Soil erosion under different harvesting managements in steep forestlands from northern Spain

In spite of the high risk of erosion, highly mechanized operations, in whic h humus layer removal and down-slope ploughing are practised, are often emp loyed to prepare steep forestlands for planting. The purpose of this study was to evaluate the effect on soil loss of three post-harvesting management systems usually used in a mountainous region of northern Spain. The techni ques evaluated were (a) conventional stem-only harvesting, (b) whole-tree h arvesting and humus layer removal and (c) whole-tree harvesting and humus l ayer removal followed by down-slope deep ploughing. Soil erosion was measur ed in a total of 39 recently clear-felled Pinus radiata plantations over th e period between the stand harvesting and establishment of understorey vege tation. They were located on slopes with gradients of between 40 and 50 per cent. 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 p roduced higher runoff. These deteriorations of soil properties delayed the establishment of pioneer vegetation thus extending the period of high erosi on. In the ploughed soils, the higher soil erodibility and the removal of s lash and vegetation cover resulted in a four-fold increase of soil losses w hen compared with no mechanical site preparation. The results indicate that , in order to maintain the soil productivity, and for conservation purposes , harvesting management should minimize the soil disturbances and maintain the slash and vegetation cover. Copyright (C) 1999 John Wiley & Sons, Ltd.