In closed-canopy forests, the availability of photosynthetically active
light has been a focal point of research, emphasizing the role of light
as a resource in limiting carbon assimilation and individual tree
growth. However, light shapes the functioning of forest ecosystems
through multiple mechanisms. Here, using a series of studies from a
network of tree diversity experiments, we explore the multifaceted ways
in which light---in terms of both quantity and quality---shapes
productivity in mixed-species forests. Spectral reflectance from remote
sensing of forest canopies is being increasingly used to detect how tree
diversity influences productivity. We demonstrate that airborne imaging
spectroscopy captures functionally important differences among canopies
related to their structure, chemistry, and underlying biological
interactions. Ground-based analyses can show in detail how
photosynthetically active light is partitioned among species in
mixed-species communities. We show that greater interception of light
and greater efficiency of light use, generated by inter- and
intra-specific differences, combine to enhance productivity in
mixed-species forests. Light may shape forest function not only as a
resource but also as a stressor and cue. Plants can perceive light at
various wavelengths, use this information to assess their neighborhoods,
and subsequently adjust their physiology and allocation. We characterize
how light quality---from the ultraviolet to shortwave infrared---varies
among and within canopies of differing diversity. We explore how these
diversity-light quality relationships arise and connect across levels of
biological organization from leaf-level trait expression to forest
function. Together these studies lend insight into light-mediated
mechanisms that drive relationships between biodiversity and
productivity in forest ecosystems---insights that are crucial to predict
how biodiversity change will affect future forest function.