Digital Cameras Improve Canopy Photography
Brent D. Johnson
A successful forester must know the best conditions under which to grow a desired type of tree. For example, managing oaks in Eastern forests requires that saplings be exposed to greater than 10 percent of full sunlight to optimize selective logging and to provide biologically meaningful measurements. One method of doing this is called hemispherical canopy photography, and a variant that employs a commercial digital camera and specialized software is helping to make the work of foresters and ecologists easier.
Hemispherical canopy photography enables foresters to calculate the amount of light reaching the forest floor during a growing season. A new variant of the technique employs a commercial digital camera and lens system. Here, the setup images a canopy in a mixed hardwood forest in northwestern Connecticut that is dominated by beech. Courtesy of Charles Canham.
Hemispherical canopy photography dates to the 1960s, when it was discovered that solar geometry could be used to predict how much light is transmitted through the forest canopy. Photographs taken from the ground with a digital camera and a fish-eye lens can be used to calculate the amount of light reaching the forest floor during a growing season, enabling the researcher to determine the relative light and shade tolerance of trees.
Commercial light sensors are inadequate for this purpose because they measure only instantaneous light, said Charles Canham of the Institute of Ecosystem Studies. Hemispherical canopy photography is concerned instead with quantum flux, or "sunfleck," the mosaic of light and dark patches on the forest floor. The light patches breaking through the trees may be as bright as in an open field, but the area immediately surrounding a patch has only 2 or 3 percent of the light radiation.
A light wand could be used to measure the average light on the ground, but the measurement could be altered by the movement of the sun or by a big gap in the canopy. Individual sensors with data loggers more accurately measure sunfleck. In the past, however, this approach has cost thousands of dollars in instruments that must be left in the study area, making security a problem.
The Nikon Coolpix 4500 has reduced the cost of these systems. The digital camera accepts supplemental lenses, including a true fish-eye. Small computer cameras with a 180° field of view have pushed the fish-eye lens technology to shorter focal lengths and have optical qualities that allow the user to get a fix on the zenith. By determining the linear function of the angle from the zenith and employing Windows-based Gap Light Analyzer software, the operator can trace the path of the sun and can accurately measure the amount of above- and below-canopy solar radiation.
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