The intra-annual intrinsic water use efficiency dynamics based on an improved model

The carbon isotope fractionation value (Δ) has been widely used to infer the intrinsic water use efficiency (iWUE) of C3 plants. Currently, the most commonly used iWUE method (expressed as iWUEtra) in tree-rings assumes that the mesophyll conductance in plants is infinite. However, many observation-based studies have pointed out that such an assumption leads to overestimating the impact of carbon dioxide (CO2) on iWUE in plants. In this study, a constant gs/gm ratio (0.79) is introduced for calculating iWUE (expressed as iWUEmes). We apply this iWUEmes model to our newly developed intra-annual (10 samples per ring) Δ13C chronologies of Cryptomeria fortunei tree for 1965–2017 at Gu Mountain Area and our annual Δ13C chronology of Pinus massoniana tree for 1865–2014 at Niumulin Natural Reserve in southeast China. Using dendrochronology techniques, our analysis shows that the current iWUEtra model overestimates the iWUE values by approximately a factor of two and that the iWUE value of trees inferred from iWUEmes modeling decreases significantly in summer-autumn time, which likely indicates that alternative factors play a role in limiting the degree of iWUE improvement under the drought-stressed forest in southeast China.