Recent studies have found that allelopathy can be an important mechanism of plant invasions. Alliaria petiolata, Lonicera maackii, Ranunculus ficaria, Celastrus orbiculatus, and Microstegium vimineum are invasive species found in the Midwestern USA. We investigated the comparative direct and indirect allelopathic effects of these five species in a laboratory setting using leaf extracts in a germination experiment and a growth experiment. Results illustrate that the effect of each invasive species varied with target species and with life stage. Extracts of L. maackii and R. ficaria had the largest overall effects (∼50% reduction) on germination across both species, but effects of extracts varied by target species. Extracts of A. petiolata and C. orbiculatus had a greater effect on germination of E. hystrix than on C. fasciculata. Extracts of L. maackii, M. vimenum, and R. ficaria had larger inhibitory germination effects on C. fasciculata than on E. hystrix. For growth of E. hystrix after 8 wk, C. orbiculatus and L. maackii extracts had the largest (>80%) reduction of biomass, followed by treatment with extracts of R. ficaria (∼ 20% reduction). Treatment with L. maackii and C. orbiculatus extracts reduced height, with extracts of L. maackii having greater effects (60% versus 40% reduction, respectively). Plants treated with L. maackii extracts allocated significantly less biomass to roots. We also found evidence of indirect impacts. Mycorrhizal inoculation overall was most negatively affected by treatment with L. maackii extracts (∼70% reduction), even at low concentrations, and least by treatment with C. orbiculatus, with intermediate effects of A. petiolata, M. vimineum, and R. ficaria. Overall, L. maackii had the strongest effects, followed closely by C. orbiculatus, which was then followed in turn by R. ficaria. These results provide further support for the allelopathic potential of L. maackii and add to the growing body of evidence on the allelopathic potential of R. ficaria and C. orbiculatus. Effects of A. petiolata and M. vimineum were less strong than in other studies, suggesting the importance of variation between populations and in experimental venue.
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