Responses of tree leaf gas exchange to elevated CO<sub>2</sub> combined with changes in temperature and water availability: A global synthesis
Zhaoguo Wang, Chuankuan Wang
Abstract
Abstract Aim An elevated CO 2 concentration (eCO 2 ) can increase photosynthesis ( A sat ) and reduce stomatal conductance ( g s ); 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 meta‐analysis on: (1) the responses of A sat , g s and intrinsic water‐use efficiency (iWUE) to eCO 2 , 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 A sat and g s acclimatize to eCO 2 in parallel. Results We found that eCO 2 stimulated A sat (β‐factor = .68) and decreased g s (β‐factor = −.36) and that iWUE increased in proportion to eCO 2 (β‐factor = 1.01), which conform with the theory of optimal stomatal behaviour. Meanwhile, conifers and older trees showed greater increases in A sat but less decreases in g s 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 A sat and g s to eCO 2 , whereas drought amplified the A sat response but attenuated the g s response. However, A sat , rather than g s , acclimatized to eCO 2 , owing to nitrogen reduction, active investment of nitrogen and sink limitation. Main conclusions Our findings suggest that eCO 2 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.