Spatiotemporal Variation in Forest Intrinsic Water-Use Efficiency and Its Sensitivity to Rising Atmospheric CO₂ in China During 1961–2012

Rising atmospheric CO₂ concentration (Cₐ) has altered terrestrial carbon–water exchange, yet long-term changes in forest responses to increasing Cₐ remain uncertain. Based on 34 tree-ring δ¹³C records from forests across China, we reconstructed forest intrinsic water-use efficiency (iWUE) from 1961 to 2012 and assessed its sensitivity to changes in Cₐ (dW/dCₐ). Forest iWUE increased significantly during 1961–2012, with the regional mean rising from 64.36 μmol mol⁻¹ in 1961 to 84.03 μmol mol⁻¹ in 2012 at a rate of 0.4366 μmol mol⁻¹ yr⁻¹ (P < 0.001). All records showed positive trends, and 33 exhibited significant increases (P < 0.05). Despite increasing iWUE, dW/dCₐ showed no significant long-term decline, with no significant temporal changes detected by moving-window analyses. Mean dW/dCₐ was numerically lower during 2000–2012 (0.237 μmol mol ⁻¹ ppm⁻¹) than during 1961–1999 (0.305 μmol mol ⁻¹ ppm⁻¹), but the difference was not significant (P = 0.509). Spatial variation in dW/dCₐ was not significantly associated with Köppen–Geiger climate regions. Environmental analyses identified MAP, MAT, and VP as relatively important factors, but their explanatory power was limited. These findings indicate that Chinese forests have increased iWUE under rising Cₐ without a detectable long-term decline in Cₐ response sensitivity.