Abstract
Aim: We examine how the climatic niches of North American tree and tall-shrub species change after their introduction to Europe and how these shifts affect their potential geographical distributions in the new range. We ask whether patterns of niche shifts differ among species confined to different biomes in North America and whether the expansions of species' climatic niches and potential distribution ranges are related to their residence time in Europe and native range size.
Location: North America and Europe.
Taxon: Vascular plants (trees and shrubs).
Methods: We used principal component analysis to quantify post-introduction shifts in climatic niches of 59 species native to North America and alien to Europe. We modelled the expansions of their potential geographical ranges using Maxent. Differences in niche shifts and geographical expansion among species introduced from different biomes were tested using the Kruskal-Wallis test. Spearman correlation was used to relate niche and geographical expansions to residence time and native range size.
Results: Alien trees and tall shrubs introduced from North America to Europe exhibited greater niche stability and unfilling than niche expansion, except for the species from Coastal Plain forests. The latter species showed the largest niche and geographical expansions. Species with a small native range in North America introduced to Europe long ago were more likely to expand to new climatic conditions and geographical areas.
Main conclusions: We show that (i) most North American tree and tall-shrub species introduced to Europe still do not occupy all areas with suitable climatic conditions in their secondary distribution range, but species from Coastal Plain forests tend to expand into areas with climates not found in their native ranges; (ii) the potential of the studied species to spread in Europe depends on the climatic conditions in the biome of origin, the size of their native range and the time since the first introduction.