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.