Interspecific competition negates CO2 benefits for most plant species.

Elevated CO2 (eCO2) frequently stimulates plant growth and is widely assumed to enhance productivity in multispecies communities1,2, yet species responses to eCO2 vary widely with biotic and abiotic context1,3,4. Whether eCO2 relaxes competition or amplifies existing competitive hierarchies, shaping how growth benefits are distributed among species, remains poorly resolved. Here we synthesize data from 19 CO2 enrichment experiments (glasshouse, chamber and field studies), encompassing 97 plant species from grass-dominated ecosystems to quantify how competitive context alters species- and mixture-level responses. Although most species respond positively to eCO2 without neighbours, over half show neutral or negative responses in mixtures. Despite this, dominant species often drive modest increases in total mixture biomass under eCO2. Trait analyses indicate that acquisitive strategies, rather than intrinsic CO2 sensitivity alone, predict success under competition. Together, our results indicate that under competition, eCO2 redistributes growth benefits unevenly among species, favouring dominant species while many subdominants experience neutral or negative responses.