| Summary: | Understanding the water-use strategy of human-planted species used in response to climate change is essential to optimize afforestation programs in dry regions. Since 2000, trees on the central Loess Plateau have experienced a shift from strengthening drought to weakening drought. In this study, we combined tree-ring δ<sup>13</sup>C and δ<sup>18</sup>O records from <i>Pinus tabuliformis</i> (syn. <i>tabulaeformis</i>) Carr. (a tree) and <i>Sophora viciifolia</i> Hance (a shrub) on the central Loess Plateau to investigate species-specific responses to rising atmospheric CO<sub>2</sub> (<i>C</i><sub>a</sub>) and drought. We found summer relative humidity controlled the fractionation of tree-ring δ<sup>18</sup>O, but the magnitude of the climate influence on δ<sup>13</sup>C differed between the species. The intrinsic water-use efficiency (<i>iWUE</i>) trends of both species suggested a strongly active response to maintain constant intercellular CO<sub>2</sub> concentrations as <i>C</i><sub>a</sub> rose. The tree-ring δ<sup>13</sup>C and δ<sup>18</sup>O of both species using first-difference data were significantly and positively correlated, with stronger relationships for the shrub. This indicated the dominant regulation of <i>iWUE</i> by stomatal conductance in both species, but with greater stomatal control for the shrub. Moreover, the higher mean <i>iWUE</i> value of <i>S. viciifolia</i> indicated a more conservative water-use strategy than <i>P. tabuliformis</i>. Based on our commonality analysis, the main driver of the increased <i>iWUE</i> was the joint effect of <i>C</i><sub>a</sub> and vapor-pressure deficit (25.51%) for the tree, while it was the joint effect of <i>C</i><sub>a</sub> and the self-calibrated Palmer drought severity index (39.13%) for the shrub. These results suggest <i>S. viciifolia</i> will be more drought-tolerant than <i>P. tabuliformis</i> and as <i>C</i><sub>a</sub> continually rises, we should focus more on the effects of soil drought than atmospheric drought on the water-use strategy of <i>S. viciifolia</i>.
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