C discrimination during CO assimilation by the terrestrial biosphere.

Estimates of the extent of the discrimination againstCO during photosynthesis (Δ) on a global basis were made using gridded data sets of temperature, precipitation, elevation, humidity and vegetation type. Stomatal responses to leaf-to-air vapour mole fraction difference (D, leaf-to-air vapour pressure difference divided by atmospheric pressure) were first determined by a literature review and by assuming that stomatal behaviour results in the optimisation of plant water use in relation to carbon gain. Using monthly time steps, modelled stomatal responses toD were used to calculate the ratio of stomatal cavity to ambient CO mole fractions and then, in association with leaf internal conductances, to calculate Δ. Weighted according to gross primary productivity (GPP, annual net CO asimilation per unit ground area), estimated Δ for C biomes ranged from 12.9‰ for xerophytic woods and shrub to 19.6‰ for cool/cold deciduous forest, with an average value from C plants of 17.8‰. This is slightly less than the commonly used values of 18-20‰. For C plants the average modelled discrimination was 3.6‰, again slightly less than would be calculated from C plant dry matter carbon isotopic composition (yielding around 5‰). From our model we estimate that, on a global basis, 21% of GPP is by C plants and for the terrestrial biosphere as a whole we calculate an average isotope discrimination during photosynthesis of 14.8‰. There are large variations in Δ across the globe, the largest of which are associated with the precence or absence of C plants. Due to longitudinal variations in Δ, there are problems in using latitudinally averaged terrestrial carbon isotope discriminations to calculate the ratio of net oceanic to net terrestrial carbon fluxes.