The direct competitive effects of exotic plants on natives are among the leading causes of plant extinctions worldwide.  Allelopathy, one type of direct plant competition, has received relatively little research, particularly in aquatic and wetland systems, even though allelopathy can be a potent mechanism through which plant communities are structured.

Typha angustifolia (narrow-leaved cattail) is an invasive exotic plant in North America that often forms monocultures in disturbed wetlands and is more invasive than native members of its genus.  We tested whether T. angustifolia was allelopathic and whether it produced different biochemicals than a native congener by growing it with the native bulrush Bolboschoenus fluviatilis (river bulrush) in soil with and without activated carbon and by qualitatively and quantitatively comparing soluble phenolics produced in the roots of T. angustifolia and the native Typha latifolia (broad-leaved cattail).

Results/Conclusions

Typha angustifolia had a strong allelopathic effect on B. fluviatilis, reducing the mean biomass of B. fluviatilis by 84%.  When the allelopathy of T. angustifolia was ameliorated by the activated carbon, however, mean biomass of T. angustifolia was reduced by almost half due to resource competition with B. fluviatilis.  Furthermore, T. angustifolia produced different, but not more, soluble phenolics than T. latifolia suggesting that the identity of the phenolics is different between the two species rather than the concentrations.  The allelopathy of T. angustifolia and its production of different root biochemicals than native congeneric species provide experimental evidence that T. angustifolia may use a novel allelochemical in its invasion of North American wetlands.  Our results also contribute to the understanding of the relative importance of allelopathy and resource competition in plant-plant interactions, and the growing literature of allelopathy in wetland plant communities.