The problem: Eastern red cedar Juniperus virginiana is a native-invasive tree that is rapidly spreading across the Midwestern United States and has encroached as far as South Dakota and Nebraska. It is a serious problem in rangelands because cattle and sheep do not eat woody plants. We investigated the role of allelopathy, the chemical inhibition of one plant by another through the release of chemical suppressants or growth inhibitors into the soil. We wished to establish whether these plants were able to suppress the germination and growth of a dominant grass, big bluestem Andropogon gerardii.

Methods: We applied root exudate of J. virginiana to seeds of A. gerardii in petri dishes and applied distilled water as a control. We also compared germination and growth rates of A. gerardii in the presence and absence of oaks as potential competitors with the eastern red cedar trees.

Results: There was significantly lower germination of A. gerardii grown in the root exudate than controls of this species (grown in distilled water). This result demonstrated that J. virginiana contained significant allelopathic chemicals. However, we found no significant difference in A. gerardii germination between oak vs. no oak for the root exudate. In a subsequent experiment, we found no significant difference in relative growth rates between those growing with and without oaks and controls. Thus, the main allelopathic effect of eastern red cedar is in suppressing germination of this grass species. Allelopathy may be an important reason for the successful dispersal of this native-invasive species.