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:
Calycanthus x raulstonii 'Hartlage Wine'
Cornus florida 'Suwannee Squat'
Mahonia 'Beijing Beauty'
Viburnum plicatum 'Nanum'