Nutrient enrichment and loss of herbivores are assumed to cause plant diversity loss in grassland ecosystems because they increase plant cover that decreases understory light. Empirical tests of the role of competition for light in natural systems are based on indirect evidence and have contributed to strong debates over the last 40 years. Using illumination by LED-lamps, we demonstrate that experimentally restoring light to understory plants in a natural grassland mitigated the loss of plant...