Land cover changes and shallow landsliding in the flysch sector of the Spanish Pyrenees

This study investigated the characteristics, triggering factors, and spatial distribution of shallow landslides in relation to historical deforestation in the sub-alpine belt of the Pyrenees, particularly in the flysch sector. Shallow landslides in this area occur on straight and concave slopes, mainly covered by mesophyte grasslands, and contribute substantially to soil erosion and landscape deterioration. The investigated landslides were variable in shape and size, although common features included delimitation by a scar or semi-circular crown (averaging 32 m long × 10.15 m wide) and a tongue with a lobe at the foot of the scar area. The sliding surface coincided with the zone of contact of the C soil horizon with the bedrock, although in 13% of the cases the sliding surface occurred within the soil. The frontal lobes frequently trigger new landslides because of water accumulation and instability, resulting in a succession of interconnected landslides that can attain 100–200 m in length. Slope gradient appeared to be the most important factor correlated with shallow landslides. Thus, with slopes > 30° the soil was unstable and tended to slide even in dry conditions, whereas with slopes < 15° the soil was stable even under saturated conditions; shallow landslides were consequently concentrated on slopes between 15° and 30°. Rapid deforestation of the sub-alpine belt during the Middle and Modern Ages caused a sudden increase in hillslope instability. The studied landslides occurred only on deforested, grassland-covered hillslopes, and not in adjacent forested areas. Therefore, deforestation is considered to be a major factor contributing to shallow landslides, because of changes in soil hydrology and reduced soil strength as a consequence of decreased root cohesion, mainly coinciding with snowmelt and large rainstorms.