most definitions of trees include some means of
distinguishing them from shrubs, generally height.

The one I am familiar with is that a shrub is a woody plant which is usually
under ten feet tall & has multiple stems. A tree is usually over ten feet tall
& usually has a single stem.
What about dwarf trees which are way under ten feet tall & might have multiple
trunks, like a dwarf birch? I would assume if the standard plant is a tree, the
dwarf form is also called a tree. Tsuga canadensis 'Minuta' is still a tree,
albeit 3" tall.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)