Analysis of light transmissivity of forest canopies with a telephoto method

Abstract

A new method using telephotographs was devised to assess the light transmissivity of forest canopies having the penumbral effect, and applied to three types of forests typical of warm temperate Japan. Photographs of forest canopies at various angular altitudes were taken with a 270-mm telephoto lens and enlarged. Each photograph was then superimposed with a transparent sheet having a circle corresponding in size to the apparent angular size of the solar disk on the print. The circle was moved over regularly distributed points, and the fraction unobstructed by the images of plant parts was read at each point and used to determine frequency distributions of the direct beam transmissivity as well as the averaged diffuse site factors. A preponderance of low direct beam transmissivity was found for all the forest canopies studied, indicating that most of the sunflecks on the forest floors receive only a minute part of the direct solar beam incident on the canopies. The average transmissivity decreased with decreasing angular altitude for each type of forest canopy. In the pine forest, the relationship between transmissivity and altitude did not significantly differ from that predicted by Beer's law. However, in the two broadleaved forest canopies, the relationship for lower altitudes differed markedly from that predicted.