Estimating photosynthetic 13C discrimination in terrestrial CO2 exchange from canopy to regional scales

[1] We determined d13C values associated with canopy gross and net CO2 fluxes from four U.S. sites sampled between 2001 and 2002. Annual mean, flux-weighted d13C 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 d13C values associated with respiratory and net (respiration minus photosynthesis) fluxes, we demonstrate a method for estimating whole-canopy photosynthetic discrimination against 13C (DA) in terrestrial ecosystems directly from field measurements. This experimental approach offers the possibility of examining interannual variability in DA from ecosystem d 13C measurements. Our estimated d13C values associated with photosynthetic fluxes are in agreement with those measured from foliar organic matter for C3 forests, and are within the range bounded by C3 and C4 grasses in a tallgrass prairie. The d 13C associated with NEE fluxes at our C3 forest sites ranges between 27.1 ± 2.7 and 28.3 ± 2.5%, and is 22.6 ± 4.0 % a