My 4 yo DD has been fascinated with "evergreens" ever since the Christmas tree lots started springing up (ohhh, about October 15th!).  When the trees arrived on the lots, she was curious to know why these trees didn't lose their needles.  

Since we were studying ancient Egypt and papermaking, we combined her interest with our history to form a unit on trees.

One part both of the children enjoyed was finding out what products are made from different parts of the trees.

This website provides some great information on "Goods from the Woods"

http://www.ncforestry.org/docs/Products/goods.htm