A forest-stepe zonal oak-wood with insignificant signs of anthropogenic transformation is remained in the cluster “Les na Vorskle” (Forest on the Vorskla River) of the strict reserve “Belogor’e”. Such plant communities are considered sustainable and primary. This forest, however, has undergone essential changes over last 60 years, that is described in the relevés of 1954, 1984 and 2018. The participance of oak Quercus robur in tree-stand was decreasing, and those of lime-tree Tilia cordata, ash Fraxinus excelsior, and especially maple Acer platanoides were increasing during the whole period of observation. Since 1954, the tree-stand has become more shadowy with more developed lower canopies. Tree fall of old upper-canopy trees has been continuing and enhancing. Heliophilous saplings of oak have almost disappeared, the saplings of elms have appeared. The glades which were previously supported by mowing have overgrown completely. The composition of undercanopy vegetation has changed significantly. Relatively heliophilous xeromesophilic species such as Prunus spinosa have disappeared from shrub layer, hygromesophilic species such as Prunus padus have ap-peared. Species diversity and total projective cover of herb layer decreased sharply in the first half of the observation period and increased in the second half of the period. Xeromesophytic and relatively heliophilous species (Agrimonia eupatoria, Astragalus glycyphyllos, Galium verum, etc.) disappeared from the herb layer in the first 30 years of the observation. Shadow-tolerant nitrophilous plants (Lamium maculatum, Sambucus nig-ra) have grown over the past 30 years; invasive species (Acer negundo, Erigeron annuum) have appeared. These changes are irreversible, i.e. they are the ref lection of ongoing succession. The main causes of the succession are following: (1) natural changes of forests with absolute predominance of oak to multidominant broad-leaved forests with minor participance of oak, predetermined by ecological features of saplings and the achievement of an average life expectancy by the main generation of oak, (2) climate humidisation, (3) ces-sation of grazing in the postwar years, (4) increasing of density of wild boars population.