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 pavia, Red Buckeye
Fagus grandifolia, American Beech
Magnolia macrophylla var asheii
Magnolia x loebneri 'Donna'
Osmanthus heterophyllus 'Goshiki'
Sideroxylon thornei, Swamp Buckthorn