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

Abstract

The roots of trees provide an important contribution towards the stability of hill slopes. Tree roots in the soil act very similarly to steel fibers in reinforced concrete and provide resistance to shear and tensile forces induced in the soil. In addition, the roots also absorb water from the soil, which reduces moisture content, again helping to increase the stability of the slope. As Iran has a long history of landslides, our research deals with the effect of tree roots on slope stability, in particular, the following species which are of economic and environmental interest: tea (Thea sinensis L.), citrus (Citrus spp.), lilaki (Gleditshia caspica Dsf.) and angili (Parrotia persica D.C.) (Mosadegh, 1996). The study was carried out in Roudsar Township in Gilan State of Iran. Of the overall surface area of 1800 ha, 288 ha were considered suitable for the purposes of this study. A large part of the area had slopes of steep gradients on which natural vegetation was present. Other parts of the same area have been cleared and planted with tea and citrus crops. Soil samples were taken from an area of approximately 70 ha for testing in the laboratory. Direct shear tests were carried out on soil samples and the factor of safety (FOS) calculated. Results showed that the FOS was increased in soils with tree roots present. The global slope FOS was then determined using Bishop’s method. We calculated the FOS in order to protect slopes where the gradient exceeds 25%. In this case study, the minimum FOS was assumed to be 1.3, which corresponds to e.g. Parrotia sp. vegetation with 40–60% crown cover, a soil internal friction angle of 15° and a slope angle of 21°. When soil internal friction angle equals 15° and slope angle is >31°, slope stability cannot be increased by any vegetation species.