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Soil erosion is one of the most severe land degradation processes in the Mediterranean region. Although badlands occupy a relatively small fraction of the Mediterranean area, their erosion rates are very high. Many studies have investigated to what extent vegetation controls soil erosion rates. This study, however, deals with the impact of erosion on vegetation establishment. In semi-arid badlands of the Mediterranean, soil water availability constitutes the main limiting factor for vegetation development. As a consequence, south-facing slopes are typically less vegetated due to a very large water stress. However, these findings do not necessarily apply to humid badlands. The main objective of this paper is to determine the topographic thresholds for plant colonization in relation to slope aspect and to assess the spatial patterns of vegetation cover and species richness. We surveyed 179 plots on highly eroded badland slopes in the Central Pyrenees. We defined four aspect classes subdivided into slope angle classes. Colonization success was expressed in terms of vegetation cover and species richness. Slope angle thresholds for plant colonization were identified for each slope aspect class by means of binary logistic regressions.The results show that a critical slope angle exists below which plants colonize the badland slopes. Below this critical slope angle, plant cover and species richness increase with a decreasing slope angle. The largest critical slope angles in humid badlands are observed on south-facing slopes, which contrasts with the results obtained in semi-arid badlands. North-facing slopes however are characterized by a reduced overall vegetation cover and species richness, and lower topographic threshold values. The possible underlying processes responsible for this slope-aspect discrepancy in vegetation characteristics are discussed in terms of environmental variables that control regolith development, weathering and erosion processes. Moreover, possible restoration strategies through the use of vegetation in highly degraded environments are highlighted.

Compared with surface soil erosion by water, subsurface erosion (piping) is generally less studied and harder to quantify. However, wherever piping occurs, it is often a significant or even the main sediment source. In this study, the significance of soil loss due to piping is demonstrated through an estimation of soil volume lost from pipes and pipe collapses (n = 560) in 137 parcels under pasture on loess-derived soils in a temperate humid climate (Belgium). Assuming a period of 5 to 10 years for pipe collapse to occur, mean soil loss rates of 2.3 and 4.6 t ha −1 yr −1 are obtained, which are at least one order of magnitude higher than surface erosion rates (0.01-0.29 t ha −1 yr −1 ) by sheet and rill erosion under a similar land use. The results obtained for the study area in the Flemish Ardennes correspond well to other measurements in temperate environments; they are, however, considerably smaller than soil loss rates due to subsurface erosion in semi-arid environments. Although local slope gradient and drainage area largely control the location of collapsed pipes in the study area, these topographic parameters do not explain differences in eroded volumes by piping. Hence, incorporation of subsurface erosion in erosion models is not straightforward.

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While most studies focus on the effect of soil pipes on hillslope stability, this present study investigates the impact of landsliding on pipe development. It is hypothesized that poorly drained active and dormant landslides change the hillslope hydrology through (i) surface flow obstruction, by changing topography, as well as (ii) subsurface flow obstruction by tilting lesspermeable clay-rich substrates. Hence, new preferential flow paths are created at reverse slopes within the landslide zone and at the boundary of the landslide, enhancing pipe formation. This study aims at a better understanding of the interaction between collapsed pipe (CP) occurrence and landslide (LS) occurrence in the Flemish Ardennes (Belgium) by comparing their respective spatial patterns. At least 24.5% of the 139 sites with CP were related to the occurrence of an observed LS. Poorly drained LS may create favourable conditions for pipe development. Outside LS, natural and anthropogenic (e.g. broken field drains, road drainage) causes may result in concentrated subsurface flow, resulting in pipe development. No evidence was found that pipe development enhanced LS, probably because the subsurface drainage discharge generated upslope of the LS is too low. Even when pipes become blocked, it is more likely that new pipes develop and new collapses occur than they trigger or reactivate LS. A conceptual model is presented summarizing all elements that influence piping erosion in the Flemish Ardennes, including the role of LS.

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