Tom's Blog

Monday, February 14, 2011

Big trees at Pleasant Valley Conservancy

Now that we have finished the tree database, it is possible to start making interesting queries using GIS. The topic today is big trees, those greater than 30 inches in diameter. It is a simple task to write the code for this query: "Diameter" >=30. Then one must go to the Symbology tab and select the "species" category. With a few clicks, it is possible to show each large tree species with its own symbol.

I am sorry this graphic isn't larger and clearer, but the Internet has massive file-size limitations. The TIFF file that was used to create this graphic was 64 MB! However, it should be a little clearer if you double click and get it in its own window.

I did a little labeling to explain the distribution of the trees. The main part of Pleasant Valley Conservancy is an E/W trending ridge, with steep north-facing and south-facing slopes. Because we are in the Northern Hemisphere, south-facing slopes are hot, dry, and prairie-like, and north-facing slopes are cool, moist, and wooded. As the graphic shows, the red oaks are primarily on the north side and the bur oaks are on the south. The black oaks are primarily on top of the ridge, which is dry, but not as dry as the south slope. And the white oaks are primarily in a basin which is lower and surrounds the Pocket Prairie. (There are a few white oaks in other places also.)

It is impressive to think about all these large trees surviving so long. Those over 40 inches in diameter, for instance, are probably over 200 years old!

There are many highly degraded sites in southwestern Wisconsin that have large trees like ours hidden among the invasive trees and shrubs. When we started restoring Pleasant Valley Conservancy, we were hardly aware of how many large trees we had. Once the invasive shrubs and the walnuts, elms, cherry, and weedier black oaks started coming down, we were able to see what our natural area could look like.

If you are interested in studying the distribution of large oaks for a particular property, look at its 1937 air photo, now available on line. As an example, take a look at the 1937 photo for Pleasant Valley Conservancy, at this link.


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