The effect of origin and resources on the competitive and allelopathic abilities of an exotic plant species, Alliaria petiolata

Background/Question/Methods
Exotic plant species may evolve changes in competitive or allelopathic abilities in response to the new species interactions and environmental conditions encountered in their invasive range. Genotypes that favor strong competitive or allelopathic abilities will only be favored if the exotic encounters strong interspecific competition or if native species are naïve to the exotic’s allelopathic chemicals. The relative importance of competition and allelopathy in invasions may depend on resource availability and it is hypothesized that allelopathy is more important when resources are low. I conducted a greenhouse experiment to test whether competitive and allelopathic ability differ in garlic mustard (Alliaria petiolata) from North America and Europe and whether nitrogen availability affects the importance of these mechanisms in invasions. Three North American forest forbs grew with Iowan and German garlic mustard in low and high nitrogen soils. Native species grew singly or in diallel competition in allelopathic soil, in competition with garlic mustard in soil with activated carbon, or in competition with garlic mustard in allelopathic soil. Relative growth rates and biomass of native seedlings grown in these three treatments were compared to that of seedlings grown singly and diallel competition in unaltered soils.
Results/Conclusions

Initial growth rates and size of native seedlings grown with garlic mustard were lower regardless of origin of garlic mustard. German garlic mustard grew faster than Iowan conspecifics suggesting that the German plants exerted negative effects on native seedlings more through competition. Growth rates of native seedlings grown in allelopathic soil from Iowan versus German garlic mustard did not differ significantly. Nitrogen significantly increased relative growth rates and size of all native and garlic mustard seedlings. However, growth rates of garlic mustard were higher suggesting that the species was better able to respond to increased nitrogen. Native seedling growth rates were greatly reduced in the competition and competition-allelopathy treatments when nitrogen was high, indicating that nitrogen increased the competitive ability of garlic mustard through increased biomass. The initial results lend support to the hypotheses that garlic mustard evolved changes in competitive and allelopathic abilities and the relative importance of these strategies in invasion depend on resource levels. Garlic mustard from the invasive range may be less competitive than European conspecifics but was still better at utilizing increased resources than native species. The relative importance of allelopathy increased with lower nitrogen, which may enable the species to invade any forest understory regardless of resource availability.