Land abandonment and slope gradient as key factors of soil erosion in Mediterranean terraced lands.

摘要

An important land use change recorded in the Mediterranean basin comprises the abandonment of agricultural lands due to economic and social changes, which is followed by significant impacts on soil erosion. Observed land abandonment may have positive or negative impacts on soil protection from erosion because fundamental ecosystem processes are influenced by changes in agricultural practices and soil resources management. Olive groves comprise a typical example of traditional, extensive cultivation, which is abandoned. The olive groves are spread on marginal areas and located mainly on sloping terraced lands with low productivity soils. A field study was conducted for two years in order to monitor the change through time of natural resources such as soil and vegetation, following land abandonment, considering three land uses, cultivation, short-time abandonment and long-time abandonment. Experimental plots were established on two specific slope gradients so that results could be comparable. The study was based on the determination of water erosivity measuring parameters such as rainfall characteristics, sediment losses and water runoff volume, and on the determination of soil erodibility, measuring parameters related to vegetation, soil, slope profile description and drystone terraces. Results show that abandonment of traditional extensive cultivation in the Mediterranean basin has different impacts on soil sediment losses according to slope gradient. When slope gradient is steep (25%), soil erosion is increasing significantly probably because the dense protective cover of annual plants decrease and shrubs' vegetation cover increases. At the same time, bare soil surface below the shrubs' foliage increases as well, while drystone terraces supporting soil material and runoff water collapse. When slope gradient is very steep (40%), soil sediment losses remain at the same high levels after cultivation abandonment because slope gradient is the main factor controlling soil erosion, although soil and vegetation properties are changing.