Post-Fire Variation in Aeolian Deposition in the Northern Great Basin

Abstract

Aeolian processes play a significant role in the redistribution of sediment and nutrients in sparsely vegetated sagebrush-steppe ecosystems. When fire is introduced to the landscape, decreased surface roughness and associated threshold friction velocities allow for the increased mobility of surface sediments and burnt organic material, mobilizing previously stable material. Once material is entrained, interactions between a dynamic atmosphere and complex topography control the spatial distribution of aeolian deposition over a landscape. Given the significant impact of fire on aeolian processes in semi-arid deserts, we posit that postfire aeolian redistribution of material is an important control on the spatial variability of soil depth and characteristics in semi-arid deserts with complex topography. Our study uses over two years of continuous passive dust trap data collected following the Soda Fire of August 2015 in the northern Great Basin. We analyze the mass flux, organic material content, grain size distribution, and geochemistry of the collected samples to trace the fingerprint of the Soda Fire through space and time. As such, the results of this study will inform research on postfire sediment and carbon redistribution, the spatial variability of soil characteristics, and landform evolution in western rangelands. Our results indicate that seasonal variation in aeolian mass flux is pronounced, with the fall months generating the highest rates of dust flux. Immediately following the Soda Fire of August 2015, the mass flux of both sediment and organic material increased by two to threefold within and proximal to the burned area. Increases v in flux lasted on the landscape until the revegetation of the burned area in the spring of 2016, leaving roughly 8 months of disturbed soil surface conditions. Samples impacted by fire contained 88% fine silt and clay-sized material while undisturbed samples averaged 94%, indicating a temporary increase in the particle size distribution within the burned area. A geochemical comparison of regional and local dust and its sources also indicates a pulse of local sediment mobility following the fire through an increase in the relative concentrations of Titanium (found in local soil) and a decrease in the relative concentrations of Barium and Strontium (found in regional soluble salts). We interpret the cessation in local mobility after revegetation to adequate surface roughness provided from spring "green up" (grasses and forbes) to return vertical fluxes of organic matter and sediment to within pre-disturbance fluctuations. Recent studies in the northern Great Basin have found aspect-controlled differences in soil depth and volumetric water content in mid-elevation sagebrush-steppe ecosystems. North-facing aspects tend to have deeper soils, with higher organic content and greater volumetric water contents throughout the water year than south-facing slopes. Our results indicate that local material is suspended and deposited over small scales (0-10 km) to spatially controlled loc...

Spatial and temporal changes in aeolian redistribution of sediments and nutrients following fire

“…Once the organic material was removed via LOI, we used a Malvern Mastersizer optical diffractometer with a 2000MU wet dispersion unit at the Utah State University Luminescence Lab to analyse the particle size distribution of the combined samples (Roehner, 2018; Roehner et al ., 2019a, b). Our study followed the guidelines for optical parameterization from Sperazza et al .…”

“…The dust traps in this study are designed following the procedures outlined by Reheis (1995) and are used commonly within the aeolian research community. Expanded methods and datasets may be found in Roehner (2018) and Roehner et al . (2019a, b).…”

“…The Upper West sites were unaffected by the Soda Fire and are also the only sites removed from the semi‐arid sagebrush ecosystem. The particle size distribution and geochemistry results were calculated from each combined subset sample population (see Roehner, 2018; Roehner et al ., 2019a, b).…”

“…While wet methods introduce error by causing a loss of soluble salts and very fine clay minerals (Mahowald et al ., 2014), sample analysis required the use of deionized water to extract the material from the dust trap. Once extracted and dried at 60 °C, samples were photographed and the coarse organic material was counted where necessary (see Roehner, 2018; Roehner et al ., 2019a, b). We weighed the samples before and after burning at 575 °C to determine the amount of organic material present following the loss‐on‐ignition (LOI) methods of Simon Fraser University Soil Science Lab (2011).…”

“…Elements analysed include Al, As, Ba, Ca, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Hf, Ho, K, La, Lu, Mg, Mn, Na, Nb, Nd, Ni, P, Pb, Pr, Rb, Sc, Si, Sm, Sr, Ta, Tb, Th, Ti, Tm, U, V, Y, Yb, Zn, Zr. Analysis of subset samples was done in triplicate with an average reproducibility of 4% relative to the standard deviation (see Roehner, 2018; Roehner et al ., 2019a, b).…”