Introduction history and species characteristics partly explain naturalization success of North American woody species in Europe

1. The search for general characteristics of invasive species has not been very successful yet. A reason for this could be that current invasion patterns are mainly reflecting the introduction history (i.e. time since introduction and propagule pressure) of the species. Accurate data on the introduction history are, however, rare, particularly for introduced alien species that have not established. As a consequence, few studies that tested for the effects of species characteristics on invasiveness corrected for introduction history. 2. We tested whether the naturalization success of 582 North American woody species in Europe, measured as the proportion of European geographic regions in which each species is established, can be explained by their introduction history. For 278 of these species we had data on characteristics related to growth form, life cycle, growth, fecundity and environmental tolerance. We tested whether naturalization success can be further explained by these characteristics. In addition, we tested whether the effects of species characteristics differ between growth forms. 3. Both planting frequency in European gardens and time since introduction significantly increased naturalization success, but the effect of the latter was relatively weak. After correction for introduction history and taxonomy, six of the 26 species characteristics had significant effects on naturalization success. Leaf retention and precipitation tolerance increased naturalization success. Tree species were only 56% as likely to naturalize as non-tree species (vines, shrubs and subshrubs), and the effect of planting frequency on naturalization success was much stronger for non-trees than for trees. On the other hand, the naturalization success of trees, but not for non-trees, increased with native range size, maximum plant height and seed spread rate. 4. Synthesis. Our results suggest that introduction history, particularly planting frequency, is an important determinant of current naturalization success of North American woody species (particularly of non-trees) in Europe. Therefore, studies comparing naturalization success among species should correct for introduction history. Species characteristics are also significant determinants of naturalization success, but their effects may differ between growth forms.