How do leaf trait values change spatially and temporally with light availability in a grassland diversity experiment?

Complementarity in light use might increase light exploitation and could be an important mechanism explaining the coexistence of multiple species in plant communities of increasing diversity. We measured vertical light profiles and leaf traits related to light acquisition and light use in 40 mixtures of varying species richness (SR, 2, 4, 8 and 16) and functional group richness (FR, 1-4) in a large grassland biodiversity experiment at five different times during the growing season. Light attenuation within the canopy differed significantly among mixtures of varying SR at peak biomass, with 40% in 2-species mixtures and up to 80% in 16-species mixtures. In contrast, increasing SR did not affect light attenuation at the beginning of the growing season or during regrowth after mowing, when large fractions of incoming radiation reached the ground level. These patterns suggested the presence of highly variable light niches over space and time. Trait expression differed among functional group...