The dual C and O isotope – gas exchange model: A concept review for understanding plant responses to the environment and its application in 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 response 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 draw inferences about changes in photosynthetic assimi-lation and stomatal conductance driven by changes in environmental parameters (CO2, water availability, air humidity, temperature, nutrients). We review the mechanistic basis for model and research to date, and discuss where isotopic observations don’t match our current under-standing of plant physiological response to environment. We demonstrate that 1) the model has been applied successfully in many, but not all studies, and 2), while originally conceived for leaf isotopes, the model has been applied extensively to tree ring isotopes in the context of tree phys-iology 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. Thus, the dual isotope model helps to interpret plant responses to a multitude of environmental factors.

Do changes in tree‐ring δ18O indicate changes in stomatal conductance?

“…In spite of the criticisms raised in this Letter, we remain optimistic that the dual isotope approach holds great promise for improving our understanding of the physiological responses of plants to the environment (Siegwolf et al, 2021). Nevertheless, it is necessary to note that previous experimental verifications of this approach have focused largely on leaf-level, 'side-by-side comparison' scenarios; in the only tree-ring-targeted, controlled experimental testing performed so far, Roden & Farquhar (2012) showed that the dual isotope framework was not fully operational.…”

“…In this case 'similar environmental conditions' refers primarily to similar δ 18 O of source water (δ 18 O sw ) and ambient relative humidity (RH), as these are two important controlling factors of δ 18 O cel . The predicted negative association of g s with δ 18 O cel has gained support from the results of several glasshouse and field tests on different plant species Scheidegger et al, 2000;Sullivan & Welker, 2006;Grams et al, 2007;Moreno-Gutiérrez et al, 2011), and consequently has resulted in the use of δ 18 O cel as a proxy for g s to aid interpretation of physiological underpinnings of iWUE in numerous investigations (Siegwolf et al, 2021).…”

Detecting long‐term changes in stomatal conductance: challenges and opportunities of tree‐ring δ18O proxy

“…We also considered differences in physiological strategies between plant functional type, physiological metrics derived from d 13 C (intercellular CO 2 concentrations, carbon isotope discrimination) and basal area increment. That is, we used the relationships found in previous studies (Barbour et al, 2000;Sullivan & Welker, 2006;Grams et al, 2007) in a conceptual framework in which environmental and physiological metrics were considered to constrain the physiological interpretation of the estimated D 18 O LW in terms of g s (Siegwolf et al, 2021). Interpreting the g s -D 18 O LW relationship as a function of changes in VPD is physiologically plausible, given the well-established relationship between g s and VPD (Grossiord et al, 2020).…”

Climate impacts on tree-ring stable isotopes across the Northern Hemispheric boreal zone

“…The interpretation of results for these regions with their complex hydrology, however, is not straight-forward due to often mixed signals recorded in trees. Mechanistic modelling of isotope ratios may help in determining more clearly the importance of the different climate variables on the observed isotope changes (Barichivich et al, 2021;Siegwolf et al, 2021Siegwolf et al, , 2022.…”