Phenotypic plasticity may be a common feature of non-native invasive plants and a potential mechanism that allows them to out-compete native species for limited resources in new environments. For example, under low light conditions species that exhibit plastic responses such as taller and wider leaves may be better able to obtain limited light resources. Furthermore, understanding the amount of variation in plasticity among invasive plant populations could help predict habitats susceptible to invasion and the potential for evolution of greater plasticity and invasiveness. We evaluated phenotypic plasticity among 12 Florida populations of the highly invasive Imperata cylindrica (cogongrass) as well as six native C4 perennial species commonly found in longleaf pine savannahs. Imperata populations representing distinct original introductions from Japan and the Philippines were collected from Pensacola to Naples, FL and evaluated in a field experiment with two light treatments, full sun and 60% shade. We sought to answer two specific questions: (1) Is there variation among Imperata populations in their plastic responses to shade; and (2) Are populations of Imperata more phenotypically plastic than co-occurring native species?
Results/Conclusions
Our results show that Imperata populations from different parts of the native range varied in their plastic response to shade. Furthermore, Imperata exhibited greater phenotypic plasticity than co-occurring native species for traits associated with foraging for limited light resources. Populations from peninsular Florida (Philippines lineage) exhibited a strong plastic response to shade where height was 69% greater in shade than sun. Conversely, populations from northwest Florida (Japan lineage) were less plastic, with average height only 17% greater in the shade. Differences among populations were also observed in biomass allocation to leaves (leaf mass ratio) and root:shoot ratios. In contrast to Imperata, native species were not plastic for traits such as leaf mass ratio or plant height. Native species, on average, experienced smaller changes in LMR (8% greater in shade) compared to Imperata (33% greater in shade). Native species, although plastic for specific leaf area, experienced higher costs of plasticity with greater herbivory and 58% lower root:shoot ratios than Imperata in the shade. Phenotypic plasticity may enhance the competitive ability of Imperata compared to native species across diverse light environments, while variation among populations suggests potential for evolution of greater plasticity and spread into novel habitats.