long way down—are carbon and oxygen isotope signals in the tree ring uncoupled from canopy physiological processes?

The carbon (δ13C) and oxygen (δ18O) stable isotope composition is widely used to obtain information on the linkages between environmental drivers and tree physiology over various time scales. The tree-ring archive can especially be exploited to reconstruct inter- and intra-annual variation of both climate and physiology. There is, however, a lack of information on the processes potentially affecting δ13C and δ18O on their way from assimilation in the leaf to the tree ring. As a consequence, the aim of this study was to trace the isotope signals in European beech (Fagus sylvatica L.) from leaf water (δ18O) and leaf assimilates (δ13C and δ18O) to tree-ring wood via phloem-transported compounds over a whole growing season. Phloem and leaf samples for δ13C and δ18O analyses as well as soil water, xylem water, leaf water and atmospheric water vapour samples for δ18O analysis were taken approximately every 2 weeks during the growing season of 2007. The δ13C and δ18O samples from the tree rings were dated intra-annually by monitoring the tree growth with dendrometers. δ18O in the phloem organic matter and tree-ring whole wood was not positively related to leaf water evaporative enrichment and δ18O of canopy organic matter pools. This finding implies a partial uncoupling of the tree-ring oxygen isotopic signal from canopy physiology. At the same time, internal carbon storage and remobilization physiology most likely prevented δ13C in tree-ring whole wood from being closely related to intra-annual variation in environmental drivers. Taking into account the post-photosynthetic isotope fractionation processes resulting in alterations of δ13C and δ18O not only in the tree ring but also in phloem carbohydrates, as well as the intra-annual timing of changes in the tree internal physiology, might help to better understand the meaning of the tree-ring isotope signal not only intra- but also inter-annually.