Study area

The PPR (41.7° to 54.7°N latitude, 92.5° to 114.5°W longitude) covers ~820,000 km² of the Great Plains in the United States and Canada and is home to millions of depressional wetlands (16). These depressions were formed during the Wisconsin glaciation, approximately 12,000 years ago (17). The underlying glacial till has low permeability, allowing depressions to fill and hold water and to ultimately develop into palustrine and lacustrine ecosystems made up of wetlands, ponds, and shallow lakes. We collectively refer to these depressional waterbodies as “wetlands” because the majority of them are less than 1 ha (0.01 km²) in size, less than 1 m deep, and seasonally (nonpermanent) ponded, meeting the definition of a wetland (sensu the Cowardin classification system) (61). In the PPR, larger wetlands often are shallow (<2 m) and have similar biogeochemical processes as smaller wetlands (62). Wetlands contain methanogenic microbial communities that are adapted to anoxic saturated soils, where they decompose organic material and produce CH₄ (9). PPR wetlands range from fresh to hypersaline (up to three times more saline than the ocean) (17). This salinity is attributable to groundwater transport of dissolved sulfate through the ion-rich glacial till. PPR wetlands with higher sulfate concentrations have suppressed CH₄ emissions (23), albeit high dissolved organic carbon concentrations in some PPR wetlands can support CH₄ production in the presence of sulfate (24). Larger wetlands in topographically lower landscape positions tend to accumulate groundwater-derived solutes, while smaller, shallower wetlands are filled with rainwater and snowmelt (45). Vegetation characteristics of PPR wetlands are influenced by water depth and chemistry, typically with concentric zones with open water with submerged aquatic vegetation toward their centers and marshes and meadows with floating and emergent macrophytes such as Typha toward their edges (37, 44).

The climate of the PPR is continental with a north to south temperature gradient ranging from ~2° to 8°C mean annual temperature and a northwest to southeast precipitation gradient ranging from ~400 to 900 mm year⁻¹ (63, 64). The PPR is centered on the confluence of tropical Pacific Ocean (El Niño–Southern Oscillation), eastern Pacific Ocean (Pacific Decadal Oscillation), and North Atlantic (Atlantic Multidecadal Oscillation) oscillations (65). Synergistic effects from synchronization of these oscillations lead to decadal periods of drought and deluge that influence groundwater levels. Wetland surface water is also extremely sensitive to variability in seasonal and annual precipitation (66). Thus, the size and hydroperiod of PPR wetlands are the result of complex interactions between long-term climate and short-term weather. An extreme multiyear drought from 1988 to 1992 led to the drying of many wetlands, with 1991 having the lowest wetland extent (45). Following the extreme drought, much of the PPR experienced a shift to a wetter climate. Annual precipitation in 2011 was one of the highest on record, and 2011 had some of the highest numbers of wetlands with ponded water (45). Therefore, we use 1991 and 2011 as extreme dry and wet years, respectively, in our modeling.

The PPR is intensively used for agricultural crop and biofuels production, which has led to extensive wetland drainage and upland conversion from prairie grassland to cropland (16). Drainage reduces CH₄ emissions and soil organic carbon stocks but increases CO₂ and nitrous oxide (N₂O) emissions (22, 67). Upland conversion from grassland to cropland can affect CH₄ emissions indirectly through increased nutrient loading, resulting in increased CH₄ emissions (38). However, wetlands nested in croplands are also subject to tillage and aeration of soils, thereby lowering CH₄ emissions (22). Wetland drainage often results in consolidation of water from multiple smaller wetlands into one larger wetland. These larger wetlands are deeper with fewer fluctuations in water levels, as well as greater coverage of invasive emergent hybrid cattail, all of which favors CH₄ production and emissions (37). Consolidation of wetlands also targets the smallest wetlands changing the distribution of wetland size classes, albeit the vast majority of wetlands are still <1 ha.