Measuring savanna canopy cover with a fish-eye lens

The oak savanna habitat is characterized by an open canopy, ranging from about 10% to 50%. As part of my work on purple milkweeds, I was interested in measuring the canopy in the locations where they are growing. A technique that has been widely used to measure canopy in plant ecology is photography with a fish-eye lens. Tom Givnish of the U.W. Botany Department generously let me use one of his digital fish-eye lens cameras. Over the past few weeks I used this lens to measure the canopy cover of all my purple milkweed sites.

The kind of image you get with a fish-eye lens is shown in the photograph. At each site I took images and then quantified the percent canopy. There is some fancy software you can buy that lets you do fairly sophisticated analyses, but my interests were much simpler. I created a digital grid that I could place over the image, and then at each grid corner I recorded whether it was touching vegetation or open sky. My grid was made so that I had about 100 points across the fish-eye lens image.

The data I obtained are given in the table below. Each code number (AP-8, etc.) stands for a purple milkweed site that has been permanently marked.

I think the data are pretty good, and the results are reasonable. The purple milkweeds do best in areas of around 50% canopy, which is fairly typical savanna habitat.

Stand	% canopy	Type of savanna	Spontaneous (S) or transplant (T)**
AP-8	64.6%	Bur oak	T
AP-11	53.7%	White oak	S
AP-12	60.0%	White oak	S
AP-13	60.6%	White oak	S
AP-16	56.3%	Bur oak	S
AP-17	61.6%	White oak	S
AP-18	34.2%	Bur oak; milkweed now gone	S
AP-19	60.4%	Bur oak	S
AP-28	50.0%	Bur oak	T
AP-29	55.0%	Bur oak	S
AP-30	53.5%	White oak	S
AP-31	37.1%	Bur oak	S
AP-33	34.4%	Black oak	S
AP-34	45.2%	White oak	T
AP-35	65.2%	Bur oak	T
AP-36	59.3%	Bur oak	T
AP-37	75.2%	Oak hickory	T
AP-38	55.1%	Bur oak	T
AP-39	53.7%	Bur oak	S
Avg	53.9%
Range	34.2 to 75.2%