Tom's Blog

Saturday, August 2, 2008

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%



4 Comments:

Blogger lalai said...

hello Mr. Toms, how to measure the canopy closure using fisheye lens? ?or what method or software?

April 25, 2012 at 12:19 AM  
Blogger lalai said...

hello Mr. Toms, how to measure the canopy closure using fisheye lens? ?or what method or software?

April 25, 2012 at 12:19 AM  
Blogger lalai said...

whats the best time to take images to measure canopy closure

April 25, 2012 at 12:36 AM  
Blogger Tom's Blog said...

Best time to take fish-eye lens images is between 10 AM and 2 PM on a clear (cloudless) day in mid summer.

No special software. Use a standard drawing program (Illustrator or Corel). Create a grid that can overlay the photo. For each point on the grid, record whether open sky or vegetation. Calculate the % open sky.

April 25, 2012 at 9:41 AM  

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