2.4. Calculation of root topology
According to the root branching theory proposed by Fitter and Stickland (1991), there are two extreme types of root branching pattern: i) herringbone branching with relatively simple branches and ii) dichotomous branching with relatively complex branches. This theory applies river branching theory to the study of root branching patterns, using topological index (TI) to describe the branching patterns of different root systems. The TI was calculated as the ratio of lgA to lgM (Fig. 2). If TI is 1, the root system is a herringbone branching, and TI is 0.5 that it is a dichotomous branching.