C-13 DISCRIMINATION DURING CO2 ASSIMILATION BY THE TERRESTRIAL BIOSPHERE

Estimates of the extent of the discrimination against (CO2)-C-13 during photosynthesis (Delta(A)) 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 to D were used to calculate the ratio of stomatal cavity to ambient CO2 mole fractions and then, in association with leaf internal conductances, to calculate Delta(A). Weighted according to gross primary productivity (GPP, annual net CO2 asimilation per unit ground area), estimated Delta(A) for C-3 biomes ranged from 12.9 parts per thousand for xerophytic woods and shrub to 19.6 parts per thousand for cool/cold deciduous forest, with an average value for C-3 plants of 17.8 parts per thousand. This is slightly less than the commonly used values of 18-20 parts per thousand. For C-4 plants the average modelled discrimination was 3.6 parts per thousand, again slightly less than would be calculated from C-4 plant dry matter carbon isotopic composition (yielding around 5 parts per thousand). From our model we estimate that, on a global basis, 21% of GPP is by C-4 plants and for the terrestrial biosphere as a whole we calculate an average isotope discrimination during photosynthesis of 14.8 parts per thousand. There are large variations in Delta(A) across the globe, the largest of which are associated with the precence or absence of C-4 plants. Due to longitudinal variations Delta(A), there are problems in using latitudinally averaged terrestrial carbon isotope discriminations to calculate the ratio of net oceanic to net terrestrial carbon fluxes.