Landscape and parental tree availability drive spread of Ailanthus altissima in the urban ecosystem of Poznan, Poland

Invasive species are drivers of urban ecosystem transformation. However, their management requires cost-demanding distributional data. Here we proposed an approach using floristic surveys, land-use maps, and field observations to reveal the patterns responsible for the spread of invasive trees using open-source software. We used the tree of heaven - Ailanthus altissima (Mill.) Swingle as a model species, one of the most common invasive tree species in cities. We conducted our study in Poznan city (W Poland). Although there were no spontaneous individuals of A. altissima recorded in the 1980s, in 2015 we found it in 18 out of 242 1 km(2) grid squares. We used a random forest model to show that the probability of A. altissima natural regeneration is positively associated with high cover of medium dense urban fabric (10-50%), recreational areas, and water. It was negatively correlated with the cover of forests, dense urban fabric (50 100%), and agricultural areas. We also found that most of the naturally regenerated individuals occurred up to 20 m from parental trees, and densities of natural regeneration were five times higher in paved surfaces and ruderal vegetation compared to lawns and bare ground. Despite model limitations, these patterns are consistent with other studies, revealing a set of rules facilitating prediction of A. altissima natural regeneration occurrence. Hence, our approach can facilitate narrowing the area of fieldwork required for mapping invasive tree species in urban ecosystems, help to design effective policy and management of invasive species in urban areas, and reduce costs of monitoring.