Fall Color Reports from the Minnesota DNR which show a state map, photos, videos and reports for each state park.
If you are wondering what is actually happening in the leaves take a look at Wikipedia's explanation with graphics showing the carotenoids and anthocyanins and how they form.
Dichotomous Tree Key (DNR Wisc.)
Tree Identification Arbor Day Foundation
Key to Wisconsin Woody Plants for the iPod
The library also has some books to help you key your trees:
A field guide to trees and shrubs Ref QK 482 .P43 1972
The illustrated encyclopedia of trees, Ref QK 475 .E29 1978
Trees of North America and Europe Ref QK 477.2 .I4 P45
Have fun observing nature and enjoy the wonderful color combinations.
Eva Hyvarinen, Visual Resource Assistant