Can oxygen isotopes in tree rings be used to detect stomatal responses to global change?

Stomatal conductance (gs) regulates CO2 and water fluxes of
plants. Although experiments have shown that gs decreases with
elevated CO2, it is unclear how gs is responding in-situ to
long-term exposures to rising CO2 and a changing climate. Tree
ring isotope analysis provides a unique method to assess tree
ecophysiological responses to long-term exposures to slowly changing
environmental conditions. In particular, it has been suggested that
changes in gs can potentially be inferred from tree ring
stable oxygen isotope ratios (δ18Otrc). Several studies have indeed used
δ18Otrc trends to conclude that gs has not significantly changed
from pre-industrial values. However, it remains unclear whether δ18Otrc is
sufficiently sensitive to detect the magnitude of change in gs expected
due to CO2 increases and climatic changes. We deposit here the
climate data and R scripts that we use to evaluate the sensitivity of
δ18Otrc trends to CO2 and climate-induced changes in gs, and to
VPD and temperature increases since the beginning of the 20th century,
using current theoretical models. We find that temporal changes in
gs only significantly affect δ18Otrc trends when the Péclet
effect is present, and then only in dry climates. In contrast to the weak
effects of gs on δ18Otrc trends, we find that temporal increases in VPD
and temperature, independent of changes in gs, have far greater
contributions to δ18Otrc trends. Thus, this increasingly popular method
should be used with caution, because it is highly challenging to
unambiguously attribute trends in δ18Otrc to changes in gs.