Management of forest-zones and soil erosion in step-slope areas of the Basque Country

Abstract

The purpose of this study is to evaluate the effect of 3 post harvesting management systems usually used in a mountainous region of Northern Spain on soil loss. The techniques evaluated were a) conventional stem-only harvesting, b) whole-tree harvesting and humus layer removal and c) whole-tree harvesting and humus layer removal followed by down-slope deep ploughing. Soil erosion was measured in a total of 39 recently clearfelled radiata pine plantations over the period between the stand harvesting and establishment of understorey vegetation. They were located on slope with gradients of between 40 and 50%. For all treatments the associated runoff-rainfall ratios were high, indicating inherent low permeability of the soil. Intense site preparation resulted in considerable decreases in soil organic matter and increases in bulk density, which decreased the hydraulic conductivity and subsequently produced higher runoff. These deteriorations of soil properties delayed the establishment of pioneer vegetation thus extending the period of high erosion. In the ploughed soils the higher soil erodibility and the lack of protective of slash and vegetation cover resulted in a four-fold increase of soil losses when compared with no mechanical site preparation. The results indicate that, in order to maintain the soil productivity and for conservation purposes, harventing management should minimize the soil disturbances and maintain the slash and vegetative cover.