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).