Estimating intra-seasonal intrinsic water-use efficiency from high-resolution tree-ring δ 13 C data in boreal Scots pine forests.

Intrinsic water-use efficiency (iWUE), a key index for carbon and water balance, has been widely estimated from tree-ring δ 13 C at annual resolution, but rarely at high-resolution intra-seasonal scale. We estimated high-resolution iWUE from laser-ablation δ 13 C analysis of tree-rings (iWUE iso ) and compared it with iWUE derived from gas exchange (iWUE gas ) and eddy covariance (iWUE EC ) data for two Pinus sylvestris forests from 2002 to 2019. By carefully timing iWUE iso via modeled tree-ring growth, iWUE iso aligned well with iWUE gas and iWUE EC at intra-seasonal scale. However, year-to-year patterns of iWUE gas , iWUE iso and iWUE EC were different, possibly due to distinct environmental drivers on iWUE across leaf, tree and ecosystem scales. We quantified the modification of iWUE iso by post-photosynthetic δ 13 C enrichment from leaf sucrose to tree-rings and by non-explicit inclusion of mesophyll and photorespiration terms in photosynthetic discrimination model, which resulted in overestimation of iWUE iso by up to 11% and 14%, respectively. We thus extended the application of tree-ring δ 13 C for iWUE estimates to high-resolution intra-seasonal scale. The comparison of iWUE gas , iWUE iso and iWUE EC provides important insights into physiological acclimation of trees across leaf, tree and ecosystem scales under climate change and improves the upscaling of ecological models.