The article presents two examples of the expansion of alien trees: Acer negundo and Padus serotina on about 10 year-old fallow lands in Poznań. Those species formed dense woods, which reached the height of 2 m. Under a canopy of box-elder trees, the ruderal species were dominant plants, whereas in the case of phytocoenoses with Padus serotina – psammophytes were dominant. In both types of ... [Show full abstract] wildwoods the segetal terophytes occurred with a high constancy. Encroachment of Acer negundo and Padus serotina on the early fallow lands indicates that those trees are pioneer species. Abandoned lands in our country are the sites where alien trees spread.