Variation for phenotypic plasticity among populations of an invasive exotic grass

Phenotypic plasticity is a common feature of plant invaders, but little is known about variation in plasticity among invading populations. Variation in plasticity of ecologically important traits could facilitate the evolution of greater plasticity and invasiveness. We examined plasticity among invasive populations of Microstegium vimineum (Japanese stiltgrass), a widespread and often dominant grass of forests in the eastern U.S. with two separate experiments. First, we exposed seven Microstegium populations to a drought treatment in growth chambers and monitored growth and physiological responses. Then, we established a greenhouse experiment using a subset of the populations; two that exhibited the most divergent responses and one intermediate population. In the greenhouse, we manipulated drought and shade and evaluated biomass production and specific leaf area (SLA). Microstegium exhibited plasticity for biomass production and SLA in the greenhouse experiment, and populations significantly varied in the degree of plasticity under drought and shade treatments. Two populations significantly increased biomass production under favorable conditions, unlike the third population. The most productive populations also responded to shade stress via greater SLA, possibly allowing for greater utilization of available light, while the third population did not. These results show that Microstegium can exhibit plastic responses to environmental conditions. Moreover, variation for plasticity among populations provides the potential for further evolution of plasticity. Future studies should focus on the relative importance of plasticity for the success of Microstegium and other plant invaders and evaluate post-introduction evolution of plasticity.