Synergistic effects of warming and elevated CO ₂ intensify drought impacts on grassland carbon and water fluxes

Understanding how rising carbon dioxide (CO ₂ ), climate warming, and drought interact to alter ecosystem functioning is critical for predicting future ecosystem resilience and land-atmosphere feedbacks. Using a long-term grassland experiment, we tested the individual and combined effects of elevated CO ₂ (eCO ₂ ), warming, and drought on ecosystem functional properties, including water vapor and CO ₂ fluxes, and water- and carbon-use efficiency (WUE and CUE, respectively). While eCO ₂ and warming had limited effects individually, their interaction synergistically amplified ecosystem respiration and drought impacts on most properties. Under ambient rainfall, this interaction did not reduce net ecosystem productivity (NEP), as higher respiration was counterbalanced by periodic increases in CO ₂ uptake. However, combined eCO ₂ and warming worsened the negative drought effects on CO ₂ fluxes and WUE and on postdrought evapotranspiration and CUE. This produced the lowest seasonal NEP, involving a fourfold stronger decline than under ambient drought. Our findings highlight that future climate conditions may decrease the capacity of drought-exposed ecosystems for water use and net carbon uptake.