If you want to read a great Children's Book about trees I highly recommend Shel Silverstein's "The Giving Tree".
It might make you cry...but it's a wonderful story about giving. My wife had a copy for her pre-school, that I got to read. I think I'll go look for it, I think we have a copy around here somewhere.
Giving a TreeGivers Gift Tree would be another way to remember someone special.