An Area Survey Method of Investigating the Distribution of Light Intensity in Woodlands, with Particular Reference to Sunflecks

Abstract

A simple, robust and portable apparatus is described capable of surveying up to 30 sq. m in an hour, using a photoelectric method. If suitable precautions are taken both the general conditions of shade light and the area, distribution, and intensity of sunflecks can be evaluated. A sample of data from two neighboring but separate plots in a Nigerian rain forest is analyzed, tests of consistency showing the results to be repeatable. Here, under the midday sun, 20-25% of the forest floor was occupied by sunflecks; taking account of spectral energy distributions and of the response of the apparatus they contributed about 80% of the light energy around noon. Allowing for mean clouding over a period and for the restriction of sunflecks to the middle of the day, they contributed about 70% of the total light energy reaching the forest floor during the months of Jan. to March. Sunflecks present a particularly favorable case for the translation of the results into absolute units, the average daily total of light for Jan. to March being about 500 cal./dm2 The difficulties of evaluating shade light in this way are discussed: here the daily average is shown not to exceed 200 cal./dm? Finally the division of the total sunfleck area between flecks of different intensity is worked out, and shown to approximate to an exponential relation. The area scale is also converted to a time one, sunflecks being visible in a particular spot for an average of about an hour a day. The meaning of these results is discussed in relation to the concept of the daylight factor, the fuller information of a sunfleck survey producing, in a case of this kind, a very different view of the spectral composition of the light reaching an undergrowth plant. The utility of the area survey method as applied to certain other types of woodland is discussed, and the new information obtained is considered from its autecological aspect.