Predicted distribution of visible and near-infrared radiant flux above and below a transmittant leaf

Abstract

The effects of background reflectance, leaf size, and leaf height above the background on upward and downward radiant flux ( φ u and φ d ) from a leaf were investigated using a computer model of a horizontal, isotropically scattering leaf. This research was conducted to determine how these variables influence the light environment above, below and adjacent to a leaf. Leaf spectral properties for big-leaf maple (Acer macrophyllum) were measured in the laboratory and used in the model. Model results were reported as relative radiant flux ( φ r ), defined as a percentage of the light entered into the model. The model showed that upward relative radiant flux φ ur from a leaf was highly dependent on the reflectance of the background and the wavelength of light. The greatest variation in φ ur was observed in the near infrared (NIR). The φ ur also varied depending upon the height of the leaf above the background and the size of the leaf. Leaves were brightest when placed the farthest distance above the background. Small leaves reached maximum brightness at lower heights than larger leaves. Finally, φ ur varied spatially. Leaf edges reflected more light than the leaf center except for leaves positioned very close to the background. Additional studies using the model showed that the intensity of light within a leaf shadow varied spatially, with the greatest downward relative radiant flux φ dr , occurring directly below the center of the leaf. Furthermore, φ dr within the shadow cast by the leaf decreased as the height of the leaf above the background increased. The rate of decrease depended upon the size of the leaf. The smaller the leaf, the greater was the change in φ dr with change in leaf height. These results imply that NIR canopy reflectance, due to leaf transmittance, may be highly dependent upon the reflectance of its background. Furthermore, architecturally different canopies may show different degrees of dependence upon background reflectance in the NIR. These results extend to closed canopies, in which leaf size and spacing may vary the reflectance of the canopy. Finally, these results suggest that the amount of light scattered to the side by leaves increases the amount of NIR light measured from adjacent, unshadowed backgrounds.