Ozone deposition degrades water use efficiency across multiple ecosystems

Atmosphere-biosphere exchange plays a key role in the global cycles of water and carbon. Air pollution can alter these processes and induce climate perturbations and feedbacks. Surface ozone (O3) is an air pollutant and greenhouse gas that is toxic to plants, reducing their growth and ability to regulate water loss. Past controlled experiments have shown that O 3 degrades a plant's water-use efficiency (WUE), which is the ratio of carbon uptake in photosynthesis to water loss in transpiration. This has potentially significant implications for terrestrial water cycle and precipitation, but no studies have evaluated the O3 effect on WUE in complete ecosystems. We aim to quantify the impact of O3 on WUE across a wide array of ecosystems.;Meteorological and biological data was obtained from 23 FLUXNET flux tower sites, which use the eddy covariance method to derive hourly fluxes of carbon dioxide (CO2), water vapor (H2O), and O 3 between the atmosphere and ecosystem. Across a broad range of sites, we find a significant negative relationship between daily anomalies of stomatal O3 flux (FS, O3) and WUE that explains 1-3% of WUE variability. The largest impacts occur in locations and species with high stomatal conductance, such as broadleaf forests, humid climates, or irrigated crops, rather than where surface O3 concentrations are highest. Past long-term studies have also found similar O3 impacts (1-3%) on WUE, indicating a consistent response across a pool species and ecosystems.