Here's what we did:
I read excerpts from pages 618-622 from The Handbook Of Nature Study to Millie. She learned about the parts of the tree, what purpose they each serve, how the roots are formed under the tree, how the tree grows, how they eat, etc. I learned a lot from this reading, too :)
We went on a nature walk down by the creek with Grandma and Grandpa. Grandpa showed Millie what tree rings look like, and how to identify various trees (oak, maple, box elder). We clipped twigs from each tree to bring home and identify using a key later, but we never got around to it. They are currently in a vase on our window sill.
Part way through our walk, Millie stopped to draw a cool tree, and identify its parts.
Millie collected lots of cool stuff on our walk! It was an absolutely fantastic March day!
Our tree nature study was inspired by The Handbook Of Nature Study blog.