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.

One of the most important drivers of the coexistence of species is the resource heterogeneity of a certain environment. Thus, many studies in different ecosystems have been carried out to test whether species richness is affected by resource heterogeneity. To date, only few studies have measured light and soil resources heterogeneity in forests to investigate its influence on plant diversity. In this study, the aim was to determine (1) which resources have major influences on forest understory plant diversity; (2) the influence of the forest canopy on the heterogeneous distribution of light and soil resources; (3) whether heterogeneity of resources increases understory plant species richness; and (4) if stand structural complexity is an indicator for understory plant species richness. Measures of stand structural complexity were obtained through inventories and remote sensing techniques in 135 study plots of temperate forests, established along a gradient of forest structural complexity. We surveyed vegetation, measured light conditions and soil properties six times in each of all plots. We calculated the standard deviations of these parameters to receive a measure of heterogeneity. Results showed that heterogeneity of light and soil C:N ratio increases with increasing stand structural complexity, increasing light heterogeneity leads to increased understory plant species richness, and finally, an increase of stand structural diversity predicts an increase in understory plant diversity. The study clearly shows that resource heterogeneity theory plays a major role in the coexistence of understory plant species and hence its diversity. These results suggest that understory plant diversity could be increased in forests managed by single tree harvesting by spatially varying the quantities of trees to be logged to create a more heterogeneous understory light environment.