Effect of pollarding on biotechnical characteristics and reinforcement amount of Lebanon oak (Quercus libani Oliv.) roots

Among positive effects of vegetation in erosion control, reinforcement of soil by roots is very important. The quantity of reinforcement depends on root biotechnical characteristics such as root density and tensile strength. The aim of current study is to assess the effect of traditional pollarding on root biotechnical characteristics, soil reinforcement and slope stability. To estimate the Root Area Ratio (RAR/Root density index) core sampling with cylinder cores was used. For this purpose, core samples were taken from four sides around each tree in three different soil depths. In each core, number of roots and total root length was determined for each diameter class. Root samples were collected from both up and down slope and their strength was measured with a standard Instron. The results showed that root distribution in relation to depth had a wide variation. The results of ANOVA revealed that pollarding had a significant negative effect on RAR. Also the results of Wald chi square showed that pollarding had a significant negative effect on tensile strength of roots. Mean RAR values of fine roots in control and pollarding samples were 47.0 and 28.0%. Mean tensile resistance in control and pollarding samples were 40.31 and 19.94 MPa, respectively. The average reinforcement of fine roots in control and pollarding samples were 2.29 kPa and 1.12 kPa. The results of this study confirmed that traditional pollarding not only have negative effect on aboveground biomass of oak trees but also significantly reduce the positive effect of vegetation on soil reinforcement.