A multi-assay comparison of seed germination inhibition by Lonicera maackii and co-occurring native shrubs

Recent work suggests that germination inhibition via allelopathy may be an important component of some species' invasion ecology. We conducted four germination assays to test the inhibitory potential of Amur honeysuckle (Lonicera maackii), a problematic invasive shrub. The subject species in the first assay, Festuca arundinacea, exhibited a significant delay in germination when treated with extract from ground honeysuckle foliage. In the second assay, two concentrations of L. maackii extract were created by soaking foliage. F. arundinacea germination was not significantly influenced by the treatments while germination in Impatiens wallerana was substantially decreased. The third assay compared the impact of foliar extract from honeysuckle, with the native shrubs Lindera benzoin and Asimina triloba using F. arundinaceae, I. wallerana, Coreopsis lanceolata and Poa pratensis as subject species. In this assay, I. wallerana was strongly inhibited by L. maackii foliage; however, none of the other species were significantly influenced by the treatments. In the fourth assay, fruit extracts from L. maackii and L. benzoin were applied to the same four subject species. L. maackii fruit extract had an inhibitory influence on seed germination of all subject species, and inhibition from L. benzoin was also noted. Across all assays, there was a mixed reaction to extract from L. maackii and co-occurring native species that was species-specific and dependent upon the extract source. Our findings provide support for the idea that L. maackii has allelopathic activity, but further work is needed to (i) understand how broad the impact may be across the wide variety of species that are found in its invasion range and (ii) substantiate that the allelopathic effect has relevance in field environments.