Abstract
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 iWUE tra ) 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 (CO 2 ) on intrinsic water use efficiency in plants. In this study, a constant g s /g m ratio (0.79) was introduced for calculating iWUE (expressedas iWUE mes ). We applied this iWUE mes model to our newly developed intra-annual (10 samples per ring) Δ 13 C chronology of Cryptomeria fortunei tree for 1965–2017 at Gu Mountain Area and our annual Δ 13 C chronology of Pinus massoniana tree for 1865–2014 at Niumulin Natural Reserve in southeast China. Using dendrochronology techniques, our analysis revealed that the current iWUE tra model overestimates the iWUE values by approximately 2 times and that the iWUE value of trees inferred from iWUE mes modelling decreased significantly in summer-autumn time, which may indicate that alternative factors play a role in limiting the degree of iWUE improvement under the drought-stressed forest in southeast China.
The Intra-Annual Intrinsic Water Use Efficiency Dynamics Based on an Improved Model
