Depends, from none to alot. I have milled trees that I know have been struck several times, one big old pecan I remember had 1/3 of the bark blown off over the years. The wood was still good. Others it kills part of the tree and bugs that won't attack live wood will go at it. Another lighting struck tree was a walnut that looked pretty good from the outside, but was hollow and full of carpenter ants. The top had been struck and killed several years before and it was dying, I got the trunk and most of it was standing dead.what affect,if any,does a lightning strike have on the wood a tree?(from a woodworking point of view)