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:
Acer saccharum 'Monumentale'
Aesculus flava, Yellow Buckeye
Aesculus pavia, Red Buckeye
Asimina triloba, Paw Paw
Fagus grandifolia, American Beech
Magnolia macrophylla var asheii
Picea omorika 'Pendula Bruns'
Picea orientalis 'Skylands'