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In the tropics, areas with year‐round high rainfall, warm temperatures, low nutrient levels (mostly derived only from rainfall), and a topography that favors retention of water, host a unique ecosystem where plant biomass accumulates to form a thick layer of partially decomposed peat. Tropical peatlands cover 243,000–300,000 km² in Southeast Asia (with the greatest extent in coastal Indonesia), 209,000 km² in Africa (especially in the Congo Basin), and 131,000–587,000 km² in the Americas (the large range reflecting uncertainty on what share of these extensive wetlands are peat‐forming systems) (Page et al., 2011; Lähteenoja et al., 2012; Gumbricht et al., 2017; Leifeld & Menichetti, 2018; Xu et al., 2018). Most contemporary tropical peatlands began formation during the Holocene, with rates of C accumulation in the range 30–270 g C/m²/yr for SE Asian sites (Page et al., 2010; Dommain et al., 2011). Depths of up to 20 m are known for some SE Asian peatlands, but most are in the range 5–7 m. Extensive peatlands on other continents are shallower: average thickness in the Peruvian Amazon is 2–3 m (max. 7.5 m; Lähteenoja et al., 2012; Draper et al., 2014) and 2 m in the Congo Basin (max. 6 m; Dargie et al., 2017). Belowground C density values average 2,775 t C/ha for SE Asia (Page et al., 2011); 2,000 t C/ha for the Congo Basin (Dargie et al., 2017), and 800 t C/ha for Peru (Draper et al., 2014). The largest store of tropical peat C is in SE Asia (69 PgC), followed by 34 PgC in Africa and 13 PgC in the Americas (Page et al., 2011; Dargie et al., 2017). Tropical peatland vegetation is dominated by either hardwood trees, palms, or a combination, and displays some level of zonation based on flooding and nutrient gradients (Anderson, 1961; Lähteenoja et al., 2012; Dargie et al., 2017; Draper et al., 2014). Prior to anthropic disturbance, peatland vegetation stored an estimated 7.1 and 1.4–2.5 PgC in SE Asia and Africa (based on 27–275 t C/ha and 67–124 t C/ha of biomass C, respectively) (Dargie et al., 2017; Wijedasa, 2020). South American peatland extent is uncertain, thereby limiting estimation of aboveground C stocks, but Peruvian peatland biomass stores 80–90 t C/ha (Draper et al., 2014). Compared to belowground C density values, aboveground C density is relatively low, but the vegetation plays an essential role in the formation, maintenance, and protection of below ground C pools.

For conversion to agricultural land uses, waterlogged swamp conditions are artificially drained, and accompanied by nutrient addition and pH control (Wijedasa et al., 2017). Drainage shifts conditions from those favoring slow peat accumulation to ones facilitating rapid decomposition, resulting in C emissions in the range 11–20 t C/ha/yr for peatlands under agriculture (Drösler et al., 2014). Due to rapid land use change, the area of peat swamp forest in SE Asia declined between 1990 and 2015 from 76% to 29% of the area (a loss of 4.6 Mha), with a corresponding increase of 11% to 50% in agricultural area (Miettinen et al., 2017), comprising both small‐scale farms and large‐scale industrial plantations (Wijedasa et al., 2018). In addition, drained peatlands are susceptible to fires that result in further, rapid C release to the atmosphere and life‐threatening air pollution. In Africa, the extensive peatlands of the Congo Basin are relatively intact, although the potential exists for future anthropic disturbance due to a lack of protection (Dargie et al., 2017); elsewhere on the continent, peatland has been exploited for agriculture (e.g., Uganda) and as a fuel source for energy generation (Rwanda). In South America, most Peruvian peatlands remain undisturbed, but as with the Congo peatlands, there is the potential for future disturbance from the development of agriculture and transport infrastructure (Roucoux et al., 2017).

The key uncertainties on tropical peatland C stocks and fluxes relate to extent, peat depth, rate of peat decomposition, and ecosystem functioning (Lawson et al., 2015; FAO, 2020). Even in the best studied tropical peatland area of SE Asia the actual extent and depth are uncertain (Vernimmen et al., 2020). This uncertainty is even more acute in Africa, where the recent study of Congo peatlands resulted in a 29% increase in known global tropical area (Dargie et al., 2017). Remote‐sensing techniques combined with ground‐truthing and field measurement campaigns are needed to improve mapping and monitoring of tropical peatlands and will also provide the basis for assessment of peatland condition and likely improvement of understanding of their role in the global C cycle (FAO, 2020). We estimate that tropical peatlands contain 8.5–9.6 PgC in aboveground biomass, 69–129 PgC in organic soil carbon (Dargie et al., 2017), and 77.5–138.6 PgC for total carbon stocks.