To Tree, or not to Tree: Sediment Storage in Forested and Non-forested Mountainous Hillslopes of the Bitterroot Mountains, MT

In steep, mountainous landscapes, interactions between soil, rock, and biotic factors combine to form complex feedbacks. Here, we explore the dynamic interplay between soil and vegetation and its influence on hillslope sediment storage and movement in the Bitterroot Range of Montana's Rocky Mountains. We focus on a set of analogous forested and non-forested hillslopes along Lost Horse Creek, where avalanche paths determine vegetative density without significant impact on other topographic variables. LiDAR, high-resolution aerial photography, and field mapping are used to determine the local and landscape variables that influence soil cover and sediment storage. We find high-resolution surface roughness is a useful remote proxy to identify bedrock and boulder outcrops, particularly beneath canopy cover. Based on this analysis and field mapping, the spatial extent of soil cover does not vary significantly between forested and non-forested regions, though soils are generally thicker under forest cover. We additionally measure fallout radiogenic nuclides (FRNs; Cs-137 and Pb-210) in soils across 40 sites to provide insight into short-term soil erosion and movement. Preliminary results show high spatial variability in FRN activities of mineral soils in both systems, which may reflect either spatially variable delivery or soil erosion. Additionally, FRN activity of surface litter and duff at the forest floor is three to four times higher than mineral soils in both the forested and non-forested sites, suggesting that FRNs may provide a novel tracer of carbon storage and export. Our results show that the coupled use of isotopic tracers and high resolution spatial data reveal quantitative insights into landscape scale hillslope soil dynamics.