Exploring the biotic homogenization and diversity resistance hypotheses. The understorey of non-native and native woodland canopies in three urban areas of Europe

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.