Climatic and anthropogenic effects on soil transport rates and hillslope evolution

Vegetation change, whether it be of anthropogenic or climatic origin, can have dramatic effects on landscape form, processes and sediment yield. General conceptual models for relationships between climate, vegetation and sediment yield exist for whole landscapes, but no rigorous analysis of the effects of vegetation change on sediment transport, in the absence of overland flow, has been done from soil-mantled hillslopes. Here we investigate the rate of erosion and sediment delivery from soil-mantled hillslopes underlain by loess, over long (27 ka) and historic (50 year) time scales. In a humid environment the change from grassland/shrubland to tall forest across the Pleistocene/Holocene boundary resulted in a 60% increase in sediment flux. We attribute this to increased soil disturbance and transport by tree-throw. In a presently subhumid landscape, average soil transport efficiency over the same 27 ka period, as quantified by the soil transport coefficient (the proportionality constant relating soil flux to hillslope gradient), was very similar to the humid area, despite a semi-arid or even drier phase prevailing during the Late Glacial in the subhumid region. This result suggests that any differences in soil fluxes and erosion rates during the Late Glacial, imbued by the very different climates in the two regions, have been overwhelmed by processes associated with the Holocene forest phase. The soil transport coefficient for the historical (last 50-year) period at the subhumid site, quantified by Cs analysis, indicates the present erosion rate under introduced grazed pasture is higher than the long-term averages at both sites, higher than that which occurs under native tussock grasslands and comparable to rates under forest. We propose a general scheme to describe the relationship between climate/vegetation and sediment yield on soil mantled hillslopes that have a trough form: the minimum coincides with subhumid grassland/shrubland, while sediment flux maxima may be associated with climatic extremes (humid forested landscapes and arid and hyperarid deserts).