Global atmospheric CO₂ concentrations and N deposition are increasing, but the consequences of these changes on species interactions remain poorly understood. It is generally hypothesized that increased atmospheric CO2 and N deposition will increase the incidence and abundance of plant invaders. We set out to determine how elevated CO₂ and N affect the establishment of invading plant species in prairies with varying levels of species and functional diversity. Our research was conducted in central Minnesota in the 10^th year of a long-term experiment examining the effects of elevated CO₂ and N enrichment on biodiversity (BioCON). Plots differed in levels of CO₂ and N (ambient and elevated), species diversity (1 to 16 species), and functional diversity (1 to 4 functional groups: N-fixing forbs, non-leguminous forbs, C3 grasses and C4 grasses). Plots were weeded several times each summer to maintain the original species composition. Weeds were divided into two species pools: those that were part of the BioCON study and therefore abundant in nearby plots (“BioCON weeds”) and those from the landscape outside the BioCON study (“general weeds”).  We collected weed species richness and abundance data on both weed classes in 251 plots (1 and 4 species plots) during the summer of 2007.

Results/Conclusions

In contrast to our hypotheses, elevated CO₂ and N did not affect the species richness or relative abundance of general weeds, but did have a significant positive effect on both the richness and abundance of the BioCON weeds.  Plot species diversity affected both general and BioCON weeds; less diverse plots had significantly higher weed richness and abundance than diverse plots.  When more than two functional groups were present in a plot, the richness and abundance of general weeds was reduced.  Neither the number nor type of functional groups in a plot had an effect on BioCON weed richness or abundance.  The presence of N-fixing forbs, however, significantly increased general weed richness and abundance in both 1- and 4-species plots.  These results suggest that different aspects of global environmental change are likely to influence invasion processes, but these effects may depend on the presence of functional groups in the invaded plant community.