Changes within the Dentario bulbiferae-Fagetum and Carici pilosae-Fagetum phytocoenoses were studied during secondary succession in a submountainous beech forest. Investigations were carried out on two different forest plots over a period of 13 years (1995–2008): the first was situated in an original beech stand (in 2008-ca 110-year-old as the control), the second was covered by a thicket (in 2008- aged ca 20 years) developed in a successional process following the clear-cut. Temporal and spatial comparison of phytocoenoses structure on the two plots revealed that more conspicuous changes occurred on the plot with a thicket. The 60% decrease in species composition detected at the end of the study period highlighted evident impoverishment. Moreover, a significant decrease in species vitality was demonstrated by the absence of the generative phase, and also by a distinct reduction in the density of the two dominant species. We suggest, that this phenomenon was most likely due to the considerably reduced light supply to the herb layer on this plot.