Statistical penetration of diffuse light into vegetative canopies: effect on photosynthetic rate and utility for canopy measurement

The approximately independent penetration of radiant flux from all sky zones into a vegetative canopy further broadens the statistical distribution of received intensities on leaf surfaces. Using standard geometric descriptions of the canopy, I show that the distribution from the diffuse skylight is still quite narrow. Thus, estimation of canopy photosynthesis is, in contrast to the case of the direct solar beam, accurate when one approximates that each leaf receives identical flux, the average flux. The width of the distribution relative to the average, however, gets large at large canopy depth. Measurements of the average flux at depth might be supplemented by measurements of the variance to estimate rapidly the cumulative leaf area index and the foliage angular distribution.