Modelling the discrimination of 13CO2 above and within a temperate broad-leaved forest canopy on hourly to seasonal time scales

Fluxes and concentrations of carbon dioxide and (CO2)-C-13 provide information about ecosystem physiological processes and their response to environmental variation. The biophysical model, CANOAK, was adapted to compute concentration profiles and fluxes of (CO2)-C-13 within and above a temperate deciduous forest (Walker Branch Watershed, Tennessee, USA). Modifications to the model are described and the ability of the new model (CANISOTOPE ) to simulate concentration profiles of (CO2)-C-13 , its flux density across the canopy-atmosphere interface and leaf-level photosynthetic discrimination against (CO2)-C-13 is demonstrated by comparison with field measurements. The model was used to investigate several aspects of carbon isotope exchange between a forest ecosystem and the atmosphere. During the 1998 growing season, the mean photosynthetic discrimination against (CO2)-C-13 , by the deciduous forest canopy (Delta(canopy)), was computed to be 22.4parts per thousand, but it varied between 18 and 27parts per thousand. On a diurnal basis, the greatest discrimination occurred during the early morning and late afternoon. On a seasonal time scale, the greatest diurnal range in Delta(canopy) occurred early and late in the growing season. Diurnal and seasonal variations in Delta(canopy) resulted from a strong dependence of Delta(canopy) on photosynthetically active radiation and vapour pressure deficit of air. Model calculations also revealed that the relationship between canopy-scale water use efficiency (CO2 assimilation/transpiration) and Delta(canopy) was positive due to complex feedbacks among fluxes, leaf temperature and vapour pressure deficit, a finding that is counter to what is predicted for leaves exposed to well-mixed environments.