Pettersson 1992 log(Kurtosis +3 )

[Silviculturalist]

For the volume distributions in Pettersson 1992, which are modelled after a Gram-Charlier-series, gamma_1 and gamma_2 parameters are V_skew and V_kurtosis, calculated by a function with the dependent variable log(kurtosis+3) and log(skew+3), respectively. Why?