Abstract. 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.

Summary1. Understanding the mechanisms driving exotic plant invasions is important for designing successful invader control strategies. Previous studies have highlighted different invasion mechanisms, including alteration of nutrient cycles through plant-soil feedback and evolutionary change toward more competitive genotypes. 2. We explored the possibility of these two mechanisms interactively affecting exotic plant invasion. Using data from recent experiments on the invasion of Phalaris arundinacea (Phalaris) in North American wetlands, we parameterized a classical resource competition model for nutrients and light, and expanded this model by including litter dynamics. We also examined the potential effects of evolutionary changes as observed in another set of recent experiments. 3. Both litter feedbacks and observed evolutionary changes may increase Phalaris' performance in invasive habitats. These mechanisms may amplify each other and this synergy may accelerate Phalaris invasion in areas where it is already present, leading to a high-litter state with Phalaris outcompeting other species. In areas with low nutrient supply where Phalaris is not yet present, a combination of litter feedbacks and evolutionary change may induce a critical transition from a low-litter, native-species-dominated state toward a high-litter, Phalaris-dominated state. 4. Our model results suggest that the importance of litter feedbacks and evolutionary change could be verified in future empirical studies through non-additive and nonlinear effects in litter addition experiments, and by increased litter : above-ground biomass ratios in biogeographical comparisons between plant communities with Phalaris in invasive and native habitats. The latter result may be caused by the accumulation of more recalcitrant litter from invasive genotypes. 5. Synthesis. Our study illustrates how exotic plant invasions may be exacerbated by the interaction between litter feedbacks and evolutionary change. This interaction may induce a positive feedback in the invasion process, suggesting that even short-term events, such as a nutrient pulse, may lead to relatively large and rapid ecosystem shifts that could be long-lasting and difficult to reverse.

Our study addresses the importance of examining competitive interactions between cytotypes across their distributional range, as competitive interactions were not consistent across sampling localities.

Abstract. Knowledge from basic plant ecology suggests that impact of one plant species on another is driven by either competition for the same limiting resources, or by unique plant traits. These processes might be context specific, explaining a differential impact of exotic plant invaders in the native vs. introduced range. With the help of a conceptual framework, we aimed at identifying the relationship between invader biomass and impact in the invasive Centaurea stoebe by conducting pairwise competition experiments with 15 European (old) and 15 North American (new) neighboring species. Old neighbors grew larger and could use available soil moisture more efficiently for growth than new neighbors. Interestingly, biomass of C. stoebe explained a substantial amount of the variation in biomass of the coevolved neighbors, but not of the new ''naı¨ve'' neighbors. Thus, impact in the home range appears to be driven by competition for the same limiting resources, but by other factors in the introduced range, possibly by exploitation of resources that are not used by the new neighbors or by interference competition. This distinction has important consequences for the management of invasive species, as in our study ecosystem recovery is less likely after simple biomass reduction.