Plant functional shifts in the invaded range: a test with reciprocal forest invaders of Europe and North America

Comparative trait analyses of native and invasive plant species have enriched our understanding of the strategies that make plants successful in particular contexts. However, it is generally unknown whether traits associated with invasiveness arise de novo in the introduced range or represent a case of ‘pre-adaptation’ of some species to become invasive. We compared the functional strategies of two invasive species, Prunus serotina, a tree native to Eastern North America (ENA) and invasive in European forests, and Rhamnus cathartica, a shrub native to Europe and invasive in ENA, in both their native and invasive ranges. We measured leaf functional traits related to plant carbon and nitrogen economics in populations across northeastern ENA (New York) and northern France. This reciprocal field approach is unique, comparing in situ physiology within and between each species' shared ranges. For both species, we found striking differences in leaf economic traits and intraspecific trait scaling relationships. Prunus serotina exhibited similar photosynthetic rates in New York (native range) and France (invasive range), but French populations had significantly greater leaf carbon investments in the form of increased leaf respiration, construction costs, and carbon concentrations. Rhamnus cathartica exhibited 50–60% higher photosynthetic rates in New York (invasive range) than France (native range), along with increased leaf nitrogen costs. New York populations also had significantly lower nitrogen resorption efficiency, with autumn leaf nitrogen concentrations 69% lower in France. Intraspecific trait differences between native and invasive ranges indicate shifts in resource-use strategies between the native and invasive range are possible in invasive species. While further investigations with additional species would be needed to determine the role of phenotypic plasticity and prevalence of post-introduction trait shifts, our results question the assumption that functional strategies of invasive species are conserved from the native range and highlight the utility of measuring in situ functional strategies of reciprocal invaders to provide mechanistic understanding of invasions.