5.1 Within-site comparisons
Degraded floodplains have the lowest carbon stocks in the majority of
sites, which supports the hypothesis although the results are not
statistically significant. Among sites with complete datasets, treatment
and reference were tied for having the highest carbon stocks, but the
reference category has more statistically significantly higher stocks
than the treatment category.
Deep Creek is the only site where reference soil carbon stocks are lower
than both treatment and degraded stocks. The reference site chosen for
Deep Creek was Gray’s Creek, a beaver meadow about 15 km away from Deep
Creek. Gray’s Creek and Deep Creek are within the North Fork Crooked
River watershed, but the two sites are underlain by different geology.
Gray’s Creek lies within Eocene- to Oligocene-aged volcaniclastic tuff
in the John Day Formation, while Deep Creek overlies the Columbia River
Basalt formation of Miocene age. Basalt weathers to clay minerals, while
tuff contains higher silica content and is more resistant to weathering
than basalt. In our dataset, floodplain soils underlain by basalt
bedrock geology contained a higher proportion of silt and clay. The
results from correlation of numerical predictor variables show that
grain size has the largest magnitude of negative correlation to carbon
stock, indicating that silt and clay content are significant
contributors to carbon stock, as demonstrated in previous work (e.g.,
Cai et al., 2016). Gray’s Creek also has a smaller drainage area than
Deep Creek, with 42 km2 and 224 km2,
respectively. Deep Creek hosts large ponderosa pine (Pinus
ponderosa) trees and is classified as evergreen forest in the National
Land Cover Dataset (Homer et al., 2012), while Gray’s Creek is
classified as emergent vegetation and contains no large trees other than
willows. Gray’s Creek is also the reference site chosen for Lost Creek.
Lost Creek is within the same geological formation as Gray’s Creek and
trends from least to greatest mean carbon stocks in degraded, treatment,
and reference sites. In hindsight, a different reference condition
should have been chosen for Deep Creek but this oversight indicates the
importance of considering the underlying geology when associating
categories of floodplain.
High variance of soil carbon stocks from our samples reflects relatively
low sample sizes per floodplain category at each site, but also aligns
with the variable nature of soil organic carbon accumulation over time.
Floodplains are highly dynamic ecosystems that undergo frequent
disturbance and include multiple stages of vegetative succession and
soil development. Floodplain heterogeneity enhances diversity of
habitats for aquatic and terrestrial species, and in turn supports more
biodiverse and resilient floodplains (Wohl, 2016). Increased
heterogeneity and river mobility within the floodplain are desirable
goals for many restoration projects, but multiple sequences of
disturbance and succession induced by frequent lateral channel migration
can also lead to a variety of patches with different soil carbon
concentrations and stocks (Lininger et al., 2018; Sutfin et al., 2021).