At the present time, forestry in Slovakia as well as in the entire Europe has to actively deal not only with the questions regarding the timber production but also with the issues of negative impacts of climate change on forest stands and the required increase of their biodiversity and ecological stability. One of the possible ways is the orientation on the close-to-nature forest management, with the selection system being the part of it. Effective management of differentiated forests in the changing environment requires better understanding of the processes responsible for the establishment and growth dynamics of the lower tree layer. To investigate this topic, the stand structure, distribution of regeneration and the light conditions in the lower layer were analysed on a series of 168 research plots in six mixed selection forest stands or stands in conversion to selection forest in Slovakia (localities Volovské vrchy, Starohorské vrchy, Oravská Magura and Oravské Beskydy). Influence of selected factors (radiation, tree height, tree species and lateral competition) on height growth and crown morphology of regeneration was studied in detail on 352 spruces and 361 firs with the height ˃ 130 cm and the diameter dbh ≤ 4 cm. Moreover, in the stands in final phase of conversion we performed the age analysis of the stems according to tree layers and reconstructed also the height growth of the trees in lower layer (dbh of 10–12 cm). The results of this study showed that the intensity of radiation in the lower layer of selection and conversion stands did not exceed 40% of the radiation over the canopy. Spatial distribution of natural regeneration was influenced by the radiation only to a limited extent. However, indirect radiation together with the height of regeneration individuals had distinctive impact on their height growth. On the contrary, the effect of lateral competition was not confirmed as a significant factor. In the shaded conditions, fir was growing faster than spruce. Better adaptation of fir to such conditions was reflected also in higher relative crown lengths in comparison with spruce. Therefore, the modification of radiation intensity has prospectively high importance for deliberate regulation of spruce and fir proportion in these stands. The results in the stands in final phase of conversion further confirmed that the majority of spruces and firs reached the lower limit of the middle layer (dbh of 12 cm) within the 60 years. With a certain degree of tolerance, the period of 60 years can be regarded as the time needed to establish the lower layer of converted fir-spruce forest stands.