Five major invasive alien tree species in European Union forest habitat types of the Alpine and Continental biogeographical regions

Invasive alien species are a threat to protected areas, species, and habitats worldwide. At the European level, the control of invasive alien species is a major goal of the EU Biodiversity Strategy, and a cornerstone, in this sense, is the Regulation (EU) no.1143/2014 on invasive alien species. In this study, we focus on the Alpine and Continental biogeographical regions of Europe (defined in art. 1 of the Habitats Directive; 92/43/EEC), and aim to give an overview of the main protected forest habitat types threatened by invasive alien species, and focus on five major invasive alien tree species (Acer negundo, Ailanthus altissima, Prunus serotina, Quercus rubra, and Robinia pseudoacacia). We considered the assessment reports under Article 17 of the Habitats Directive for the period 2007–2012, and six documents on the interpretation of habitats, to identify those most threatened by invasive alien species and in particular by the five invasive alien trees. We also assessed selected examples of impacts by these alien tree species on different species groups within Natura 2000 sites to highlight potential effects on biodiversity. Riparian forest habitats and oak dominated woodlands are among the most prone to invasion within the two biogeographical regions. A. altissima and R. pseudoacacia are the most reported invasive alien trees in these protected forest habitat types. Furthermore, examples of the impacts of these tree species have indicated potential detrimental effects on forest habitats structure and functions, and on single species listed in the Habitats and Birds Directives. In conclusion, we emphasise the need for effective management and systematic monitoring of these five invasive alien tree species within Natura 2000 sites.