Competitive interactions between native and invasive exotic plant species are altered under elevated carbon dioxide

We hypothesized that the greater competitive ability of invasive exotic plants relative to native plants would increase under elevated CO2 because they typically have traits that confer the ability for fast growth when resources are not limiting and thus are likely to be more responsive to elevated CO2. A series of competition experiments under ambient and elevated CO2 glasshouse conditions were conducted to determine an index of relative competition intensity for 14 native-invasive exotic species-pairs. Traits including specific leaf area, leaf mass ratio, leaf area ratio, relative growth rate, net assimilation rate and root weight ratio were measured. Competitive rankings within species-pairs were not affected by CO2 concentration: invasive exotic species were more competitive in 9 of the 14 species-pairs and native species were more competitive in the remaining 5 species-pairs, regardless of CO2 concentration. However, there was a significant interaction between plant type and CO2 treatment due to reduced competitive response of native species under elevated compared with ambient CO2 conditions. Native species had significantly lower specific leaf area and leaf area ratio under elevated compared with ambient CO2. We also compared traits of more-competitive with less-competitive species, regardless of plant type, under both CO2 treatments. More-competitive species had smaller leaf weight ratio and leaf area ratio, and larger relative growth rate and net assimilation rate under both ambient and elevated CO2 conditions. These results suggest that growth and allocation traits can be useful predictors of the outcome of competitive interactions under both ambient and elevated CO2 conditions. Under predicted future atmospheric CO2 conditions, competitive rankings among species may not change substantially, but the relative success of invasive exotic species may be increased. Thus, under future atmospheric CO2 conditions, the ecological and economic impact of some invasive exotic plants may be even greater than under current conditions.