Stable carbon and oxygen isotopes in tree rings and basal area increment from mature temperate forests within the AmeriFlux network.

Data were used to investigate long-term changes in tree intrinsic
water use efficiency (iWUE, i.e., the ratio between CO2 assimilation
and stomatal conductance) and the underlying physiological mechanisms.
We used delta18O to estimate the 18O enrichment in leaf water above
the source water, Delta18OLW. Moreover we assessed the relationship
between isotope-derived parameters and atmospheric CO2 (ca) and
climate factors. Isotope-related parameters included in the dataset
are: alpha-cellulose delta13C, carbon isotope discrimination
(Delta13C), intercellular CO2 concentration (ci) and the ratio of
intercellular to atmospheric CO2 concentrations (ci/ca),
alpha-cellulose delta18O, estimated delta18O in precipitation (see
Method), oxygen isotope discrimination above the source water
(Delta18O). The dataset includes also the following climate
parameters: growing season temperature (Tgrs), precipitation (Pgrs)
and vapor pressure deficit (VPDgrs) and mean annual temperature (Ta),
precipitation (Pa) and vapor pressure deficit (VPDa), and standard
precipitation-evaporation index relative to August, with 3 months lag
(SPEI8_3) from the global database. Finally, we also include the ca
values that were used to calculate delta13C, iWUE and ci/ca. All the
equations used to calculate the isotope-derived parameters, including
the leaf water Delta18O (see Figure 3 in Guerrieri et al. 2019 PNAS)
are also provided.