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Our understanding of how climate influences species distributions and our ability to assess the risk of introduced species depend on the assumption that species' climatic niches remain stable across space and time. While niche shifts have been detected in individual invasive species, one assessment of ~50 plants in Europe and North America concluded that niche shifts were rare, while another concluded the opposite. These contradictory findings, limited in species number and geographic scope, leave open a need to understand how often introduced species experience niche shifts and whether niche shifts can be predicted. We found evidence of climatic niche shifts in 65-100% of 815 terrestrial plant species introduced across five continents, depending on how niche shifts were measured. Individual species responses were idiosyncratic, but we generally saw that niche shifts reflected changes in climate availability at the continent scale and were largest in long-lived and cultivated species. Smaller intercontinental niche shifts occurred within species' native ranges. Overall, the climatic niches of terrestrial plant species were not conserved as they crossed continents. These results have major consequences for applying environmental niche models to assess the risk of invasive species and for predicting species responses to climate change. Our findings challenge the tenet that species' niches are conserved aspects of their ecology.

Invasive plant species should be evaluated and prioritized for management according to their impacts, which include reduction in native diversity, changes to nutrient pools, and alteration of fire regimes. However, the impacts of most invasive species have not been quantified and, when measured, those impacts are based on a limited number of response metrics. As a result, invasion ecology has been overwhelmed by speculation and bias regarding the ecological consequences of invasive plants. We propose a quantitative mathematical framework that integrates any number of impact metrics as a function of groundcover and geographic extent. By making relative comparisons between invaded and uninvaded landscapes at the population scale, which results in a percent change for each metric, we overcome previous limitations that confounded the integration of metrics based on different units. Our model offers a quantitative approach to ecological impact that may allow identification of the transition from benign introduction to impactful invader, while also allowing empirical comparisons at the species and population levels that will be useful for management prioritization.

Several volatile allelochemicals were identified and characterized from fresh leaf tissue of three distinct populations of the invasive perennial weed, mugwort (Artemisia vulgaris). A unique bioassay was used to demonstrate the release of volatile allelochemicals from leaf tissues. Leaf volatiles were trapped and analyzed via gas chromatography coupled with mass spectrometry. Some of the components identified were terpenes, including camphor, eucalyptol, alpha-pinene, and beta-pinene. Those commercially available were tested individually to determine their phytotoxicity. Concentrations of detectable volatiles differed in both absolute and relative proportions among the mugwort populations. The three mugwort populations consisted of a taller, highly branched population (ITH-1); a shorter, lesser-branched population (ITH-2) (both grown from rhizome fragments from managed landscapes); and a population grown from seed with lobed leaves (VT). Considerable interspecific variation existed in leaf morphology and leaf surface chemistry. Bioassays revealed that none of the individual monoterpenes could account for the observed phytotoxicity imparted by total leaf volatiles, suggesting a synergistic effect or activity of a component not tested. Despite inability to detect a single dominant phytotoxic compound, decreases in total terpene concentration with increase in leaf age correlated with decreases in phytotoxicity. The presence of bioactive terpenoids in leaf surface chemistry of younger mugwort tissue suggests a potential role for terpenoids in mugwort establishment and proliferation in introduced habitats.

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(Houtt.) Ronse Decr.]. Can. J. Plant Sci. 86: 887-905. Polygonum cuspidatum (Japanese knotweed) is an introduced perennial geophyte in the buckwheat family (Polygonaceae). The phytogeographic distribution of P. cuspidatum in North America suggests a large number of intentional introductions via ornamental plantings from 1870 to 2000, followed by secondary spread from these foci. This species is most pernicious along riparian corridors and road and railroad rights-of-way, reducing visibility, displacing native species, negatively affecting native wildlife, and causing alterations in natural hydrologic processes. Although non-hybrid seed recruitment has not been observed in Europe because of the presence of male-sterile clones only, dispersal of seeds and stem and rhizome fragments by flowing water does occur in North America and populations are readily established from these sources. The primary means of local and regional range expansion is human-mediated transport of rhizome-infested soil. Hybridization is common with the congener P. sachalinense in the introduced ranges of North America and Europe resulting in the equally noxious P. × bohemicum.

The North American historic phytogeographic distribution of mugwort (Artemisia vulgaris) and Japanese knotweed (Polygonum cuspidatum), two invasive perennial species introduced from Eurasia and East Asia respectively, was recreated using herbarium records.