Purpose of Review: Non-native tree species (NNT) raise a range of different associations and emotions—to many citizens they are just an exotic curiosity in parks, to many conservationists they are an evil to native ecosystems that should be eradicated, to a rising group of foresters they are part of the solution to climate change and an increasing timber demand, and to others they are already daily forestry business. In this review, where we also summarise the findings of the recent COST Action FP1403 (NNEXT) ‘Non-native tree species for European forests: experiences, risks and opportunities’, we highlight opportunities and challenges in the light of climate change, ecological risks and legislative limits of growing non-native tree species in Europe.Recent Findings: Few NNT in Europe show invasive behaviour and are listed as prohibited species or as species to be monitored. A larger number of NNT is utilised in productive forestry and forest restoration due to their superior growth, valuable timber properties and good performance under harsh growing conditions. Current species distribution, experiences with success and failures and environmental concerns differ profoundly across Europe, with Western Europe overall revealing higher shares in NNT and showing a stronger interest of forestry related stakeholder groups to continue planting NNT.Summary: Many more NNT are already used in forestry than previously thought, but relatively few species have major importance in terms of area, mainly in western European countries. Diversification, mixing and avoidance of invasion in relation to NNT are necessities that are relatively new on the agenda. In contrast, provenance research of major NNT has been going on for many decades and now provides important information for climate change adaptation. Despite the limitations to the use of NNT either through legal restrictions or forest certification that differ considerably across Europe, the careful integration of a range of tested NNT also into future forest management planning shows a high potential for climate change adaptation and mitigation.

In the context of global change, the integration of non-native tree (NNT) species into European forestry is increasingly being discussed. The ecological consequences of increasing use or spread of NNTs in European forests are highly uncertain, as the scientific evidence is either constraint to results from case studies with limited spatial extent, or concerns global assessments that lack focus on European NNTs. For either case, generalisations on European NNTs are challenging to draw. Here we compile data on the impacts of seven important NNTs (Acacia dealbata, Ailanthus altissima, Eucalyptus globulus, Prunus serotina, Pseudotsuga menziesii, Quercus rubra, Robinia pseudoacacia) on physical and chemical soil properties and diversity attributes in Europe, and summarise commonalities and differences. From a total of 103 publications considered, studies on diversity attributes were overall more frequent than studies on soil properties. The effects on soil properties varied greatly among tree species and depended on the respective soil property. Overall, increasing (45%) and decreasing (45%) impacts on soil occurred with similar frequency. In contrast, decreasing impacts on biodiversity were much more frequent (66%) than increasing ones (24%). Species phylogenetically distant from European tree species, such as Acacia dealbata, Eucalyptus globulus and Ailanthus altissima, showed the strongest decreasing impacts on biodiversity. Our results suggest that forest managers should be cautious in using NNTs, as a majority of NNT stands host fewer species when compared with native tree species or ecosystems, likely reflected in changes in biotic interactions and ecosystem functions. The high variability of impacts suggests that individual NNTs should be assessed separately, but NNTs that lack European relatives should be used with particular caution.

Key message: Douglas-fir growth correlates with the climate, the soil moisture regime, and the soil nutrient status, reflecting a broad physiological amplitude. Even though planting this non-native tree species is suggested as a viable strategy to improve adaptiveness of European forests to a more extreme climate and to assure future productivity, the expected temperature increase may induce a decline in forest stand productivity for Douglas-fir in already warm and dry regions.Context: Tree species selection is one of the most important forest management decisions to enhance forest productivity and stand stability on a given site. Douglas-fir (Pseudotsuga menziesii(Mirb.) Franco var.menziesii), a non-native species from north-western America, is seen as an important additional species option for adapting Central European forests to a changing climate.Aims: This study assesses Douglas-fir forest productivity derived from site conditions. We investigate climatic and physico-chemical soil characteristics and productivity of 28 mature Douglas-fir stands growing on siliceous, as well as carbonate bedrock material in southern Germany and north-eastern Austria.Methods: The importance of climatic and physico-chemical soil characteristics was analyzed with the machine learning methodRandom Forests.Results: The results show that Douglas-fir growth correlates with climate, soil moisture, and soil nutrient availability derived from ten climatic and physico-chemical soil parameters.Conclusion: The broad pH optimum between 4.5 and 7.2 reflects the broad physiological amplitude of Douglas-fir, and no significant differences were detectable between carbonate and siliceous bedrock. We also conclude that climate change may induce a forest stand productivity decline, because lower productivity with the highest mean summer temperature across our study range was observed at the warmest sites in Eastern Austria.

The study reviews scientific literature on the topic of old-growth forests and held a workshop with around 20 external scientific experts in February 2021 to discuss preliminary findings.

Europe has a history rich in examples of successful and problematic introductions of trees with a native origin outside of Europe (non-native trees, NNT). Many international legal frameworks such as treaties and conventions and also the European Union have responded to the global concern about potential negative impacts of NNT that may become invasive in natural ecosystems. It is, however, national and regional legislation in particular that affects current and future management decisions in the forest sector and shapes the landscapes of Europe. We identified all relevant legal instruments regulating NNT, the different legal approaches and the regulatory intensity in 40 European countries (no microstates). Information on hard and effective soft law instruments were collected by means of a targeted questionnaire and consultation of international and national legislation information systems and databases. In total, 335 relevant legal instruments were in place in June/July 2019 to regulate the use of NNT in the investigated 116 geopolitical legal units (countries as well as sub-national regions with their own legislation). Countries and regions were empirically categorized according to ad hoc-defined legislation indicators. These indicators pay respect to the general bans on the introduction of non-native species, the generally allowed and prohibited NNT, approval mechanisms and specific areas or cases where NNT are restricted or prohibited. Our study revealed a very diverse landscape of legal frameworks across Europe, with a large variety of approaches to regulating NNT being pursued and the intensity of restriction ranging from very few restrictions on species choice and plantation surface area to the complete banning of NNT from forests. The main conclusion is that there is a clear need for more co-ordinated, science-based policies both at the local and international levels to enhance the advantages of NNT and mitigate potential negative effects.

Forests are a key player within the global carbon cycle and reforestation is an important climate change mitigation mechanism. In this study, we identify potentially suitable areas for reforestation to assess the carbon sequestration potential in the highly deforested and degraded Amhara region of Ethiopia. We apply biogeochemical mechanistic ecosystem modelling to predict the amount of carbon that can be potentially sequestered within different time horizons. Since human intervention plays a key role within the Amhara region, three different forest management scenarios and five different rotation periods following reforestation are tested: (i) unthinned; (ii) removal of 5% of the stem carbon every 20 years (thinning 1); and (iii) removal of 10% stem carbon every 20 years (thinning 2), as well as a rotation period of 10, 30, 50, 100, and 150 years. Sustainable management of reforested land is addressed by implementing the so called ‘Normal-forest’ system (equal representation of every age class). This ensures the long term sequestration effect of reforested areas. The study shows that 3.4 Mha (Mha = Million hectare) of land, including bare land (0.7 Mha), grass land (1.2 Mha), and shrub land (1.5 Mha) can be considered as ecologically potentially suitable for reforestation. Assuming a 100 year rotation period in a ‘Normal-forest’ system, this shows that a total net carbon sequestration potential of 177 Tg C (10.8 Tg C in the soil and 165.9 Tg C aboveground; Teragram = 10¹² g) is possible, if all 3.4 Mha are replanted. The highest total net carbon sequestration (soil and aboveground) was evident for the Highland-wet agro-ecological zone, whereas the lowest values are typically in the Midland-dry zone. The highest net aboveground carbon sequestration was predicted for reforestations on current grass land and shrub land versus bare land, whereas the highest net soil carbon sequestration was predicted on current bare land, followed by grass land and shrub land.