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:
Sequoia sempervirens 'Mount Loma Prieta Spike'
Sideroxylon thornei, Swamp Buckthorn
Smilax biflora var biflora
Taxus baccata 'Dovastoniana Pendula'
Thujopsis dolobrata 'Jurassic Park'