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

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 ¹²C/¹³C 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 CO₂ 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.