Introduced garden plants are strong competitors of native and alien residents under simulated climate change

Most invasive plants have been originally introduced for horticultural purposes. Still, most alien garden plants have not naturalized yet, probably due in part to inadequate climatic conditions. Climate change may alter this, but few experimental studies have addressed this for non-naturalized alien garden plants, and those that have, addressed only singular aspects of climate change. In a greenhouse experiment, we examined the performance of nine non-naturalized alien herbaceous garden plants of varying climatic origins in response to simulated climate warming and reduced water availability, in a factorial design, as projected for southern Germany. To assess their invasion potential, we grew the species in competition with resident native and already-naturalized alien species. Reduced watering negatively affected non-naturalized garden plants, as well as the native and naturalized competitors, particularly at higher temperatures. However, non-naturalized aliens performed better relative to competitors when temperatures increased. Naturalized and native resident competitor responses to climate change were both negative, but across climate treatments, non-naturalized aliens, irrespective of their climatic origins, performed better against native than against naturalized competitors. Synthesis. We conclude that relative performance compared to resident species may increase for non-naturalized alien garden plants under climate change, as resident species become less competitive. Ongoing climate change is therefore likely to promote naturalization of commonly planted alien herbaceous species.