Urbanization and black locust dominance effects on understory plant biodiversity in three European cities compared.

The worldwide city growth is increasing the number of people who benefit from the provision of services by semi-natural urban ecosystems. The plant species living in semi-natural "wild" urban woodlands, is one of the main structural component giving ecosystem complexity and stability. Plant biodiversity is mainly affected by anthropogenic disturbances like habitat modification, destruction and fragmentation. Alien plants are shaping their plant composition. Urban woodland canopy is rich in alien trees. To assess whether this is resulting in the development of novel ecosystems, comparisons of plant communities between ecosystems and cities with different levels of invasion are needed. The aim of this study is to disentangle the effects of urbanization, plant invasion, and climate, on urban woodland plant communities. We present their similarity patterns in three European cities distributed along a latitudinal gradient. Sampling relied on a paired comparison design through which seventy-six 100 m2 paired plots in stands invaded and not invaded by the alien black locust tree (Robinia pseudoacacia) were identified in Berlin, Padua and Rome. A relative high proportion of alien species was recorded. This abundance may be interpreted both as a driver of and a response to anthropogenic disturbances. Differences in plant communities were explained by different levels of urbanization and landscape components of the urban matrix. The understory composition among different cities showed higher homogeneity within woodlands dominated by black locust than within native woodlands. This research enabled us to explore joint impacts of urban matrix components and dominance of black locust tree on alpha and beta diversity and confirmed the importance of multiple-scale studies to better understand how effects on plant biodiversity can be modulated by multiple factors.