Encroachment of the black cherry (Padus serotina) into swamp forests (alder carrs and riparian ash-alder forests) was poorly recognised. Most papers claim that P. serotina prefers less moist and less fertile soils, so many studies were limited to these habitats. The aims of this paper were to assess the density of P. serotina natural regeneration and the relationship between its density and species richness and diversity of invaded plant communities. The study was conducted in the Bogdanka River Valley, in Poznań (West Poland). 40 experimental plots (0.01 ha) were established. On each plot the density of P. serotina was determined and phytosociological relevé was conducted. There were four plant communities distinguished: alder carrs Carici elongatae-Alnetum, riparian ashalder forests Fraxino-Alnetum, plant communities transitional between alder carrs and ash-alder riparian forests and secondary forest community with ash on drained sites. There were no statistically significant differences between densities of P. serotina natural regeneration between plant communities except the secondary community with ash, where P. serotina was not recorded. There was a statistically significant (p < 0.01) positive relationship between density of P. serotina natural regeneration and forest floor species richness, as well as Shannon’s diversity index. P. serotina is able to colonize and reproduce in the studied plant communities, except the secondary community on drained sites, where competition with dense Sambucus nigra undergrowth does not allow P. serotina to establish. The relationship between diversity and invasibility has shown that P. serotina is able to invade well-preserved sites and is a potential threat to these plant communities.

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