Raw data on vegetation survey of Prunus mahaleb and Prunus serotina dominated plots (5 x 5 m2 each) with associated natural regeneration of both non-native trees and understory species abundance expressed in their percentage cover in plots, collected in urban pine forests near Toruń city. These data were used in the publication: „Community assembly and co-invasion jointly shape the regeneration of two non-native tree species”

Numerous studies demonstrated that invasive trees modify resource availability and their usage by understory vegetation. Success of invasive trees regeneration depends on competition within the understory, but it is not clear whether and how the relationship between invasive trees adult and young generations is mediated by understory assembly. We assessed how understory functional diversity shapes the regeneration of two pioneer non-native trees co-occurring in temperate forests (<i>Prunus mahaleb </i>L., and <i>P. serotina </i>Ehrh.). We hypothesized that their abundance in the canopy will shape their regeneration density by modifying understory assembly and that the two non-natives will affect each other’s regeneration in species-specific ways, mediated by their differential impact on understory. Canopy cover of both non-natives affected assembly rules of understory vegetation: <i>P. serotina </i>increased community-weighted mean specific leaf area, while <i>P. mahaleb</i> - leaf dry mass content. However, none of them affected functional richness. For <i>P. serotina</i>, the positive effect of functional richness on natural regeneration increased with increasing abundance of parental trees. In contrast, the natural regeneration of <i>P. mahaleb</i> did not shift its response to functional richness over the canopy cover gradient. Even under the high canopy cover of <i>P. serotina</i>, <i>P. mahaleb </i>effectively utilized unoccupied niches in the understory. We provided the first explanations of understory community assembly-mediated modification of invasive trees overstory-understory abundance, besides direct propagule pressure-mediated effects. Our results expand understanding of ecological processes shaping the spread of the studied non-native trees, as well as their impact on understory biodiversity and functioning.