Tree Canopies and Sun Flecks

Forrest M. Mims III, Editor

Plants that grow on the floor of a forest receive very little sunlight. Such plants are often categorized as subcanopy vegetation.

I have studied the sunlight received under forest canopies in the woods adjacent to the Texas field called Geronimo Creek Observatory and in Brazil and Hawaii. This work has established that cumulus clouds and haze play major roles in the availability of both ultraviolet and photosynthetic sunlight at the forest floor.

Figure 1 is a photograph that shows a forest canopy very near my office. The trees are predominantly cedar elms (Ulmus crassifolia). The camera, a Nikon 990 with a resolution of 3.3 megapixels, has been placed so that a bright beam of sunlight passing through a small opening in the canopy strikes its 186-degree fisheye lens. This beam of sunlight is known as a sun fleck.

Many subcanopy plants receive only a few sun flecks a day as a bright ray of sunlight briefly sweeps across their foliage as the sun moves past the openings in the canopy high above.

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