The studies were conducted in the Białowieża primeval forest on a study area of 28,000 m2, divided into two plots and 280 quadrats. In 1979, all Carpinus betulus individuals were cut off within the experimental plot (E, 7800 m2), whilst the control plot (C, 13000 m2) remained unchanged. Each plot contained a phytocoenose of Potentillo albae-Quercetum and an adjacent community of Tilio-Carpinetum.
By 1983, hornbeam invasion into the oak-forest habitat within C plot had resulted in: 1) a decrease in species number by more than 10 per 100 m2 on average (c.a. 30%); 2) a twofold greater deletion rate of heliophil oak-forest species than for Fagetalia and Querco-Fagetea; 3) a diminution of the area of oak-forest phytocoenose by more than 100 m2 per year; 4) formation of a community with a species combination corresponding to Tilio-Carpinetum. Hornbeam removal had the opposite effect: 1) the number of species increased by more than 100%; 2) the number and frequency of oak forest species rose considerably; 3) those parts of the phytocoenose colonized previously by Carpinus betulus regenerated, and thence the area of Potentillo albae-Quercetum community increased.