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Modelling the discrimination of 13 CO 2 above and within a temperate broad‐leaved forest canopy on hourly to seasonal time scales
Author(s) -
BALDOCCHI D. D.,
BOWLING D. R.
Publication year - 2003
Publication title -
plant, cell and environment
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.646
H-Index - 200
eISSN - 1365-3040
pISSN - 0140-7791
DOI - 10.1046/j.1365-3040.2003.00953.x
Subject(s) - canopy , vapour pressure deficit , temperate deciduous forest , photosynthetically active radiation , environmental science , atmospheric sciences , deciduous , temperate forest , transpiration , tree canopy , growing season , carbon dioxide , ecosystem , photosynthesis , ecology , botany , biology , geology
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.