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Biogeographic experiments that test how multiple interacting factors influence exotic plant abundance in their home and recipient communities are remarkably rare. We examined the effects of soil fungi, disturbance and propagule pressure on seed germination, seedling recruitment and adult plant establishment of the invasive Centaurea stoebe in its native European and non-native North American ranges., Centaurea stoebe can establish virtual monocultures in parts of its non-native range, but occurs at far lower abundances where it is native. We conducted parallel experiments at four European and four Montana ( USA) grassland sites with all factorial combinations of ± suppression of soil fungi, ±disturbance and low versus high knapweed propagule pressure [100 or 300 knapweed seeds per 0.3 m × 0.3 m plot (1000 or 3000 per m2)]. We also measured germination in buried bags containing locally collected knapweed seeds that were either treated or not with fungicide., Disturbance and propagule pressure increased knapweed recruitment and establishment, but did so similarly in both ranges. Treating plots with fungicides had no effect on recruitment or establishment in either range. However, we found: (i) greater seedling recruitment and plant establishment in undisturbed plots in Montana compared to undisturbed plots in Europe and (ii) substantially greater germination of seeds in bags buried in Montana compared to Europe. Also, across all treatments, total plant establishment was greater in Montana than in Europe., Synthesis. Our results highlight the importance of simultaneously examining processes that could influence invasion in both ranges. They indicate that under 'background' undisturbed conditions, knapweed recruits and establishes at greater abundance in Montana than in Europe. However, our results do not support the importance of soil fungi or local disturbances as mechanisms for knapweed's differential success in North America versus Europe. [ABSTRACT FROM AUTHOR]

The greater abundance of some exotic plants in their nonnative ranges might be explained in part by biogeographic differences in the strength of competition, but these competitive effects have not been experimentally examined in the field. We compared the effects of neighbors on the growth and reproduction of spotted knapweed (Centaurea stoebe) in Europe, where it is native, and in Montana, where it is invasive. There were strong negative competitive effects of neighboring vegetation on C. stoebe growth and reproduction in Europe. In contrast, identical experiments in Montana resulted in insignificant impacts on C. stoebe. Although the mechanisms that produce this dramatic biogeographic difference in competitive outcome remain unknown, our results indicate that differences in net competitive interactions between ranges may contribute to the striking dominance of C. stoebe in parts of North America. [ABSTRACT FROM AUTHOR]

1. Exotic invasive plants can have strong effects on native communities. Centaurea maculosa, a forb that is native to Eurasia, has created near-monocultures in many parts of its invaded range in western North America and produces the root exudate (±)-catechin. Controlled laboratory experiments suggest that the phytotoxic effects of (±)-catechin may be stronger on some North American species than on some European species. 2. We conducted experiments in the field in two different years in the native (Romania) and invaded (MT, USA) ranges of C. maculosa, testing the effects of (±)-catechin on species that co-occur with C. maculosa in both ranges. 3. (±)-Catechin reduced the growth of native plant species in Montana in both years, although there was some variability between species in the effect of (±)-catechin on leaf growth in 2005. There was no effect of (±)-catechin on plants in Romania. 4. This first in situ test of the novel weapons hypothesis supports the notion that novel biochemical constituents of some invasive species may contribute to their success. 5. Synthesis. In addition to providing information useful for understanding invasions, our results indicate that some species in the native range of C. maculosa may be adapted to its particular biochemical traits, raising the possibility that interactions among plant species may be affected by a common evolutionary history. [ABSTRACT FROM AUTHOR]

Recent studies suggest that the invasive success of Centaurea maculosa may be related to its stronger allelopathic effects on native North American species than on related European species, one component of the “novel weapons” hypothesis. Other research indicates that C. maculosa plants from the invasive range in North America have evolved to be larger and better competitors than conspecifics from the native range in Europe, a component of the “evolution of increased competitive ability” hypothesis. These hypotheses are not mutually exclusive, but this evidence sets the stage for comparing the relative importance of evolved competitive ability to inherent competitive traits. In a competition experiment with a large number of C. maculosa populations, we found no difference in the competitive effects of C. maculosa plants from North America and Europe on other species. However, both North American and European C. maculosa were much better competitors against plants native to North America than congeners native to Romania, collected in areas where C. maculosa is also native. These results are consistent with the novel weapons hypothesis. But, in a second experiment using just one population from North America and Europe, and where North American and European species were collected from a broader range of sites, competitive interactions were weaker overall, and the competitive effects of C. maculosa were slightly stronger against European species than against North American species. Also consistent with the novel weapons hypothesis, (±)-catechin had stronger effects on native North American species than on native European species in two experiments. Our results suggest that the regional composition of the plant communities being invaded by C. maculosa may be more important for invasive success than the evolution of increased size and competitive ability. [ABSTRACT FROM AUTHOR]