Multiple climate change factors jointly increase the competitiveness of a C ₄ grass against its C ₃ competitor

C 3 and C 4 plants are the two major terrestrial plant groups that respond differently to temperature, water, and CO 2 . Although much research has focused on the individual effect of warming, water availability, and elevated CO 2 on biomass production and competitive interactions between C 3 and C 4 plants, the joint effects of these factors remain underexplored. We grew naturally co-occurring C 3 ( Oplismenus composites ) and C 4 ( Paspalum conjugatum ) grass under three temperature warming scenarios (control, +2°C, and +4°C), two water supply intervals (normal 2 days vs. prolonged 7 days [drought]), and two CO 2 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 CO 2 together enhanced the above- and/or below-ground biomass of both C 3 and C 4 grass. Moreover, temperature, water, and CO 2 interacted to increase the below-ground biomass of the C 4 grass. Surprisingly, the C 3 grass performed worse under interspecific competition (mixture biomass) relative to intraspecific competition (monoculture biomass) at elevated CO 2 compared to ambient CO 2 , while the reverse is true for the C 4 grass. Furthermore, the C 4 grass was most competitive against the C 3 grass under simultaneous 4°C warming, drought, and elevated CO 2 . Taken together, our results suggest that the competitive balance could potentially shift in favor of C 4 plants under the projected temperature warming, increased drought frequencies, and rising atmospheric CO 2 , with an increase in C 4 relative abundance in future plant communities.