<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.
13.2.1 Wetland Regulations
During the settlement of North America, wetlands were viewed as unproductive areas that were impediments to transportation and development, as well as a breeding ground for disease. That sentiment lasted for over 150 years, during which draining of wetlands for agriculture, forestry, and urban development was routine to make these ecosystems productive for commercial use. Once drained, wetlands generally have very productive soils because of their high organic matter and associated nutrients. Not until the mid-1900s did the effects of wetland drainage on both inherent wetland values and larger landscape impacts begin to be identified. Wetlands are now known to provide critical habitats for many rare species, serve as filters for pollutants and sediment, store water to prevent flooding, and sequester and store carbon, but those ecosystem services were not broadly recognized until relatively recently.
Currently, vegetation removal, surface hardening (e.g., pavement and soil compaction), and drainage are identified as the most common physical stressors on U.S. wetlands (U.S. EPA 2016). To address the threats and subsequent losses of wetlands, wetland policies have been developed to avert further wetland conversion, degradation, or loss. The United States has an overarching policy of “no net loss” of wetlands adopted in 1989. This policy has dramatically slowed U.S. wetland losses and led to the development of wetland banking programs whereby losses due to development are offset by wetlands restored or created elsewhere. In Canada, the main causes for wetland losses are from land conversion to urban or agriculture, water-level control including flooding from hydroelectric development, and climate change (Federal Provincial and Territorial Governments of Canada 2010). In 1991, the Canadian government enacted the Federal Policy on Wetland Conservation (Canadian Wildlife Service 1991). Similarly, the Natural Protected Areas Commission of Mexico announced a national wetland policy in 2014 designed to protect wetlands and avert losses. Recent research in Mexico indicates that drainage for agriculture and conversion to aquaculture are two major threats to wetlands (De Gortari-Ludlow et al., 2015).
These national-level policies are not the only regulations in place designed to protect wetlands. The United States and Canada have wetland-focused state and provincial regulations, as well as other federal regulations that, while not focused on wetlands, do protect wetland habitat. Migratory bird agreements among the United States, Mexico, and Canada often have wetland protection implications. In 1986, the United States and Canada adopted the North American Waterfowl Management Plan and were later joined by Mexico in 1994 (North American Waterfowl Management Plan Committee 2012). This plan establishes strategies to protect wetland habitat for the primary purpose of sustaining migratory bird populations with the associated benefit of protecting carbon pools.
Competing land uses and economic development will continue to threaten wetlands in North America. Multiple policies have been designed to protect against, and mitigate for, wetland loss. However, while losses are greatly stemmed, the United States continues to experience net losses of wetlands in terms of absolute acreage in spite of the no net-loss policy. Canada and Mexico currently have no nationwide wetlands inventory, limiting the ability to estimate wetland conversion or function, including carbon fluxes and pools. It is important to remember that no net-loss policies do not protect against reduced functionality in restored versus natural wetlands.
13.2.2 Change in Wetland Area
As a result of socioeconomic drivers, there have been massive disturbances and conversions of wetlands over the past 150 or more years in North America. The latest assessment of the status and trends of wetlands in the conterminous United States (CONUS) estimates that there are 445,000 km2 of wetlands, which includes 395,197 km2 of terrestrial wetlands (USFWS 2011). In colonial America, there were an estimated 894,000 km2; between 1870 and 1980, the United States experienced a 53% loss of wetland area (Dahl 1990). From 2004 to 2009, increased wetland restoration on agricultural lands occurred; however, wetland losses continued to outpace gains, leading to a total wetland area decline of 0.06% (USFWS 2011). The current rate of loss is 23 times less than that of the historical trend (e.g., 1870 to 1980), an indication of changing attitudes toward wetlands and the effectiveness of policies to protect them (USFWS 2011).
Although Canada does not have a national wetlands inventory, estimated losses are approximately 14% of the country’s original 1,470,000 km2 of wetlands (Environment Canada 1991). Similarly, an estimated 62% of wetland area has been lost from Mexico’s original 112,166 km2 of wetlands (Casasola 2008; Landgrave and Moreno-Casasola 2012). Mexico’s small area of peatlands covers about 20,000 km2 generally found in high-elevation ecosystems and near-coastal freshwater marshes (Instituto Nacional de Estadística y Geografía 2010). The country has another 15,000 km2 of mineral soil wetlands.
In CONUS, about 468,000 km2 of wetlands have been lost, 96% of which have been mineral soil wetlands and 4% peatlands (Bridgham et al., 2007). Similarly, in Canada, of the 212,000 km2 of wetlands lost, 94% have been mineral soil wetlands and 6% peatlands (Bridgham et al., 2007). However, Canadian peatlands are now being lost in large numbers due to urban development, hydroelectric development, and energy production (Chimner et al., 2016), including in the oil sands region where nearly 300 km2 have been destroyed by mining (Rooney et al., 2012). In the United States, forested wetlands are undergoing the most rapid losses among terrestrial wetland types. From 2004 to 2009, 1.2% of forested wetlands were lost (2,562 km2) per year, compared to gains of 1,084 km2 per year for emergent wetlands and 729 km2 per year for shrub wetlands (Dahl 2011).
The change in wetland area is quite high in the U.S. Midwest where Iowa, Missouri, Illinois, Ohio, and Indiana have experienced a greater than 85% loss of their wetlands. California has lost 96% of its original wetlands (Dahl 2011; Garone 2011). Other notable ecosystem examples include bottomland hardwood forests of the Lower Mississippi River Alluvial Plain (i.e., southern Illinois to the Gulf of Mexico); these forests, once comprising an area of approximately 85,000 km2, were reduced to about 20,000 km2 by 1990, primarily through agricultural conversion and alterations to the hydrological system for flood protection (Stanturf et al., 2000). Major federal flood-control projects that began following a significant flood in 1927 contributed to more than 30% of wetland losses and subsequent agricultural conversions in the Mississippi River Valley (King et al., 2006; Stavins and Jaffe 1990). Similarly, the Prairie Pothole Region (see Section 13.3.3) of the United States and Canada included 200,000 km2 of wetland area prior to European settlement but has since decreased to 70,000 km2 of intact (i.e., not drained) wetland area (Dahl 2014; Euliss et al., 2006). In contrast, Alaska is reported to have had negligible wetland loss (Bridgham et al., 2007), although the state does not have a completed assessment under the U.S. Fish and Wildlife Service (USFWS) National Wetlands Inventory.
Areal extent alone does not indicate the ecosystem function and services that wetlands deliver. In 2011, the U.S. Environmental Protection Agency (EPA) released the first national assessment of the condition of U.S. wetlands. Findings indicated that 48% of wetlands were in good condition, 20% were in fair condition, and 32% were in poor condition (U.S. EPA 2016). While wetlands may remain intact, their alterations by humans are still affecting the ability of wetlands to function similarly to an unaltered state. Carbon sequestration is one of those important functions affected by wetland condition. Connecting wetland condition to carbon stocks and fluxes will be an important next step for assessing impacts on the carbon cycle.
13.2.3 Overview of Disturbance Effects on Carbon Stocks and Fluxes
Wetlands have been sequestering carbon from the atmosphere for thousands of years. Following the end of the last glacial period about 12,000 years ago, wetlands developed over much of the northern part of North America. Low areas or areas with less permeable soils tended to pond water and create the anoxic environment critical for peatland and mineral soil wetland formation. In undisturbed wetlands, carbon pools are relatively stable over short time intervals, but carbon fluxes may be quite variable due to complex interactions of climate, vegetation, soils, and hydrology. For example, annual CO2 fluxes ranged from a sink of 2 to 112 grams of carbon (g C) per m2 per year, and CH4 fluxes ranged from a source of 2.8 to 4.4 g C per m2 per year during a 6-year study in a peatland in southern Ontario (Roulet et al., 2007). Carbon dioxide fluxes generally decrease (i.e., sinks or lesser sources) and CH4 fluxes generally increase (i.e., sources or lesser sinks) as water tables get nearer to the surface (Olson et al., 2013). During droughts or high-water events, CO2 and CH4 fluxes can vary greatly, even in undisturbed wetlands. Changes in carbon fluxes resulting from disturbance lead to changes in carbon pools. Drainage is the main human-caused disturbance that has led to a variety of local- to landscape-level impacts. Wetland drainage causes an abrupt change from anaerobic conditions during flooding to aerobic conditions subsequent to drainage, resulting in rapid acceleration of decomposition through microbial oxidation of organic matter (Drexler et al., 2009). As a result, wetland drainage generally leads to lower carbon stocks, lower CH4 fluxes, and a long-term increase in CO2 fluxes (Bridgham et al., 2006). In peatlands, drainage also can result in significant land-surface subsidence (Drexler et al., 2009). Other human-caused disturbances include filling of wetlands for development, construction of dams that permanently flood wetlands, stream channelization and road construction that can disconnect wetlands from their water source, removal of vegetation (including forest harvesting), and agricultural conversion of surrounding uplands.
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