Investigating the novel weapons hypothesis from the perspective of a native understory plant, American ginseng (Panax quinquefolius L.)

Background/Question/Methods The novel weapons hypothesis (NWH) postulates the success of invasive plants could be attributed to the release of novel chemical compounds to which plants in the invasive range are naïve. We tested the predictions of NWH from the perspective of American ginseng, a long-lived North American native understory species. Ginseng has been observed growing near the invasive allelopathic tree-of-heaven (Ailanthus altissima), the native allelopathic tree black walnut (Juglans nigra), and the native nonallelopathic tree sugar maple (Acer saccharum). We hypothesized there would be little to no evidence for allelopathic effects of black walnut and sugar maple; however, allelochemicals from tree-of-heaven would negatively affect ginseng performance. Additionally, we hypothesized that the younger, earlier phase ginseng seeds and seedlings would be more negatively impacted by the allelochemicals released by tree-of-heaven than the older, later phase ginseng adult plants. Ginseng seeds, seedlings and adult plants were planted under individual canopies each tree species at two sites and observed for two growing seasons. Activated carbon was mixed into the soil for half of the ginseng and used to reduce allelopathic effects. Data were analyzed using a full factorial 2-way ANOVA with presence of carbon and tree species as main effects.

Results/Conclusions The effect of activated carbon addition on ginseng performance measures did not differ for plants growing under the three canopy tree species, suggesting no differential allelochemical effects. There was also no evidence suggesting seeds and seedlings were more negatively impacted by allelochemicals than the older, later phase adult plants. However, there was a significant main effect of canopy tree species on some ginseng seed, seedling, and adult plant performance variables. Surprisingly, plants grown under sugar maple canopies performed significantly worse than the plants around tree-of-heaven and black walnut, which was opposite from the anticipated pattern and also does not support the NWH. Previous research involving tree-of-heaven and black walnut strongly suggested inhibitory chemical compounds are present in tissues of both trees. However our results do not support NWH and instead indicate the net effect of growing under a tree-of-heaven canopy is not more negative than growing under native species canopies. Therefore, if tree-of-heaven is releasing novel chemical compounds, their effects are not a net detriment to this particular understory species. Despite this, the potential allelopathic effects of tree-of-heaven, clearly shown in bioassays, should not be discounted entirely as these effects could be species specific.