Impacts of invasive plant species on riparian plant assemblages: interactions with elevated atmospheric carbon dioxide and nitrogen deposition.

Resource competition is commonly invoked to explain negative effects of invasive plants on native plant abundance. If invasives out-compete natives, global changes that elevate resource availability may interact with invasives to exacerbate impacts on native communities. Indeed, evidence is accumulating that elevated CO(2) and N deposition decrease native biomass and simultaneously increase invasive biomass. However, superior competitive ability, and a relative increase in the magnitude of invasive impacts under elevated resource availability, remain to be definitively proven. Using model, multi-species, multi-individual riparian plant communities, where planting density was maintained by replacement of native with exotic individuals, we conducted a greenhouse, competition experiment using native (to the UK) and invaded communities exposed to ambient and elevated CO(2) (CO(2) experiment) or N availability (N experiment). We tested two hypotheses: (1) invasives are superior competitors to natives at ambient atmospheric CO(2) and N deposition; (2) negative effects of invasives on natives are exacerbated under elevated CO(2) or N availability. Our results provide some support for the first hypothesis: in the CO(2) experiment native biomass was significantly lower in invaded communities. In the N experiment, native biomass was unaffected by the presence of exotics but other characteristics (e.g. root:shoot ratios) were altered. Differences in light availability between the experiments may have modified the effects of the invasives on the native assemblages but our design did not permit us to determine this definitively. The hypothesis that elevated CO(2) and N availability benefit invasives at the expense of natives was not supported by our results. This may be explained either because the invasives showed minor responses to the resource manipulations or because native and exotic species were differentially limited by CO(2) and N. Our results confirm the expectation that invasives alter the characteristics of native assemblages but lead us to question whether elevated resource availability will magnify these effects.