Non-native Douglas fir (Pseudotsuga menziesii) in Central Europe: ecology, performance and nature conservation.
Against the background of the ongoing climate change, forest science and Central-European forest management need information about tree species that are suitable of forming resistant and resilient multispecific and multipurpose forest stands. Non-native species are also considered for this purpose. One of these species may be the Douglas fir (Pseudotsuga menziesii), which has been introduced from western North America to Europe in the 19th century. The aim of this review is to compile recent scientific results that are relevant in the context of the use of the Douglas fir in future Central-European forestry to create a foundation for science-based decision-making, and to formulate future research tasks on that basis. Due to its high growth rates and low susceptibility to needle cast fungi, the Douglas fir's coastal variety (P. menziesii var. menziesii, syn.: var. viridis) is being preferred in Central-European silviculture. It is competitively superior to all indigenous Central-European forest tree species due to rapid height growth and efficient shading of competing plants; high drought tolerance, water use efficiency and resilience after drought stress; and efficient water and nutrient uptake and high nutrient use efficiency. In Central Europe, the Douglas fir is currently less threatened by pests and pathogens than are the indigenous Norway spruce and Scots pine. Its litter is better decomposable compared to native conifers, but increased nitrification, especially at sites with former agricultural use or under anthropogenic nitrogen deposition, or lower nitrate uptake rates due to lower nitrogen demand of the species can result in enhanced soil acidification, aluminum mobilization and leaching of nitrate, "base" cations and aluminum compounds. Mixtures of Douglas fir and native trees may be particularly effective in sequestrating carbon and nitrogen in the soil. Negative effects of the Douglas fir on the plant diversity of a given community seem to be small or even non-existent, but its interactions with the fauna is more ambiguous. The majority of nature conservation organizations recommend avoiding Douglas-fir monocultures and restricting the fraction of Douglas-fir admixtures to stands of native tree species to 30% at maximum in considering current regulations for nature protection. Future research tasks comprise monitoring of the Douglas-fir provenances in cultivation and of introduced pests and pathogens, investigations of responses to consecutive and combined stress factors and of the species' invasiveness at dry sites as well as comparative long-term studies on interactions with animal communities and on matter flux and turnover in the ecosystems.