The planning and implementation of the majority of professional measures and activities in forest production management requires the knowledge of the basic quantitative elements (tree diameter, height, volume) and value elements (value of wood volume). In that sense, the aim of this research was to define the relationship between the value of timber and tree dimensions in beech forest thinning in Serbia (regression models, average growth rates, etc.). This will make it possible to predict in advance some value parameters, to raise the quality level of thinning operation planning, as well as to achieve better management of financial flows in the forestry sector (e.g. assessment of efficiency and profitability). The sample consisted of 7,815 beech trees harvested by thinning (5,973 in high forests and 1,842 in coppice forests), and the measurements included total volume and the volume and value of timber assortments.

The main scientific method applied in the study was the modelling method, and the analysis of causality between the study elements was performed using mathematical-statistical methods, such as descriptive statistics and regression and correlation analysis.

Based on the study results, the conclusions were as follows: (a) there is a high variability in timber value depending on forest origin and assortment category, (b) the effect of the diameter at breast height on timber value is stronger (the coefficient of linear regression R=0.52-0.83) than the effect of height (R=0.47-0.68), (c) the obtained regression models are a very good description of the relation between diameter at breast height and total timber value (R=0.58-0.97 in high forests and R=0.64-0.97 in coppice forests). An especially high quality model is the model which refers to the effect of diameter at breast height on the value per tree (R=0.97 in high forests and coppice forests), (d) all models show unequal distribution of the error along regression line (heteroskedasticity), but as this occurrence is more related to large trees, which are relatively few in thinning operations, this does not affect the accuracy of timber value assessment in thinning operations. The study conclusions point out that the timber value in beech forest thinning can be assessed with relatively high reliability.

The obtained models can be used when it is necessary to know the total value of thinned material (aiming at the determination of financial assessment of thinning efficiency), or when it is necessary to create the plans for selling the felled timber (estimation of the potential revenue).

Key words: diameter, height, timber value, beech, thinning, Serbia