tallest stands. Mangrove height is driven by temperature and precipitation and so the largest forests are found close to the equator, with notably large stands on the west coast of Colombia and in Gabon, where the world’s tallest stands of 63 m are found (Simard et al., 2019). Shorter trees are found towards the extremes of their geographic limits, such as in higher‐latitude countries like Japan and New Zealand. However, the maximum height that mangroves are able to achieve is often limited by the occurrence of tropical cyclones and hurricanes (Simard et al., 2019). These extreme weather events limit the height of mangroves in regions where taller mangrove forests would be expected, such as in equatorial East Africa. Mangrove forest height is determined by a number of local scale variables which include salinity, hydraulic conductivity, wave exposure, and soil type, which lead to local and regional variation in height and AGC.
At national and regional levels, total mangrove carbon stocks are also driven by mangrove areal extent. Asia contains 40% of the total global mangrove area, therefore it contains over half (52%) of the total global mangrove AGB. Indonesia alone contains 22% of the global mangrove AGC stock, due to its vast mangrove area and mean mangrove AGB of 215 Mg/ha. The total AGB in mangrove forests globally is estimated at 1.52 (Hu et al., 2020) to 1.75 PgC (Simard et al., 2019), with 20.7% stored in Africa, 11.9% in Oceania, 39.3% in Asia, 27.9% in the Americas, and 0.2% stored in the Middle East. The total mangrove ecosystem carbon, however, is dominated by that which is stored in the soils. The soil C alone has been estimated to range from 1.93 PgC (Ouyang & Lee, 2019) to 6.4 PgC (Sanderman et al., 2018), although the overlap of uncertainty between these estimates is approximately equal. Lower estimates of mangrove soil carbon tend to be more common, from 1.93 to 2.96 PgC (Ouyang & Lee, 2019; Hamilton & Friess, 2018). Africa stores 13–19% of the soil C to 1 m depth with 28% stored in the Americas, 10% stored in Oceania, 42–49% stored in Asia, and no more than 0.2% in the Middle East. The controls on this total soil C are driven by coastal environmental settings (such as Holocene sea level rise and current tidal regimes), which can be divided into classes of deltas (river dominated), estuaries (tide dominated), lagoons (wave dominated), composite (river and wave dominated), bedrock (drowned bedrock valley), and carbonate settings, which contain different mangrove soil C:N:P stoichiometry (Rovai et al., 2018). The use of a model that contained a range of climatic variables (e.g., temperature), biological estimates (e.g., PET) and information on geophysical setting (e.g., runoff), determined global soil C to be controlled by tidal range and temperature (Sanderman et al., 2018). However, although mangrove area is also a determinant in the total quantity of mangrove soil C per region, expressed by the large areal extent of mangrove in Asia, soil C is controlled at the local level by geomorphological settings (Rovai et al., 2018). Based on this review of the literature, for mangroves we therefore estimate aboveground stocks to be 1.52–1.75 PgC and belowground soil carbon to range from 1.93–6.4 PgC, for a total of 3.45–8.15 PgC.
1.4.2. Tidal Salt Marshes
Tidal salt marshes are dynamic ecosystems that can be found in a range of sedimentary coastal settings from the tropical to the arctic climate zone (Mcowen et al., 2017). Marsh plants consist of halophytic herbs, grasses, and low shrubs that are adapted to frequent or occasional inundation by tides (saltwater) and lay approximately between mean high‐water neap and mean high‐water spring tides (Mcowen et al., 2017). Located in the intertidal zone, they can share a similar ecological niche and have overlapping distributions with mangroves in temperate regions (Saintilan et al., 2014; Kelleway et al., 2017).
Table 1.1 Summary of local rates and global estimates of carbon burial and stock in salt marshes
(Source: Based on Chmura et al., 2003, and Duarte et al., 2013.)
| Local carbon burial rate (g C/m2/yr) | Carbon stock in soil (Mg C/ha) | Global area (km2) | Reference |
|---|---|---|---|
| 151 | 380,000 | Woodwell et al. ( 1973 ) | |
| 218 (18–1,713) | 22,000 | Chmura et al. ( 2003 ) | |
| 218 (18–1,713) | 400,000 | Duarte et al. (2005) | |
| 218 ± 24 | 162 | 22,000–400,000 | Mcleod et al. ( 2011 ) |
| 244.7 | 41,657 | Ouyang and Lee ( 2014 ) | |
| 218 (18–1,713) | 55,000 | Mcowen et al. ( 2017 ) | |
| Global carbon burial (TgC/yr) | Global carbon stock in soil (PgC) | Global carbon burial (reference) | |
| 60.4 | none available | Duarte et al. (2005) | |
| 60.4–70 (max. 190) | Nellemann et al. ( 2009 ) | ||
| 4.8 | Mcleod et al. ( 2011 ) | ||
| 87.3 | Mcleod et al. ( 2011 ) | ||
| 4.8–87.3 | 0.4–6.5 | Duarte et al. ( 2013 ) | |
| 10.2 ± 1.1 (0.9–31.4) | none available | Ouyang and Lee ( 2014 ) | |
| 12 ± 1.3 | Al‐Haj and Fulweiler ( 2020 ) | ||
As highly productive ecosystems (primary production exceeds respiration), tidal salt marshes sequester large amounts of carbon within their underlying sediments, within their living aboveground biomass (leaves, stems) and belowground biomass, including live and dead roots (Chmura et al., 2003; Ouyang & Lee, 2014). Marsh plants further trap and accumulate litter and allochthonous material/carbon from upstream rivers and tidal exchange (Saintilan et al., 2013). Consequently, carbon burial rates and carbon stocks in salt marshes are exceptionally high, exceeding those of terrestrial forests (Duarte et al., 2005a, 2013; Mcleod et al., 2011).
The global net primary production of tidal salt marsh is estimated at 0.01–0.18 PgC/yr (Duarte et al., 2013). Local burial rates vary from 18 to 1,713 g C/m2/yr with an average of ~230 g C/m2/yr (Table 1.1). Global estimates of carbon burial depend on accurate estimates of the global extent of coastal marsh. Historically, there have been high uncertainties