Effects of invasive alien species on riparian vegetation over a 20-year time-lapse: a case study from the Tiber river in Central Italy

Invasive alien species (IAS) have a severe impact on indigenous vegetation globally, including in riparian ecosystems, which are already subject to severe natural and anthropogenic modifications. This paper presents a case study of the long-term impact of IAS that was carried out on the banks of the Tiber River near Lake Alviano in Central Italy. We compared the vegetation composition from 1998 and 2020, to record IAS invasion after a time-lapse of 22 years. We performed 48 phytosociological relevés in 2020, using the same method that had been used in 1998 (with 46 relevés). The importance of IAS in determining the changes in floristic composition and vegetation dynamics was quantified using chorology, life forms and Ellenberg’s ecological indicator values. Results of our multivariate analysis revealed that Paspalum distichum L. and Amorpha fruticosa L. are the most impactful species, due to their ecological traits that make them highly competitive in disturbed environments. Ellenberg’s indicator values show that the habitat has shifted towards increased eutrophication and warming. The multivariate analysis confirmed a change in species composition and therefore in vegetation, leading to a more monotonous habitat with completely different vegetation types than before. Finally, the consequences for Natura 2000 habitats were assessed. Key policy insights IAS drastically changed and reduced the cover of native vegetation species between 1998 and 2020, changing the environment from an herbaceous marshy vegetation type to a more homogeneous shrubland. The analysed habitat increased in eutrophication driven by 22 years of overgrazing and by the establishment of a nitrogen-fixing IAS (Amorpha fruticosa L.). The most impactful IAS were Paspalum distichum L. and Amorpha fruticosa L. Some IAS (e.g. Paspalum distichum L.) are currently considered characteristic species of some N2000 habitats (e.g. 3280 and 3290), presenting a paradox of the negative effects of invasions and evidencing a gap in the inclusion of important native communities in the EU Habitats Directive worthy and in need of protection.