Abstract
We test whether the invasive earthworm Lumbricus terrestris and leaf litter of the invasive herbaceous plant Alliaria petiolata interact to influence the native plant, Podophyllum peltatum, using both observational field data and a multi-year experiment. We hypothesized invader interactive effects on the native plant might result from either changes in allelochemical distribution in the soil or nutrient availability mediated by the invasive earthworm pulling leaf litter down into the soil. Within the field data we found that Alliaria petiolata presence and higher soil nitrogen correlated with reduced Podophyllum peltatum cover, and no evidence for an invader-invader interaction. Within the factorial experiment, we found a super-additive effect of the two invaders on plant biomass only when activated carbon was present. In the absence of activated carbon, there were no differences in Podophyllum peltatum biomass across treatments. In the presence of activated carbon, Podophyllum peltatum biomass was significantly reduced by the presence of both Lumbricus terrestris and Alliaria petiolata leaf litter. The absence of an effect of Alliaria petiolata leaves without activated carbon, combined with a failure to detect arbuscular mycorrhizal colonization, suggests that indirect effects of allelochemicals on arbuscular mycorrhizal fungi were not the primary driver of treatment responses. Rather direct nutrient availability might influence a potential interaction between these invaders. Leaf nitrogen content was higher and leaf CO(2) concentration was lower in the presence of Lumbricus terrestris, but treatment did not influence maximum photosynthetic rate. While the field data do not suggest a negative interaction between these invaders, the experiment suggests that such an interaction is possible with greater environmental stress, such as increasing nitrogen deposition. Further, even plants with rapid physiological responses to increased nitrogen availability may have other physiological limits on growth that prevent them from compensating from the harm caused by multiple invaders.