The line between whether a plant is a "tree" or a "shrub" is sometimes very thin and quite blurry! We use the following criteria to decide which type applies to a certain plant:
Asimina triloba, Paw Paw
Fagus grandifolia, American Beech
Magnolia macrophylla var asheii
Poncirus trifoliata var. monstrosa
Viburnum plicatum 'Nanum'