with
\(z_{\text{ij}}=\left\{\par \begin{matrix}1,\ \ \ \ \ \ \ \ \ \alpha_{j}<\alpha_{0}\text{\ \ \ \ \ \ }\\ 0,\ \ \ \ \ \ \ \ \ otherwise\\ \end{matrix}\right.\ \) and 0≦\(W_{i}\)≦1 (3)
\(W_{i}\)=0 indicates that the trees in the vicinity of the reference tree are positioned in a regular manner, whereas \(W_{i}\)=1 points to an irregular or clumped distribution.
The value range and meaning of the contagion index \(W_{i}\) were further clarified in Fig.1. In this study, the average of the contagion index \(\overset{\overline{}}{W}\) calculated by each standard tree was used as the comprehensive contagion index of each runoff plot: