SPODOSOLS DEVELOPMENT AND SLOPE STABILITY IN OLD-GROWTH TEMPERATE RAINFOREST OF SE ALASKA

The quantification of soil redistribution rates (erosion and deposition) has been crucial for understanding erosion processes and landscape evolution in natural areas worldwide. Soil redistribution assessments have mostly focused on the effect of topography, climate, and vegetation, but few explored how soil development influences soil redistribution rates and slope stability. In Southeast Alaska, deep well-developed Spodosols formed in colluvial deposits are found on steep backslope positions (37- 60% slope gradient), where a shallower and less-developed soils would be expected due to high erosion rates from flowing water. We hypothesized that deep well-developed Spodosols formed on different parent materials in mid backslopes in SE Alaska have negligible soil redistribution rates and surfaces are stable. To test this hypothesis, we selected Spodosols in forested hilly and mountainous areas in Juneau (SE Alaska) formed on distinct lithologies of the region (i.e. tonalite, slate, greywacke, phyllite). We quantified soil erosion and deposition rates using profile patterns of ^239+240Pu radionuclides in slopes with similar slope form, aspect, inclination, and length. Pedons show a sequence of O, E, Bhs, and Bs horizons and share similar morphological properties: depth varying from 87 to 158 cm deep, thick O horizons (5-15 cm), gravelly textures, moderate medium subangular blocky structure, and abundant fine to coarse roots in surface horizons. Results from ^239+240 Pu measurements reveal negligible soil redistribution rates for all pedons, ranging from erosion rates of 0.42 t/ha/y to deposition rates of 0.36 t/ha/y. These results suggest that, regardless of soil parent material, well-developed Spodosols are in a steady-state condition in terms of soil redistribution processes and indicates a geomorphological stability of steep, forested slopes in SE Alaska. These findings shed light on the co-evolution of soils and slopes towards a geomorphological stability in a temperate rainforest environment. Moreover, it also provide forest managers additional information, based on soil properties, to differentiate stable and unstable areas for better soil management.