Conditions for feedbacks between geomorphic and vegetation dynamics on lateral moraine slopes: a biogeomorphic feedback window

International audienc

Debris-flow activity and snow avalanches in a steep watershed of the Valais Alps (Switzerland): Dendrogeomorphic event reconstruction and identification of triggers

Debris flows and snow avalanches are common processes in the headwaters of steep watersheds worldwide. In forested areas, dendrogeomorphic analyses of trees affected by debris flows and snow avalanches have regularly been used to date past events. Previous studies have, however, almost never focused on both processes at once, as snow avalanche impacts cannot easily be distinguished from debris-flow scars. In a similar way, tree-ring studies have often been limited to conifers, and sites colonized with broad-leaved forests have been widely disregarded. We report on a case from the Valais Alps (Switzerland) where past debris-flow and snow avalanche activity was dated with intraseasonal precision using different broad-leaved and conifer trees. In total, the analysis of 171 cores, 34 wedges, and 11 crosssections from 93 trees allowed identification of 20 debris-flow and 3 snow avalanche events between A.D. 1930 and 2008. Results also indicate that some of the events would have been missed without the sampling of broad-leaved trees

Catenary soil development influencing erosion susceptibility along a hillslope in Uganda

Systematic variations of soil properties occur along the hillslope due to differences in water and energy conditions. Such catenary soil variations are often assumed, in turn, to influence erosion processes, but quantitative investigations dealing with the reciprocal relationship between catenary soil development and erosion processes are limited. This study models the influence of catenary soil development on erosion processes on a hillslope in Uganda. The Water Erosion Prediction Project (WEPP)- model was selected to determine the impact of spatial distribution of soil types on hillslope soil loss. A detailed soil survey confirmed a well-developed catenary sequence at the study site. Soils at the summit position had a thick solum due to the stable soil formation on the flat surface, whereas soils at the shoulder position had shallow A-horizons due to active erosion processes. Valley and footslope soils showed hydromorphic features and accumulation of soil material from upslope. The performance of the WEPP- model was evaluated by a sensitivity analysis, which proved that the model was sensitive to vertical changes in soil properties to a depth of 40 cm. High sensitivity to soil texture indicated that the catenary sequence at the study site may have a strong influence on model simulations. When the hillslope was modelled as a uniform soil-landscape unit using each individual soil profile separately, simulated outputs showed high variations with annual soil loss ranging between 2.5 and 9 t/ha. This variation was reduced by including an increasing number of distinct soil-landscape units representative for the individual slope sections. Simulations considering a catenary soil sequence showed a clear spatial demarcation between erosion and sedimentation zones, which was verified by soil investigations. This implies that simulations including a higher number of soil-landscape units generate a more realistic spatial distribution of erosion–sedimentation processes at a hillslope