Exploring the climatic and non-climatic fingerprints of the hydrogen isotope signals in tree rings.

Abstract

The analysis of a Europe-wide network of tree-ring stable isotopes has shown that the climatic signal of δ2H in tree-ring cellulose (C6H10O5), is far weaker compared to those recorded in carbon (δ13C) and oxygen (δ18O)isotopes. Furthermore, the δ2H and δ18O relationships were shown to be site dependent and significantly deviated from the Global Meteoric Water Line. These results suggest that non-climatic effects are modifying the hydrological signature of δ2H. Recent experiments have underlined the potential of δ2H in tree-ring cellulose as a physiological indicator of shifts in autotrophic versus heterotrophic processes. However, the impact of these processes has not yet been quantified under natural conditions.Defoliating insect outbreaks can disrupt photosynthetic production and carbon allocation, stimulating the remobilization of stored carbohydrates. Such disturbance events, therefore, provide unique opportunities to evaluate the impact of changes in the use of fresh versus stored non-structural carbohydrates, i.e., of non-climatic signals stored in δ2H. By exploring a 700-year tree-ring record from Switzerland, we assess the impact of 79 larch budmoth (LBM, Zeiraphera griseana) outbreaks on the growth of its Larix decidua host trees.LBM outbreaks significantly altered the tree-ring isotopic signature, creating a 2H-enrichment and a depletion in 18O 13C. Changes in tree physiology during outbreak years are shown by the decoupling of δ2H and δ18O (O–H relationship), in contrast to the positive correlation in non-outbreak years. The O–H relationship in outbreak years was not significantly affected by temperature, indicating that non-climatic physiological processes dominate over climate in determining δ2H variations. We conclude that the combination of these isotopic parameters may serve as a metric for assessing changes in physiological mechanisms over time and that hydrogen isotopes can be considered as a proxy for non-climatic disturbance signals in dendrochronological research.