The Stable Hydrogen Isotopic Signature: From Source Water to Tree Rings

Abstract

AbstractThe hydrogen isotopic signature (δ2H) of water in trees contains information on plant functional responses to climatic changes and on the origin of the water. This is also true for the non-exchangeable hydrogen isotopic signature (δ2HNE) of plant organic matter, which contains additional physiological and biochemical information that can be dated to specific years if extracted from annual rings of trees. Despite this potential for gaining unique insights from δ2HNEof tree-ring cellulose (δ2HTRC), it has not been widely used compared to other isotope signals, likely due to challenging methodological constraints and interpretations of these isotopic signals. In this chapter, we first summarize hydrogen isotope (2H-) fractionation that occurs between source water and tree rings and review methods (e.g. nitration, equilibration, position-specific applications) and calculations to determine δ2HNE in tree material. Building upon a summary of the current state of knowledge, this chapter also provides an exhaustive synthesis of δ2HTRC papers, applications, and associated data from approximately 180 sites across the globe (paired with modelled precipitation δ2H values and climate data). The data allow us to investigate the hydrological-climatic effects driving δ2HTRC pattern on a global scale, the relationship of hydrogen with oxygen isotopes in the same tree-ring material, as well as the influence of physiological-biochemical effects (e.g., species differences, tree growth) that appear to be more important on local or temporal scales  than on a large spatial scales. Thus, when local hydro-climatic influences on source water δ2H can be isolated, δ2HTRC gives novel insights on tree physiological responses to abiotic and biotic stresses. We conclude that the growing constellation of tree-ring metrics, including advancements in 2H-processing (i.e., equilibration techniques allowing rapid determinations of δ2HNE) and further refinements to the understanding of post-photosynthetic 2H-fractionations will together provide many new opportunities to understand past climates and ecophysiology by using δ2H in tree rings.