Wetlands (Inland)
Draining wetland soils for agriculture, forestry, or development leads to emissions of a large quantity of carbon that had been sequestered in the soils for centuries (Kolka et al., 2018; Moomaw et al., 2018). Wood harvesting and wildland fires are additional examples of disturbances that release soil carbon from wetlands (Moomaw et al., 2018). Wetland soils can contain more than 40% organic carbon by weight, but agricultural soils typically have a soil organic carbon range from 0.5–2%, and disturbances are shown to significantly reduce wetland soil carbon even when a full land conversion has not taken place (Nahlik & Fennessy, 2016). This drastic difference between agricultural and wetland SOC density gives an indication of the magnitude of soil carbon that could be lost over time if wetland soils are converted to agricultural or other uses. Disturbances to wetlands and conversions in favor of other land uses cannot be easily undone because restored and created wetlands in their early years have significantly lower rates of biogeochemical functioning than reference wetlands (Moreno‐Mateos et al., 2012).
An analysis of CONUS wetland class change shows the approximate amount of sequestered soil carbon vulnerable to changes in land use. The loss of wetlands to agriculture and development is explored in this chapter through an analysis of Sleeter et al. (2018) on contemporary land use change. From 1974 to 2007, nearly 17,000 km2 of CONUS land transferred into or out of an inland wetland classification, with a net loss of nearly 5,000 km2 of inland CONUS wetland area.
Some of the changes documented by Sleeter et al. (2018) are due to changes in moisture availability. For example, droughts in the late 1980s and early 1990s followed by a rapid change from drought to wet conditions in 1993 (Huang et al., 2011) resulted in the wettest period seen in study area in North Dakota in 130 years (Winter & Rosenberry, 1998). Much of the change in wetland land cover may be due to natural wetland expansion and contraction due to changes in soil moisture. It is common for some wetlands, including prairie pothole wetlands, to expand during wet years and contract in dry years. These wetlands are linked by groundwater hydrology or aboveground flow, and may even be combined during wet periods. One such period was from 1993 to 1998, when record high ground and surface water levels were recorded due to high precipitation. These wet‐dry cycles (also called oscillatory fluctuations) are common and have been documented over thousands of years in prairie pothole wetlands through proxy data such as tree rings. These cycles can cause changes in species presence and abundance and occur over long periods of time, estimated to range from 4–35 years (Valk, 2005). Moisture trends may be caused in part by increased snowmelt and warmer air temperatures in the Prairie Pothole region (McKenna et al., 2017).
Wetland gains and losses in the last several decades (1974–2007) in the United States were not distributed uniformly. A small number of ecoregions represented a large amount of the total change, with rates of change over 100 km2 per year in some areas (Sleeter et al., 2018). Some of these ecoregions are at a higher risk of pressure from agriculture and development, while others benefit from land changes and land use changes conducive to wetland development such as wetland restoration. Some of the larger changes documented are in the Southern Coastal Plain ecoregion, which lost the most wetland area during the years studied: 4,700 km2 net loss. Of this, 4,600 km2 were lost to development. The largest gains were in the Western Corn Belt Plains, which gained the most wetland area of any ecoregion. There was a gain of 880 km2 from agricultural lands that became wetlands, however other wetland areas were lost to agricultural expansion or development, leaving a net gain of 750 km2 of wetland area.
Table 2.3 Burned areas in wetlands are compared to burned areas in all CONUS landcover classes in select years
| Year | Percent of CONUS Area Burned | Percent of Wetland Area Burned | Total Burned Area km2 | Wetland Burned Area km2 | Wetland Burned Area as Percent of Total Burned Area |
|---|---|---|---|---|---|
| 1984 | 0.08 | 0.06 | 5,600 | 300 | 4.42 |
| 1990 | 0.23 | 0.44 | 15,700 | 1,900 | 12.24 |
| 2000 | 0.72 | 0.78 | 52,700 | 3,400 | 6.45 |
| 2011 | 0.85 | 1.0 | 62,000 | 4,400 | 7.03 |
| 2015 | 0.37 | 0.27 | 27,100 | 1,200 | 4.33 |
Burned areas were determined through MTBS.
2.4. IMPACT OF WILDFIRE ON WETLAND CARBON
Wildfires are a regular feature of many wetland ecosystems in the United States, such as pine barrens (Clark et al., 2006), pocosins (Bailey et al., 2007), northern spruce peatlands (Granath et al., 2016), and Alaskan lowlands (Jafarov et al., 2013). Charcoal, a type of pyrolyzed or black carbon, can be seen in cores taken from peatland cores in the Southeast, Midwest, and boreal regions, indicating a long‐standing fire history in these areas (Neary et al., 2005). Fires in Alaskan wetlands can be particularly destructive, 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 |