Effects of vapour pressure deficit, temperature and soil drought on triple isotope patterns of assimilates and tree-ring cellulose

Stable isotope compositions of carbon (δ¹³C) and oxygen (δ¹⁸O) in plant carbohydrates such as photosynthetic assimilates or cellulose are widely applied tools to reconstruct climate and plant physiological responses. In contrast, applications of hydrogen isotope composition (δ²H) in plant carbohydrates are limited because of previous methodological constrains and limited knowledge on processes causing hydrogen isotope fractionations. To better understand the individual climatic drivers of isotopic variations in tree rings, particularly for δ²H, we performed a controlled experiment over one growing season in climate chambers with saplings of broadleaf and conifer tree species. The growing conditions resembled conditions that can be typically found in the field: vapor pressure deficit (VPD; 1.0, 1.6, and 2.2 kPa), air temperature (T; 25 and 30 °C), and soil drought (D, well-watered and extreme dry).  After 5 months of treatment, δ¹³C, δ¹⁸O, and δ²H of water, sugars and starch in stems and leaves, as well as in cellulose of the recent year tree rings were measured. For δ²H analyses of plant carbohydrates, we applied a newly developed hot water vapour equilibration method (Schuler et al., 2022, doi.org/10.1111/pce.14193). Across all species, first results show that the three elements in tree-ring cellulose respond differently to the climatic drivers: both δ²H and δ¹³C values increased with increases in D and VPD, but the VPD responses were more pronounced under high than under low T conditions. In contrast, δ¹⁸O values were affected by T and VPD, while the VPD response was more pronounced under wet than under dry soil D conditions. Thus, the combination of δ²H and δ¹³C values could be used to identify D occurrences independent of VPD conditions, while δ¹⁸O value is a better indicator for T and VPD responses. In the following steps, the isotopic variations in tree-ring cellulose will be linked to those in water, sugars, and starch and their concentrations in leaf and stem material, as well as to various other structural and functional traits which have been measured throughout the experiment (Schönbeck et al., 2022, doi.org/10.1111/pce.14425). With this unique experimental design, we aim to provide new knowledge facilitating the interpretation of stable isotope patterns in tree rings under field conditions.