A controlled test of the dual-isotope approach for the interpretation of stable carbon and oxygen isotope ratio variation in tree rings

Seedlings of a conifer (Pinus radiata D. Don) and a broad leaf angiosperm (Eucalyptus globulus Labill.) were grown for 100 days in two growth cabinets at 45 or 65 relative humidity. The seedlings were exposed to treatments designed to modify carbon assimilation rates and capacities, stomatal conductance and transpiration to test conceptual models that attempt to clarify the interpretation of carbon isotope discrimination (Δ¹³C) by using oxygen isotope enrichment (Δ¹⁸O). Differences in relative humidity and within-cabinet treatments (including lower irradiance, lower nitrogen inputs, higher leaf temperature and lower moisture status than control seedlings) produced significant differences in assimilation rates, photosynthetic capacities, stomatal conductance, leaf transpiration rates and leaf evaporative enrichment. The dual-isotope approach accurately interpreted the cause of variation in wood cellulose Δ¹³C for some of the treatments, but not for others. We also tested whether we could use Δ¹³C variation to constrain the interpretation of ω¹⁸O variation. Carbon isotope discrimination appears to be influenced by transpiration (providing information on leaf evaporative enrichment), but the results did not provide a clear way to interpret such variation. The dual-isotope approach appears to be valid conceptually, but more work is needed to make it operational under different scenarios.