5.2 Modeled treatment stocks
Predicted treatment stocks were lower than measured treatment stocks in four sites when models using degraded category data were used to estimate SOC stocks in treatment floodplains. Models made separately for each site are intended to minimize variability in climate, geology, and soil formation processes. The input of degraded data to make these models utilizes the assumption that treatment sites were similar to degraded sites before restoration took place. To truly test this concept, a before-after-control-impact study design is appropriate. In our data, magnitudes of differences in carbon stocks in treated versus degraded, divided by the number of years since restoration, suggest carbon sequestration rates that seem unrealistically high. If the difference between measured and predicted carbon stocks serves as an estimate of carbon sequestered since treatment, under the assumption that pre-restoration conditions are well represented by degraded sites, the magnitude of carbon stored since restoration in Staley Creek, South Fork McKenzie River, Whychus Creek, and Kimball Creek is 354 Mg ha-1, 132 Mg ha-1, 56 Mg ha-1, and 118 Mg ha-1, respectively. These four sites also contained higher treatment stocks than degraded stocks. Divided by the number of years since restoration, the per-year carbon sequestration approximations for the four sites are 118 Mg ha-1 year-1 for Staley Creek, 66 Mg ha-1 year-1 for South Fork McKenzie, 14 Mg ha-1 year-1 for Whychus Creek, and 59 Mg ha-1 year-1 for Kimball Creek. Table 3 in Sutfin et al. (2016) lists accumulation rates ranging from 0.03-8 Mg C ha-1 year -1, which is an order of magnitude lower than the estimated differences from this study. We cannot accurately estimate carbon accumulation rates in the sites for this study because there are no measurements of antecedent conditions, but the substantial difference between our inferred rates and the range of published rates for diverse environments around the world suggests that our inferred rates are too high. Thus, we infer that the sites with measured treatment stocks that were higher than degraded stocks, or higher than modeled treatment stocks, likely contained more carbon than degraded floodplains before treatment, facilitated by historic conditions prior to degradation that likely factored into the choice to select the area for stream restoration. Laurel and Wohl (2019), for example, demonstrated that relatively high soil organic carbon stocks can persist in beaver-modified floodplains even after beavers abandon a site and the floodplain becomes drier.
In future studies, it would be beneficial to consider time since degradation, specific manner of degradation, and further information about historic conditions prior to degradation when comparing the categories of degraded, treatment, and reference. Although floodplains such as South Fork McKenzie River and Staley Creek underwent large scale regrading of the floodplain as part of restoration, it is promising that their soil carbon stocks were not destroyed by the disturbance within the organic-rich upper layer. Instead, these sites retained their existing carbon stocks and/or sequestered carbon since treatment. For purposes other than research, such as carbon offset verification, we recommend that direct comparisons to estimate magnitude of carbon stored since restoration be made on repeat pre-post data rather than assuming degraded conditions can directly reflect pre-treatment conditions.
5.3 Across-site comparisons
Two Colorado floodplains and all Utah sites show significant carbon stocks compared to other regions. This is likely explained by high elevation and low mean annual temperature compared to other areas. It is pertinent to consider mountain valleys of the interior western USA as zones of high potential carbon stock. Floodplain drainage, development for agriculture, and associated degradation of wide, wet valley bottoms with potential for high carbon stock (i.e., beaver meadow complexes and other wetlands) could have disproportionately high impacts on carbon sequestration (Wohl et al., 2018). Considering that US states such as Colorado likely have lower total budgets for stream restoration compared to states in the Pacific Northwest that are greatly driven by funding from fisheries conservation, carbon sequestration can serve as an additional added benefit and enhanced return on investment in stream restoration within the Intermountain West.
Both sites within the Cascades ecoregion, Staley Creek and South Fork McKenzie River, contained higher carbon stocks at treatment sites than at degraded or reference sites. Because the data in this study do not directly compare pre-and post-restoration stocks, our ability to quantify carbon sequestered directly as a result of restoration is limited. However, both projects in this region utilized similar methods of stream restoration, in which the valley bottom was regraded to fill incised channels and to lower high-elevation surfaces, and large wood was laid across the valley bottom to maintain hydraulic roughness as vegetation reestablishes. In both cases, surface water was spread across the valley bottom, and field observations of declining upland species and early succession wetland vegetation suggest water tables were raised. Whether observed increases in soil carbon stock took place at each site or persisted from former valley conditions, the manner of restoration sampled in this ecoregion facilitated the development of river corridors with processes and planforms that support carbon sequestration. In addition, carbon stocks in the form of large wood were greatly increased at both sites via the project designs.
The most influential factors contributing to carbon stock are climate and geology, as outlined in the conceptual framework for floodplain carbon stock in Hinshaw and Wohl (2021) and further illuminated by correlations between this dataset and environmental variables. Correlations between grain size, temperature, and elevation support patterns of carbon stocks described in existing literature (Wang et al., 2013; Cai et al., 2016; Qi et al., 2016). Generally, SOC stock increases with (i) elevation and associated climate trends toward cooler temperatures in all study areas and (ii) higher proportion of silt and clay. Geographically, SOC stock increases toward the center of the continent. Potential SOC stock depends primarily on intermediate to long term processes such as soil formation from weathering of underlying lithology and gradual organic matter input from vegetation, but local hydrologic and geomorphic conditions, especially those influenced by floodplain restoration, can set the stage for soil carbon emissions versus soil carbon sequestration. Elevated concentrations of SOC can persist for decades after degradation or drying (Laurel and Wohl, 2019), but rather than optimizing carbon stock potential, dry, degraded floodplains gradually decrease in SOC capacity over time (Ferre et al., 2014; Hanberry et al., 2015; Limpert et al., 2020; Lininger and Polvi, 2020).