Abstract
We investigate the utility of an improved isotopic method to partition the net ecosystem exchange of CO 2 ( F ) into net photosynthesis ( F A ) and nonfoliar respiration ( F R ). Measurements of F and the carbon isotopic content in air at a high‐elevation coniferous forest (the Niwot Ridge AmeriFlux site) were used to partition F into F A and F R . Isotopically partitioned fluxes were then compared with an independent flux partitioning method that estimated gross photosynthesis (GEE) and total ecosystem respiration (TER) based on statistical regressions of night‐time F and air temperature. We compared the estimates of F A and F R with expected canopy physiological relationships with light (photosynthetically active radiation) and air temperature. Estimates of F A and GEE were dependent on light as expected, and TER, but not F R , exhibited the expected dependence on temperature. Estimates of the isotopic disequilibrium D , or the difference between the isotopic signatures of net photosynthesis ( δ A , mean value −24.6‰) and ecosystem respiration ( δ R , mean value −25.1‰) were generally positive ( δ A >δ R ). The sign of D observed here is inconsistent with many other studies. The key parameters of the improved isotopic flux partitioning method presented here are ecosystem scale mesophyll conductance ( g m ) and maximal vegetative stomatal conductance ( g cmax ). The sensitivity analyses of F A , F R , and D to g cmax indicated a critical value of g cmax (0.15 mol m −2 s −1 ) above which estimates of F A and F R became larger in magnitude relative to GEE and TER. The value of D decreased with increasing values of g m and g cmax , but was still positive across all values of g m and g cmax . We conclude that the characterization of canopy‐scale mesophyll and stomatal conductances are important for further progress with the isotope partitioning method, and to confirm our anomalous isotopic disequilibrium findings.