The study was based on the empirical material collected from 78 beech−pine stands located in the territory of the Miłomłyn (46 plots) and Stare Jabłonki (32 plots) Forest Districts. On the basis of the collected material, a breast height diameter growth model has been developed for the beech growing in the understorey of pine−beech stands. The developed function is an integral part of the growth model for mixed beech−pine stands.
The studies were based on the empirical material collected from 88 sample plots located in 78 beech−pine stands and 10 pine stands with beech undergrowth. The effect of the competition of beech trees from the understorey and pine trees on the radial increment (r) and basal area increment (g) was investigated. The strength of the competition was determined using distance−independent indices, which did not take into account the position of a tree in the stand. The effect of the competition on the basal area growth of both beech and pine trees was found to be greater than on the radial growth.