Estimating Photosynthetic C-13 Discrimination In Terrestrial CO2 Exchange From Canopy To Regional Scales

We determined δ¹³C values associated with canopy gross and net CO₂ fluxes from four U.S. sites sampled between 2001 and 2002. Annual mean, flux-weighted δ¹³C values of net ecosystem CO₂exchange (NEE) were estimated for four contrasting ecosystems (three forests and one grassland) by analyzing daytime flask measurements collected at the top of canopies. Combining δ¹³C values associated with respiratory and net (respiration minus photosynthesis) fluxes, we demonstrate a method for estimating whole-canopy photosynthetic discrimination against ¹³C (Δ_A) in terrestrial ecosystems directly from field measurements. This experimental approach offers the possibility of examining interannual variability in Δ_A from ecosystem δ¹³C measurements. Our estimated δ¹³C values associated with photosynthetic fluxes are in agreement with those measured from foliar organic matter for C₃forests, and are within the range bounded by C₃ and C₄ grasses in a tallgrass prairie. The δ¹³C 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‰ at the prairie site. These estimates differ from a previous study, particularly for C₃ ecosystems at comparable latitudes. Sensitivity analyses indicate that our estimates of δ¹³C 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 δ¹³C measurements and calculations of Δ_A are also discussed. Our measurement-based analyses are particularly useful when both latitudinal and longitudinal variations in Δ_A are to be considered in the global inversion modeling studies.