Comparison of the effect of elevated CO2 on an invasive species (Centaurea solstitialis) in monoculture and community settings

The ongoing increase in atmospheric CO2 concentration ([CO2]) is likely to change the species composition of plant communities. To investigate whether growth of a highly invasive plant species, Centaurea solstitialis (yellow starthistle), was affected by elevated [CO2], and whether the success of this species would increase under CO2 enrichment, I grew the species in serpentine soil microcosms, both as a monoculture and as a component of a grassland community. Centaurea grown in monoculture responded strongly to [CO2] enrichment of 350 mumol mol(-)1, increasing aboveground biomass production by 70%, inflorescence production by 74%, and midday photosynthesis by an average of 132%. When grown in competition with common serpentine grassland species, Centaurea responded to CO2 enrichment with similar but nonsignificant increases (+69% aboveground biomass, +71% inflorescence production), while total aboveground biomass of the polyculture increased by 28%. Centaurea's positive CO2 response in monoculture and parallel (but non-significant) response in polyculture provoke questions about possible consequences of increasing CO2 for more typical California grasslands, where the invader already causes major problems.