I have two white pines in the backyard that I hang from for 1-2 nights a month. They are very familiar with my 1-inch webbing. There is some glossiness on the bark where the straps go, which I'm sure means that at least the outer layers of bark have been affected in some way. I held a straight edge up and down along the tree today and it doesn't appear that all of those hangs have created an indentation in the bark. Plus, if you don't actually wrap the webbing fully around the tree but instead use a Dutch clip, caribiner, or something similar, you're only putting pressure on about 80% of the circumference. Still, I wouldn't want to put regular wear on bark that didn't belong to a tree on my property. Girdling a tree a good thing for us all to be thinking about, but I'm thinking we're OK in most cases.