Multiple climate change factors jointly increase the competitiveness of a C4 grass against its C3 competitor

C3 and C4 plants are the two major terrestrial plant groups that respond differently to temperature, water, and CO2. Although much research has focused on the individual effect of warming, water availability, and elevated CO2 on biomass production and competitive interactions between C3 and C4 plants, the joint effects of these factors remain underexplored. We grew naturally co-occurring C3 (Oplismenus composites) and C4 (Paspalum conjugatum) grass under three temperature warming scenarios (control, +2{degrees}C, and +4{degrees}C), two water supply intervals (normal 2 days vs. prolonged 7 days [drought]), and two CO2 concentrations (ambient 400 ppm vs. elevated 800 ppm) in growth chambers and measured their above- and below-ground dry biomass in monoculture and mixture to quantify their biomass performances and competitive responses. Warming and elevated CO2 together enhanced the above- and/or below-ground biomass of both C3 and C4 grass. Moreover, temperature, water, and CO2 interacted to increase the below-ground biomass of the C4 grass. Surprisingly, the C3 grass performed worse under interspecific competition (mixture biomass) relative to intraspecific competition (monoculture biomass) at elevated CO2 compared to ambient CO2, while the reverse is true for the C4 grass. Furthermore, the C4 grass was most competitive against the C3 grass under simultaneous 4{degrees}C warming, drought, and elevated CO2. Taken together, our results suggest that the competitive balance could potentially shift in favor of C4 plants under the projected temperature warming, increased drought frequencies, and rising atmospheric CO2, with an increase in C4 relative abundance in future plant communities.