Understanding the interactions between plant canopies and the environment is important for elucidating past, current, and future carbon cycle dynamics. Developments in instrumentation and modeling present new opportunities for quantifying the processes controlling photosynthesis at a variety of spatial and temporal scales, and thus are vital for estimating terrestrial carbon assimilation globally. The remote sensing based photochemical reflectance index (PRI) represents one such methodological advance. Leaf- and canopy-level PRI observations were combined with leaf optical and radiative transfer simulation models to elucidate the interactions between phenological changes in canopy structure, pigments, and physiology and reflectance-based estimates of photosynthetic radiation use efficiency. Simulation modeling results demonstrated that the PRI is significantly influenced by the carotenoid/chlorophyll ratio, photosynthetic acclimation, and changes in canopy structure. Equations describing the relationship between leaf pigments and spectral vegetation indices were developed with simulation models and used for prediction of carotenoid content. Empirically-based relationships between sky diffuse fraction and forest carbon assimilation were used to estimate the potential consequences of changing global radiation regimes on biosphere-atmosphere exchange of CO2 . Simulations showed that a 1% increase/decrease in forest carbon assimilation occurs for every 1% increase/decrease in shortwave radiation that results from changes in sky diffuse fraction. Simulation results also showed no significant advantage of moderately diffuse skies compared to clear skies for total growing season carbon assimilation. The results of this dissertation demonstrate that the PRI should be considered to be more broadly useful for understanding photosynthetic efficiency and photoprotection than previously assumed. Such measurements may be useful for national efforts to understand the influence of climate change on terrestrial ecosystem carbon cycling.