I have a few areas on my farm that could be considered living fences.
What they end up being is a combination of trees and spreading shrubs. So the under-story is things like lilac, chokecherry, hansen's cherry, bridal wreathe, wild roses, cotoneaster, etc. The over-story ends up being standard apples and cherries, nuts, maples, boxelder, poplar, cottonwood, elm,
ash, etc. If intentionally planted, I might include things like apples, nuts, cherries, pear, apricots, raspberries, grapes, locus, Siberian
pea, etc.
There are also a few impenetrable areas that consist of spruce trees that have grown together.