Prevalence of alien versus native species of woody plants in Berlin differs between habitats and at different scales

Cities are hotspots for plant invasions and woody plants that have escaped from cultivation contribute significantly to this phenomenon. Yet whether the richness of alien species in the floras of woody plants in urban areas also corresponds to a prevalence of alien species at the habitat and population levels is an open question. To explore the scale and context dependence of invasions by woody plants of urban environments, we analysed the occurrence of alien and native species of trees, shrubs and vines at the city, habitat and community scales in Berlin, Germany. The percentage of alien species in the flora of spontaneously occurring woody plants increased from 16% at the end of the 18th century to 67% two hundred years later. Of the 181 species of alien woody plants in Berlin's flora 32% have become naturalized. Species from other parts of Europe, the Mediterranean and western Asia escaped and became naturalized more frequently than species from other areas. Escape from cultivation did not increase the share of evergreens in the total flora of woody plants. All habitats other than wetlands had more alien than native species, and the percentage of alien species was significantly higher in green spaces, wastelands and residential areas than in forests and wetlands. However, native species were more frequent at the habitat scale. Overall, the trees most likely to be found in all habitats were native Acer platanoides, Betula pendula, Quercus robur and alien Robinia pseudoacacia, Acer negundo and Prunus serotina, and the most frequent shrubs the native Sambucus nigra and alien Mahonia aquifolium. At the community scale, counts of the numbers of individual trees in two selected study areas revealed that native species prevailed in residential areas and alien species in urban wasteland. The results demonstrate that invasion success of alien woody species in urban environments is strongly scale- and context-dependent. The clear dominance of alien species in the total urban species pool was not similar at both the habitat and community scales, particularly when the frequency of species is considered. In conclusion, assemblages of woody species in urban areas are not only characterized by high numbers of aliens but also by an increase in the abundance of native species such as the formerly rare Acer platanoides and A. pseudoplatanus, which now prevail due to enhanced propagule pressure and the eutrophication of urban ecosystems.