regions; 1970-2015 time period), we analysed the species pool and frequency of alien vascular plants with respect to geographic origin and life-forms, and the levels of invasion across the European Nature Information System (EUNIS) woodland habitats.
Results:We found a total of 386 alien plant species (comprising 7% of all recorded vascular plants). Aliens originating from outside of and from within Europe were almost equally represented in the species pool (192 vs. 181 species) but relative frequency was skewed towards the former group (77% vs. 22%) due, to some extent, to the frequent occurrence of Impatiens parviflora (21% frequency among alien plants).Phanerophytes were the most species-rich life-form (148 species) and had the highest representation in terms of relative frequency (39%) among aliens in the dataset. Apart from Europe (181 species), North America was the most important source of alien plants (109 species). At the local scale, temperate and boreal softwood riparian woodland (5%) and mire and mountain coniferous woodland (<1%) had the highest and lowest mean relative alien species richness (percentage of alien species per plot), respectively.
Main conclusions:Our results indicate that European woodlands are prone to alien plant invasions especially when exposed to disturbance, fragmentation, alien propagule pressure and high soil nutrient levels. Given the persistence of these factors in the landscape, competitive alien plant species with a broad niche, including alien trees and shrubs, are likely to persist and spread further into European woodlands.
K E Y W O R D SEUNIS, exotic, forest, invasive plants, life-form, neophyte, non-native, origin, tree
| INTRODUCTIONGlobalization has triggered a massive spread of plant species to areas outside their native distribution ranges (van Kleunen et al., 2015).Some alien species persist only temporarily as casuals in the new area, while others can overcome local abiotic and reproductive barriers to establish self-sustaining populations (Richardson et al., 2000). Some naturalized aliens become invasive, that is they can spread in large numbers and across considerable distances (Richardson et al., 2000) or can have detrimental environmental and socio-economic impacts Woodlands cover a third of Europe's terrestrial area (Forest Europe, 2015; note that we use "woodland" as a synonym of "forest" in our article). In the past, they were logged and transformed to cropland andother open landscape types on a massive scale (Behre, 1988). Today, most European woodlands are composed of stands where the mean tree age is only 60 years (Vilén et al., 2012). Woodlands-and stands with old trees in particular-are generally thought to be resistant to alien plant invasions given the specific abiotic conditions in their herb layer, such as a dense canopy cover and a thick litter layer (Rejmánek, 2015). However, an increasing number of studies has questioned this assumption (e.g., Essl, Mang, & Moser, 2012;Kohli, Jose, Pal Singh, & Batish, 2009;Martin, Canham, & Marks, 2009;Re...
