Plants with Crassulacean Acid Metabolism (CAM) outcompete C4 grasses under CO2 enrichment and drought

Background/Question/Methods

Plants with Crassulacean Acid Metabolism (CAM) are increasing in drylands worldwide but the underlying drivers remain unknown. Studies have evaluated the response of C₃ or C₄ plants to increased atmospheric CO₂ concentrations. In general, an increase in CO₂ increases intercellular CO₂ concentration and suppresses oxygenase activity of RuBisCo, thereby improving C₃ photosynthesis and productivity. In contrast, C₄ species are more readily saturated as CO₂ increases because phosphoenolpyruvate carboxylase in C₄ plants is insensitive to the CO₂ to O₂ ratio relative to RuBisCo in C₃ plants. An empirical evaluation of competition between CAM and C₄ plants under increased atmospheric CO₂, however, is still missing. Moreover, the competitive relationship between CAM and C₄ plants in response to increased atmospheric CO₂ also depends on rainfall. CAM plants have an advantage in dry environments because they feature water storage and are better adapted to drought.  In this study, we investigated the competitive relationships between seedlings of Cylindropuntia imbricata (CAM) and Bouteloua eriopoda (C₄ grass), which coexist in semiarid ecosystems across the Southwestern United States.  To this end, we implemented a set of greenhouse experiments at the Duke Phytotron where seedlings of C. imbricata and B. eriopoda in both monoculture and multispecies associations were subjected to two CO₂ concentration gradients under drought and well-watered conditions. Measurements of gas exchange, concentrations of titratable acid, carbon isotopes, and plant biomass and productivity were conducted to evaluate plants’ response to these treatments.

Results/Conclusions

Our experiments under altered CO₂ and water conditions show that C. imbricata positively responded to CO₂ enrichment under drought conditions, while B. eriopoda died from water stress.  Conversely, in well-watered conditions B. eriopoda had a strong competitive advantage on C. imbricata, thereby reducing the photosynthetic rate and biomass (per individual) of C. imbricata grown in mixture with grasses, when compared to C. imbricata alone. These results suggest that under drought and elevated CO₂ concentrations the dominance of CAM plants is likely to continue increase in semiarid ecosystems. More interestingly, we found that although CO₂ enrichment increased the photosynthesis and biomass of both C. imbricata and B. eriopoda when grown alone, in mixture CO₂ enrichment benefited C. imbricata more than B. eriopoda and thus indirectly disfavored B. eriopoda under drought conditions. This suggests that interspecific competition between CAM plants and C₄ grasses may counteract the favorable direct effect of CO₂ enrichment and drought on plant community structure and function in arid C₄-dominated grasslands.