I use to be a forestry major. To be honest it doesnt really hurt the tree that much. Its not as bad as taking a increment bore or something like that. If you dont plan on harvesting the tree for timber then you really dont have to worry about how much damage a couple screws do. Ur only piercing the a little bit of working tissue in the tree. There is a chance that disease could set into the hole but not real likely. Unless ur a strait up tree hugger and that tree is ur G/F then i wouldnt worry about it. LOL