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).