Joining the locals: Plant invaders shift leaf defenses to match native neighbors.

Local adaptation is common in invasive plants, but there is no consensus as to whether shifts in functional traits between invader "home" and "away" ranges contribute to their success in competition with native species. Theory based on enemy release suggests that invaders should reallocate limiting resources away from nutritive-based defenses and toward high growth potential. However, empirical studies of home-away trait shifts are typically conducted on a single invader and fail to consider how environmental differences across regions may impact leaf trait syndromes. We measured nine defense-related leaf traits for 27 invasive species across their home and away ranges in France, Japan, and the United States, and compared them to distributions of those same traits for co-occurring native species in both their home and away ranges. Our study included a total of 21 woody species sampled under forest canopies, and 23 herbaceous species sampled in fields and roadsides. Traits included toxic leaf alkaloids and cyanogenic glycosides; structural attributes including cell wall mass and fiber content; carbon (C), nitrogen (N), C:N, and total protein content; and specific leaf area. We found significant overall shifts in both qualitative (alkaloids) and quantitative (fiber, cell wall, N content) defense traits, particularly in woody species that are hypothesized to be more apparent to herbivores. However, the direction of trait shifts was not consistent across regions. Rather, for seven of nine traits, trait means of invaders shifted toward the means of native species in the recipient communities, likely reflecting environmental differences among regions rather than a shift in allocation from defense to growth. We suggest this "join the locals" pattern, whereby trait shifts in invaders match regional differences in native trait syndromes due to environmental variation, is a reasonable null model for studies of adaptive evolution in invasive species. Although the "join the locals" pattern is not mutually exclusive with shifts in functional traits caused by enemy release, our study involving multiple species and habitats suggests environmental gradients override trait shifts driven by varying herbivore communities.