Direct and indirect effects of Mediterranean vegetation on runoff and soil loss

Vegetation cover acts in a complex way in influencing runoff and soil loss and a great deal of information is needed to model these effects. In the Mediterranean, the abandonment of land is important under extensive land-use. Abandoned lands typically have a rolling landscape with steep slopes, and are dominated by herbaceous communities that grow on pasture land interspersed by shrubs. To characterize communities of vegetation such as these, which grow in central Spain, and to evaluate their direct and indirect effects on runoff and soil loss, we carried out experiments with simulated rain. We assessed separately the effects of pasture land and of four species of shrubs (Dorycnium pentaphyllum Scop., Medicago strasseri Greuter et al., Colutea arborescens L. and Retama sphaerocarpa, L.). The infiltration rates under herbaceous vegetation were 7.9 times greater than those obtained on bare land (92.2 mm hour(-1) compared with 11.7 mm hour(-1)), and 88% of these differences could be attributed to direct effects. On the pasture land, as the proportion of covered land increased, the runoff decreased linearly, whereas the soil loss decreased exponentially. On the land covered by shrubs, the average infiltration rate was 82.5 mm hour(-1). Under D. pentaphyllum and M. strasseri infiltration rates were greater than 105 mm hour(-1), whereas for R. sphaerocarpa the infiltration rate was 57 mm hour(-1). For D. pentaphyllum and M. strasseri soil loss was less than 4.5 g m(-2), whereas for C. arborescens soil loss was 61.4 g m(-2). Unlike the results for the pasture land, for the shrub-type vegetation the increases in infiltration rates could be attributed to indirect effects: they explained 47% of the increase in infiltration for C. arborescens, 69% for R. sphaerocarpa, 75% for D. pentaphyllum and 100% for M. strasseri.