Thank you, Brian. I will cut the trunks at about 6" and leave the wood
untreated as you suggested.

At 15' it is a good specimen of that species~~though others might
disagree.

I believe it is at least 25 years old. I have two of them and they both
seem to have peaked at about the same height. Growing conditions may
not be ideal though.

Its growth suggests that at some time in the past it has either
been cut or died, down to ground level.

It was probably pruned or trained so the trunks grew in a more-or-less
straight line to fit in the small space. I believe that it always had
multiple trunks (stems, branches?) though. They grow natively here, and
I see some in the woods that have a single trunk and others with
multiple trunks. Both have identical leaves and samaras so I am not
sure is they are varieties of the species or if some condition triggers
the different behavior.

Best regards,
Dean