Fall is a great time for nature walks and for learning about trees...
Lapaz Farm has a very interesting post on the American Chestnut Tree with lots of links about general tree and leaf studies as well.
Measure the height of a tree using its shadow.
One of my personal favorite children's books is Why Do Leaves Change Color? by Betsy Maestro. Interesting content and great illustrations!
Dead Log Alive! by Jo Kittinger describes the mini-ecosystem that can be present in a fallen tree. We especially enjoyed learning about mosses, lichens and fungi in this book. My son wrote about it this spring (with some interesting pictures!)
We've seen lots and lots of interesting fungi, mosses and lichens on trees or stumps on our nature walks lately. Though we've pretty much learned to distinguish between these three, we're not very good at identifying particular types yet. Here are some fun and/or interesting sites on fungi (If I understand it right, lichens are really a union of algae and fungi - those Lyrical Life Science CDs are starting to have an impact!)...
Giant Puffballs (We really enjoyed the humor here, but aren't brave enough to try eating them - besides which we haven't seen them on our own property before)
Identifying Tree Fungi
MacBeth's Book and Activity List for the Fall
Elizabeth's Fall Reading Lists for All Ages