Microstegium vimineum is an aggressive annual grass that is invading bottomland forests and disturbed areas in the eastern U.S. Invasions can reduce native tree regeneration and cause significant reductions in native herbaceous species diversity and abundance. Here we specifically investigate the effects of invasion on the native woodland herb Senna hebecarpa (Caesalpiniaceae). We used an experimental setup consisting of 32 5.25 m x 5.25 m plots that was established in the fall of 2005. All plots were seeded with 12 native herbaceous species including approximately 193 seeds of Senna per plot. Half of the plots were then randomly selected to be seeded with Microstegium. In the summer of 2008, we quantified the number and size of adult Senna plants, the proportion of plants that were reproductive, and the number of seeds produced per plant. The field experiment was followed up with greenhouse experiments to determine if changes in soil properties contributed to the decline of Senna and the success of Microstegium.

Results/Conclusions

We found nearly four times more Senna plants in un-invaded (mean ± SE , 21.3 ± 3.4) than in invaded (5.6 ± 0.94) plots.  Senna was significantly taller and weighed more in the un-invaded plots (129.1 ± 5.3cm and 21.3 ± 3.8g) than in the invaded plots (102.9 ± 4.6 cm and 7.6 ± 1.1g).  In addition, a greater proportion of Senna plants in un-invaded plots produced seed and seed production was over 4 times greater in un-invaded than invaded plots. In the greenhouse experiment the biomass of Senna was not significantly greater in soil from invaded (105 ± 12.8 mg) than invaded (90 ± 8.1mg) plots.  Microstegium biomass was also similar in invaded (203 ± 29mg) and un-invaded (204 ± 29 mg) soils. Our field experiment shows that Microstegium significantly reduces the abundance, size, and reproduction of a notable native woodland species. However, we found no evidence that changes in soil properties are the mechanism underlying the effects of Microstegium on Senna.