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

ABSTRACT Fluxes and concentrations of carbon dioxide and 13 CO 2 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 13 CO 2 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 13 CO 2 , its flux density across the canopy–atmosphere interface and leaf‐level photosynthetic discrimination against 13 CO 2 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 13 CO 2 , by the deciduous forest canopy (Δ canopy ), was computed to be 22·4‰, but it varied between 18 and 27‰. 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 Δ canopy occurred early and late in the growing season. Diurnal and seasonal variations in Δ canopy resulted from a strong dependence of Δ canopy on photosynthetically active radiation and vapour pressure deficit of air. Model calculations also revealed that the relationship between canopy‐scale water use efficiency (CO 2 assimilation/transpiration) and Δ 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.