as fires that burn the organic soil layer can destabilize or permanently thaw permafrost (Jafarov et al., 2013). The largest fire in CONUS began in the Okefenokee Swamp in Georgia in 2007 and is one of more than 300 fires that have burned there since 1937, demonstrating that fire is a long‐standing part of this ecosystem (U.S. Fish and Wildlife Service Fire Management, 2020). Although fire can release stored carbon and threaten developed areas and some animals, it is an important component of the ecosystem with many regulatory benefits.
Wetland fires have complex relationships to wetland hydrology, including changes to soil moisture and aeration. Fire frequency may change between wetlands with different hydroperiods, with long hydroperiods associated with more frequent fires. Although the relationship between hydroperiod and fire regime is complex, and may also involve vegetation type and SOC content, longer hydroperiods are also associated with wetlands containing more organic soils. Wetlands that are unable to drain due to frozen soils may see more SOC bunt in fires occurring later in the season, after drainage. Fire type is also key, with differences between fires that burn surface vegetation versus fires which burn underground and remove carbon from the soil – this second type is common in wetlands with lower water tables (Neary et al., 2005).
Table 2.4 Burned areas in wetlands are compared to all CONUS landcover classes
Source: Based on Eidenshink et al., 2007.
| Fire Type | Wetland Burned Area km2 | Total Burned Area km2 | Wetland Burned Area as Percent of Total Burned Area |
|---|---|---|---|
| Other | 6,800 | 76,900 | 8.84 |
| Prescribed Fire | 11,500 | 48,300 | 23.73 |
| Wildfire | 23,300 | 440,000 | 5.29 |
Burned areas determined through MTBS.
Maps from the Landsat Burned Area Essential Climate Variable (BAECV) program as published by Hawbaker et al. (2017) for the years 1984, 1990, 2000, 2011, and 2015 were analyzed using the wetland types and locations available in NLCD 2016 (Dewitz, 2019). This analysis shows that wildfires are a consistent feature in wetlands, representing 4–12% of all burned areas in the CONUS, and that wetlands are slightly more likely to burn than the average of all CONUS soils from NLCD, 2016 (Table 2.3).
An analysis of total burned areas from 1984–2017 from Monitoring Trends in Burn Severity (MTBS) project (Eidenshrink, et al., 2007) shows the proportion of wetland areas affected by wildfires and prescribed fires for wetland classes and all CONUS land (Table 2.4).
Fig. 2.2 shows the area burned in five wetland regions (the four inland NWCA regions and the tidal region) in the year studied, showing that the Coastal Plains experience the largest area of wildfires but that all regions experience fires.
Fig. 2.3 shows that fires are common in all wetland regions, but are most often seen in the Coastal Plains region, which also contains the most wetland area. Fires are not confined to a single region or wetland landcover type.
Figure 2.2 Area burned separated by wetland regions in the years studied. Wetland regions are Coastal Plains (CPL), Eastern Mountains Upper Midwest (EMU), Interior Plains (IPL), Tidal, and West (W).
Figure 2.3 Inland wetland area burned by year and vegetation type, showing the area burned in each of the years studied.
Peat contains high SOC density and can extend to several meters in depth throughout regions in the United States, representing a large stock of carbon that is vulnerable to fire. Lost carbon is often studied through simulation of the fire’s effects over areas burned; however, Reddy et al. (2015) used pre‐and post‐fire LiDAR surface elevation data along with soil bulk density and soil carbon content to estimate the volume of carbon lost in the Great Dismal Swamp. This study determined that the Lateral West fire in the Great Dismal Swamp National Wildlife Refuge in Virginia in 2011 burned an average of 47 cm deep and removed a mean of 44 kg C/m2. These previously drained peat soils have a high carbon density, which may be due to historical compaction. High carbon density peatlands, are found in the United States in areas of Alaska, the Midwest (Eastern Mountains Upper Midwest Region), Virginia, Florida, and North Carolina (Coastal Plains region), can combust when moist or dry during drought or drainage (Reddy et al, 2015). Estimates of carbon lost due to emissions from fire must account for loss of aboveground biomass, soil carbon, and future emissions or stock changes by a state change in the wetland system (e.g., vegetation type, newly open water). Organic soils (a classification of wetland soils which contain a high percentage of organic carbon) vary in burn depth and severity, complicating the process of calculating carbon emissions from fire. (Hiraishi et al., 2013). IPCC guidelines previously did not account for emissions from below ground carbon (Mickler, 2013). This guidance has important effects on our understanding of wetland carbon, as emissions from organic soils are high compared to emissions from aboveground biomass (Hiraishi et al., 2013). Fire management practices can affect fire characteristics (intensity, duration, frequency) as well as ecosystem characteristics such as vegetation and microtopography, thus affecting carbon emissions (Hiraishi et al., 2013).
Prescribed burning is used to manage wetlands in the United States to reduce the risk of catastrophic wildfires. Prescribed burns in wetlands can increase nutrient cycling, benefiting some plants and animals, increasing plant growth, and changing plant community structure (Venne et al., 2016). Accumulation and storage of recalcitrant carbon deposited from the burned vegetation can enhance the carbon sequestration potential of wetland vegetated soils over longer terms. Overall, the labile soil carbon cycle and plant productivity is enhanced by prescribed fires in wetlands (Wang et al., 2019a). Intermediate levels of disturbances related to fire can be used to manage taller vegetation and increase biodiversity (Middleton, 2013). Fire may also be prescribed in wetlands to preserve existing habitat conditions (Osborne et al., 2013). Special considerations must be taken when prescribing fire in wetland areas. This includes avoiding draining the wetland through construction of fire lines, avoiding complete burning of organic soils, and controlling intensity to minimize runoff and erosion (U.S. Environmental Protection Agency, 2015).
2.5. U.S. WETLAND MANAGEMENT AS A CARBON‐RELEVANT LANDCOVER CHANGE
Much of the research on wetland carbon dynamics focuses on conversion to agriculture, settlements, or preservation of wetland functions, such as through conservation easements. Development of wetlands for agricultural production of upland crops tends to reduce soil carbon stocks, particularly in drained peatlands, and although methane emissions may be reduced, the
