The unknown third - Hydrogen isotopes in tree-ring cellulose across Europe

Area of Science:

• Dendrochronology
• Stable Isotope Biogeochemistry
• Plant Physiology
• Climate Science

Background:

• Tree-ring cellulose isotopes (δ²H, δ¹³C, δ¹⁸O) and tree-ring width (TRW) are proxies for past environmental conditions.
• Hydrogen isotope ratios in tree-ring cellulose (δ²H_c) integrate climate and physiological information.
• Previous studies have not comprehensively assessed δ²H_c across Europe for climatological and physiological signals.

Purpose of the Study:

• To conduct the first Europe-wide assessment of climatological and physiological information from δ²H_c.
• To compare δ²H_c signals with δ¹³C_c, δ¹⁸O_c, and TRW.
• To investigate the influence of climate, hydrology, physiology, and growth on δ²H_c variability.

Main Methods:

• Analysis of annually resolved 100-year tree-ring records from *Pinus* and *Quercus* at 17 European sites.
• Comparison of high-frequency climate signals in δ²H_c, δ¹³C_c, δ¹⁸O_c, and TRW chronologies.
• Investigation of relationships between δ²H_c, δ¹⁸O_c, TRW, temperature, and precipitation.
• Evaluation of mechanistic δ²H_c models at a continental scale.

Main Results:

• δ²H_c chronologies showed weaker high-frequency climate signals than δ¹³C_c and δ¹⁸O_c, but similar to TRW.
• δ²H_c climate signal strength varied geographically and was stronger in *Pinus* than *Quercus*.
• Dry summer conditions led to significant ²H-enrichment in tree-ring cellulose for both genera.
• δ²H_c inter-annual variability was site-specific, influenced by climate, hydrology, physiology, and growth.
• Significant negative relationships were found between δ²H_c and TRW, and positive relationships between δ²H_c and δ¹⁸O_c.
• Mechanistic models accurately simulated average δ²H_c values but failed to capture year-to-year variations.

Conclusions:

• δ²H_c provides distinct information compared to δ¹⁸O_c, with a stronger physiological component potentially linked to carbohydrate reserve utilization.
• Further research on ²H-fractionation, climate interactions, physiology, and species-specific traits is crucial for improving δ²H_c modeling and interpretation.
• Advancements in δ²H_c analysis can offer new insights into trees' carbon allocation, and responses to environmental stressors.