Canopy structure mediates the light environment, and thus the photosynthetic activity of leaves that underpins forest productivity. Directly illuminated leaves have more light available to power photosynthesis, but those higher light levels can also increase leaves' stress. In contrast, shaded leaves often have less stress and more photosynthetic efficiency. Light distribution within the canopy is critical to understanding overall photosynthesis rates and the detection of stress responses. Structure-mediated light distribution is not static within a given day or season. We are particularly interested in studies with the following foci: (1) refinement or use of multi-layer advanced radiative transfer models that explicitly handle vegetation vertical heterogeneity, (2) imaging spectroscopy studies in which direct versus diffuse illumination is explicitly addressed via an appropriate observational framework across days and seasons, and (3) new advances in ecophysiological theory at the interface of structure and function.