From ornamental to detrimental? The incipient invasion of Central Europe by Paulownia tomentosa

The invasion of Paulownia tomentosa (Paulowniaceae), a new alien tree species in Central Europe, native to China, is analysed. By using its distribution in Austria, the invasion of this country is analysed in detail. The first reports of P. tomentosa in Austria were in the 1960s in Vienna. Since then, the number of sites has increased exponentially, with a total of 151 sites in 27 grid cells of the Floristic Mapping project of Austria. The number of sites per grid cell is strongly positively correlated with the minimum residence time in grid cell, which explains 86% of the deviance in the general linear model (GLM). The localities are confined to warm lowland areas (below 450 m altitude) and are concentrated in cities, with 90% of all localities recorded in cities with > 100,000 inhabitants. Paulownia tomentosa typically occurs in small populations of less then 10 individuals (83% of all records) and behaves as a pioneer species colonizing mainly disturbed urban habitats. Near-natural habitats, e.g. forest clearings and riparian shrubberies are rarely colonized. In extremely disturbed areas, the average number of vascular plant species is low (8.9 species), as is total plant cover (17%). As P. tomentosa is currently mostly confined to synanthropic habitats in urban areas, the invasion is not yet a nature conservation issue. In the future, predicted climate change might allow P. tomentosa to spread beyond its current distribution. The habitat preference in the eastern USA indicates that further spread of P. tomentosa in Central Europe might be accompanied by a switch to more natural habitats, e.g. forest clearings and forest margins. Thus, the future spread of this species should be closely monitored.