Mechanisms of competitive effects of an invasive grass on native vegetation in a restored upland Mississippi forest

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Griffin L. Williams and J. Stephen Brewer, Department of Biology, University of Mississippi, University, MS

Background/Question/Methods: Japanese stiltgrass (Microstegium vimineum) is a highly problematic, non-native invasive grass in the eastern United States. Native vegetation responds negatively to Microstegium by a reduction in native plant species richness, but no studies specifically partitioned its’ competitive effects into their above- and belowground effects. While the grass is competitively dominant over native plants, the specific mechanism of this competition is not known. We studied the aboveground, belowground, and interactive effects of Microstegium on the growth, survival, and reproduction of two native plant species previously shown to respond positively to the restoration of fire and open canopies following a prolonged period of fire exclusion in an oak forest. Data were collected from open-oak woodland restoration plots at the Strawberry Plains Audubon Center throughout the 2020 growing season. We used a randomized 2x2x2 factorial arrangement of aboveground vegetation reduction (pinning back) x soil origin (Microstegium soil v. native soil) x native plant species (either the Common cinquefoil (Potentilla simplex) or the regionally endemic Ozark sunflower (Helianthus silphioides)) at two field sites to elucidate the mechanism(s) of the competitive effects of Microstegium on these native species.
Results/Conclusions: Results revealed a weak negative effect of shade (aboveground competition) on the survival of both species. There was no effect of soil origin (the belowground effect) on transplant survival. The interaction of above- and belowground competition was not significant; indicating that any negative effects of shade were not amplified by Microstegium soil. Helianthus had significantly higher survival than Potentilla across both sites. When treating site as a fixed factor, the lower survival of Potentilla was especially pronounced at site 2 (the more mesic, and therefore more shaded location). The trend towards a negative effect of shade from Microstegium on survival suggests that running a longer experiment and calculating growth rates may eventually allow us to detect an aboveground, shading effect of Microstegium on native vegetation. From these preliminary findings, we can conclude that potential allelopathic compounds in the rhizosphere soil of Microstegium did not affect the survival of the two native species studied here. We plan to monitor spring 2021 emergence, survival, and the transplant growth rate through the 2021 growing season.