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  1. 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 (Anet) to stomatal conductance (gs), over the last century. We show a ∼40% increase in tree iWUE globally since 1901, coinciding with a ∼34% increase in atmospheric CO2(Ca), although mean iWUE, and the rates of increase, varied across biomes and leaf and wood functional types. While Cawas a dominant environmental driver of iWUE, the effects of increasing Cawere 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 inAnetdominated the observed increased iWUE in ∼83% of examined cases, supporting recent reports of global increases inAnet, whereas reductions ingsoccurred 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 Caand other environmental factors have important implications for the coupled carbon–hydrologic cycles under future climate. Our results furthermore challenge the idea of widespread reductions ingsas 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.

     
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  2. Abstract

    Carbon use efficiency (CUE) represents how efficient a plant is at translating carbon gains through gross primary productivity (GPP) into net primary productivity (NPP) after respiratory costs (Ra). CUE varies across space with climate and species composition, but how CUE will respond to climate change is largely unknown due to uncertainty inRaat novel high temperatures. We use a plant physiological model validated against global CUE observations and LIDAR vegetation canopy height data and find that model‐predicted decreases in CUE are diagnostic of transitions from forests to shrubland at dry range edges. Under future climate scenarios, we show mean growing season CUE increases in core forested areas, but forest extent decreases at dry range edges, with substantial uncertainty in absolute CUE due to uncertainty inRa. Our results highlight that future forest resilience is nuanced and controlled by multiple competing mechanisms.

     
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  3. Abstract

    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 (Anet), and stomatal conductance to water (gs) 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 (NOx) 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(Ci), particularly in wetter, recent years. Modeled estimates of seasonally integratedAnetandgsrevealed a 43%–50% stimulation ofAnetwas responsible for increasing iWUE in both tree species throughout 79%–86% of the chronologies with reductions ingsattributable to the remaining 14%–21%, building upon a growing body of literature documenting stimulatedAnetoverwhelming reductions ingsas 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.

     
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