Every neighborhood needs some trees, and every front yard, garden or landscape needs a tree or two. Trees take in carbon dioxide and provide us with oxygen, cleaning and freshening the air around us. They soften the sharp edges of buildings and lend curves to the straight lines man has imposed on our view. Even a small tree creates some shade. The dense layered shade created by the overlapping leaves and boughs of trees is much cooler than, say, an awning.