Key points are not available for this paper at this time.
J. M. Norman, P. G. Jarvis, Photosynthesis in Sitka Spruce (Picea sitchensis (Bong.) Carr.). III. Measurements of Canopy Structure and Interception of Radiation, Journal of Applied Ecology, Vol. 11, No. 1 (Apr., 1974), pp. 375-398
Norman et al. (Mon,) studied this question.