Disaster Mitigation of Debris Flows, Slope Failures and Landslides 535 Soil Fixation by Tree Roots: Changes in Root Reinforcement Parameters with

Abstract The number of catastrophic incidents due to slope instability problems in China is a major problem, with> 2400 major events occurring in recent years, resulting in the destruction of urban and rural environments and the loss of human life. Cost-effective and ecologically friendly methods can be used to avoid certain potential catastrophes. Such techniques involve the use of vegetation to stabilize a slope or arrest erosion, but can also include for example, the management of protection forests against avalanches and landslides. Shallow landslides can be avoided by planting species which improve soil cohesion, either through having an increased Root Area Ratio (RAR), a high root tensile strength, or a combination of both. A large number of species, both woody and non-woody, have been identified as useful in improving soil cohesion. However, with regard to tree species, it is not known whether tree age affects soil cohesion through differences in either RAR or root mechanical properties. Therefore, we carried out a study on root distribution and mechanical properties of trees at different ages, growing in plantation forests in Sichuan province, China. Soil cores (150 × 450 mm) were extracted at different distances around trees of Japanese cedar (Cryp-tomeria japonica D. Don) growing in plantations aged 9, 20 and 30 years old. All plantations were close together on the same type of soil and the same slope angle. Each soil core was divided into three different depth classe