Introduction and naturalisation of key non-native forest trees in Europe: Results from the COST Action FP1403 NNEXT

Non-native tree species are widely used in forest plantations and agroforestry systems worldwide for their often-high productivity and performance compared to native trees. In addition, they have been and are introduced and used for multiple reasons, such as gardening, protective functions, arboreta, erosion control and for increasing the forest area through afforestation of abandoned or derelict land. However, these advantages may be compromised in situations where non-native trees establish outside plantation sites or start to become invasive in sensitive habitats. As such, the use of non-native tree species creates both opportunities and risks. Information regarding the initial introduction of non-native tree species to Europe is very important for the evaluation of its present status in the introduction-naturalisation-invasion continuum. The transition from introduction to invasion often spans many decades (a lag phase), or it may never occur at all. Introductions may date back to the beginning of the 17th century (e.g., for Robinia pseudocacia) as well as to more recent times (e.g., Cryptomeria japonica), with significant differences among countries concerning the period, the total area, the habitats, the biogeographic region and the human-mediated disturbances and management. This provides us with a series of “natural” (i.e., nonmanipulative) experiments presenting opportunities for gaining useful insights on central issues in invasion ecology (Richardson et al. 2012). To these aims, in the framework of the COST Action FP1403 NNEXT (Non-native Tree Species for European Forests – Experiences, Risks and Opportunities) we collected information on a selected set of 15 non-native tree species in 16 European countries. We determined the date (or period) of first introduction, the present status (in the introduction-naturalisation-invasion continuum), the date of the first detection in the wild as casual and/or naturalised, the period in which the species was used more intensively in forestry and the total area of plantations.