Reconciling water‐use efficiency estimates from carbon isotope discrimination of leaf biomass and tree rings: nonphotosynthetic fractionation matters

Summary Carbon isotope discrimination (∆) in leaf biomass (∆ BL ) and tree rings (∆ TR ) provides important proxies for plant responses to climate change, specifically in terms of intrinsic water‐use efficiency (iWUE). However, the nonphotosynthetic 12 C/ 13 C fractionation in plant tissues has rarely been quantified and its influence on iWUE estimation remains uncertain. We derived a comprehensive, ∆ based iWUE model (iWUE com ) which includes nonphotosynthetic fractionations ( d ) and characterized tissue‐specific d ‐values based on global compilations of data of ∆ BL , ∆ TR and real‐time ∆ in leaf photosynthesis (∆ online ). iWUE com was further validated with independent datasets. ∆ BL was larger than ∆ online by 2.53‰, while ∆ BL and ∆ TR showed a mean offset of 2.76‰, indicating that ∆ TR is quantitatively very similar to ∆ online . Applying the tissue‐specific d ‐values ( d BL = 2.5‰, d TR = 0‰), iWUE estimated from ∆ BL aligned well with those estimated from ∆ TR or gas exchange. ∆ BL and ∆ TR showed a consistent iWUE trend with an average CO 2 sensitivity of 0.15 ppm ppm −1 during 1975–2015. Accounting for nonphotosynthetic fractionations improves the estimation of iWUE based on isotope records in leaf biomass and tree rings, which is ultimate for inferring changes in carbon and water cycles under historical and future climate.