The aim of this research was to answer the question of whether the floristic- phytosociological characteristics of the forest communities described in the mid-twentieth century in the Zielonka Experimental Forest (Poland) have changed, and if so, what were the reasons for these changes and what were their environmental and economic consequences. The basis of the research was a comparison of plant community descriptions drawn up in 1957-1961 to descriptions of the states of those communities in 2010-2017. The study showed a significant decrease in the number of plant species. The reasons for the disappearance of some plant communities are seen in the natural or artificial changes of pine in sandy habitats, which evolved towards richer communities. The disappearance of xero-thermophile oak woods had two main causes: a natural cause, which was a consequence of regeneration towards oak-hornbeam woods after the anthropogenic phenomena responsible for their management, and anthropogenic disturbance related to the planting of Fagus sylvatica trees or Prunus serotina. The second case is described more broadly and considers both the positive and negative roles of changing the floristic compositions of forest ecosystems.