> I never heard of a trunk being referred to as a stem

I had a tree surgeon come to check on the health of a huge tree (a Tuart FWIW - one of only two in the suburb, and probably the tallest at 35m/115ft+) that's growing on my boundary a year ago. He referred to the tree's main stem, secondary stems and, overall, its "architecture". Obviously there's a whole language in tree surgery!

stales