I revised the Basic Safety Protocol list to include Treeman's and TreeWeasel's comments. That idea of this list is to cover the most basic and important safety issues and is not intended to be a comprehensive tree climbing safety guide.

1. Do a thorough inspection and assessment of the tree, if you are not qualified to do so, find someone who is.
2. Use rated rope and gear designed for the task.
3. Set your rope on a live, structurally sound branch no less than 6" diameter.
4. Set your rope close to the branch union with the trunk.
5. Wear a helmet rated for impact from above.
6. Inspect all gear and life support connections before leaving the ground. Continue inspecting your gear and life support connections during the climb. Inspect your gear after the climb.
7. Always remain tied in while in the tree.
8. Do not climb above your tie-in (see item 11).
9. If you perform a switchover from one tie-in to another, do not detach from the first tie-in until you have inspected your new attachment points (harness and tree) and fully loaded the second tie-in. That means vertical hang with your feet off the tree. The first tie-in is slacked just enough that you can fully load the second system.
10. Be aware that if you move horizontally out on a limb that a swing back to the trunk can generate force similar to falling to the ground.
11. Arborist semit-static and static climbing ropes and harnesses (that we generally use) are not designed to absorb a fall, they are rated for "work positioning". Do not climb in such a way that exposes you to a fall onto a slack rope. Always remove slack from the rope when you climb with your hands and feet on the tree.
12. Respect fatigue, your brain doesn't work very well when you are tired.
13. Stay hydrated in the tree, always carry water when you climb up into a tree, even in winter.