In order to consider the effects of light quality on the growth of understory plants, both the direct and diffuse components of solar radiation must be evaluated. However, the basis for choosing the threshold between the two components of radiation has not been systematically determined. We evaluated the basis and methods for separating direct and diffuse radiation under field conditions. Daily courses of direct and diffuse photosynthetic photon flux densities (PPFDs) over and under canopies were measured with quantum sensors and were predicted from hemispherical photographs. We proposed a new variable, called the diffuse coefficient of canopy (dc), to compensate for the increase in the diffuse radiation component as the sunlight passes through the overstory canopy. dc was highly correlated with canopy coverage (cc) (dc = 0.01cc - 0.3, r2 = 0.95). For woodland sites, PPFD can be predicted from hemispherical photographs according to the following equations: Wdir = Sout(τ - dc)M sinα and Wdif = Sout (0.271 - 0.294(τ - dc)M) sinα, where Wdir and Wdif are the direct and diffuse radiation components in woodland sites, Sout is the extraterrestrial PPFD normal to the solar beam, τ is the atmospheric transmittance, M is the optical airmass number and α is the solar elevation. The daily courses of PPFD and sunflecks under canopies can be estimated from hemispherical photographs when the effects of dc are considered.
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