The effect of Douglas-fir on biodiversity in European forests – What do we know and what do we not know?

1. In recent years, climate change has led to drought and severe bark beetle infestations, affecting Norway spruce ( Picea abies ) across Europe, with detrimental consequences for forest owners, the forestry sector and associated industries. As a result, silviculture now faces the challenge of identifying tree species more resilient to these stressors to mitigate the impacts on forest management, forest-dependent economies and rural livelihoods. The North American Douglas-fir ( Pseudotsuga menziesii ) has emerged as a promising conifer species, better suited to future climate conditions and capable of producing high timber yields. 2. Non-native tree species may affect native biodiversity, yet the impacts of Douglas-fir on native forest biodiversity are not clear. A comprehensive review evaluating the impact of Douglas-fir on faunal and floral biodiversity in European forests is lacking. 3. Here, we present the results of a systematic literature review on Douglas-fir effects on native biodiversity, focusing on studies conducted in Europe. For arthropods, sufficient studies were found to do more detailed quantitative assessments. For fungi, birds, plants and soil fauna some studies existed, but only qualitative evaluations could be made. Other taxa were not investigated. 4. In the present literature, the effects of Douglas-fir inclusion in stands on native biodiversity, compared to stands of solely native tree species, were mostly non-significant (78.6%, based on 32 studies). Positive effects were noted in 12% of cases, while negative effects were observed in 9.4% (total of 1,936 effects). Above-ground fauna was more extensively studied than below-ground fauna. Mechanisms proposed to explain taxa responses were often discussed but not always formally tested. For arthropods, there were varying effects on diversity between studies evaluating different scales (i.e., tree-scale vs. stand-scale). In general, differences in effects depended on a range of factors, including stand composition and structure, season, and sampling site and period. 5. Our review indicates limited evidence of adverse effects of Douglas-fir on biodiversity in European forests, highlighting a significant knowledge gap due to the scarcity of studies. Douglas-fir's impact on biodiversity likely varies depending on the forest type and management practices. Further research in diverse contexts is crucial to determine optimal levels of admixture and guide forest management. • A systematic literature review on the impact of Douglas-fir on native biodiversity in European forests. • The review encompasses published evidence including birds, arthropods, soil fauna, plants, and fungi. • Demonstration of inconsistent responses of taxa with mostly non-significant effects. • Outline of research gaps that need to be filled to assess the risk of large-scale Douglas-fir introduction to Europe.