Soil Fixation by Tree Roots: Changes in Root Reinforcement Parameters with Age in Cryptomeria Japonica D. Don. Plantations

摘要

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 x 450 mm) were extracted at different distances around trees of Japanese cedar (Cryptomeria 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 classes and roots < 10 mm in diameter were removed from each class. Root tensile strength was then determined on > 150 roots using a tensile testing machine. Initial results suggest that young trees provide greater soil cohesion than in older plantations through increased RAR. The increase in RAR of young trees may be due to a higher planting density. In older plantations, RAR was extremely low at distances of > 1.0 m from the tree stem, therefore soil cohesion is highly heterogeneous in older plantations, thus leading to an increased likelihood of soil slippage. It was also observed that removal of trees through thinning can also decrease soil cohesion temporarily, as roots of harvested trees die and remaining trees have not yet had time to exploit the newly available space.