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

This is the first Europe-wide comprehensive assessment of the climatological and physiological information recorded by hydrogen isotope ratios in tree-ring cellulose (delta H-2(c)) based on a unique collection of annually resolved 100-year tree-ring records of two genera (Pinus and Quercus) from 17 sites (36 degrees N to 68 degrees N). We observed that the high-frequency climate signals in the delta H-2(c) chronologies were weaker than those recorded in carbon (delta C-13(c)) and oxygen isotope signals (delta O-18(c)) but similar to the tree-ring width ones (TRW). The delta H-2(c) climate signal strength varied across the continent and was stronger and more consistent for Pinus than for Quercus. For both genera, years with extremely dry summer conditions caused a significant H-2-enrichment in tree-ring cellulose. The delta H-2(c) inter-annual variability was strongly site-specific, as a result of the imprinting of climate and hydrology, hut also physiological mechanisms and tree growth. To differentiate between environmental and physiological signals in delta H-2(c), we investigated its relationships with delta O-18(c) and TRW. We found significant negative relationships between delta H-2(c), and TRW (7 sites), and positive ones between delta H-2(c) and delta O-18(c) (10 sites). The strength of these relationships was nonlincarly related to temperature and precipitation. Mechanistic delta H-2(c) models performed well for both genera at continental scale simulating average values, but they failed on capturing year-to-year delta H-2(c) variations. Our results suggest that the information recorded by delta H-2(c) is significantly different from that of delta O-18(c,) and has a stronger physiological component independent from climate, possibly related to the use of carbohydrate reserves for growth. Advancements in the understanding of H-2-fractionations and their relationships with climate, physiology, and species-specific traits are needed to improve the modelling and interpretation accuracy of delta H-2(c). Such advancements could lead to new insights into trees' carbon allocation mechanisms, and responses to abiotic and biotic stress conditions.