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Aim: Exploring the biotic homogenization and diversity resistance hypotheses by assessing the effect of non‐native black locust canopy on understorey species turnover. Location: Berlin, the Venetian metropolitan area, and Rome. Methods: We modelled the zeta (ζ) expression of diversity to compare the understorey species turnover between the non‐native black locust and native woodland canopies across multiple sites and through predictors of anthropogenic pressure (road and built‐up density) and interior conditions (tree basal area and mean height). Results: In Rome, black locust showed the lowest survivability and cover and did not produce any homogenization of the understorey. In Berlin and in the Venetian metropolitan area, black locust caused understorey homogenization, although with a lower intensity in Berlin. Under black locust canopies, distance between sites and road density was more consistently associated with species turnover, across urban areas and multiple sites. Under native canopies in Berlin, factors prominently associated with species turnover were road and built‐up density and mean tree height, while in the Venetian metropolitan area it was road density. Main conclusions: Evidence in support of the biotic homogenization in contrast to the diversity resistance hypothesis varied across urban areas. Species turnover was influenced by land use patterns more evidently under native tree canopies and where the non‐native tree had higher survivability. Similar analyses in other urban areas may confirm these relationships with other types of landscapes. [ABSTRACT FROM AUTHOR]

• Riparian forests are particularly vulnerable to non-native tree invasion. • Robinia pseudoacacia and Ailanthus altissima are by far the most frequent invaders. • Eucalyptus camaldulensis, Populus × canadensis and Ulmus pumila have spread recently. • Potentially invasible forests can provide information on the future threat posed by non-native trees. • Tailored strategies are needed for the management of non-native tree species. Many forest tree species have been moved outside their native range to provide goods and services elsewhere, but some of them have become invasive, causing negative impacts on biodiversity and human activities. The assessment and knowledge on the degree and scale to which forest ecosystems are invaded by non-native trees is of paramount importance for tailored policies and strategies aiming at forest conservation. By analyzing main databases and literature and applying a four-level scale of invasion (not currently invaded and with low invasibility; potentially invasible; moderately invaded; massively invaded), we assessed the current and potential occurrence of twenty-five invasive non-native trees across forest ecosystems in Italy. Vulnerability to invasion substantially differed across forest categories, with riparian forests being particularly vulnerable to invasion. Acacia dealbata , Juglans nigra , Populus × canadensis and Prunus laurocerasus invaded a relatively small number of forest categories but showed their potential to widen their impact on others. Furthermore, some non-native species showed a recent increase of their invasive status in Italy. This study identified the Italian forest categories and non-native tree species that deserve primary attention, and can support proper management and allocation of funding to protect forests from the spread of invasive tree species. [ABSTRACT FROM AUTHOR]