On the convertibility of different microsite light availability indices, relative illuminance and relative photon flux density

1. To reveal the convertibility between different indices of microsite light environment, relative photosynthetically active photon-flux density (R-PPFD) and relative illuminance (RI) were measured with the ratio of red light to far-red light (R/FR) simultaneously below various types of vegetation: temperate riparian deciduous broadleaf forest, coniferous forest, mixed forest of evergreen broadleaf and coniferous trees and cool-temperate deciduous broadleaf forest, and in rice paddy in summer. 2. Within and throughout the vegetation, R-PPFD and RI were strongly correlated, but the slopes of the regression equations significantly exceeded 1, and differed among the vegetation types and varied seasonally. 3. R/FR decreased with decreasing R-PPFD. The difference between RI and R-PPFD increased with decreasing R/FR: the darker the microsite, the larger the 'error' of the evaluated light availability if RI was used. 4. RI data can be converted to R-PPFD. Conversion from RI to R-PPFD would allow more reliable data comparisons among the different vegetation types and among studies that use different methods to measure the light environment.