Ok, so the shape is not really a part. Still, it is a distinguishing feature of a tree and another way to help in its identification.
Naturalist Roger Tory Peterson says that unlike the precise silhouette of birds, a tree is not so consistent in form or shape. "The beginner, learning his trees, yearns for a book that will give him shapes and field marks by which he can make snap identification. But it isn't that easy...within limits one can with practice, recognize by shape and manner of growth quite a few trees".
A yellow-poplar will always look like a yellow-poplar in a very general sense. However, a young tree may look entirely different from the parent tree. A forest grown tree may grow tall and slender while his field-grown cousin develops a maximum crown in the open sun.
The shape chart above describes Broadly Conical as being illustrations B and E; Broadly Columnar as A,C and F; Narrowly Conical as D,G and I; Narrowly Columnar as F and K; Broadly Spreading as H,J and L. My point is, even with these tree shapes, you obviously need more information to identify these trees by species.