Various stand features, such as canopy closure, crown size, biosocial status and site conditions, have a significant impact on the development of the breast height form factor (f_1.3). The regularities of the formation of f_1.3 of pedunculate oak Quercus robur developing in open conditions are known, but this parameter in the case of oak growing under the canopy of Scots pine stands has not been analysed in detail so far. We examined the properties of the f_1.3 of pedunculate oaks that grew for a long time under the canopy of a Scots pine stand, and after removal of the upper layer, they created a new stand, which was formed from underwood and underbrush in the territory of north−eastern Poland (Polish variant). The breast height form factor was also calculated for oaks that grew in the second layer of the Scots pine stand and were cut down together with trees of the main species in western Ukraine (Ukrainian variant). 100 model trees from 20 stands (Polish variant) and 110 trees from 4 stands (Ukrainian variant) were selected for the analysis. All stands grew in the same type of forest site conditions – fresh mixed broadleaf forest. For all model trees, the volume of the trunk in the bark was determined (in one−meter sections) and the value of the breast height form factor in the bark was calculated using the Complex Smalian’s Formula. The aim of the work was to compare the features of the breast height form factor of pedunculate oak trees under the canopy of a Scots pine stand in various growth conditions and to model the relationship between f_1.3 and the value of the diameter at breast height (dbh) (d_1.3) and height (h) of trees. It was found that the number of model trees is sufficient to model the defined relationships, and the parameters of these trees were determined with sufficient statistical accuracy. In general, 9 types of formulas were used to model the dependence of f_1.3on d_1.3 and h. The search for optimal solution was carried out using the least squares method. The decisive statistic for assessing the goodness of fit of the tested formulas was the coefficient of determination R². On the basis of this indicator, the formula that best describes the dependence of f_1.3on d_1.3 and h was selected. Based on the results of substitutions to the selected formulas, tab− ular data on the dependence of f_1.3 on d_1.3 and h for the Polish and Ukrainian variants were developed. The obtained f_1.3 values of the Polish and Ukrainian variants were relatively lower than the normative data. The trees of the Polish variant had lower values of f_1.3 compared to the Ukrainian variant, which proved their higher degree of taper.

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