Protection roles of forest and non-forest woody species on slopes in Iran

Abstract

The roots of trees provide an important contribution towards the stabilityof hill slopes. Tree roots in the soil act very similarly to steelfibers in reinforced concrete and provide resistance to shear andtensile forces induced in the soil. In addition, the roots also absorbwater from the soil, which reduces moisture content, again helpingto increase the stability of the slope. As Iran has a long historyof landslides, our research deals with the effect of tree roots onslope stability, in particular, the following species which are ofeconomic and environmental interest: tea (Thea sinensis L.), citrus(Citrus spp.), lilaki (Gleditshia caspica Dsf.) and angili (Parrotiapersica D.C.) (Mosadegh, 1996). The study was carried out in RoudsarTownship in Gilan State of Iran. Of the overall surface area of 1800ha, 288 ha were considered suitable for the purposes of this study.A large part of the area had slopes of steep gradients on which naturalvegetation was present. Other parts of the same area have been clearedand planted with tea and citrus crops. Soil samples were taken froman area of approximately 70 ha for testing in the laboratory. Directshear tests were carried out on soil samples and the factor of safety(FOS) calculated. Results showed that the FOS was increased in soilswith tree roots present. The global slope FOS was then determinedusing Bishop's method. We calculated the FOS in order to protectslopes where the gradient exceeds 25%. In this case study, the minimumFOS was assumed to be 1.3, which corresponds to e.g. Parrotia sp.vegetation with 40-60% crown cover, a soil internal friction angleof 15 degrees and a slope angle of 21 degrees. When soil internalfriction angle equals 15 degrees and slope angle is > 31 degrees,slope stability cannot be increased by any vegetation species.