Changes in vegetation of oak "Forest on Vorskle" of Belogorie reserve for 60 years.
Forest-steppe oak forests with insignificant signs of anthropogenic transformation have been preserved on the "Les na Vorskla" site of the "Belogorye" reserve. These plant communities were considered stable, indigenous, but over the past just over 60 years, significant changes have occurred in them, recorded by geobotanical descriptions in 1954, 1984 and 2018. During the entire observation period, the participation of the oak Quercus robur in the forest stand decreased, and the participation of the linden Tilia cordata, the ash of Fraxinus excelsior, and, especially, the maple Acer platanoides, increased. Since 1954, the stand has become more shady with better defined lower layers. The felling of old trees of the first tier is taking place at an accelerating pace. The light-loving oak undergrowth has almost completely disappeared, and the elm undergrowth has appeared. There was a complete overgrowth of the fields, which were previously supported by mowing. The composition of the vegetation under the forest has changed dramatically. Relatively light-loving xeromesophilic species, such as the blackthorn Prunus spinosa, disappeared from the underbrush, and hygromesophilic species such as the bird cherry Prunus padus appeared. The species diversity and richness and the total projective cover of the herbage sharply decreased in the first half of the observation period and increased in the second half of the period. During the first 30 years of observations, xeromesophytic and relatively light-loving species (Agrimonia eupatoria, Astragalus glycyphyllos, Galium verum, etc.) disappeared from the grass stand. Over the past 30 years, primarily shade-tolerant nitrophils (Lamium maculatum, Sambucus nigra) have grown, invasive species (Acer negundo, Erigeron annuum) have appeared. These changes are irreversible, i.e. are a reflection of the ongoing succession. The main reasons for the succession: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 appeared. 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 nigra) have grown over the past 30 years; invasive species (Acer negundo, Erigeron annuum) have appeared. These changes are irreversible, ie they are the reflection 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) cessation of grazing in the postwar years, (4) increasing of density of wild boars population.