Environmental, Physiological and Biochemical Processes Determining the Oxygen Isotope Ratio of Tree-Ring Cellulose

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Abstract
Analysis of the oxygen isotope ratioof tree-ring cellulose (δ18Ocell) is a promising tool for reconstructingpast climatic variations and their influence on terrestrial ecosystems, but control mechanisms of δ18Ocell are multi-faceted, involving a number of fractionation steps along the oxygen transfer pathway from precipitation water to the site of cellulose formation. The goal of the current chapter is to provide an overview of the current knowledge concerning fractionation mechanisms related to δ18Ocell. The review is organized by using the currently widely-used δ18Ocell model as a reference context, and is focused on three main determinants of δ18Ocell: source water isotope ratio (δ18Osw), leaf water isotope enrichment (Δ18Olw), and biochemical fractionations downstream of Δ18Olw. For each component, we summarize environmental, physiological, and/or biochemical processes underlying 18O fractionations, and provide explanations of how these processes are critically relevant for linking δ18Ocell to climatic factors in real-world scenarios. We identify knowledge gaps in mechanistic controls of δ18Ocell, and highlight opportunities for more research to improve upon the existing model.

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