(A nail driven into a tree can kill it.) Say What? What kind of tree and what kind of nail? In the pacific northwest "green" activists "spike" trees, i.e. drive in a big nail that gets covered over by new growth so that later loggers trying to harvest trees will hit the nail with disastrous results thus "in theory" protecting the trees from logging. Nailing trees is done to "protect" trees.

A copper spike is a different story of course and can severely damage a tree.

I log nuisance trees and take them to a band mill to be cut. Trees that were near old home sites are dangerous as they may contain lots of nails, staples, and the like from having been used as fence posts and the like. The tree's growth covers over the debris and may not be noticeable. I have counted the annual rings and found that the healthy appearing tree had wire and other metal debris in it from over 50 years ago.

I'm not recommending driving nails into trees but don't think it is so much a danger to the tree as it is to anyone in the future who has to prune, trim, or remove a tree.

Patrick