Traits of British alien and native urban plants

1. As urbanization accelerates, interest is growing in the traits of urban plants. We classified 822 UK vascular plants on the basis of their occurrence along an urban-rural gradient in 26 710 samples of vegetation from 2508 UK 1-km grid squares, including a wide range of rural habitats and Sheffield and Birmingham, two of the UK's largest cities. 2. Both alien and native species were classified with respect to mean proportion of urban land cover in the 1-km grid squares in which the species occurs (urbanity), and absolute frequency in highly urban grid squares (urban frequency). Bayesian regression models were then developed for both measures, with a wide range of plant traits as explanatory variables. 3. Results for aliens and natives and for both urban measures were remarkably similar: the single, coherent picture of 'successful urban species' that emerges from our analysis is of robust plants of relatively fertile, dry, unshaded, base-rich habitats. Only seed mass behaved very differently for natives and aliens; seed mass was related positively to success of urban natives, and negatively to success of urban aliens. Neither clonality, seed dispersal nor seed persistence in the soil were strongly linked to success in urban habitats. 4. Synthesis. Cities can provide opportunities for surprisingly rich floras, but the traits of species that can persist in cities are quite narrowly circumscribed. More generally, it is clear that analysis of traits reveals important patterns in floristic data that would be far from obvious from a purely floristic analysis.