Invasive species can interact with native species through multiple pathways, which can be mediated through different environmental conditions such as shade and moisture. To evaluate the effects of interference competition and disease transmission dynamics of an invasive grass on its own population and on native grasses, we conducted two greenhouse experiments using Microstegium vimineum litter collected from field sites with prior foliar fungal epidemics. In Experiment 1, we used differing amounts of infected litter and analyzed plant establishment rate, final biomass, and infection of Elymus virginicus and M. vimineum seedlings planted alone and in competition. In Experiment 2, we manipulated moisture content during the initial inoculation period of M. vimineum seedlings to observe changes in infection rate.
Results/Conclusions
M. vimineum litter inhibited the establishment of both E. virginicus and M. vimineum, but there was no significant effect of the species treatment or shade on establishment. Litter presence reduced E. virginicus establishment by 9%, and had a significant nonlinear effect on M. vimineum, reducing establishment by 5% (low), 12% (medium), and 28% (high). Presence of both live and dead M. vimineum significantly reduced E. virginicus biomass, with litter reducing biomass by 0.73 g, and live M. vimineum reducing biomass by 4.96 g. Shade reduced M. vimineum biomass by 22.14 g, E. virginicus presence reduced it by 4.62 g, and each gram of litter reduced it by 1.20 g. Live and dead M. vimineum increased the percentage of E. virginicus leaves with lesions (evidence of foliar fungal infection). Live M. vimineum increased the percentage of E. virginicus leaves with infection by 15%, dead M. vimineum increased E. virginicus infection by 10%, and the presence of both increased E. virginicus infection by 28%. M. vimineum developed very little infection, with only 2% of individuals displaying lesions across all treatments.
Results from experiment 2 demonstrate that moisture, and likely leaf wetness, plays a significant role in the formation of lesions on M. vimineum, with none of the dry treatment pots showing infection. However, 80% percent of hydrated pots had lesions present at the end of the experiment.
These results demonstrate the negative impacts of M. vimineum invasion on E. virginicus populations and indicate an increased competitive ability of M. vimineum infected with Bipolaris through litter from the previous generation. Additionally, our findings show the importance of moisture for M. vimineum to become infected, suggesting that wetter environments will have higher levels of infection.