Plant invasions often involve rapid evolutionary change. Founder effects, hybridization, and adaptation to novel environments cause genetic differentiation between native and introduced populations and may contribute to the success of invaders. An influential idea in this context has been the Evolution of Increased Competitive Ability (EICA) hypothesis. It proposes that after enemy release plants rapidly evolve to be less defended but more competitive, thereby increasing plant vigour in introduced populations. To detect evolutionary change in invaders, comparative studies of native versus introduced populations are needed. Here, we review the current empirical evidence from: (1) comparisons of phenotypic variation in natural populations; (2) comparisons of molecular variation with neutral genetic markers; (3) comparisons of quantitative genetic variation in a common environment; and (4) comparisons of phenotypic plasticity across different environments. Field data suggest that increased vigour and reduced herbivory are common in introduced plant populations. In molecular studies, the genetic diversity of introduced populations was not consistently different from that of native populations. Multiple introductions of invasive plants appear to be the rule rather than the exception. In tests of the EICA hypothesis in a common environment, several found increased growth or decreased resistance in introduced populations. However, few provided a full test of the EICA hypothesis by addressing growth and defence in the same species. Overall, there is reasonable empirical evidence to suggest that genetic differentiation through rapid evolutionary change is important in plant invasions. We discuss conceptual and methodological issues associated with cross-continental comparisons and make recommendations for future research. When testing for EICA, greater emphasis should be put on competitive ability and plant tolerance. Moreover, it is important to address evolutionary change in characteristics other than defence and growth that could play a role in plant invasions.

Invasive plants are often more vigorous in their introduced ranges than in their native ranges. This may reflect an innate superiority of plants from some habitats or an escape from their enemies. Another hypothesis proposes that invasive plants evolve increased competitive ability in their introduced range. We present the results of a 14‐year common garden experiment with the Chinese Tallow Tree (Sapium sebiferum) from its native range (Asia), place of introduction to North America (Georgia) and areas colonized a century later (Louisiana and Texas). Invasive genotypes, especially those from recently colonized areas, were larger than native genotypes and more likely to produce seeds but had lower quality, poorly defended leaves. Our results demonstrate significant post‐invasion genetic differences in an invasive plant species. Post‐introduction adaptation by introduced plants may contribute to their invasive success and make it difficult to predict problem species.

Despite years of accumulating scientific evidence that fire is critical for maintaining the structure and function of grassland ecosystems in the US Great Plains, fire has not been restored as a fundamental grassland process across broad landscapes. The result has been widespread juniper encroachment and the degradation of the multiple valuable ecosystem services provided by grasslands. Here, we review the social–ecological causes and consequences of the transformation of grasslands to juniper woodlands and synthesize the recent emergence of prescribed burn cooperatives, an extensive societal movement by private citizens to restore fire to the Great Plains biome. We discuss how burn cooperatives have helped citizens overcome dominant social constraints that limit the application of prescribed fire to improve management of encroaching woody plants in grasslands. These constraints include the generally held assumptions and political impositions that all fires should be eliminated when wildfire danger increases.

Summary The first trees establishing in grasslands may indirectly favour their seedlings in competition with neighbouring herbaceous vegetation by increasing soil fertility with nitrogen‐rich litter and by reducing light levels under their canopies. It is predicted that increasing soil nitrogen availability will accelerate invasion of trees by stimulating their growth more than that of herbaceous species. Decreasing light availability is predicted to increase tree invasion by limiting the growth of herbaceous vegetation more than that of trees (competitive release). We tested these predictions using Chinese tallow tree (Sapium sebiferum), which is an aggressive alien invader of grasslands in the southern USA, and Hackberry (Celtis laevigata), which is a native tree common in these grasslands. Nitrogen and light were manipulated in two factorial field experiments in grasslands in Texas, USA. In the first, nitrogen was increased and light was decreased in plots with planted Celtis or Sapium seedlings. In the second experiment, light availability to planted Celtis or Sapium seedlings was increased by holding back prairie vegetation. In the first experiment, growth of Celtis and Sapium seedlings increased with nitrogen fertilization while the above‐ground biomass of prairie vegetation did not change. Prairie vegetation biomass decreased and tree seedling growth increased under shading. Sapium's growth increased dramatically in the treatment with combined nitrogen and shade. Sapium survival decreased when shade was applied. In the second experiment, Sapium growth increased with increased light. Thus, increased Sapium growth at low light levels in the shade experiment was probably a consequence of decreased competitive interference from prairie vegetation, rather than better absolute performance of Sapium in low light levels. These results provide evidence for facilitation as a mechanism involved in tree invasions of grasslands. Changes in resource levels, perhaps in combination with other factors, may explain rapid conversion of grassland communities to woodlands after the first pioneer trees are established. The marked response of Sapium to the combination of nitrogen and shade suggests that these positive feedbacks may be particularly strong for this alien plant species.