UC Berkeley Library

Plants with crassulacean acid metabolism (CAM) are increasing in distribution and abundance in drylands worldwide, but the underlying drivers remain unknown. We investigate the impacts of extreme drought and CO₂ enrichment on the competitive relationships between seedlings of Cylindropuntia imbricata (CAM species) and Bouteloua eriopoda (C₄ grass), which coexist in semiarid ecosystems across the Southwestern United States. Our experiments under altered water and CO₂ water conditions show that C. imbricata positively responded to CO₂ enrichment under extreme drought conditions, while B. eriopoda declined from drought stress and did not recover after the drought ended. Conversely, in well‐watered conditions B. eriopoda had a strong competitive advantage on C. imbricata such that the photosynthetic rate and biomass (per individual) of C. imbricata grown with B. eriopoda were lower relative to when growing alone. A meta‐analysis examining multiple plant families across global drylands shows a positive response of CAM photosynthesis and productivity to CO₂ enrichment. Collectively, our results suggest that under drought and elevated CO₂ concentrations, projected with climate change, the competitive advantage of plant functional groups may shift and the dominance of CAM plants may increase in semiarid ecosystems.