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Abstract

We investigate the utility of an improved isotopic method to partition the net ecosystem exchange of CO₂ (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 (delta(A), mean value -24.6 parts per thousand) and ecosystem respiration (delta(R), mean value -25.1 parts per thousand) were generally positive (delta(A)>delta(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. [References: 107]