I was curious so I searched and found this definition of a tree.
"woody plant having one erect perennial stem (trunk) at least three inches in diameter at a point 4-1/2 feet above the ground, a definitely formed crown of foliage, and a mature height of at least 13 feet."
http://treesandshrubs.about.co...
Of course that means that all young trees are at one point shrubs, and then grow into trees doesn't it? But not all shrubs will grow into trees. So it looks like maybe if it has the potential to become a tree it is classified as a tree?
