Estimating intraseasonal intrinsic water-use efficiency from high-resolution tree-ring δ13 C data in boreal Scots pine forests

Opposing seasonal trends in source water and sugar dampen intra-annual variability in tree rings oxygen isotopes

Variations of oxygen isotopes δ18O in tree rings provide critical insights into past climate and tree physiological processes, yet the mechanisms shaping the intra-annual δ18O signals remain incompletely understood. To address this gap, we investigated how seasonal changes in source water, leaf water, and sugars influence δ18O recorded along the tree rings of Pinus sylvestris in Finland. We conducted a seasonal analysis measuring δ18O from needle water, source water, and phloem sugars and investigated the fraction of oxygen isotope exchange during wood formation. We found that seasonal δ18O amplitudes are significantly reduced from leaf water to tree rings, driven by opposing seasonal patterns in increasing source water δ18O and decreasing evaporative enrichment as relative humidity increases. Additionally, the isotope exchange between source water and phloem sugars further dampens seasonal δ18O signals in the rings. Our findings show that oxygen isotope exchange is critical in shaping δ18O signals, influencing the role of source water and relative humidity recorded on intra-annual resolution. This refined understanding helps interpret tree physiological responses under changing conditions and improves climate reconstructions based on tree rings using intra-annual resolution.

Analysis of 18O/16O Isotope Ratios in Organic Matter by Laser Ablation IRMS

In recent years, the application of laser ablation isotope ratio mass spectrometry has revolutionized the field of carbon isotope analysis, enabling routine analyses at high spatial resolution (30-40 μm). Until now, an equivalent analytical method for oxygen isotope ratio (18O/16O) measurements has been lacking. In this article, we describe a preparatory system for analysis of the oxygen isotope composition of carbon monoxide produced from organic samples. The system couples a UV laser platform and automated cryofocusing unit with an isotope ratio mass spectrometer. The method is tested on cellulose and wood (tree rings) and is shown to produce data with analytical precision typically better than 0.4‰ with a sampling resolution of 100 μm (laser beam diameter). Coupled with spatially accurate and minimally invasive laser sampling, the ability to measure stable oxygen isotopes in this way represents a significant advance as it opens up new research opportunities in plant sciences, ecology, paleoclimatology, and science-based archeology.

Resin acid δ13C and δ18O as indicators of intra-seasonal physiological and environmental variability

Understanding the dynamics of δ13C and δ18O in modern resin is crucial for interpreting (sub)fossilized resin records and resin production dynamics. We measured the δ13C and δ18O offsets between resin acids and their precursor molecules in the top-canopy twigs and breast-height stems of mature Pinus sylvestris trees. We also investigated the physiological and environmental signals imprinted in resin δ13C and δ18O at an intra-seasonal scale. Resin δ13C was c. 2‰ lower than sucrose δ13C, in both twigs and stems, likely due to the loss of 13C-enriched C-1 atoms of pyruvate during isoprene formation and kinetic isotope effects during diterpene synthesis. Resin δ18O was c. 20‰ higher than xylem water δ18O and c. 20‰ lower than δ18O of water-soluble carbohydrates, possibly caused by discrimination against 18O during O2-based diterpene oxidation and 35%-50% oxygen atom exchange with water. Resin δ13C and δ18O recorded a strong signal of soil water potential; however, their overall capacity to infer intraseasonal environmental changes was limited by their temporal, within-tree and among-tree variations. Future studies should validate the potential isotope fractionation mechanisms associated with resin synthesis and explore the use of resin δ13C and δ18O as a long-term proxy for physiological and environmental changes.

Reconciling water-use efficiency estimates from carbon isotope discrimination of leaf biomass and tree rings: nonphotosynthetic fractionation matters

Carbon isotope discrimination (∆) in leaf biomass (∆BL) and tree rings (∆TR) provides important proxies for plant responses to climate change, specifically in terms of intrinsic water-use efficiency (iWUE). However, the nonphotosynthetic 12C/13C fractionation in plant tissues has rarely been quantified and its influence on iWUE estimation remains uncertain. We derived a comprehensive, ∆ based iWUE model (iWUEcom) which includes nonphotosynthetic fractionations (d) and characterized tissue-specific d-values based on global compilations of data of ∆BL, ∆TR and real-time ∆ in leaf photosynthesis (∆online). iWUEcom was further validated with independent datasets. ∆BL was larger than ∆online by 2.53‰, while ∆BL and ∆TR showed a mean offset of 2.76‰, indicating that ∆TR is quantitatively very similar to ∆online. Applying the tissue-specific d-values (dBL = 2.5‰, dTR = 0‰), iWUE estimated from ∆BL aligned well with those estimated from ∆TR or gas exchange. ∆BL and ∆TR showed a consistent iWUE trend with an average CO2 sensitivity of 0.15 ppm ppm-1 during 1975-2015. Accounting for nonphotosynthetic fractionations improves the estimation of iWUE based on isotope records in leaf biomass and tree rings, which is ultimate for inferring changes in carbon and water cycles under historical and future climate.

Progress in high-resolution isotope-ratio analysis of tree rings using laser ablation

Stable isotope ratio analysis of tree rings has been widely and successfully applied in recent decades for climatic and environmental reconstructions. These studies were mostly conducted at an annual resolution, considering one measurement per tree ring, often focusing on latewood. However, much more information could be retrieved with high-resolution intra-annual isotope studies, based on the fact that the wood cells and the corresponding organic matter are continuously laid down during the growing season. Such studies are still relatively rare, but have a unique potential for reconstructing seasonal climate variations or short-term changes in physiological plant properties, like water-use efficiency. The reason for this research gap is mostly technical, as on the one hand sub-annual, manual splitting of rings is very tedious, while on the other hand automated laser ablation for high-resolution analyses is not yet well established and available. Here, we give an update on the current status of laser ablation research for analysis of the carbon isotope ratio (δ13C) of wood, describe an easy-to-use laser ablation system, its operation and discuss practical issues related to tree core preparation, including cellulose extraction. The results show that routine analysis with up to 100 laser shot-derived δ13C-values daily and good precision and accuracy (ca. 0.1‰) comparable to conventional combustion in an elemental analyzer are possible. Measurements on resin-extracted wood is recommended as most efficient, but laser ablation is also possible on cellulose extracted wood pieces. Considering the straightforward sample preparation, the technique is therefore ripe for wide-spread application. With this work, we hope to stimulate future progress in the promising field of high-resolution environmental reconstruction using laser ablation.