Riparian ecosystems are one of the most invasible habitats due to frequent disturbances and high resource availability, creating conditions for pioneer species. In urban areas, richer in alien species than other areas, the risk of invasion is higher than elsewhere. The richness of both alien and native woody plants in urban riparian areas allows examination of the relationship between native and alien species richness, which has not been previously done for woody species as a separate guild. The aims of study were to compare current flora of woody species to that from the 1980s and to test whether the hypothesis of biotic acceptance can explain richness of alien woody species. The study was conducted in the Warta River Valley in Poznań city (Poland). During field work we compiled lists of woody species in 31 grid squares (1 × 1 km). Regression analysis was used to examine the relationship between number of native and alien woody plant species. From the 1980s to 2013, the number of woody species increased from 76 to 116. The share of alien species also increased from 29.3% to 45.7%. We found a positive relationship between richness of alien and native woody species, which explains 46% of variance. The abundance of the most successful invaders is connected with high initial propagule pressure from the high number of alien ornamental species in urban green areas. The positive relationship between number of native and alien woody species shows the wider base of “the rich get richer” concept, so treated separately, woody species may be determinants of dynamic processes in vegetation. Due to recent manifestations of invasive potential of some species, their invasion probability may be underestimated. Therefore, alien species should be avoided in urban green areas near natural and semi-natural urban forests, especially riparian forests, which are vulnerable to biological invasion.