where aij is the angles measured clockwise between a line connecting
the location of tree i and a nearest neighbour j and a reference
bearing (e.g. due north or line between tree i and the 1st nearest
neighbour), n is the number of neighbours, in our study n = 4.
Mi ¼
1
n
Xn
j¼1
1ðspeciesi–speciesjÞ ð3Þ
1() is an indicator function that returns the value of 1 if the condition
in the brackets is fulfilled, otherwise it returns the value of 0.