6 Conclusions
Our study design was constrained by the difficulty of finding exact environmental matches when substituting space for time in comparing reference, treatment, and degraded reaches, and by the short time since restoration was completed at the study sites. However, the results show that floodplains in reference conditions tend to contain higher carbon stocks, and therefore river restoration offers an opportunity to sequester more carbon. An important consideration is that the continuum of degraded, treatment, and reference alternative states is not linear, and does not always follow the assumed temporal order of degraded, treatment, and reference. Disturbance associated with stream restoration construction can reset floodplain SOC stock in treatment sites to lower values than carbon stock of degraded conditions, or the disturbance may not affect persistent carbon stocks with the floodplain chosen for restoration. Uncertainties regarding the potential for persistent floodplain SOC stocks that remain from conditions prior to restoration, along with the challenges of substituting space for time in a complex natural system with multiple interacting variables, strongly indicate that the effects of river restoration on floodplain SOC stocks can be most accurately assessed by (i) measuring stocks prior to restoration and repeating these measurements over a period of years following restoration and (ii) conducting analogous measurements on an adjacent portion of the river corridor not undergoing restoration or on carefully chosen degraded and reference sites.
The current estimated fluxes of carbon into and out of floodplain-wetland corridors show carbon release through methane emissions from wetlands (e.g., Saarnio et al., 2009), carbon dioxide emissions to the atmosphere (Butman and Raymond, 2011), and export of carbon out of floodplains via dissolved carbon in water (e.g., Whitworth et al., 2014) and transport of large wood (Benda and Sias, 2003). The magnitude of carbon sequestration versus carbon transport within individual river corridors or on regional to global scales remains poorly constrained (Hilton and West, 2020), but the potential for net carbon sequestration in river corridors is likely to be notable in the context of climate change.
Political and economic pressure to reduce carbon emissions and develop additional ways to measure and store carbon is likely to increase (e.g., Lindstad and Bø, 2018). Carbon offsets within the carbon market currently fall into two categories: emission reduction (e.g., Sinha and Chaturvedi, 2019) and carbon sequestration (e.g., Lal, 2007). We suggest that stream restoration can offer both. By revitalizing hydrologic conditions that limit the decomposition and extend the residence time of soil organic carbon, stream restoration involving hydrologic reconnection prevents gradual or rapid loss of carbon that is stored in soil and released during floodplain degradation. By enhancing organic matter input from regenerated riparian vegetation and creating conditions for fine sediment deposition, the potential for new carbon sequestration increases.
Despite the variations in floodplain SOC stock relative to potential restoration effects in the data analyzed here, restoration has the potential to enhance organic carbon sequestration and stocks by enhancing floodplain water tables, deposition, and wetland formation. This study shows that reference carbon stocks in anastomosing grassed wetlands and anastomosing wet woodlands are generally higher than degraded and treatment stocks within the same regions, giving the restoration community something to work toward as we strive for resilient, functioning floodplains and creative solutions to climate change.