The Role of Forest-and Nonforest Species on Slope Stability in the North of Iran

Slope instability due to landslides is particularly common in Iran. It is possible to fight against these hazards thanks to the roots of trees, which provide an important contribution towards the stability of hillslopes. However, our knowledge of the ef fectiveness of roots for slope stability needs improvement. Therefore this study was carried out on the ef f ect of tree roots on slope stability, in particular for the following species: tea (Thea sinensis L.) and citrus (Citrus spp.), which are of economic interest; and Lilaki (Gleditshia caspica Dsf .) and Angili (Parrotia persica D. C.), without economic interest. The study area was located at Roudsar Township in Gilan Province of Iran. 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 with and without roots of the mentioned species were taken, on an area covering approximately 70 ha, for testing in the laboratory. Soil shear tests were carried out on these samples and the Factor of Saf ety (FS) was calculated. Results showed that the FS was increased in soils with tree roots present. The global slope FS was then determined using Bishop’s method. We calculated FS in order to protect slopes where the gradient exceeds 25%. In this case study, minimum FS corresponds to e.g. Parrotia sp. vegetation with 40-60% crown cover, a soil internal friction angle of 15° and slope angle of 21°. When the soil internal friction angle equals 15° and the slope angle is >31°, slope stability cannot be increased by any mentioned vegetation species. The most important results show that the main contributory factors which af fect slopes’ stability are: angle of slope, angle of internal friction, and percentage of vegetation cover