I might be branching out a bit by saying this, I'm known to stick to my roots after-all, but this is a tree's way of saying "Leaf me alone." Typically their bark is worse than their bite but you still wood not want to mess with one.
I grew up in rural arizona & not sure if they are true, but there were all these legends about rednecks getting pinned under saguaros they would fire at at, then dying of exposure.