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:
Buxus microphylla var japonica 'Borderline'
Distylium x 'Spring Frost'
Ilex opaca 'Maryland Dwarf'
Sequoia sempervirens 'Albospica'
Smilax biflora var biflora
Viburnum x burkwoodii 'Conoy'
Weigela florida 'Pink Poppet'