[pkghost]

Not having read the much more authoritative response above (or any such), I'll foolishly offer the one fact about trees I recall from boy scouts or outdoor school:

The core of a tree is much less alive than the bark—so much less, in fact, that if you walk in a circle around a tree scraping off a thin strip of bark and make sure to stop where you started, the tree will die.

Having googled this just to be sure, I also learned that trees can only lose up to about a quarter of a circumference of bark (in the fashion describe above) before facing mortal peril.

[mogrim]

Cork trees are an exception to this, and are harvested all the way round without harming the tree.