Invasive plants may impact native species through novel weapons, such as allelochemicals, either directly or indirectly. We tested the Novel Weapons Hypothesis by examining if invasive Lonicera japonica (Japanese honeysuckle) represents a greater allelopathic threat than a native vine, Lonicera sempervirens (trumpet honeysuckle), to a native grass species, Elymus hystrix (bottlebrush grass). We compared the effects of invasive L. japonica and native L. sempervirens on the performance of E. hystrix in sterilized and unsterilized soils, using varying levels of impact. We also determined the effects of leaf extracts of varying concentrations of L. japonica and L. sempervirens on germination of E. hystrix.
Native L. sempervirens exhibited greater effects than L. japonica on percent germination of E. hystrix, with greater effects at increasing concentrations of both extracts overall. For growth, as measured by total biomass, plants grown with L. japonica were larger compared to plants grown with L. sempervirens. Plants growing in sterilized soil were significantly larger and had a larger root to shoot ratio compared to plants growing in unsterilized soil. There was a significant interaction between species and soil sterilization. Plants grown with L. japonica were similar in biomass between the two sterilization treatments, while plants grown with L. sempervirens decreased from the sterilized to the unsterilized treatments. Our experiments therefore are not consistent with the Novel Weapons Hypothesis for L. japonica allelopathy and show evidence of direct and indirect effects of L. sempervirens on germination and growth of E. hystrix.