Black locust (Robinia pseudoacacia L.), a species native to the eastern North America, was introduced to Europe probably in 1601 and currently extends over 2.3 × 10⁶ ha. It has become naturalized in all sub-Mediterranean and temperate regions rivaling Populus spp. as the second most planted broadleaved tree species worldwide after Eucalyptus spp. This wide-spreading planting is because black locust is an important multipurpose species, producing wood, fodder, and a source of honey as well as bio-oil and biomass. It is also important for carbon sequestration, soil stabilization and re-vegetation of landfills, mining areas and wastelands, in biotherapy and landscaping. In Europe, black locust is drought tolerant so grows in areas with annual precipitation as low as 500–550 mm. It tolerates dry, nutrient poor soils but grows best on deep, nutrient-rich, well-drained soils. It is a fast-growing tree and the height, diameter and volume growth peak before the age of 20. It mostly regenerates vegetatively by root suckers under a simple coppice system, which is considered the most cost-effective management system. It also regenerates, but less frequently, by stool sprouts. Its early silviculture in production forests includes release cutting to promote root suckers rather than stool shoots, and cleaning-respacing to remove low-quality stems, reduce the number of shoots per stool, and adjust spacing between root suckers. In addition, early, moderate and frequent thinning as well as limited pruning are carried out focusing on crop trees. The species is regarded as invasive in several European countries and its range here is expected to expand under predicted climate changes.

Northern red oak (Quercus rubra L. syn. Q. borealis F. Michx.) is a valuable broadleaved tree species originating from the eastern half of the USA and Canada. It was introduced to Europe in 1691 and currently covers over 350 000 ha, being found all over the continent, except the coldest part of Scandinavia. It is a fast-growing and valuable broadleaved tree due to its ecological characteristics, good wood properties and high economic value. Northern red oak prefers deep, loose, moderately humid and acid soils, without compact horizons and of at least moderate fertility. It does not grow well on dry, calcareous soils as well as waterlogged or poorly drained soils. It is either naturally regenerated using a group shelterwood system or planted using seedlings of European provenance, collected in certified seed stands. As northern red oak is light-demanding, its management should be ‘dynamic’ and includes heavy interventions (cleaning–respacing and thinning from above), in order to minimize crown competition between the final crop trees. These should produce large diameter trees for valuable end uses (e.g. veneer, solid furniture, lumber, etc.) within a rotation period generally of 80–100 years. The necessity for pruning (both formative and high) depends on the stand stocking at establishment, the subsequent silvicultural interventions as well as the occurrence of forking. The adaptation potential of northern red oak to predicted climate change, especially drought, seems to be higher than for European native oaks, the importance of the species is expected to increase in the future.

Douglas-fir (Pseudotsuga menziesii(Mirb.) Franco), one of the most commercially important tree species in western North America and one of the most valuable timber trees worldwide, was introduced to Europe in 1827. It became a major species for afforestation in Western Europe after WWII, currently grows in 35 countries on over 0.83 million ha and is one of the most widespread non-native tree species across the continent. A lower sensitivity to drought makes Douglas-fir a potential alternative to the more drought-sensitive Norway spruce so its importance in Europe is expected to increase in the future. It is one of the fastest growing conifer species cultivated in Europe, with the largest reported dimensions of 2.3 m in diameter and 67.5 m in height. Pure stands have high productivity (up to 20 m³ ha⁻¹a⁻¹) and production (over 1000 m³ ha⁻¹). The species is generally regenerated by planting (initial stocking density from less than 1000 seedlings ha⁻¹to more than 4000 ha⁻¹), using seedlings of European provenance derived from seed orchards or certified seed stands. As the range of end-uses of its wood is very wide, the rotation period of Douglas-fir is highly variable and ranges between 40 and 120 years. When the production of large-sized, knot-free timber is targeted, thinnings are always coupled with pruning up to 6 m. There is an increasing interest in growing Douglas-fir in mixtures and managing stands through close-to-nature silviculture, but the species’ intermediate shade tolerance means that it is best managed through group selection or shelterwood systems.

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.

Sessile oak ( Quercus petraea (Matt.) Liebl.) is widely distributed across most of Europe particularly the hills and lower mountain ranges, so is considered “the oak of the mountains”. This species grows on a wide variety of soils and at altitudes ranging from sea level to 2200 m, especially in Atlantic and sub-Mediterranean climates, and it is sensitive to low winter temperatures, early and late frosts, as well as high summer temperatures. Sessile oak forms both pure and mixed stands especially with broadleaves such as European beech, European hornbeam, small-leaved lime and Acer spp. These form the understorey of sessile oak stands, promoting the natural shedding of lower branches of the oak and protecting the trunk against epicormic branches. Sessile oak is a long-lived, light-demanding and wind-firm species, owing to its taproot and heart-shaped root system. Its timber, one of the most valuable in Europe, is important for furniture-making (both solid wood and veneer), construction, barrels, railway sleepers, and is also used as fuelwood. It is one of the few major tree species in Europe that is regenerated by seed (naturally or artificially) and by stump shoots in high forest, coppice-with-standards and coppice forests. Sessile oak forests are treated in both regular and irregular systems involving silvicultural techniques such as uniform shelterwood, group shelterwood, irregular shelterwood, irregular high forest, coppice-with-standards and simple coppice. Young naturally regenerated stands are managed by weeding, release cutting and cleaning-respacing, keeping the stands quite dense for good natural pruning. Plantations are based on (1) 2–4-year old bare-root or container-grown seedlings produced in nurseries using seeds from genetic resources, seed stands and seed orchards. The density of sessile oak plantations (mostly in rows, but also in clusters) is usually between 4000 and 6000 plants ha ⁻¹ . Sessile oak silviculture of mature stands includes crown thinning, focusing on final crop trees (usually a maximum of 100 individuals ha ⁻¹ ) and targeting the production of large-diameter and high quality trees at long rotation ages (mostly over 120 years, sometimes 250–300 years). In different parts of Europe, conversion of simple coppices and coppice-with-standards to high forests is continuing. Even though management of sessile oak forests is very intensive and expensive, requiring active human intervention, the importance of this species in future European forests will increase in the context of climate change due to its high resistance to disturbance, superior drought tolerance and heat stress resistance.