Invasiveness of Douglas fir in south-western Germany: an assessment based on forest inventory data.
Douglas fir (Pseudotsuga menziesii [Mirb.] Franco) is the most significant tree species of non-native origin in south-western Germany from a commercial point of view. Its importance is expected to increase in the effort to adapt forests to climate change because Douglas fir is considered to be more drought-resistant than, for instance, Norway spruce. Currently, the use of Douglas fir in silviculture and the assessment of its invasiveness are controversially discussed between representatives of nature conservation and forestry. Evidence-based evaluations of the current extent of the establishment and spread of the tree species provide an improved basis for the assessment of invasiveness, as they add objective facts to the discussion and increase the transparency of the process. In this article, we analyze the natural regeneration of Douglas fir in different forest types in the State of Baden-Wurttemberg (south-western Germany) with the help of three systematically collected inventory datasets. The existing natural regeneration serves as a measure for the successful establishment and spread of the species and provides information on its potential invasiveness, i.e. presumable negative impacts on native species and ecosystems. According to the inventory data, the natural regeneration of Douglas fir occurs at a low level (on only 0.3% of the forest surface in Baden-Wurttemberg). However, it naturally regenerates in special protected rare forest habitats, especially on open rocky sites and bolder fields and in sessile oak forest communities on dry sites with acidic soils and mixed broadleaved forests dominated by sycamore. Here, negative impacts might be possible. Thus the species can be considered potentially invasive in these specific conditions. Since Douglas fir natural regeneration was only reported on a proportion of 0.2% of the whole area of monitored protected forest habitats, it should still be possible to successfully eradicate this tree species from these habitats. Data on natural regeneration across a range of different forest types provide an improved basis for the assessment of invasiveness, and thus for identification of appropriate management approaches, such as designating buffer zones around valuable protected habitats.