Plant invasion impacts on fungal community structure and function depend on soil warming and nitrogen enrichment.

The impacts of invasive species on biodiversity may be mitigated or exacerbated by abiotic environmental changes. Invasive plants can restructure soil fungal communities with important implications for native biodiversity and nutrient cycling, yet fungal responses to invasion may depend on numerous anthropogenic stressors. In this study, we experimentally invaded a long-term soil warming and simulated nitrogen deposition experiment with the widespread invasive plant Alliaria petiolata (garlic mustard) and tested the responses of soil fungal communities to invasion, abiotic factors, and their interaction. We focused on the phytotoxic garlic mustard because it suppresses native mycorrhizae across forests of North America. We found that invasion in combination with warming, but not under ambient conditions or elevated nitrogen, significantly reduced soil fungal biomass and ectomycorrhizal relative abundances and increased relative abundances of general soil saprotrophs and fungal genes encoding for hydrolytic enzymes. These results suggest that warming potentially exacerbates fungal responses to plant invasion. Soils collected from uninvaded and invaded plots across eight forests spanning a 4 °C temperature gradient further demonstrated that the magnitude of fungal responses to invasion was positively correlated with mean annual temperature. Our study is one of the first empirical tests to show that the impacts of invasion on fungal communities depends on additional anthropogenic pressures and were greater in concert with warming than under elevated nitrogen or ambient conditions.

CO(2) enrichment reduces reproductive dominance in competing stands of Ambrosia artemisiifolia (common ragweed).

Plants growing in dense stands may not equally acquire or utilize extra carbon gained in elevated CO(2). As a result, reproductive differences between dominant and subordinate plants may be altered under rising CO(2) conditions. We hypothesized that elevated CO(2) would enhance the reproductive allocation of shaded, subordinate Ambrosia artemisiifolia L. (Asteraceae) individuals more than that of light-saturated dominants. We grew stands of A. artemisiifolia at either 360 or 720 muL L(-1) CO(2) levels and measured the growth and reproductive responses of competing individuals. To test whether elevated CO(2) altered size and reproductive inequalities within stands, we compared stand-level coefficients of variation (CV) in height growth and final shoot, root, and reproductive organ biomasses. Elevated CO(2) enhanced biomass and reduced the CV for all aspects of plant growth, especially reproductive biomass. Allocation to reproduction was higher in the elevated CO(2) than in the ambient treatment, and this difference was more pronounced in small, rather than large plant positive relationships between the CV and total stand productivity declined under elevated CO(2), indicating that growth enhancements to smaller plants diminished the relative biomass advantages of larger plants in increasingly crowded conditions. We conclude that elevated CO(2) stimulates stand-level reproduction while CO(2)-induced growth gains of subordinate A. artemisiifolia plants minimize differences in the reproductive output of small and large plants. Thus, more individuals are likely to produce greater amounts of seeds and pollen in future populations of this allergenic weed.