I figured that there are antifreeze chemicals in trees. Anything in solution will depress the freezing point. Pure water freezes/ thaws at 32 degrees F, but a saline solution, or car antifreeze solution depresses the freezing point (as well as raises the boiling point).
Only trees I've cut below freezing were some mountain mahogany, which are small, and some fir snags in snow shoes for a cabin that I was caretaking in the Sierras. The previous caretaker didn't put up any wood, when I got there at Thanksgiving to start my duties, the propane was out, the pipes were freezing, and I was cutting up wood chairs with a carpenter's handsaw to try to fight any more freezing pipes, ergo the snowshoe felling. While cumbersome, they were a lot safer than trying to exit the stump and post-holing to my thighs.