Riparian vegetation, which performs many key ecological functions, has been modified or lost at an alarming rate during the past century as a result of human activity. The aims of this study are (a) to investigate the effects of poplar plantations on plant diversity in riparian zones; and (b) to estimate the ecological implications of extending cover by poplar plantations. For this purpose, we assessed species richness, habitat indicator species and functional diversity based on Grime’s C-S-R strategies. We used non-metric multidimensional scaling to examine the role of environmental factors such as soil properties, forest structure and management. Disturbance, in particular the frequency of harrowing, led to a decline in species richness and modified the indicator species and functional diversity by favoring Ruderal (R) species at the expense Stress-Tolerant (S) and Competitor (C) species, which are better suited to riparian forest conditions. Poplar plantations should not be used as surrogates for riparian forests, and minimizing harrowing in poplar plantations promotes vascular plant diversity. Furthermore, reintroduction of herbs, ferns and geophytes with a high conservation value and low seed dispersal capacity is advisable from the sixth year after establishment, once harrowing for weed control has been completed.