Synergistic effects of warming and elevated CO2 intensify drought impacts on grassland carbon and water fluxes.

Understanding how rising carbon dioxide (CO2), 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 CO2 (eCO2), warming, and drought on ecosystem functional properties, including water vapor and CO2 fluxes, and water- and carbon-use efficiency (WUE and CUE, respectively). While eCO2 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 CO2 uptake. However, combined eCO2 and warming worsened the negative drought effects on CO2 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.