Covariation between oxygen and hydrogen stable isotopes declines along the path from xylem water to wood cellulose across an aridity gradient

Abstract

Summary Oxygen and hydrogen isotopes of cellulose in plant biology are commonly used to infer environmental conditions, often from time series measurements of tree rings. However, the covariation (or the lack thereof) between δ18O and δ2H in plant cellulose is still poorly understood. We compared plant water, and leaf and branch cellulose from dominant tree species across an aridity gradient in Northern Australia, to examine how δ18O and δ2H relate to each other and to mean annual precipitation (MAP). We identified a decline in covariation from xylem to leaf water, and onwards from leaf to branch wood cellulose. Covariation in leaf water isotopic enrichment (Δ) was partially preserved in leaf cellulose but not branch wood cellulose. Furthermore, whilst δ2H was well‐correlated between leaf and branch, there was an offset in δ18O between organs that increased with decreasing MAP. Our findings strongly suggest that postphotosynthetic isotope exchange with water is more apparent for oxygen isotopes, whereas variable kinetic and nonequilibrium isotope effects add complexity to interpreting metabolic‐induced δ2H patterns. Varying oxygen isotope exchange in wood and leaf cellulose must be accounted for when δ18O is used to reconstruct climatic scenarios. Conversely, comparing δ2H and δ18O patterns may reveal environmentally induced shifts in metabolism.

Finding balance: Tree‐ring isotopes differentiate between acclimation and stress‐induced imbalance in a long‐term irrigation experiment

“…The temperature response curves in our study show how δ 18 O and δ 2 H had opposite responses to increasing temperature (Figure S14), inducing a stronger O–H decoupling (lower R 2 values) in control trees, which were more negatively affected by the high temperatures due to their small needles and the low water availability. Furthermore, during organic synthesis, many water‐availability‐dependent reactions can induce additional variation in both δ 2 H and δ 18 O (Holloway‐Phillips et al., 2023; Luo & Sternberg, 1992; Martínez‐Sancho et al., 2023).…”

“…Meanwhile, the irrigation‐induced increase in total biomass of the stop trees created an increased respiratory demand for sugars, which could not be sustained due to the decline in assimilation rate after irrigation stopped (Schönbeck et al., 2022). In this case, the sugar latency time in the stem might have been shorter and the glucose C–H groups might have exchanged only weakly with the xylem water, thus retaining the δ 2 H values established in the leaf sugars, reflecting different metabolic reactions (Augusti et al., 2008; Holloway‐Phillips et al., 2023).…”

“…Gessler et al, 2004) and metabolic processes (Kagawa & Battipaglia, 2022;Wieloch et al, 2022;Yakir & DeNiro, 1990), including both kinetic and equilibrium isotope exchange with xylem water, resulting in a dampening of the initial leaf-level hydrological signal. Importantly, the metabolic-associated isotope effects are different for O and H (Hayes, 2001;Holloway-Phillips et al, 2022), leading to differential isotope patterns in cellulose (Holloway-Phillips et al, 2023).…”

“…Gessler et al., 2004) and metabolic processes (Kagawa & Battipaglia, 2022; Wieloch et al., 2022; Yakir & DeNiro, 1990), including both kinetic and equilibrium isotope exchange with xylem water, resulting in a dampening of the initial leaf‐level hydrological signal. Importantly, the metabolic‐associated isotope effects are different for O and H (Hayes, 2001; Holloway‐Phillips et al., 2022), leading to differential isotope patterns in cellulose (Holloway‐Phillips et al., 2023). Carbon (δ 13 C) and oxygen (δ 18 O) isotopic ratios in tree rings have been used extensively as proxies for canopy functioning (Farquhar et al., 1982; Scheidegger et al., 2000; Vitali et al., 2021), and to investigate past climatic conditions (e.g.…”

“…δ 18 O and δ 2 H have often been assumed to have a strong positive relationship inherited from the source water (Dansgaard, 1964). However, isotope fractionations during the biosynthesis of organic compounds (heron metabolic share of the isotope signals) are not coupled for δ 18 O and δ 2 H (Holloway‐Phillips et al., 2022, 2023). As a result, the relationship between δ 18 O and δ 2 H (hereafter O–H relationship) is weaker than expected if it were based only on the hydrological signal and was shown to be dependent on site conditions and year‐to‐year variation (Nabeshima et al., 2018; Vitali et al., 2022).…”