Comparison of seasonal variations in water-use efficiency calculated from the carbon isotope composition of tree rings and flux data in a temperate forest

Tree-ring delta 13C is often interpreted in terms of intrinsic water-use efficiency (WUE) using a carbon isotope discrimination model established at the leaf level. We examined whether intra-ring delta 13C could be used to assess variations in intrinsic WUE (W-g, the ratio of carbon assimilation and stomatal conductance to water) and variations in ecosystem WUE (W-t, the ratio of C assimilation and transpiration) at a seasonal scale. Intra-ring delta 13C was measured in 30- to 60-mu m-thick slices in eight oak trees (Quercus petraea). Canopy W-g was simulated using a physiologically process-based model. High between-tree variability was observed in the seasonal variations of intra-ring delta 13C. Six trees showed significant positive correlations between W-g calculated from intra-ring delta 13C and canopy W-g averaged over several days during latewood formation. These results suggest that latewood is a seasonal recorder of W-g trends, with a temporal lag corresponding to the mixing time of sugars in the phloem. These six trees also showed significant negative correlations between photosynthetic discrimination Delta calculated from intra-ring delta 13C, and ecosystem W-t, during latewood formation. Despite the observed between-tree variability, these results indicate that intra-ring delta 13C can be used to access seasonal variations in past W-t.