PREDICTING EFFECTS OF ATMOSPHERIC CO2 PARTIAL-PRESSURE ON FOREST PHOTOSYNTHESIS

A mechanistic (i.e. hierarchic or explanatory) model of forest canopy mass and energy exchange that has been previously tested with eddy-cor relation measurements in the field - albeit only at present ambient CO 2 partial pressure - was used to predict photosynthetic response of a deciduous Quercus-Acer forest in eastern North America to atmospheric CO2 partial pressure. Four partial pressures of CO2 were used in simul ations: 28 (pre-industrial), 36 (present), 54 and 72 Pa. This is (one of) the first set(s) of predictions of forest photosynthetic response to CO2 partial pressure made by a mechanistic forest physiology model shown to accurately predict independent field measurements of whole-fo rest CO2 exchange at the hourly time scale. The model includes a bioch emically based Farquhar-type model of leaf mesophyll CO2 assimilation, which is central to its ability to predict photosynthetic response to different CO2 partial pressures. Whole-forest photosynthesis was posi tively related to CO2 partial pressure, as expected. This was the case under both clear and cloudy skies, but the relative response to CO2 w as greater under a clear sky compared to a cloudy sky (the clear sky d ay was also warmer). Instantaneous water use efficiency (mel CO2 assim ilated per mol H2O transpired) was positively related to atmospheric C O2 partial pressure for all conditions included in the simulations. Mo del predictions indicate that (1) present forest photosynthesis and wa ter use efficiency may be significantly greater than they were in pre- industrial times (per unit ground area of forest) and (2) future highe r CO2 partial pressures could further stimulate forest photosynthesis and water use efficiency, unless future climatic changes have signific ant negative effects on photosynthesis or acclimation and adaptation p rocesses markedly downregulate photosynthesis in response to greater C O2 partial pressure.