Invasive alien plants and the city: distribution patterns in response to urban elements and urbanization in three of the largest cities in Italy

Cities are extensively colonised by invasive alien plants, and their spread can negatively impact urban ecosystems including remaining natural habitats. It is therefore crucial to understand how invasive alien plants spread across urban landscapes and which anthropogenic drivers favour their persistence. Accordingly, in three of the largest Italian cities (Milan, Rome and Turin), a pool of alien species highly invasive in Europe was selected and their distribution was studied. Using a stratified sampling design defined along a gradient of urbanization, we analysed the distribution and abundance of 26 target species in relation to urban elements (e.g. linear infrastructures, gardens and buildings). With almost 20,000 occurrences, the analysis highlighted that, within the species’ pool, the most widespread spontaneous alien plants were Ailanthus altissima (Mill.) Swingle, Parthenocissus agg., Phytolacca americana L., Robinia pseudoacacia L. and Sorghum halepense (L.) Pers. Notably, Ulmus pumila L. was widespread in Milan and Turin. In response to the gradient of urbanization, a clear distinction emerged between species adapted to highly disturbed central areas, i.e. A. altissima, and those more suited to suburban contexts, i.e. R. pseudoacacia. Along the entire gradient, physical constraints limited the occurrence of species on buildings and other sealed areas, whereas gardens and road sides were the most receptive urban elements in terms of taxa richness and frequency. This comparative study of urban alien flora in three large Italian cities contributes to the understanding of distribution patterns and secondary spread of alien species in highly urbanised contexts of Southern Europe. Our study suggests that strategies for preventing and managing the colonisation of invasive alien plants in cities should be tailored to certain species, depending on levels of urbanisation and fine-scale land use.