Laser ablation tree-ring δ13C for estimating intra-seasonal intrinsic water-use effificiency

Intrinsic water-use efficiency (iWUE) is a key index for evaluating carbon and water balance of terrestrial ecosystems. A common approach to estimate iWUE is via measuring tree-ring stable carbon isotope composition (δ13C), which has been widely applied at annual resolution, but rarely at high-resolution intra-seasonal scale. In this study, we examined the reliability and accuracy of tree-ring δ13C in estimating intra-seasonal iWUE (iWUEiso), with the aid of laser ablation IRMS analysis of tree-rings, by comparing iWUEiso with iWUE derived from leaf gas exchange (iWUEgas) and eddy covariance (iWUEEC) data for two Pinus sylvestris L. dominated boreal forests from 2002 to 2019. By carefully timing iWUEiso via modelled yearly tree-ring growth, intra-seasonal variability of iWUEiso aligned well with that of iWUEgas and iWUEEC. However, absolute values of iWUEiso were apparently higher than that of iWUEgas and iWUEEC, which could be ascribed to post-photosynthetic 13C-enrichment from leaf assimilates to tree-rings and to nonexplicit inclusion of mesophyll and photorespiration terms in iWUEiso estimation. These two factors overestimated iWUEiso by up to 11% and 14%, respectively. Our study demonstrates that tree-ring δ13C data can be used to trace intra-seasonal variability of iWUE, and thus provides a valuable tool for detecting short-term dynamics in carbon and water trade for areas and periods with no instrumental data. We encourage more comparisons between iWUEgas, iWUEiso and iWUEEC, which can deepen our understanding of how trees physiologically adapt to climate change across leaf, tree and ecosystem levels.