A new conceptual framework for the use of hydrogen isotopes in tree rings

Carbon and oxygen stable isotopes in tree-rings are successfully used in climate and environmental research, for instance for the reconstruction of past climatic conditions and corresponding physiological responses of trees to local climate. In contrast, hydrogen isotope ratios (δ²H), available also in the cellulose molecule of tree-rings, have been largely neglected. Mostly due to methodological reasons, but also because various studies found a rather poor climate information in the δ²H of tree rings. Recent studies show that the latter might be caused by isotope fractionation mechanisms that are related to plant physiological and biochemical processes rather than to climate or hydrological changes. These results also suggest that a relative use of carbon reserves and photosynthetic assimilates may explain δ²H variations in tree-rings. We therefore investigated the literature and observed strong relationships between δ²H and tree growth chronologies across various species in Switzerland, Germany, Norway, China, and India. The relationships between tree-ring growth and δ²H show a dependence to site-specific factors, climatic conditions (e.g. temperature, precipitation), and competition/light effects. Based on our findings we set up a novel conceptual framework that may allow the reconstruction of physiological responses such as carbon use strategies under varying environmental conditions. This new tool may find widespread application to identify and date, with high resolution, stressful conditions or stress-release phases that a tree or a forest ecosystem has experienced in the past.