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We conducted a meta-analysis of carbon and oxygen isotopes from tree ring chronologies representing 34 species across 10 biomes to better understand the environmental drivers and physiological mechanisms leading to historical changes in tree intrinsic water use efficiency (iWUE), or the ratio of net photosynthesis (A_net) to stomatal conductance (g_s), over the last century. We show a ∼40% increase in tree iWUE globally since 1901, coinciding with a ∼34% increase in atmospheric CO₂(C_a), although mean iWUE, and the rates of increase, varied across biomes and leaf and wood functional types. While C_awas a dominant environmental driver of iWUE, the effects of increasing C_awere modulated either positively or negatively by climate, including vapor pressure deficit (VPD), temperature, and precipitation, and by leaf and wood functional types. A dual carbon–oxygen isotope approach revealed that increases inA_netdominated the observed increased iWUE in ∼83% of examined cases, supporting recent reports of global increases inA_net, whereas reductions ing_soccurred in the remaining ∼17%. This meta-analysis provides a strong process-based framework for predicting changes in tree carbon gain and water loss across biomes and across wood and leaf functional types, and the interactions between C_aand other environmental factors have important implications for the coupled carbon–hydrologic cycles under future climate. Our results furthermore challenge the idea of widespread reductions ing_sas the major driver of increasing tree iWUE and will better inform Earth system models regarding the role of trees in the global carbon and water cycles.

Trees continuously regulate leaf physiology to acquire CO₂while simultaneously avoiding excessive water loss. The balance between these two processes, or water use efficiency (WUE), is fundamentally important to understanding changes in carbon uptake and transpiration from the leaf to the globe under environmental change. While increasing atmospheric CO₂(iCO₂) is known to increase tree intrinsic water use efficiency (iWUE), less clear are the additional impacts of climate and acidic air pollution and how they vary by tree species. Here, we couple annually resolved long‐term records of tree‐ring carbon isotope signatures with leaf physiological measurements ofQuercus rubra(Quru) andLiriodendron tulipifera(Litu) at four study locations spanning nearly 100 km in the eastern United States to reconstruct historical iWUE, net photosynthesis (A_net), and stomatal conductance to water (g_s) since 1940. We first show 16%–25% increases in tree iWUE since the mid‐20th century, primarily driven by iCO₂, but also document the individual and interactive effects of nitrogen (NO_x) and sulfur (SO₂) air pollution overwhelming climate. We find evidence forQuruleaf gas exchange being less tightly regulated thanLituthrough an analysis of isotope‐derived leaf internal CO₂(C_i), particularly in wetter, recent years. Modeled estimates of seasonally integratedA_netandg_srevealed a 43%–50% stimulation ofA_netwas responsible for increasing iWUE in both tree species throughout 79%–86% of the chronologies with reductions ing_sattributable to the remaining 14%–21%, building upon a growing body of literature documenting stimulatedA_netoverwhelming reductions ing_sas a primary mechanism of increasing iWUE of trees. Finally, our results underscore the importance of considering air pollution, which remains a major environmental issue in many areas of the world, alongside climate in the interpretation of leaf physiology derived from tree rings.