What are they trying to accomplish in their yard?

This sounds like some variant of coppicing being practiced, if only they hadn’t retained that much trunk.

Feet! How the hell would I notice something 8 inches high while driving by?

I’m guessing that the trees actually do have bark, and are just of a species which looks bald due to not having much cork.

Vertical elements “placed in a straight line, a few feet apart” suggest posts for a fence to me, with the panels/pickets/whatever to come later. Maybe I’m just not seeing it.

Have you seen the triptychs at Stonehenge?