Some alien woody species used in commercial forestry become invasive and, as invaders, cause major problems in natural and semi-natural ecosystems. However, the deliberate introduction of aliens can bring unintended negative changes also within areas of their cultivation. This paper presents the effects of the intentional introduction of the North-American Quercus rubra in European mixed Scots pine-Pedunculate oak forests (POFs): Querco roboris-Pinetum (W. Mat. 1981) J. Mat. 1988. Phytosociological data from field research combined with GIS data analysis of the current distribution of Northern Red oak in the studied habitat were used to determine the composition and structure of forest communities in plots with and without Q. rubra participation. The results show that Q. rubra significantly reduces native species richness and abundance, both in old-growth and in secondary (post-agricultural) forests. Not one resident vascular plant benefits from the introduction of Northern Red oak and only a few are able to tolerate its co-occurrence. The natural restocking of all native woody species is also strongly limited by this alien tree. The introduction of Northern Red oak significantly limits the environmental functions of the POF ecosystem and weakens its economic and social aspects. However, its further cultivation is justified from an economic point of view, as the essential function of the studied forests is commercial timber production, and the introduction of this fast growing alien tree supports the provisioning ecosystem services. A clear description of the level of trade-off between the accepted negative and positive effects of the introduction of Q. rubra on forest ecosystem services requires further interdisciplinary studies.

This account presents information on all aspects of the biology ofQuercus rubraL. (Red Oak, Northern Red Oak; syn.Q. borealis,Q. maxima) that are relevant to understanding its ecological characteristics and behaviour. The main topics are presented within the standard framework of theBiological Flora of the British Isles:distribution; habitat; communities; responses to biotic factors; responses to environment; structure and physiology; phenology; floral and seed characters; herbivores and disease; history; and conservation. Quercus rubrais a large deciduous tree native to eastern North America. In its native range this species is one of the most important mid‐successional tree species. Since the 18th centuryQ. rubrahas been introduced into forest management in Europe and elsewhere. Currently,Q. rubrais a naturalized species in 23 countries. It is an important source of wood, and this was one of the most important reasons for its introduction in forests. In forest management this species is valued for its high growth rate and drought resistance. Red Oak is also widely cultivated in Europe for its ornamental value—red leaves in autumn and smooth, grey bark. Red Oak is a threat to native species within its introduced range as it can alter environmental conditions beneath its canopy, especially by decreasing light availability and litter decomposition rate. Such conditions also contribute to biotic homogenization of the understorey vegetation.Q. rubrais widely dispersed by small mammals and birds, up to 1,500 m from seed sources, thereby increasing its potential invasiveness. In EuropeQ. rubraseems to be less prone to herbivores and fungi, in contrast to its native range, where it is grazed by ungulates and insects, and infected byPhytophthoraand oak wilt. Due to its higher tolerance to drought than native European oaks,Q. rubrais claimed to be one of the species replacing native trees which are decreasing due to climate warming. However, consideration of all possible consequences is needed in wide‐scale introduction of alien species, such asQ. rubra, due to negative impacts on native ecosystems.