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.

Plant diversity has often been linked to increased productivity. However, this apparent diversity-productivity relationship for plants may rely on interactions with microbes. Furthermore, these relationships are likely to be context dependent in response to resource availability. We used a biodiversity-ecosystem function experiment with trees exposed to high and low water availability treatments to determine if soil fungal communities mediate the diversity-productivity relationship. We found that richness and community composition of soil fungi and the interaction between plant pathogen and ectomycorrhizal richness were significant drivers of above-ground productivity and supported indirect, positive effects of tree species richness and functional diversity on productivity. Soil fungi also mediated tree water relations, reducing the effect of water limitation on productivity. Fungal communities were important drivers of positive net diversity and complementarity effects on productivity. Our study provides evidence that soil fungi play an important role in diversity-productivity relationships under variable water availability.