The contagion index \(W_{i}\) described the degree of regularity of the spatial distribution of the four trees nearest to a reference tree i.\(W_{i}\) was based on the classification of the angles between these four neighbors. A reference quantity was the standard angle\(\alpha_{0}\), which was expected in a regular point distribution. The binary random variable \(z_{\text{ij}}\) was determined by comparing each \(\alpha_{j}\) with the standard angle \(\alpha_{0}=90\), and the contagion index \(W_{i}\) was then defined as the proportion of angles \(\alpha_{j}\) between the four neighboring trees which were smaller than the standard angle\(\alpha_{0}\):