Does hitting nails in the tree bark hurt the tree?
Well, technically anything you do to injure the tree will hurt it, but over the long haul of the trees life, the fact that one nail is hammered into its bark should not endanger the tree. It has more important things that will hurt it — like lack of water, sunlight etc. I have cut down many large trees (I worked for Asplundh) and found nails way inside the tree that were hammered in there 50 or 60 years ago and the tree stayed healthy and strong.