RADIOMETRIC MEASUREMENTS OF GAP PROBABILITY IN CONIFER TREE CANOPIES

Measurements of gap probability, defined as the probability that a photon of direct illumination at a given zenith angle will penetrate a tree canopy, were made for some moderate-sized, open-grown conifers of varying species in the National Arboretum, Washington, D.C. The measurements were collected by rationing the standardizing readings in red and near-infrared bands of two radiometers with 6-degrees fields of view that were pointed directly at the sun. One radiometer was located within the shadow of the conifer tree; the other was placed outside of the shadow. The path length associated with each radiometric measurement was calculated by fitting a simple conical geometric model to each tree from measurements of tree height and width. The radiometric measurements are best fit by a binomial model in which sunlight is either unobstructed or fully attenuated, and analysis shows that the gap probability, which is taken as the mean of the binomial, fits a negative exponential function of path length quite well. The conifer shadow, then, is an object of almost uniform darkness with some bright holes or gaps that are found near the shadow's edge but rapidly disappear toward the shadow's center.