Competitive interactions between native plants and nonnative, invasive plant species have been extensively studied; however, within degraded landscapes, the effect of interspecific interactions among invasive plants is less explored. We investigated a competitive interaction between two sympatric, invasive mustard species that have similar life history strategies and growth forms: garlic mustard and damesrocket. Greenhouse experiments using a full range of reciprocal density ratios were conducted to investigate interspecific competition. Garlic mustard had a negative effect on the final biomass, number of leaves, and relative growth rate in height of damesrocket. Survival of damesrocket was not negatively affected by interspecific competition with garlic mustard; however, garlic mustard showed higher mortality because of intraspecific competition. These results indicated that although garlic mustard has been observed to be the dominant species in this landscape, it may not completely outcompete damesrocket in all situations. Studies of invasive species in competition are important in degraded landscapes because this is the common situation in many natural areas.
Nomenclature: Damesrocket, Hesperis matronalis L.; garlic mustard, Alliaria petiolata (Bieb.) Cavara & Grande.
Management Implications: In degraded habitats, nonnative, invasive plant species not only compete against the resident, native plant species but also compete against the other invasive species present. To take the first steps in understanding the dynamics of invasive plant competition, we conducted a greenhouse study in which we grew two species in competition that regularly occur together in northwest Indiana: garlic mustard [Alliaria petiolata (Bieb.) Cavara & Grande] and damesrocket (Hesperis matronalis L.). These two species are closely related (both are in the Brassicaceae [mustard] family) and share similar life histories (both are biennials). These species were grown both alone and together in a wide range of densities to determine which was the better competitor. Final aboveground biomass, final leaf number, height growth rate, and mortality were all monitored. At the end of the study, it was determined that when garlic mustard was grown in mixture with damesrocket, it had a negative effect on the aboveground biomass, final leaf number, and height growth rate of damesrocket. Garlic mustard, however, had greater mortality when grown in mixture with damesrocket because of intraspecific competition. Thus, based on this study, it is unclear which species would become dominant in areas where they coexist. Other factors, such as availability of light and moisture as well as differences in phenology, are most likely important in the final outcome and need to be researched further in field studies. The ability of damesrocket to survive in competition with garlic mustard indicates that despite the domination by garlic mustard in the northeastern U.S. landscape, damesrocket can compete with this species, at least in the context presented in the greenhouse study. Thus, it is possible that in certain scenarios, damesrocket may prove to be a threat similar to garlic mustard.