Terrestrial Wetlands

An important concern globally is how wetlands will respond to a changing climate. Climate change has the potential to affect carbon cycling of natural, degraded, created, and restored wetlands. However, there is considerable uncertainty regarding the likely responses, including how warming and variations in precipitation regimes will influence the balance between plant productivity and organic matter decomposition. An example pattern might be warming followed by drier conditions leading to wetland carbon losses, as has occurred in simulated peatland droughts (Fenner and Freeman 2011). Altered precipitation regimes also may shift the hydrological balance in the absence of warming. Even on an annual timescale, individual wetlands can alternate between a carbon sink in wet years to a carbon source in dry years, illustrating the sensitivity of wetlands to biotic and abiotic conditions. However, the direct correspondence of increased peat oxidation with a lowered water table is not universal. Instead, Makiranta et al. (2008) showed soil temperature controlled more of the variability in peatland soil respiration than did the water-table position. Similarly, CH₄ fluxes in high-latitude wetland ecosystems with high water tables were more sensitive to soil temperature than were those ecosystems with lower water tables, which were more sensitive to water-table position (Olefeldt et al., 2013). Accordingly, changes in carbon pools and fluxes in response to changes in temperature and precipitation regimes will vary greatly based on wetland type and interactions with hydrology because carbon cycling may be different under warmer and wetter conditions than under warmer and drier conditions. For example, CH₄ fluxes from PPR wetlands were four times higher under warmer and wetter conditions than the fluxes were under warmer and drier conditions (Bansal et al., 2016). Northern seasonally frozen peatlands already are undergoing rapid changes, and increased carbon fluxes are likely to continue over the coming decades to centuries as conditions continue to warm (Schuur et al., 2015). Another general pattern is that drier conditions will facilitate and exacerbate fires, especially in peatlands, resulting in large fluxes from the oxidized peat (Turetsky et al., 2011b; see also Ch. 11: Arctic and Boreal Carbon).

The response of mineral soil wetlands to changes in temperature and precipitation regimes is uncertain, largely because of the wide range in properties and geomorphic setting. Histic-mineral soil wetlands (“histic” refers to soils with a 20- to 40-cm organic horizon) may be expected to respond similarly to peatlands. For other types, such as mineral soil wetlands in floodplains where the surface organic layer is thin due to high turnover rate, the changes in that layer associated with climate change are likely small. Changes in the hydrological regime also are expected to alter the carbon balance. Increased periods of a high water table or flooding may be expected to reduce productivity (Trettin et al., 2006) and increase CH₄ fluxes (Sharitz and Pennings 2006). The effect of climate change on organic matter decomposition and carbon export from the wetland is an important uncertainty and feedback to adjoining aquatic ecosystems. The uncertainty in mineral soil wetland response is high, largely because there are far fewer studies on mineral soil wetlands than on peatlands.

Rising atmospheric CO₂ is considered likely to increase GHG fluxes from wetlands due to increased CH₄ fluxes offsetting gains from increased plant carbon sequestration (Bridgham et al., 2007; Hyvonen et al., 2007). Hyvonen et al. (2007) suggest that soil carbon in the temperate and boreal zones will increase because of increased litter input, but the magnitude of the response will depend on available nitrogen and land management. Little is known about interactions between changes in water regime and plant productivity. In upper Michigan, lowered water tables led to increased productivity in vascular plants (e.g., shrubs and sedges) and Polytrichum; higher water tables led to higher Sphagnum production (Potvin et al., 2015). Demonstrating the importance of field experimentation, Dijkstra et al. (2012) measured increases in CH₄ in both mineral soil wetlands and peatlands following manipulation of the water regime. Understanding these interactions with CH₄ fluxes is fundamental to considering the feedback associated with rising atmospheric CO₂ (Petrescu et al., 2015; Zhang et al., 2017b).