Competitive Ability of Alliaria petiolata (Garlic Mustard, Brassicaceae), an Invasive, Nonindigenous Forest Herb

Alliaria petiolata (Bieb.) Cavara & Grande (garlic mustard, Brassicaceae) is a Eurasian herb that is currently invading forested areas throughout portions of the northern United States and southern Canada. Alliaria petiolata enters plant communities in an aggressive fashion, often displacing resident understory species. One possible mechanism that may enable garlic mustard to successfully invade these areas is hypothesized superiority as a competitor. In order to determine the competitive potential of A. petiolata, a multiple deWit replacement series was conducted in the greenhouse between A. petiolata and three native species: Impatiens capensis, an herbaceous annual, and Acer negundo and Quercus prinus, two woody perennials. Each target species was grown in mixture with A. petiolata, and all species were grown in monoculture. After 21 wk, plants were harvested and dried. Aboveground dry‐weight biomass (yield) was then used to calculate the relative yield per plant, relative yield total, and mean aggressivity index. Impatiens capensis and A. negundo experienced greater intraspecific competition than interspecific competition with A. petiolata, while A. petiolata grown with A. negundo experienced more interspecific competition. Impatiens capensis and A. petiolata were approximately equal in aggressivity, while A. negundo was more aggressive than A. petiolata. In contrast, Q. prinus experienced more interspecific competition when grown with A. petiolata and had a lower aggressivity value. Alliaria petiolata rosettes had a greater competitive ability and aggressivity than Q. prinus, indicating that oak forest understories may be more vulnerable to A. petiolata invasion and that A. petiolata may negatively affect oak regeneration.