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

The carbon (delta C-13) and oxygen (delta O-18) 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 delta C-13 and delta O-18 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 (delta O-18) and leaf assimilates (delta C-13 and delta O-18) to tree-ring wood via phloem-transported compounds over a whole growing season. Phloem and leaf samples for delta C-13 and delta O-18 analyses as well as soil water, xylem water, leaf water and atmospheric water vapour samples for delta O-18 analysis were taken approximately every 2 weeks during the growing season of 2007. The delta C-13 and delta O-18 samples from the tree rings were dated intra-annually by monitoring the tree growth with dendrometers. delta O-18 in the phloem organic matter and tree-ring whole wood was not positively related to leaf water evaporative enrichment and delta O-18 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 delta C-13 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 delta C-13 and delta O-18 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.