Giant Ragweed Invasion is Not Well Controlled by Biotic Resistance

The effect of native plant restoration on invasion by giant ragweed (Ambrosia trifida), an invasive species, is currently unknown. We hypothesized that (1) functional group identity would be a good predictor of biotic resistance to A. trifida, and (2) mixtures of species would be more resistant to invasion than monocultures. Using seven functional traits, 37 native and non-native plants were divided into three functional groups that differed primarily in longevity and woodiness. We conducted a competition experiment using an additive competition design with A. trifida and monocultures or mixtures of 14 species. Biotic resistance was evaluated by calculating a relative competition index (RCI_avg) based on the average performance of A. trifida in treatments compared with that in control. In monocultures, RCI_avg of resident plants did not significantly differ among the three functional groups or within each functional group. The highest RCI_avg (40%) was observed for some fast-growing annuals (FG1) such as Zea mays and Secale cereal, which were strong competitors. RCI_avg of resident plants was not significantly greater in mixtures than in monocultures. Taken together, the results show that plant diversity did not control invasion by A. trifida and that giant ragweed invasion cannot be well controlled by biotic resistance.