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.