Interpretation of intra-annual tree-ring δ13C profiles of control, droughted and re-watered Scots pines

Intra-annual tree-ring δ13C record has the potential to provide deep insights into past plant performance and environmental conditions. With concomitant high temporal resolution δ13C analysis of non-structural carbohydrates, the processes behind observed low- and high-frequency δ13C changes in tree-ring record could be interpreted more reliably. This is essential for predicting forest response to impacts of climate change, such as more frequent and severe drought episodes.

To better understand post-photosynthetic 13C-fractionation processes and drought associated changes in tree function, we conducted a drought experiment with Scots pine saplings in a greenhouse during a growing season. Scots pine clones were exposed to two treatments: control trees, and trees exposed to drought and subsequent re-watering. We analysed δ13C of individual sugars (compound-specific isotope analysis) and starch in leaves, phloem and roots at weekly resolution, to study how environmental changes were recorded in leaf assimilates and modified in down-stem transport. Tree physiological response was monitored to support interpretation. High resolution δ13C profiles of tree-rings were obtained using laser ablation isotope ratio mass spectrometry, which enabled us to identify how the δ13C signal of assimilates was finally recorded in stem xylem, and to determine which processes distorted this signal.

The results shed new insight into the widely reported 13C-enrichment of sink organs relative to leaves and demonstrate the potential of using high-resolution tree-ring δ13C-profiling to understand drought associated processes in xylem formation.