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'
Buxus sempervirens 'Elegantissima'
Chaenomeles x 'Bonita'
Danae racemosa, Poet's Laurel