Comparison of the non‐native herb Alliaria petiolata with dominant native herbs in microhabitats of a Midwestern forest

A mechanistic understanding of the causes of naturalization in terrestrial plant communities, and the consequences for associated ecosystems, relies on understanding whether and how native and non‐native plant traits differ. Traits of non‐native plants may either help them compete or avoid competition with natives, and depending on their interaction with the native community, the non‐native may act as more or less of an active driver vs. a passive passenger of change in the native ecosystem. Trait comparisons between native and non‐native plants are often laboratory‐based, providing insights that can be difficult to apply to natural communities. Studies of the impacts of non‐native plants, by contrast, are generally performed in natural communities, but usually do not compare observed impacts of the non‐natives with those exerted by dominant natives on each other and the community. Thus, the likely relative disruption of the non‐native to the community is unclear. In a four‐year observational study conducted at the microhabitat scale in the herbaceous layer of a Midwestern oak forest, we compared the non‐native herb Alliaria petiolata (garlic mustard) with dominant native herbaceous species in terms of correlational patterns related to three broad categories of traits: (1) environmental correlates (light, topography, soil nutrients) with presence and cover, (2) inter‐annual regeneration and phenology, and (3) associations with other species (competition and herbivory). We found that Alliaria differed strongly from dominant native herbs in all three categories. Compared with native herbs, Alliaria was more strongly associated with south‐facing slopes and high nutrient levels, displayed earlier phenology and enhanced dispersal ability, and was strongly avoided by the generalist native herbivore, Odocoileus virginianus (white‐tailed deer). Despite well‐described allelopathic effects of Alliaria on mycorrhizae and mycorrhizal plants, we found little evidence to suggest that Alliaria had negative interactions with native plants that were stronger than those of native community members. Overall, our data suggest that naturalization of Alliaria in this site is primarily due to differences from native plants in phenology, dispersal capability, and avoidance of herbivory, and that Alliaria is therefore more likely to be acting as a passenger than as a driver of change in our site.