Intra-annual tree-ring cellulose δ2H as an indicator of soil drought

Temporal variability of tree-ring cellulose δ2H (δ2Hring-cel) can be a unique tool for understanding tree physiology and climate. However, we do not fully understand the drivers of temporal variability in δ2Hring-cel. Investigating seasonal δ2Hring-cel in boreal forests is particularly challenging. Previous studies on intra-annual tree-ring δ18Ohave shown that tree-ring isotope variability can result from the combined but opposing effects of source water and leaf assimilates, a dynamic likely relevant for δ2Hring-cel as well. To be able to use δ2Hring-cel as a standalone and reliable bioindicator, it is important to understand the variable hydrogen isotope fractionation between source water and tree rings. Our study aimed to provide context to this variability in a natural forest by tracing intra-annual δ2Hring-cel to the δ2H of its sources (water, sugars & starch), and comparing δ2Hring-cel to physiological and climatic factors.The δ2H of source water, leaf water and carbohydrate pools (i.e. water-soluble carbohydrates, starch) were analysed from five pine (Pinus sylvestris) trees during 2019 at Hyytiälä forest, Finland. Their δ2H were used to model continuous δ2H of source water (δ2Hsource) and bulk leaf water (δ2Hleaf-water) and photosynthetic water (δ2Hphoto-water). Intra-annual δ2Hring-cel were analysed in 2018 and 2019 at a resolution of 5-10 timepoints per year, and they were allocated to xylogenetic timepoints. They were then compared to time-integrated δ2Hsource, δ2Hleaf-water, δ2Hleaf-sug, net assimilation rate, and various other physiological and climatic factors.Carbohydrate δ2H was significantly different among leaves, branches and stems. δ2Hring-cel had strong time-integrated relationships to modelled δ2Hsource, net leaf assimilation rate and evapotranspiration, but the direction of their relationships was different between years. At monthly resolution, water-soluble carbohydrate δ2H measured from one year-old needles had a strong, positive relationship to δ2Hring-cel. δ2Hring-cel also had strong relationships to Standardized Soil Moisture Index (SSMI) in both years.We show that δ2Hring-cel has a potential as an indicator of soil drought conditions, and that this signal is likely mediated by the leaf-level response to soil drought. This clearly support the growing body of evidence that δ2Hring-cel is strongly mediated by physiological processes, while also opening a new avenue for δ2Hring-cel interpretations. Our results show promise for δ2Hring-cel functioning as a bioindicator of soil drought related physiological stress signals in long-term tree ring chronologies.