Modelling diameter distributions of Quercus suber L. stands in “Los Alcornocales” Natural Park (Cádiz-Málaga, Spain) by using the two parameter Weibull functions

Abstract

Aim of study: The aim of this work was to model diameter distributions of Quercus suber stands. The ultimate goal was to construct models enabling the development of more affordable forest inventory methods. This is the first study of this type on cork oak forests in the area. Area of study: The area of study is “Los Alcornocales” Natural Park (Cádiz-Málaga, Spain). Material and methods: The diameter distributions of 100 permanent plots were modelled with the two-parameter Weibull function. Distribution parameters were fitted with the non-linear regression, maximum likelihood, moment and percentile-based methods. Goodness of fit with the different methods was compared in terms of number of plots rejected by the Kolmogorov-Smirnov test, bias, mean square error and mean absolute error. The scale and shape parameters in the Weibull function were related to the stand variables by using the parameter prediction model. Main results: The best fitting was obtained with the non-linear regression approach, using as initial values those obtained by maximum likelihood method, the percentage of rejections by the Kolmogorov-Smirnov test was 2% of the total number of cases. The scale parameter “b” was successfully modelled in terms of the quadratic mean diameter under cork (R2 adj = 0.99). The shape parameter “c” was modelled by using maximum diameter, minimum diameter and plot elevation (R2 adj = 0.40). Research highlights: The proposed model diameter distribution can be a highly useful tool for the inventorying and management of cork oak forests.