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

Abstract

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

Tree diversity does not always improve resistance of forest ecosystems to drought

“…Latewood sections from a given species, a given year, and a given stand were bulked and analyzed for δ 13 C with a mass spectrometer. By selecting only the latewood, whose δ 13 C mainly characterizes the functioning of the trees during the second part of the growing season, we avoided potential effects related to the remobilization of stored photosynthates from the previous growing season (26) or to a favorable spring climate.…”

Topography alters tree growth–climate relationships in a semi‐arid forested catchment

“…Alternatively, the lack of change in P. ponderosa foliar d 13 C could also be due to 2012 foliage being formed from carbon stores from previous years. Both Kagawa et al (2006) and Offermann et al (2011) found that stored carbohydrates assimilated in previous years are used in current year foliage formation. This process may be more pronounced during drought and pine species may rely heavily on carbon reserves during drought seasons for radial growth (Oberhuber et al 2011).…”

Progress and challenges in using stable isotopes to trace plant carbon and water relations across scales

“…2.3), the potential limitations of using the bulk isotope signature need to be kept in mind and sampling strategies need to be adapted. For trees, the intra-annual analysis of tree ring, whole wood or cellulose can provide a tool to study periods during the growing season when the isotopic signature in this archive is directly coupled to leaf physiology (Helle and Schleser, 2004;Offermann et al, 2011).…”