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The Loess Plateau is a fragile ecological zone that is sensitive to climate change. The response, adaptation, and feedback of tree growth in forest ecosystems to global warming and CO₂ enrichment are urgent scientific issues. Intrinsic water use efficiency (iWUE) is an important indicator for understanding forest ecosystem adaptability to climate change and CO₂ enrichment. In this study, tree-ring width, tree-ring stable carbon isotope ratio (δ¹³C), and iWUE of <i>P. tabulaeformis</i> Carr. were established. Climate response analysis showed that temperature was the main limiting factor affecting radial tree growth and that relative humidity significantly affected the stable carbon isotope fractionation of tree rings. During 1645–2011, the iWUE increased by 27.1%. The responses of iWUE to climate factors and atmospheric CO₂ concentrations (C_a) showed that the long-term variation in iWUE was affected by C_a, which could explain 69% of iWUE variation, and temperature was the main factor causing iWUE interannual variation. The ecosystem of <i>P. tabulaeformis</i> showed a positive response to rising C_a, as its carbon sequestration capacity increased. In response to global warming and CO₂ enrichment, rising C_a promoted increases in iWUE but ultimately failed to offset the negative impact of warming on tree growth in the study area.