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Although interactions between alien and native plant species are well studied, data on interactions between two co-existing alien species with respect to their invasibility are scarce. The aims of this study were (1) to evaluate three factors shaping abundance of the alien shrub species Cornus alternifolia: abundance of another alien (invasive) shrub species ( Prunus serotina), type of tree stand (coniferous vs. broadleaved) and distance to propagule sources and (2) to assess the potential dispersal distance of the species studied. Densities of both species were assessed within 194 experimental plots (located in experimental plantations of trees) in Rogów Arboretum (Central Poland). P. serotina occurred on 79 and C. alternifolia on 33 of the 194 plots. The furthest distance of C. alternifolia from the propagule source was 338 m. C. alternifolia reached higher densities in coniferous than broadleaved tree stands. Density of C. alternifolia depended on tree stand type and distance from the propagule source, but did not depend on density of P. serotina. Density of C. alternifolia decreased with increasing distance from the propagule source; however, this relationship was modified by the type of tree stand: densities were lower in broadleaved than in coniferous stands. The presence of the invasive species seems to neither facilitate nor limit the dispersal distance of C. alternifolia, as these two species differ in shade tolerance. The study also provided the first information about C. alternifolia potential invasiveness, because earlier this species was noticed only as casually escaping from cultivation in Slovakia. [ABSTRACT FROM AUTHOR]

Density of natural regeneration of black cherry ( Padus serotina ) depends on distance from the propagule source. Ecological success of this species is higher in coniferous than deciduous forests. The main aim of this study was to assess the interaction between the distance from propagule source and tree stand type (coniferous, deciduous and mixed) on occurrence and density of natural regeneration of black cherry. The study was conducted on 202 experimental plots in Rogów Arboretum (Central Poland), consisting of plantations of alien and native tree species, growing on potential habitats of fertile deciduous forest. The density of natural regeneration was measured in four height classes: 0–0.5 m, 0.5–2 m, 2–5 m and over 5 m. Natural regeneration of black cherry occurred on 79 of the 202 plots, and its density varied from 0 to 25,660 ind. ha −1 . The mean density of black cherry was statistically significantly higher ( p < 0.001) in coniferous (1275.8 ± 374.2 ind. ha −1 ) than in deciduous (138.3 ± 48.3 ind. ha −1 ) and mixed (29.3 ± 12.3 ind. ha −1 ) stands. There was also a negative relationship between distance from propagule source (stand of P. serotina established in 1932 in the central part of the Arboretum) and density of natural black cherry regeneration ( R 2 = 0.19, p < 0.001). Tree stand type modified the relationship between distance from propagule source and probability of natural black cherry regeneration occurrence ( p < 0.001), as well as density of natural regeneration ( p < 0.001). The study proved that effects of propagule pressure were strongly modified by habitat features, which confirms that plantation of conifers on sites suited for deciduous forests increases the risk of P. serotina invasion. [ABSTRACT FROM AUTHOR]