Competitive interactions of Microstegium vimineum on two native forest herbs

Background/Question/Methods

Multiple factors contribute to a successful biotic invasion including:  specific traits of the invading species, propagule pressure, and invasibility of the community. Here, we conducted a field experiment to examine the competitive ability of Microstegium vimineum (an annual C₄ plant) on two native perennial, rhizomatous, C₃ species; Polygonum virginianum or Boehmeria cylindrica. Twenty paired-plots were established across four sites based on the presence of Microstegium. Each plot pair consisted of a focal plant (either Polygonum virginianum (3 sites) or Boehmeria cylindrica (1 site) with or without Microstegium. Each month of the growing season, plots were cleared to ensure only Microstegium and the focal plant (treatment) or only the focal plant (control) were present. We predicted that each focal species would respond differently to Microstegium and that each focal would grow more in the absence of Microstegium. For each focal plant, we examined the lnRR (response ratio) to determine whether competition or facilitation was occurring using height as the productivity measure. We also assessed differences in leaf number and flower production for control vs. treatment plots for each species using t-tests.

Results/Conclusions

There was a significant difference in lnRR values across sites for Polygonum, indicating that one site showed a facilitative response (lnRR= -0.095) when grown with Microstegium; whereas all Boehmeria plots showed a competitive response (lnRR= 0.055) in the presence of Microstegium. However, the site that showed a facilitative response for Polygonum was very similar in attributes to the other two sites; further data need to be examined to determine the factors contributing to this relationship. These findings were corroborated by individual two-sample t-tests for each focal plant. There were no differences in control vs. treatment plots for species height (p = 0.658 for Polygonum; p = 0.609 for Boehmeria, respectively) or number of leaves (p = 0.430 for Polygonum; p = 0.173 for Boehmeria, respectively). Furthermore, there was no significant difference for flower production for either species growing with or without Microstegium (p = 0.698 for Polygonum; p = 0.881 for Boehmeria, respectively). Our results indicate that Microstegium may be utilizing unused resources not assessed in this study. Further work will examine nutrient concentrations, light levels, and soil moisture in control and treatment plots to assess how Microstegium utilizes resources differently.