Soil degradation, restoration and management in abandoned and afforested lands

Abstract

Soil degradation is currently one of the major worldwide environmental problems, being land abandonment one of its main causes. In the last decades an important number of researchers from five continents have studied the interactions between land abandonment and soil degradation. Mediterranean mountain areas and semi-arid environments constitute the areas with the largest amount of abandoned lands, where soil degradation processes act most effectively. The hydro-geomorphological evolution of abandoned lands presents variable results depending on the field type (e.g. terraced, sloping, flat), climate conditions (which regulate the revegetation processes), land management and the age of abandonment. In general, if the revegetation processes result in a dense vegetation cover, infiltration increases and runoff and soil erosion decrease. Hence, in sloping fields in mountain areas, the problem is observed in the first years after land abandonment when plant colonization is progressing but not complete. Then, a dense shrub cover protects the soil, reducing both runoff and sediment yield to minimum values. In terraces, the most important erosion process is the collapse of outer walls due to small mass movements, forming scars that facilitate gully development. The lack of maintenance of the drainage systems, created during the cultivation period, favors soil saturation; in these cases, the plant colonization cannot avoid the collapses of the walls. In semi-arid environments, plant colonization is very slow and spatially heterogeneous. Soils tend to crust and are poor in soil organic matter, and consequently the infiltration capacity is very low. Therefore, in some lithologies (as marls and gypsums) soil erosion processes (rills, gullies, piping) are very active, leading in a few years to the total degradation of abandoned fields. All these results suggest that land abandonment should be programmed, and that those already abandoned lands should be managed, emphasizing the role that extensive livestock could produce in these environments.