Background/Question/Methods Coastal mangrove forests are renowned for providing a broad array of ecosystem services including fisheries production, sediment regulation, wood production, and protection from storms and tsunamis—yet conversion rates of mangroves to other land uses are among the highest of any forest type (e.g., 1.5% yr^-1 in Southeast Asia).  With deforestation accounting for ~20% of global anthropogenic greenhouse gas emissions, recent international climate accords have put forth “Reduced Emissions from Deforestation and forest Degradation (REDD+)” as one important climate mitigation option.  This program would offer economic incentives for conserving forests and associated carbon (C) stores in developing countries.  Mangroves may be well suited to such strategies, but while known for high rates of C assimilation and export (fluxes), their total C storage—the amount which may be emitted upon conversion—has not been well quantified.  We measured total ecosystem C storage (above- and below-ground) in mangroves across a broad tract (spanning 30° of latitude, 73° of longitude) of the Indo-Pacific region, the global epicenter of mangrove forest area, development, and diversity.  We also assessed variations in C pools and their vulnerability to sea-level rise and land use along transects running inland from the ocean edge.
Results/Conclusions Total carbon storage in Indo-Pacific mangroves is exceptionally high relative to most forest types, with a mean of 1043 Mg C ha^-1 and range of 437 to 2186 Mg C ha^-1.  These carbon stocks result from a combination of large-stature forest (up to ~2-meter diameter trees) and organic-rich peat/muck soils up to 5 m deep or more.  Aboveground C mass varies widely depending on stand composition and history, but belowground pools compose a large portion of ecosystem C storage in most sites.  Although mangrove composition is often zonated with distance from the ocean edge, C storage does not consistently vary along this gradient.  Projected sea-level rise is several times higher than typical rates of mangrove soil surface elevation change, suggesting high but variable susceptibility and a potential positive feedback via loss of C stocks.  Mangrove removal/conversion also reduces C storage, but the mechanisms of loss are not yet all clear, especially belowground.  Thus, a combination of very high C stocks, susceptibility to land use activities, and numerous ecosystem services makes tropical mangroves potentially strong candidates for REDD+ strategies, particularly if such strategies can be applied to facilitating adaptation to climate change (e.g., local migration) as well as mitigation.