Simulation of carbon isotope discrimination of the terrestrial biosphere

We introduce a multistage model of carbon isotope discrimination during C3 photosynthesis and global maps of C3/C4 plant ratios to an ecophysiological model of the terrestrial biosphere (SiB2) in order to predict the carbon isotope ratios of terrestrial plant carbon globally at a 1 degrees resolution. The model is driven by observed meteorology from the European Centre for Medium-Range Weather Forecasts (ECMWF), constrained by satellite-derived Normalized Difference Vegetation Index (NDVI) and run for the years 1983 - 1993. Modeled mean annual C3 discrimination during this period is 19.2 parts per thousand; total mean annual discrimination by the terrestrial biosphere (C3 and C4 plants) is 15.9 parts per thousand. We test simulation results in three ways. First, we compare the modeled response of C3 discrimination to changes in physiological stress, including daily variations in vapor pressure deficit ( vpd) and monthly variations in precipitation, to observed changes in discrimination inferred from Keeling plot intercepts. Second, we compare mean delta(13) C ratios from selected biomes ( Broadleaf, Temperate Broadleaf, Temperate Conifer, and Boreal) to the observed values from Keeling plots at these biomes. Third, we compare simulated zonal delta(13) C ratios in the Northern Hemisphere (20 degrees N to 60 degrees N) to values predicted from high-frequency variations in measured atmospheric CO2 and delta C-13 from terrestrially dominated sites within the NOAA-Globalview flask network. The modeled response to changes in vapor pressure deficit compares favorably to observations. Simulated discrimination in tropical forests of the Amazon basin is less sensitive to changes in monthly precipitation than is suggested by some observations. Mean model delta C-13 ratios for Broadleaf, Temperate Broadleaf, Temperate Conifer, and Boreal biomes compare well with the few measurements available; however, there is more variability in observations than in the simulation, and modeled delta C-13 values for tropical forests are heavy relative to observations. Simulated zonal delta(13) C ratios in the Northern Hemisphere capture patterns of zonal delta C-13 inferred from atmospheric measurements better than previous investigations. Finally, there is still a need for additional constraints to verify that carbon isotope models behave as expected.