Suitability of the selected statistical distributions for fitting diameter data in distinguished development stages and phases of near-natural mixed forests in the Świętokrzyski National Park (Poland)

The purpose of this study is to evaluate goodness of fit of empirical and theoretical DBH distributions in near-natural forests with fir (Abies alba Mill.) and beech (Fagus sylvatica L.) representing different development stages and phases.The goodness of fit was tested using the χ2 test and the error index (e) of Reynolds et al. [Reynolds, M.R., Burk, T., Huang, W.-H., 1988. Goodness-of-fit tests and model selection procedures for diameter distribution models. For. Sci. 34, 373–399].The Weibull and gamma distributions approximated the actual DBH distributions in 14 and 8 stands, respectively (out of 17 analysed). The results suggest a relationship between the stage and phase of stand development and the type of distribution. The normal distribution most closely approximated the actual DBH distributions for stands in the growing up/break-up stage, one-storied structure phase [younger generations I and II]. The stands in the break-up stage, two-storied structure phase [upper story—younger generation II] were characterised by the DBH distribution close to gamma distribution. The exponential, gamma, and Weibull distributions were best for stands in the growing up stage, selection structure phase [upper layer—younger generation II].