Abstract
Due to the transition of Ukraine in 2019 to new regulatory documents on standardisation regarding the classification of round timber by quality classes, it became necessary to develop standards for evaluating volumes in accordance with the new requirements. The purpose of the study was to develop a mathematical model of the forming stem of Scots pine for trees of two groups of diameters. The study is based on the use of empirical material collected in different years by researchers of the Department of forest taxation and forest management of the National University of Life and Environmental Sciences of Ukraine, which is 583 model pine trees. The Kozak model was use as the basic model for modelling the forming Scots pine. A mixed effect model was calibrated to characterise the variability in the tree stem shape of Scots pine. Parameters for the role of random effects were selected based on their coefficients of variation using the bootstrap process. Data were grouped into two groups of diameters by analysing modelling residues and evaluating the quality of models based on the use of the Akaike information criterion (AIC). By dividing the data array into two groups of thickness degrees and developing models of the creator for each of the groups separately, a decrease in the indicator of the Akaike information criterion (AIC) was achieved to the level of AIC=7433.02 for a group (8-28 cm) and AIC=13154.67 for the group (32-76 cm), which increased the accuracy of the developed models. Enabling random effect parameters in model by A. Kozak for the developed mathematical equations allowed more accurately describing the variability of the tree stem of Scots pine. The obtained models give a clear and fairly accurate prediction of the determination of tree stem diameters along the entire length. The use of the developed models can serve as a basis for improving forest traction standards for estimating the volume of tree stem in the process of forestry production and predicting the yield of roundwood volumes by quality classes