In the field, photosynthesis of Acer saccharum seedlings was rarely light saturated, even though light saturation occurs at about 100 μmol quanta m-2 s-1 photosynthetic photon flux density (PPFD). PPFD during more than 75% of the daylight period was 50 μmol m-2 s-1 or less. At these low PPFD's there is a marked interaction of PPFD with the initial slope (CE) of the CO2 response. At PPFD-saturation CE was 0.018 μmol m-2 s-1/(μl/l). The apparent quantum efficiency (incident PPFD) at saturating CO2 was 0.05-0.08 mol/mol. [Formula: see text] and PPFD-saturated CO2 exchange was 6-8 μmol m-2 s-1. The ratio of internal CO2 concentration to external (C i /C a ) was 0.7 to 0.8 except during sunflecks when it decreased to 0.5. The decrease in C i /C a during sunflecks was the result of the slow response of stomates to increased PPFD compared to the response of net photosynthesis. An empirical model, which included the above parameters was used to simulate the measured CO2 exchange rate for portions of two days. Parameter values for the model were determined in experiments separate from the daily time courses being sumulated. Analysis of the field data, partly through the use of simulations, indicate that the elimination of sunflecks would reduce net carbon gain by 5-10%.