This paper shows the results of solar flux measurements routinely collected within boreal conifer forests during BOREAS 1994. Such vegetation have open canopies with a high level of clumping and covered by crowns which yields strong directional effects on the spectral hemispherical transmittances, as well as absorbed PAR. The crown intercepts the light more effectively at large sun angles and thus influences the daily estimates of downwelling solar fluxes measured beneath the canopy. This fact explains the seasonal effect of insolation, in particular the observed time variability of main radiative terms in late summer. Indeed, the present findings support the fact that the shape of the crown causes vertical distribution of the radiation field within conifer species to be sigmoidal, thus deviating from the classical Beer’s law extinction.