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Aim To assess the population genetic consequences of the colonization of two species with contrasting mating systems, Solidago canadensis and Lactuca serriola, along altitudinal gradients in both their native and introduced ranges. Location Allegheny Mountains, West Virginia and Wallowa Mountains, Oregon, USA; Valais, southern Switzerland. Methods Leaf material was collected from populations along altitudinal gradients and genotyped at seven microsatellite loci for each species. Differences in variability between native and introduced areas and in relation to altitude were analysed using linear models. Differences in the genetic, geographical and altitudinal structure of populations between areas were analysed by AMOVA, cluster analysis and Mantel tests. Results Genetic variation within and across populations of S. canadensis was significantly reduced, while populations of L. serriola were significantly more variable, in the introduced area. Genetic diversity decreased significantly with altitude for S. canadensis but not L. serriola. Genetic structure of S. canadensis was similar in both areas, and populations were isolated by geographical but not altitudinal distance. By contrast, population structure of L. serriola was much weaker in the introduced area, and populations were not isolated by distance in either area. Main conclusions Solidago canadensis has experienced a strong genetic bottleneck on introduction to the Valais, but this has not prevented it from colonizing a wide altitudinal range. Variation in neutral markers is therefore not necessarily a good measure for judging the ecological behaviour of a species. By contrast, the greater variability of L. serriola in the introduced area, where it also occurs over a greater altitudinal range, can be explained by increased outcrossing among admixed populations. This suggests that the ecological amplitude of alien species might be enhanced after population admixture in the new range, especially for species with highly structured native populations. However, even genetically depauperate introduced populations can be expected to colonize the same environmental range that they occupy in the native area. [ABSTRACT FROM AUTHOR]

Aim Since ecological and evolutionary context changes when a plant species is introduced to a new area, it can be assumed that responses of alien plants to changing conditions along environmental gradients differ from those in their native range. Even if seed availability is not limited, the distribution of alien plants along such a gradient might still be restricted by their ability to germinate and establish as seedlings. In the present study, we aim at testing what factors promote or limit plant invasions during early establishment by using altitude as a model gradient. Location Altitudinal gradients in the Wallowa Mountains (Oregon, USA) and the Swiss Alps (Valais, Switzerland). Methods In transplant experiments along altitudinal gradients, we investigated the early establishment success of eight invasive alien Asteraceae species in their native and introduced ranges in the Wallowa Mountains and the Swiss Alps. Results Seedling recruitment was not restricted to relatively lower altitudes in the introduced range. In addition, we found no evidence for genetic adaptation along the altitudinal gradient in the introduced range, highlighting the importance of phenotypic flexibility for invasions. Furthermore, seedling recruitment was only enhanced by disturbance in the native range where vegetation was comparably dense but not in the introduced range. However, plant development was strongly delayed in the introduced range, probably due to low seasonal water availability. Main Conclusions We conclude that introduced plants, due to their ability to tolerate a wide range of environmental conditions, are not necessarily more restricted in their altitudinal limits than they are in their native range. Furthermore, due to other interacting factors (e.g. different competition situations among ranges), attempts to predict distributions of alien plants in the introduced range that are based on their distributions in the native range may be misleading. [ABSTRACT FROM AUTHOR]

Nonnative species richness typically declines along environmental gradients such as elevation. It is usually assumed that this is because few invaders possess the necessary adaptations to succeed under extreme environmental conditions. Here, we show that nonnative plants reaching high elevations around the world are not highly specialized stress tolerators but species with broad climatic tolerances capable of growing across a wide elevational range. These results contrast with patterns for native species, and they can be explained by the unidirectional expansion of nonnative species from anthropogenic sources at low elevations and the progressive dropping out of species with narrow elevational amplitudes--a process that we call directional ecological filtering. Independent data confirm that climatic generalists have succeeded in colonizing the more extreme environments at higher elevations. These results suggest that invasion resistance is not conferred by extreme conditions at a particular site but determined by pathways of introduction of nonnative species. In the future, increased direct introduction of nonnative species with specialized ecophysiological adaptations to mountain environments could increase the risk of invasion. As well as providing a general explanation for gradients of nonnative species richness and the importance of traits such as phenotypic plasticity for many invasive species, the concept of directional ecological filtering is useful for understanding the initial assembly of some native floras at high elevations and latitudes.