Effects of elevated CO2 on competiton between native and invasive grasses

We tested competitive outcomes for four native and four invasive perennial C3 and C4 grasses under ambient (390 ppm) and elevated (700 or 1000 ppm) CO2 concentrations in a closed-chamber greenhouse experiment with non-limiting water and nutrients, predicting that elevated CO2 would increase the competitive suppression of native grasses by invasive grasses. To test this, we determined the relative interaction intensity of biomass allocation for natives grown alone vs. those grown in native-invasive species pairs and measured photosynthetic traits that contribute to plant invasiveness and may be affected by elevated CO2 concentrations. We found no effect of CO2 for plant production measures of interaction intensity or for relative performance for most of the measured photosynthetic traits. In competition, invaders nearly always produced more biomass and tillers than natives, regardless of CO2 level. The results suggest that increasing CO2 concentration alone has little effect on grass competitive outcomes under controlled conditions.