An arboretum is a collection of woody plants (especially trees, but may include shrubs and vines) used for study. 

This is a geo-arboretum! As geocachers, we obviously love the outdoors, but that doesn't mean that we always recognize what we're seeing. I'll try to point out the key features that will help you learn a few common and not-so-common trees that you may discover in Wisconsin's landscapes, both natural and planted.

Black Walnut

If there were a prince of Wisconsin's forest, it would have to be black walnut. On good sites this tree can quickly and easily top a hundred feet. Not only is it one of our tallest trees, it's also one of our strongest, and therefore one of our most commercially valuable hardwoods. Because it resists warping even under great stress, and looks good doing so, it's often used for high-quality woodworking. A single large tree can fetch ten grand for its wood alone. During its lifetime it also produces literally tons of nuts. And it isn't just the squirrels that go crazy for these nuts--we can eat them too, and who doesn't like walnuts? The thick flesh around the nut produces a dark brown dye, too. And this prince takes no guff from young trees--in fact, walnuts produce a substance that keeps other trees from growing in their shade.

And these are all reasons that people resist planting walnuts. Instead of reveling in the bounty, most people simply get annoyed at the fallen fruits and complain that they can't grow anything in walnut shade. This is no way to treat royalty.

Bark

The bark is deeply ridged and dark brown. Nothing unusual here.

Leaves

The leaves of a walnut can't be mistaken for many other things. For one, walnut leaves are compound--they're made of leaflets that are each several inches long, creating a complete leaf that is about two feet from end to end. That's a big leaf! Also, the leaves are opposite each other on the branch. (Most trees around here have alternate leaves.) And the leaflets have a toothed edge. 

Fruit

If you live near one of these guys, you know it. The green fruits are the size of golf balls, and each one has a nut at the center. If you don't see them, you'll smell them. They have a distinctive odor (no, it's not bad, just distinctive), and you might even hear them hit the ground. The squirrels will also make sure you know about the prince's presence! The fruits may leave a temporary brown stain on your pavement, but the stain is harmless.

Tolerate this sovereign's foibles, and you will be treated to a valuable, desirable shade tree that can outlive your house.