This is a great lesson from the book Dynamic Art Projects for Children. When I googled it, I found quite a few teachers that had done it with their class. However, I kept seeing the same comment in the blogs and comments: this project took too long to complete and the kids lost interest, mainly because the tree was done with paints and had to dry.
What is the solution to this? Draw your tree on black paper! When the tree is drawn lightly on black paper, it not only eliminates the step of painting, but it also puts more emphasis on creating something by focusing only on the negative space behind the trees. I had several students finish within the hour (and they were working on HUGE pieces of cardstock the size of their desk!) If you want to have most to all students finish within the hour, I would suggest cutting a piece of poster board into 4 pieces, and that should be the right size to get it done, while still being big enough to pack a punch.
I can post step by step instructions here if anyone is interested - it's just a bit of work to do if people don't need them! If you would like step by steps, please comment and let me know, and I'll make some up and post them asap!