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

<p>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 (&#948;¹³C), 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 &#948;¹³C in estimating intra-seasonal iWUE (iWUE_iso), with the aid of laser ablation IRMS analysis of tree-rings, by comparing iWUE_iso with iWUE derived from leaf gas exchange (iWUE_gas) and eddy covariance (iWUE_EC) data for two <em>Pinus</em> <em>sylvestris </em>L. dominated boreal forests from 2002 to 2019. By carefully timing iWUE_iso via modelled yearly tree-ring growth, intra-seasonal variability of iWUE_iso aligned well with that of iWUE_gas and iWUE_EC. However, absolute values of iWUE_iso were apparently higher than that of iWUE_gas and iWUE_EC, 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 iWUE_iso estimation. These two factors overestimated iWUE_iso by up to 11% and 14%, respectively. Our study demonstrates that tree-ring &#948;¹³C 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 iWUE_gas, iWUE_iso and iWUE_EC, which can deepen our understanding of how trees physiologically adapt to climate change across leaf, tree and ecosystem levels.</p>