Long-term biogenic soil mixing and transport in a hilly, loess-mantled landscape: Blue Mountains of southeastern Washington

a r t i c l e i n f o In soil-mantled landscapes, downslope sediment transport occurs via disturbance-driven processes that vary with climate and vegetation change. To help constrain the long-term (≫10 yr) pattern and rate of soil mixing and transport in forests, we analyzed the distribution of tephra grains in soil along a hillslope transect in the Blue Mountains, SE Washington. Deposited within a loess mantle, tephra associated with Mt. Mazama (7.7 cal. kyr B.P.) serves as a marker bed for estimating erosion and transport rates. Moving downslope, the buried tephra horizon is progressively exhumed and becomes increasingly mixed in the upper soil layer, reflecting disturbance and transport via tree root growth and turnover. This pattern also implies increasing erosion rates downslope and our hillslope transect becomes increasingly convex coincident with progressive exhumation of the tephra layer. This systematic correspondence between topographic form, specifically, local convexity, and surface lowering is consistent with theoretical models for which soil transport rates depend on slope inclination. From our analysis, calibrated coefficients for a linear, slope-dependent transport model are on the order of 10 − 3 m 2 yr − 1 , consistent with previous work in forested loess-mantled landscapes. In addition, our results reveal both the high degree of soil mixing over millennial timescales and the local variability of mixing in forested landscapes. Furthermore, the results enable us to quantify the amount of energy expended by trees in mixing and transporting soil and the net sediment transport fraction of the net primary productivity NPP of the ecosystem. During the Quaternary, climate change occurs over thousand-year timescales and many soil-mantled hillslopes have thus fluctuated between grassy and forested vegetative regimes. The morphologic legacy of different ecosystems, however, has not been addressed in the literature. Biotic processes are thought to play a major role in the but few studies have succeeded in quantifying the biotic role in sediment transport (Yoo et al., 2005). If vegetation influences transport processes, which in turn control landscape evolution, then the form of landscapes responds to and reflects climatic variation. The transport of soil in the absence of overland flow has been attributed to processes such as soil creep, rain splash, variations in soil moisture (wet/dry) and temperature (freeze/thaw), tree throw The depth to which these processes occur, the soil above which is termed the active soil layer or biomantle (Johnson, 1990), limits disturbance and transport of …