Risk is in the eye of the assessor: comparing risk assessments of four non-native tree species in Germany

Non-native tree species (NNT) that pose risks to biodiversity are classified as `invasive' in some European countries. However, country-specific risk assessment methods may lead to different results for the same NNT between countries of comparable growth conditions, raising doubts about the reliability of risk classifications. Here, we analysed six risk assessment tools used in Germany and adjacent countries for their practical applicability and consistency using four NNT (Fraxinus pennsylvanica Marsh., Paulownia tomentosa (Thunb. ex Murray), Pseudotsuga menziesii (Mirb.) Franco and Quercus rubra L.) as case studies. Using these tools to classify risks for the same NNT and reference area (Germany) yielded inconsistent results for all NNT. The reasons for this were (1) differences in classification and weighting of criteria, (2) a lack of data to quantify invasion risks and (3) uncertainties related to assessment methodologies. Moreover, the tools fail to distinguish between risks posed by NNT in different sites. We suggest that instead the risks should be assessed for different ecosystem types by using site-specific inventory data covering the establishment, spread and potential impact of NNT. Our recommendations provide a foundation for developing a consistent, systematic Pan-European approach to assess invasiveness while addressing both risk and forest management aspects.