertainty propagation technique, combining our C storage values with other global data (Kristensen et al., 2008; Komiyama, Ong, J. E. & Poungparn, S., 2008) and applying a range of assumptions regarding land-use effects on above and below-ground pools (see Supplementary Information). This approach yields a plausible estimate of 112-392MgC released per hectare cleared, depending in large part on how deeply soil C is affected by different land uses. 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 estimate leads to global emissions on the order of 0.02-0.12 Pg Cyr-1” (295).

Works Cited:

Duke, N. C. et al. A world without mangroves? Science 317, 41_42 (2007).

Polidoro, B. A. et al. The loss of species: Mangrove extinction risk and geographic areas of global concern. PLoS ONE 5, e10095 (2010).

Alongi, D. M. Present state and future of the world’s mangrove forests. Environ. Conserv. 29, 331_349 (2002).

Food and Agriculture Organization of the United Nations (FAO). The World’s Mangroves 1980-2005 (FAO Forestry Paper 153. FAO, 2007).

Giri, C. et al. Status and distribution of mangrove forests of the world using earth observation satellite data. Glob. Ecol. Biogeogr. 20, 154-159 (2011).

van der Werf, G. R. et al. CO2 emissions from forest loss. Nature Geosci. 2, 737_738 (2009).

Page, S. E., Rieley, J. O. & Banks, C. J. Global and regional importance of the tropical peatland carbon pool. Glob. Change Biol. 17, 798-818 (2011).

Kristensen, E., Bouillon, S., Dittmar, T. & Marchand, C. Organic carbon dynamics in mangrove ecosystems. Aquat. Bot. 89, 201-219 (2008).

Komiyama, A., Ong, J. E. & Poungparn, S. Allometry, biomass, and productivity of mangrove forests. Aquat. Bot. 89, 128-137 (2008).

Twilley, R. R., Chen, R. H. & Hargis, T. Carbon sinks in mangroves and their implications to carbon budget of tropical coastal ecosystems. Water Air Soil Pollut. 64, 265-288 (1992).

Bouillon, S. et al. Mangrove production and carbon sinks: A revision of global budget estimates. Glob. Biogeochem. Cycles 22, GB2013 (2008).

Alongi, D. M. et al. Sediment accumulation and organic material flux in a managed mangrove ecosystem: Estimates of land-ocean-atmosphere exchange in peninsular Malaysia. Mar. Geol. 208, 383-402 (2004)

Chmura, G. L., Anisfeld, S. C., Cahoon, D. R. & Lynch, J. C. Global carbon sequestration in tidal, saline wetland soils. Glob. Biogeochem. Cycles 17, 1111 (2003).

Eong, O. J. Mangroves-a carbon source and sink. Chemosphere 27, 1097-1107 (1993).