Direct and Indirect Effects of Conditioned Soils and Tissue Extracts of the Invasive Shrub, Lonicera Maackii, on Target Plant Performance

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Lonicera maackii is an invasive Asian shrub naturalized in North America that has negative effects on forest understory vegetation. Aqueous extracts of leaves and roots of this plant have been shown to inhibit germination of several species. In this study, we examined the extent to which field-collected soils conditioned by L. maackii growth, with and without additions of L. maackii extracts, had effects on growth, morphology, and reproduction of Arabidopsis thaliana in a greenhouse setting. We also examined the effects of nutrient addition to the same soils on the performance of A. thaliana, whether plant responses to nutrients varied among L. maackii-conditioned and -unconditioned soils, and whether the effects of L. maackii extracts varied in soils with different histories and fertility. Plants grown in forest soil collected from a site where L. maackii was present grew slower and flowered later than plants grown in an unconditioned soil, but ended up being larger, with more leaves, branches and a higher total seed output. Nutrient addition had a positive effect on performance of A. thaliana, but when nutrients were added with extracts of L. maackii roots and leaves, the positive effects of nutrients were greatly diminished. Inhibiting plant responses to resource opportunities is a potentially important indirect effect of allelochemicals. Where direct effects of extracts on growth were seen, leaf extracts had a greater effect than root extracts. While the most important impacts of L. maackii in the field are undoubtedly due to competition for light and moisture, evidence for allelopathic effects of this plant is growing.



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