<b>Kolka</b>, R., C. <b>Trettin</b>, W. Tang, K. Krauss, S. Bansal, J. Drexler, K. Wickland, R. Chimner, D. Hogan, E. J. Pindilli, B. Benscoter, B. Tangen, E. Kane, S. Bridgham, and C. Richardson, 2018: Chapter 13: Terrestrial wetlands. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 507-567, https://doi.org/10.7930/ SOCCR2.2018.Ch13.
Generally, terrestrial wetlands are managed for one or more of the ecosystem services they provide. In many cases, wetlands are managed as set-aside areas used as natural filters for water quality, areas for rare species, and land for hunting and trapping due to their faunal diversity. For example, several international conservation organizations consider the PPR of the midwestern United States and Canada as the most important waterfowl habitat in North America. Management decisions and development that change the hydrology, soils, or vegetation will affect carbon dynamics, often leading to enhanced decomposition, decreased CH4 flux, and reduced carbon sequestration, particularly when wetlands are drained. In contrast, restoration of drained wetlands (or avoided loss of wetlands through easements) increases carbon sequestration and CH4 production. Policies using wetlands as carbon banks and using the carbon gained through wetland restoration to trade in carbon markets are becoming increasingly common globally.
13.4.1 Effects of Wetland Management, Restoration, and Creation on Carbon
This section considers wetland management that does not convert wetlands to another land use. Wetland management occurs on a gradient from very intensive management to preservation. As they have been for thousands of years, wetlands managed for preservation or their intrinsic ecosystem services generally are carbon sinks, although there are some indications that rising temperatures from climate change may be changing wetlands from sinks to sources. For example, an undisturbed bog in Canada was a carbon source for 3 years of a 6-year study (Roulet et al., 2007). Even if wetland sinks are smaller than they once were, management or restoration practices could have dramatic feedbacks to atmospheric concentrations of CO2 and CH4. In a management example, there are approximately 658 km2 of terrestrial wetlands under “moist-soil” management in the U.S. National Wildlife Refuge System, where lands are flooded for wintering and migrating waterfowl. Research has demonstrated that seasonal drainage in moist soil regimes leads to major losses of soil carbon (Drexler et al., 2013). The practice of deeply flooding marshes is not as common in the national wildlife refuges as seasonal drainage, but deep flooding may be an option for increasing carbon sequestration rates (Bryant and Chabreck 1998).
The effect of altered hydrology does not necessarily cause a loss of ecosystem carbon from managed wetlands. Studies of carbon pool response to managed peatlands in Finland have shown that increased forest productivity may offset losses due to water management resulting in a net increase of carbon, but this response is site dependent (Minkkinen et al., 2008). Similarly, forest harvesting only had a transient effect on the soil carbon pool of a mineral soil wetland (Trettin et al., 2011). In contrast, peat utilization, as in peat mining for fuel or horticultural purposes, is the extreme where the peat itself is removed from the wetland. Although peat mining is not common in North America, Canada is the third largest producer of horticultural peat in the world, with much of the peat originating from the peatlands in the St. Lawrence Lowlands on the Canadian side of the Great Lakes (Van Seters and Price 2001). For production agriculture where wetlands remain wetlands, water levels are typically controlled to maximize production, usually at the expense of carbon pools. Prairie potholes and other hydrologically isolated wetlands are often nested within agricultural lands but remain undrained. These cropped, undrained wetlands can be major sources of GHGs due to increased nutrient loading and associated nitrous oxide (N2O) fluxes. In addition, temporarily ponded wetlands that dry down during the growing season can be tilled and farmed, increasing decomposition rates. Approximately 6,500 km2 of U.S. peatlands are being used for crop production (ICF International 2013). The converted peatlands are usually highly productive for agriculture, but they also have high potential as GHG mitigation sites if the land is restored to vegetated wetlands (Richardson et al., 2014; Wang et al., 2015). Specific GHG mitigation benefits accrue from 1) decreases in CO2 fluxes related to the oxidation of soil carbon while in crop production, 2) decreases in the use of nitrogen fertilizers, 3) decreases in lime application amendments, and 4) increases in carbon sequestered in soils and perennial vegetation (ICF International 2013). Crops such as sugarcane lead to large losses of carbon through enhanced decomposition (Baker et al., 2007). Paddy rice production systems are well-known sources of CH4 (Lindau et al., 1993) and N2O. Other crops such as sugar beet, radish, cranberry, blueberry, lettuce, celery, carrot, potato, onion, and mint are grown in wetlands, but little data exist on their influence on ecosystem carbon balance. Similarly, aquaculture has altered wetlands in North America, but, again, little data exist on the impact on carbon storage or fluxes. Although forest harvesting causes short-term changes in carbon sequestration during the period of stand regeneration, it generally has little impact on long-term wetland soil carbon balance (Roulet 2000; Trettin et al., 2011).
Wetland restoration usually includes the re-establishment of hydrological regimes to support hydrophytic vegetation. Wetland restoration and creation of new wetlands (where none existed previously) and small ponds have counteracted much of the wetland losses in CONUS (Dahl 2011). For instance, from 1998 to 2004 and 2004 to 2009, areas reclassified as wetlands in the United States increased by 17%, meaning that 802 km2 of new wetlands were created, but this figure does not indicate how many additional square kilometers of the restored wetlands were still classified as wetlands. In addition, creation of small ponds has increased over the last few decades, with 838 km2 per year created from 2004 to 2009 (Dahl 2011).
Wetland restoration can lead to the opposite effects of drainage, with increases in carbon pools and in CH4 fluxes and lower CO2 fluxes (Wickland et al., 2014). Research has found that restoring wetlands by rewetting them increases soil carbon storage (Lucchese et al., 2010). IPCC guidelines for mineral soil wetlands state that cultivation leads to losses of up to 71% of the soil organic carbon in the top 30 cm of soil over 20 years and that restoration increases depleted soil carbon pools by 80% over 20 years, and by 100% after 40 years (Wickland et al., 2014). Rewetting also may increase CH4 fluxes, not only above the previously drained levels, but also above reference levels temporally (Badiou et al., 2011). However, some studies have found that restoration did not increase CH4 fluxes (Richards and Craft 2015). In the long term, restoring degraded wetlands appears to be a positive for GHG mitigation.
Creating new wetlands and small ponds also can affect both long-term soil carbon storage and gaseous fluxes. Created wetlands tend to have carbon accumulation rates higher than those of natural wetlands (Bridgham et al., 2006). In addition, created wetlands often have similar or lower CH4 fluxes (Mitsch and Hernandez 2013; Winton and Richardson 2015). However, assessments have found that small ponds are large sources of CH4 (Holgerson and Raymond 2016). Similar to created wetlands and some riparian zones, small ponds may sequester carbon at high rates due to high sediment deposition rates from the surrounding land.
Many restored wetlands do not provide the level of ecosystem services they did before their degradation, usually a result of inadequate hydrology restoration. One survey found that only 21% of wetland restoration sites have ecologically equivalent natural functions (Turner et al., 2001). Post-restoration monitoring is critical to determining restoration success and providing opportunities to modify restoration techniques if necessary. Assessment of success usually occurs over relatively short periods (1 to 3 years) and with relatively simple protocols because of time, resource, and technical constraints. Determining success over the short term is difficult because wetland processes, such as soil formation or forest recovery, occur over decades. Also, most current assessment techniques are fairly simple and may not adequately characterize the condition of a wetland, especially if critical functions such as hydrology or processes such as carbon and nutrient cycling are not fully understood. Moreover, inadequate study of many wetland types challenges efforts to understand both the processes that lead to carbon accumulation and fluxes and the impact of wetland restoration on carbon. Furthermore, due to the developmental trajectory of restored wetlands, their capacity to store carbon may change through time, with considerable storage initially and then much less storage thereafter once vegetation has fully colonized and root systems have developed (Anderson et al., 2016).
13.4.2 Processes and Policies that Affect Wetland Management, Restoration, and Creation
Recognition of the values that wetlands provide has led to changes in federal policies aimed at protecting, restoring, and creating wetlands over the past four decades. Four significant policies are 1) Section 404 of the Clean Water Act (1972); 2) the Highly Erodible Land Conservation and Wetland Conservation Compliance provisions of the 1985 Food Security Act and subsequent amendments, commonly known as the “Swampbuster program”; 3) President George H. W. Bush’s “no net-loss” policy (1989); and 4) the U.S. Army Corps of Engineers and EPA compensatory mitigation rule (USACE 2008). Initially passed as part of the Federal Water Pollution Control Act of 1972, the Clean Water Act focused on nonagricultural wetland conversions (U.S. EPA 2015). In its initial form, the Swampbuster program discouraged farmers from converting wetlands by withholding federal farm program benefits if conversion occurred on nonexempt wetlands. Farm Bill 1990 amendments created the Wetland Reserve Program, which was later consolidated with other easement programs into the Agricultural Conservation Easement Program (ACEP). Rather than withholding incentives, the USDA NRCS incentivizes farmers to restore, protect, and enhance wetlands by purchasing wetland reserve easements via ACEP (USDA 2014). The Agricultural Act of 2014 (i.e., Public Law 113-79, commonly referred to as the 2014 Farm Bill) provided NRCS with the authority to enroll wetlands in 1) permanent easements, with 100% of the easement value and 75% to 100% of restoration costs covered, 2) 30-year easements funded at 50% to 75% of the easement value with 50% to 75% of the restoration costs covered, and 3) term easements with stipulations dependent on state laws.
The no net-loss policy, which sought to replace lost wetland habitat with new habitat by restoring and creating wetlands, is now the cornerstone of U.S. wetland conservation (Mitsch and Gosselink 2015). As a result, numerous federal and state agencies, non-governmental organizations, and private landowners are engaged in wetland restoration and creation across the United States with a keen focus on establishing the proper hydrological conditions needed to support flora and fauna specific to a certain wetland type. Such activities often result in preserving or expanding the carbon pool of wetlands, but little attention has been given to ensuring the long-term sustainability of such newly formed carbon sinks. Wetland restoration is still a relatively new field, and management approaches for maintaining the sustainability of carbon sinks are still being developed, tested, and refined.
The Federal Policy on Wetland Conservation in Canada (Canadian Wildlife Service 1991) also encourages no net-loss of wetlands. The regulation is focused largely on activities undertaken by the Canadian government on its federal land. Although the policy discourages wetland destruction or degradation, the Canadian government does not require compensatory mitigation. Though currently limited, the Natural Protected Areas Commission of Mexico has a national wetland policy to protect wetlands and avert losses.
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