Estimating photosynthetic 13C discrimination in terrestrial CO2 exchange from canopy to regional scales -: art. no. GB1041

[1] We determined delta(13)C values associated with canopy gross and net CO2 fluxes from four U.S. sites sampled between 2001 and 2002. Annual mean, flux-weighted delta(13)C values of net ecosystem CO2 exchange (NEE) were estimated for four contrasting ecosystems (three forests and one grassland) by analyzing daytime flask measurements collected at the top of canopies. Combining delta(13)C values associated with respiratory and net (respiration minus photosynthesis) fluxes, we demonstrate a method for estimating whole-canopy photosynthetic discrimination against C-13 (Delta(A)) in terrestrial ecosystems directly from field measurements. This experimental approach offers the possibility of examining interannual variability in Delta(A) from ecosystem delta(13)C measurements. Our estimated delta(13)C values associated with photosynthetic fluxes are in agreement with those measured from foliar organic matter for C-3 forests, and are within the range bounded by C-3 and C-4 grasses in a tallgrass prairie. The delta(13)C associated with NEE fluxes at our C-3 forest sites ranges between -27.1 +/- 2.7 and -28.3 +/- 2.5parts per thousand, and is -22.6 +/- 4.0parts per thousand at the prairie site. These estimates differ from a previous study, particularly for C-3 ecosystems at comparable latitudes. Sensitivity analyses indicate that our estimates of delta(13)C values of net CO2 fluxes are robust with respect to measurement errors, but can vary depending on the selection of background atmospheric values. Other factors (e.g., drought and sampling footprint) that might have influenced our delta(13)C measurements and calculations of Delta(A) are also discussed. Our measurement-based analyses are particularly useful when both latitudinal and longitudinal variations in Delta(A) are to be considered in the global inversion modeling studies.