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:
Aesculus flava, Yellow Buckeye
Cephalotaxus harringtonia 'Gold Rush'
Danae racemosa, Poet's Laurel
Illicium parviflorum 'Forest Green'