Alien tree species and plant diversity in urban and peri-urban environments: the case of black locust

Forest patches are an important feature for increasing and maintain plant diversity in the urban and peri-urban landscapes. However, in such environments, the number of alien trees has increased over time due to the invasion of those species planted for ornamental and soil stabilization purposes. These species are able to invade abandoned areas whether forming patches of forests or linear features of different dimensions that may be considered novel urban ecosystems and a sort of urban wilderness. However, invasive alien tree species can also cause several detrimental effects. From the conservation and management perspective, the impacts on plant assemblages are of extreme importance. Alien tree species can have an effect on native communities and also can favour the establishment of other alien plants. Interestingly, their spread and consequences on plant diversity may be different in urban and peri-urban areas highlighting that their negative impacts are not an axiom. Here we discuss all these aspects by taking as an example black locust (Robinia pseudoacacia L.) in both urban and peri-urban environments. We conclude highlighting the need of further investigations on the impacts of alien tree species in urban and peri-urban environments distinguishing these from those due to urbanization per se, on the ecosystem services and social benefits related to such forests and, hence, the importance of considering invasive alien tree species in urban planning.