A combined geometric-optical and radiative transfer (GORT) model allows incorporation of multiple scales of clustering in conifer canopies on the estimation of radiation transmission. Consideration of clustering of branches into whorls is the latest addition to this model. Modification of the GORT model to include whorl orientation improves the ability to model the observed patterns of solar radiation transmission as a function of solar zenith angle and height in the canopy. Whorl orientation distributions are derived from multidirectional measurements using a geometric optical mutual shadowing model. For BOREAS test stands, model estimates and vertical measurements of photosynthetically active radiation transmittance within the canopy show (1) general decreases in transmission as solar zenith angles increase in the range of solar zenith angles dominated by beam irradiance, (2) increases in PAR transmission at very high solar zenith angles where diffuse skylight is dominant, (3) maximum scattering and absorption occur in the middle of the canopy. Model estimates match measurements from the forest floor, indicating the value of the model for providing radiation inputs to snowmelt models in forested landscapes. ¿ 1997 American Geophysical Union