Measurements of PAR transmittance within boreal forest stands during BOREAS

This paper shows the results of solar flux measurements routinely collected within boreal conifer forests during BOREAS 1994. Such vegetation have ope n canopies with a high level of clumping and covered by crowns which yields strong directional effects on the spectral hemispherical transmittances, a s well as absorbed PAR. The crown intercepts the light more effectively at large sun angles and thus influences the daily estimates of downwelling sol ar fluxes measured beneath the canopy. This fact explains the seasonal effe ct of insolation, in particular the observed time variability of main radia tive terms in late summer. Indeed, the present findings support the fact th at the shape of the crown causes vertical distribution of the radiation fie ld within conifer species to be sigmoidal, thus deviating from the classica l Beer's law extinction. (C) 1999 Elsevier Science B.V. All rights reserved .