Abstract
Four North American trees are becoming invasive species in Western Europe: Acer negundo, Prunus serotina, Quercus rubra, and Robinia pseudoacacia. However, their present and future potential risks of invasion have not been yet evaluated. Here, we assess niche shifts between the native and invasive ranges and the potential invasion risk of these four trees in Western Europe. We estimated niche conservatism in a multidimensional climate space using niche overlap Schoener's D, niche equivalence, and niche similarity tests. Niche unfilling and expansion were also estimated in analogous and nonanalogous climates. The capacity for predicting the opposite range between the native and invasive areas (transferability) was estimated by calibrating species distribution models (SDMs) on each range separately. Invasion risk was estimated using SDMs calibrated on both ranges and projected for 2050 climatic conditions. Our results showed that native and invasive niches were not equivalent with low niche overlap for all species. However, significant similarity was found between the invasive and native ranges of Q. rubra and R. pseudoacacia. Niche expansion was lower than 15% for all species, whereas unfilling ranged from 7 to 56% when it was measured using the entire climatic space and between 5 and 38% when it was measured using analogous climate only. Transferability was low for all species. SDMs calibrated over both ranges projected high habitat suitability in Western Europe under current and future climates. Thus, the North American and Western European ranges are not interchangeable irrespective of the studied species, suggesting that other environmental and/or biological characteristics are shaping their invasive niches. The current climatic risk of invasion is especially high for R. pseudoacacia and A. negundo. In the future, the highest risks of invasion for all species are located in Central and Northern Europe, whereas the risk is likely to decrease in the Mediterranean basin.
Highlights
Exotic trees have been broadly planted for economic and ornamental purposes all over the world
Ecology and Evolution published by John Wiley & Sons Ltd
Characteristics of the climatic niches in the native and introduced ranges The climatic space of the North American and Western European backgrounds is represented in a principal component analysis (PCA) analysis (Fig. 1): The two first axes explain 71.7% of the total variance Table 1
Summary
Exotic trees have been broadly planted for economic and ornamental purposes all over the world. We would hypothesize that those species would potentially share their climatic niche between the east coast of North America, where the core of these species distributions is located, and Western Europe, where they are spreading. These trees were mostly introduced into Europe during the 17th and 18th centuries for ornamentation and forestry purposes (Lowe et al 2000; Starfinger et al 2003; Medrzycki 2007; Richardson and Rejmanek 2011; Cierjacks et al 2013; Woziwoda et al 2014), and some of them still have an economic importance.
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