To give the tree texture I decided to use thread. I built a spindle, tied the end of the thread around the base, and gradually wound it up around the trunk.
This secured the pin to the wire even if the glue should break. It also gave the tree trunk texture (and some shinyness - oh well).
I tied the thread off at the top and added a tiny drop of super glue. This secured the thread and gave the tree top a different color (an unexpected but desired look).
Then a snip of the extra thread, and the trees are ready for foliage.