I have studied the light climate and canopy geometry of a young bigtooth aspen (Populus grandidentata) stand at UMBS located in northern lower Michigan. The objectives of this study are: (1) to determine the diurnal patterns and the spatial variations of photosynthetic photon flux density (PPFD) in the forest canopy, (2) to develop a semi-empirical radiation model for predicting the PPFD in the canopy, (3) to develop a method for accessing the light climate and canopy photosynthesis from hemispherical photographs, (4) to determine the physiological significances of light climate and canopy geometry from the viewpoint of canopy productivity, and (5) to access the seasonal changes of whole canopy photosynthesis. In order to accomplish these goals, I investigated the diurnal pattern of canopy radiation in the canopy and developed a semi-empirical canopy radiation model. The model was validated by graphically comparing the time plots of canopy radiation predicted by the model with those of canopy radiation measured in the field. Based on the validated model, photosynthesis of leaves with different orientations have been simulated. From the simulated plots, the photosynthetic signficance of leaf orientation in the canopy was discussed. Finally, I integrated all the above results to estimate the photosynthesis of the whole canopy and its seasonal changes. From the above analyses, I conclude that: (1) the technique and the model predict the diurnal patterns of PPFD in the canopy on clear and overcast days which agree well with the measured ones; (2) the sunflects (direct radiation) are not of as much signficance in photosynthetic production of the lower canopy and understory plants as suggested before; (3) the distribution of leaf orientation (inclination and azimuthal angles) is not significant in photosynthetic production within a canopy level; (4) the canopy maximizes its photosynthetic productivity by having near vertical leaves in the upper canopy and near horizontal leaves in the lower canopy; (5) the PPFD is not the limiting factor for canopy photosynthesis during a clear hot summer day but rather that the mid-day drought that drives the stomatal closing may be a limiting factor and, (6) in order to access the canopy photosynthesis, it is necessary to develop a canopy photosynthesis model which will account for the mid-day stomatal closing.