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'
Metasequoia glyptostroboides 'Miss Grace'
Picea omorika 'Pendula Bruns'
Picea orientalis 'Skylands'