The impact of soil water repellency on soil hydrological and erosional processes under Eucalyptus and evergreen Quercus forests in the Western Mediterranean

Forest areas of the Mediterranean regions of Portugal, Morocco, and Tunisia are suffering major land use changes, with the replacement of traditional evergreen Quercus species (i. e. Quercus suber and Quercus ilex) by fast-growing Eucalyptus species. Since Eucalyptus species are amongst those with a higher impact on soil water repellency, this study examined the effect of the replacement on soil properties, water repellency, and on soil hydrological processes and erosion rates. Measurements were performed in areas that correspond to the climatic distribution of evergreen Quercus suber: at Macao and Portel in Portugal; Ben Slimane in Morocco; and Cap Bon, Sousse, and Ain Snoussi in Tunisia. Soil superficial characteristics, including vegetation and litter cover, organic matter content, soil compaction and shear strength, and water repellency were measured for evergreen oak and Eucalyptus stands and related to soil erosion rates and soil hydrological processes. The data are based on the spatial distribution of properties assessed through the use of intensive spatial sampling and on rainfall simulations to address soil hydrological and erosional processes. The results show very different wetting patterns for some of the Eucalyptus stands during dry and moist periods, as a result of strong hydrophobic characteristics following dry spells. Nevertheless, the Eucalyptus stands in semi-arid climate show no sign of water repellency, which contradicts the theory that water repellency is purely a result of dry conditions. The experiments show no significant increases on overland flow amounts and erosion rates as direct result of soil water repellence (hydrophobicity) characteristics.