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Responses of tree leaf gas exchange to elevated CO
 2
 combined with changes in temperature and water availability: A global synthesis

Handling Editor: Benjamin Poulter Abstract Aim: An elevated CO2 concentration (eCO2) can increase photosynthesis (Asat) and reduce stomatal conductance (gs); hence, it can benefit tree growth directly by increasing carbon availability and/or indirectly by saving water. However, uncertainties remain regarding the direction and magnitude of these effects, particularly in combination with changes in temperature and water availability. Location: Global. Time period: 1986– 2020. Major taxa studied: Trees. Methods: We compiled 3,391 observations from 270 studies and performed a metaanalysis on: (1) the responses of Asat, gs and intrinsic wateruse efficiency (iWUE) to eCO2, in addition to variations in these responses between species (phylogeny and leaf habit), within species (tree age) and within individuals (canopy position and leaf age); (2) the effects of changes in temperature and/or water availability on these responses; and (3) whether Asat and gs acclimatize to eCO2 in parallel. Results: We found that eCO2 stimulated Asat (βfactor = .68) and decreased gs (βfactor = −.36) and that iWUE increased in proportion to eCO2 (βfactor = 1.01), which conform with the theory of optimal stomatal behaviour. Meanwhile, conifers and older trees showed greater increases in Asat but less decreases in gs than angiosperms and young trees, respectively, with a proportional increase in iWUE being independent of phylogeny and tree age, which support the theory further. Warming had little effect on the responses of Asat and gs to eCO2, whereas drought amplified the Asat response but attenuated the gs response. However, Asat, rather than gs, acclimatized to eCO2, owing to nitrogen reduction, active investment of nitrogen and sink limitation. Main conclusions: Our findings suggest that eCO2 can enhance carbon uptake and reduce water loss at the leaf scale, which may be modified by water stress. This study improves our physiological understanding of tree gas exchange in a changing climate.