The aim of the research was to study 20-year changes in the floristic richness of the vegetation and soil diaspore bank of agroceonoses and adjacent midfield woodlots, as a result of the intensification of agricultural production, and the role of the ecotone in the preservation of field weeds. The studies were conducted in two periods, the first between 1996 and 1998 and the second between 2016 and 2018, on 12 permanent plots located in a transition zone between agroceonoses and woodlots. The width of the ecotone zone was determined with a linear transect that crossed both communities. The study of vegetation was conducted using the Braun-Blanquet method. During each period, soil diaspore reserves were determined. A large decrease of approximately 30% in the number of species was noted in the agroceonoses, while there were minor changes in the wood communities. With only slight variation during the research, floristic diversity of the soil diaspore bank in those habitats was poorer than vegetation diversity. The ecotone phytoceonoses had the highest species richness. They showed greater floristic similarity to the wood phytoceonoses than to agroceonoses. In the transition zone a clear boundary was observed, dividing those two communities. The ecotone was a refuge for field species, including rare and endangered ones. The species composition of the ecotone soil diaspore bank was poorer than the composition of the flora and showed greater similarity to the species diversity of the agroceonoses. Additionally, a significant relationship between species richness and habitat conditions was noted.