Soil storage on steep forested and non-forested mountain hillslopes in the Bitterroot Mountains, Montana

Abstract

Mountain hillslopes are dynamic settings with discontinuous soils affected by a suite of variables including climate, lithology, hydrology, and vegetation. Our study seeks to understand how forest cover influences soil and rock distribution at decadal to century timescales. We focus on a series of post-glacial hillslopes in Lost Horse Creek of the Bitterroot Mountains, Montana. In this system, avalanche paths maintain parallel, topographically similar swaths of forested and non-forested slopes with uniform aspect, lithology, and climate. We combine field observations, fallout radionuclide analysis (210 Pbex & 137 Cs), and remote sensing data to understand both landscape- and fine-scale patterns in soil and rock distribution. Local soil and rock measurements indicate more extensive soil cover (forest = 94.4 + or = 2.6%; non-forest = 88.3 + or = 1.9%) and thicker soils (6cm greater median) in the forested system. We compare landcover-classified rock to topographic metrics from LiDAR data and find a doubling of rock cover (from 40% to 80%) as hillslope angles transition across slopes of ~24-42 ?. Topographic roughness, calculated as the standard deviation of slope, is predictive of only ~60% of total landscape rock cover, but can identify large boulders and coarse-scale outcrops with higher accuracy (79%). These calibrated remote sensing metrics indicate higher rock cover in non-forested regions (34%, compared to 20% in forested areas), though with high uncertainty. Additionally, we measure fallout-radionuclide inventories in soils to explore variations in decadal transport processes and soil residence times. We find distinct 210 Pb and 137 Cs behaviors in forested and non-forested systems, controlled both by unique partitioning of each nuclide within organic and mineral soil horizons, but also due to depth-driven differences in their physical mobility. Average 210 Pb ex inventories in non-forested soils are 33% lower, and half as variable as soils in the forested region (10.45 + or = 0.97 and 15.49 + or = 1.91 kBq/m 2 respectively), while 137 Cs inventories are indistinguishable (4.04 + or = 0.34 and 3.73 + or = 0.42 kBq/m 2). Together, our spatial, field, and isotope analyses suggest forested systems have greater soil storage and longer residence times than non-forested soils, mediated by differences in surface erosion processes within a larger fire disturbance landscape.