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Temperate wetlands include those found in the conterminous United States, Europe, India, Asia, Japan, and other temperate regions. These wetlands are generally classified into mineral‐soil based (“freshwater mineral‐soil wetlands”) or peat‐based organic soil wetlands (Mitsch & Gosselink, 2007). Examples of temperate mineral‐soil wetlands include freshwater swamps, freshwater marshes, and riparian forests (Mitsch & Gosselink, 2015). Total global areal estimate for mineral‐soil temperate wetlands is 2,315,000 km² (Bridgham et al., 2006). Total carbon stock estimates for freshwater mineral‐soil wetlands are 31–46 PgC (Kolka et al., 2018; Bridgham et al., 2006). Peat‐accumulating wetland soils (histosols) occupy 1.3% of the global land area but store approximately 30% of the world’s soil organic carbon (Trettin et al., 2003; Bridgham et al., 2006). Through sequestration and storage, peatlands provide critical ecosystem services that help regulate global climate and mitigate climate change (McLeod et al., 2011). However, these systems are shrinking due to human land‐use modifications and sea level rise (McLeod et al., 2011; Pendleton et al., 2012).

Most peatlands (>50%) occur in northern latitudes; the southern limits of these regions coincide approximately 30–40°N latitude in North America and 50°N latitude in Europe and Asia (Craft et al., 2008; Tarnocai & Stolbovoy, 2006; Yu et al., 2011), where low temperatures are partially responsible for inhibiting organic matter decomposition and limited productivity (Clymo, 1984; Roulet et al., 2007). In the case of wetlands in more temperate or subtropical climates, this temperature effect seems to be more complex and less understood. In a study of temperate freshwater peatlands across a latitudinal gradient in the US, Craft et al. (2008) found that, like terrestrial ecosystems, organic C accumulation in freshwater peatlands is linked to climate through the effects of temperature. Higher C accumulation was measured in cooler and moister climates with lower accumulation in warmer and drier climates. However, other factors, such as species composition (Stricker et al., 2019) also play an important role in the variability in C sequestration and storage. Temperate peatlands in North America cover 861,000 km² (compared to 3,443,000 km² peatland coverage globally) (Bridgham et al., 2006; Kolka et al., 2018). In the United States, temperate peatlands cover an area of approximately 93,000 km² and can be found from Minnesota to Florida and California (Brigham et al., 2006; Craft et al., 2008; Reddy et al., 2015; Drexler et al., 2017). In the southeastern US, temperate peatlands once covered approximately 15,000 km² (Richardson et al., 2003). In the Southeast US, forested peatlands include peat swamps and shrub‐scrub and pond pine (Pinus serotina)‐dominated bogs referred to as pocosins (Richardson, 1983). In North Carolina alone, peatlands covered 9,079 km² of the coastal plain, but by 1979 only 2,810 km² remained (Richardson, 1983). Much of the landscape was deforested at the beginning of the twentieth century, and in the 1970–1980s large canals and ditches were built to facilitate agriculture (Carter, 1975).

In temperate freshwater peatlands, the largest carbon pool occurs in their organic peat soils, with average organic C accumulation ranging from 40–80 g C/m²/yr (Roulet et al., 2007; Frolking et al., 2011). Craft et al. (2008) found accumulation rates ranging from 49±11 (standard error, SE) g C/m²/yr to 86 (maximum) g C/m²/y for temperate freshwater peatlands across a latitudinal gradient in the US. However, C accumulation rates can vary from 7–300 g C/m²/yr (Gorham, 1991; Turunen et al., 2002; Kolka et al., 2011). In these systems, soil accretes slowly, with accretion rates ranging from 0.3 to 10.3 mm/yr. Total biomass of aboveground and belowground components of vegetation in peatlands is variable, depending on the species composition of the vegetation present. In relatively open peatlands, vegetation biomass can be small (e.g., 760 g/m²) but in forested peatlands, the aboveground and belowground components of vegetation biomass can be much higher (e.g., 13,800 g/m² to 20,000 g/m²)(Gorham, 1991; Brinson & Blum, 1995). The authors assume a carbon content between 0.441 (for herbaceous‐dominated) and 0.501 (for woody‐dominated); this equates to approximately 334 g C/m² to 10,000 g C/m² in carbon mass (Martin et al., 2018; Byrd et al., 2018). Increased productivity aboveground, however, does not necessarily imply gains in soil C pool since soil response depends on the interaction of both soil and plant functions, many of which are still understudied (Trettin & Jurgensen, 2003).

For the Northern Hemisphere, estimates of the total C stored in peatlands ranged between 270–604 PgC (Gorham, 1991, Turunen et al., 2002; Frolking et al., 2011; Yu et al., 2014). In North America (including Alaska and Canada), peatlands store approximately 125 PgC (Bridgham et al., 2006). For the conterminous United States, the most recent estimate (Kolka et al., 2018) for total temperate wetland carbon stock from the Second State of the Carbon Cycle Report (SOCCR2) was 13.5 PgC (mainly from peatlands). The China Second Wetlands Survey was used to derive aboveground wetland carbon stocks of 0.22 PgC and soil wetland carbon of 16.65 PgC (Xiao et al., 2019). The SOCCR2 report provided estimates ~1 PgC for Puerto Rico and Mexico. Estimates for other temperate countries were unavailable, i.e., for Europe, India, Japan. Thus, studies are needed to represent these regions and their wetland carbon stocks.

In temperate zones, land‐use activities like water management and drainage are significant contributors to wetland carbon loss (Armentano & Menges, 1986; Richardson et al., 2003; Reddy et al., 2015). Catastrophic wildfires in hydrologically altered temperate peatlands are also common drivers of wetland carbon loss (Page et al., 2009). In one study, emissions during a single fire event measured approximately 3,600 g C/m² (Turetsky et al., 2011). Many freshwater peatlands are also vulnerable to impacts from sea level rise. It is estimated that approximately 150,000 km² of freshwater peatlands are located less than 5 m in elevation (Henman & Poulter, 2008), highlighting the need to understand the impacts on carbon pools and fluxes. In a coastal freshwater peatland in North Carolina, USA, total existing carbon storage (peat and vegetation) ranged between 155.5 TgC and 201.0 TgC, with potential losses between 99.4–128.0 TgC by the end of the century due to inundation from sea level rise (Henman & Poulter, 2008). In low‐lying coastal ecosystems, restoring peatland structure and function can improve climate resiliency as peatlands play a critical role in ecosystem adaptation to sea level rise, by preventing soil loss through oxidation while allowing soil accretion to resume. Restoring peatlands through reintroduction of wetland hydrology (rewetting) can prevent soil carbon loss and ensure these peatlands continue to serve as carbon sinks (Limpens et al., 2008; Lamers et al., 2015; Chimner et al., 2017). Rewetting also has the benefit of reducing the likelihood of catastrophic wildfires (Wurster et al., 2016).

There is a significant amount of uncertainty in temperate wetland carbon stock estimates. Most studies reporting carbon stock estimates are generally focused on either boreal or tropical peatlands, leaving temperate freshwater peatlands largely understudied. Difficulty in differentiating between mineral‐soil and organic‐soil wetlands at landscape‐scales also increases the uncertainty in the areal extents and estimates of carbon stores for temperate wetlands. Drainage and conversion have fragmented and decreased temperate wetland areal extents, making measurements even more difficult. However, with the increasing availability of aerial‐ and satellite‐remote sensing data from both active and passive sensors, opportunities exist to address this knowledge gap. Studies that combine remote‐sensing data, repeat field measurements, and intensive inventories would improve our understanding of these systems and provide a critical baseline for monitoring and forecasting future changes related to climate and anthropogenic pressures in these ecosystems.

Table 1.2 Summary of global wetland carbon stocks. The temperate wetland stock estimate includes the conterminous United States and China

Wetland Type	Aboveground stocks (PgC)	Belowground stocks (PgC)	Total Ecosystem Stocks (PgC)
Mangroves	1.52–1.75	1.93–6.4	3.45–8.15
Salt marshes	–	0.4–6.5	0.4–6.5
Seagrass	–	–	4.2–8.4
Tropical peatlands	8.5–9.6	69–129	77.5–138.6
Without permafrost, Boreal wetlands (mineral and organic combined)	10.0–15.0	400–500	410–515
With permafrost, Boreal wetlands	10.0–15.0	1672	1682–1687
Temperate wetlands (for China, US (lower 48) mineral and organic combined)	1.2–3.2	27.3–38.1	29.5–41.3
Global total	21.2–29.6	498.6–680	519.8–709.6
Global total w. permafrost (1672 PgC)	21.2–29.6	1770.6–1852.0	1791.8–1881.6