Updating the dual C and O isotope – gas exchange model: A concept to understand plant responses to the environment and its implications for tree rings

Abstract

The combined study of C and O isotopes in plant organic matter has emerged as a powerful tool for understanding plant functional responses to environmental change. The approach relies on established relationships between leaf gas exchange and isotopic fractionation to derive a series of model scenarios that can be used to infer changes in photosynthetic assimilation and stomatal conductance driven by changes in environmental parameters (CO2, water availability, air humid-ity, temperature, nutrients). We review the mechanistic basis for a conceptual model, in light of recently published research, and discuss where isotopic observations don’t match our current understanding of plant physiological response to environment. We demonstrate that 1) the mod-el was applied successfully in many, but not all studies, 2), while originally conceived for leaf isotopes, the model has been applied extensively to tree ring isotopes in the context of tree physiology and dendrochronology. Where isotopic observations deviate from physiologically plau-sible conclusions, this mismatch between gas-exchange and isotope response provides valuable insights on underlying physiological processes. Overall, we found that isotope responses can be grouped into situations of increasing resource limitation versus higher resource availability. The dual isotope model helps to interpret plant responses to a multitude of environmental factors.

Seasonal and Species‐Level Water‐Use Strategies and Groundwater Dependence in Dryland Riparian Woodlands During Extreme Drought

“…In a recent review, Siegwolf et al. (2023) note the potential for reallocation of stored carbohydrates to contribute to a dampening effect on 13 C that could obscure dual isotope (δ 18 O and Δ 13 C) interpretations in earlywood. In addition, the early season use of shallow soil moisture, which is more isotopically variable, may have overshadowed the influence of leaf water enrichment and resulted in a partial decoupling of tree‐ring δ 18 O and stomatal conductance.…”

“…Therefore, the greater use of deeper (and more isotopically stable) moisture sources during the hot, dry summer likely resulted in a more consistent δ 18 O signal of source water and allowed for a more coordinated response of ∆ 13 C res and δ 18 O that reflected leaf‐level processes. Furthermore, greater drought stress during the late season could similarly reduce the influence of source water relative to leaf‐level enrichment, as stomatal closure weakens the Péclet effect (Siegwolf et al., 2023). In this context, our interpretations are consistent with assumptions and predictions under the dual isotope approach of Scheidegger et al.…”

Uncoupling of stomatal conductance and photosynthesis at high temperatures: mechanistic insights from online stable isotope techniques

“…Stable isotopes in carbon dioxide (CO 2 ) and H 2 O have been used to improve our understanding of various processes of leaf gas exchange, because of the isotopic fractionation that co‐occurs during these processes (Farquhar et al ., 1982; Cernusak et al ., 2016; Siegwolf et al ., 2023). The combined instrumentation of infrared gas analysers and isotope laser spectrometers has facilitated measurements of C and oxygen (O) isotopic composition (δ 13 C and δ 18 O) in CO 2 and H 2 O entering and leaving a leaf gas exchange chamber in real‐time under controlled conditions (Evans et al ., 1986; Barbour, 2017).…”

Half of the 18O enrichment of leaf sucrose is conserved in leaf cellulose of a C3 grass across atmospheric humidity and CO2 levels

“…The 18 O enrichment of cellulose above source water (Δ 18 O Cel , for definitions and specifications of symbols, see Table 1) is believed to contain important environmental and physiological information (Barbour, 2007; Gessler et al, 2014; Helliker & Richter, 2008; Roden & Lin, & Ehleringer, 2000; Song et al, 2022; Werner et al, 2012), such as past atmospheric relative humidity (RH) (Barbour et al, 2004; Helliker & Ehleringer, 2002a, 2002b; Hirl et al, 2021; Liu et al, 2016, 2017b) or differences in transpiration or stomatal conductance among plant species and genotypes in the same environment (e.g., Baca Cabrera et al, 2021; Barbour et al, 2000a; Lin et al, 2022; Scheidegger et al, 2000; Siegwolf et al, 2023). These relationships are grounded in the fact that virtually all of the oxygen in cellulose originates from water (DeNiro & Epstein, 1979; Liu et al, 2016) and evaporative conditions lead to an 18 O enrichment of leaf water above source water (Δ 18 O LW , Table 1) (Cernusak et al, 2016, 2022; Dongmann et al, 1974; Farquhar & Cernusak, 2005; Farquhar et al, 2007; Flanagan et al, 1991; Roden & Ehleringer, 1999).…”