What is the best way to avoid harming trees when hanging a hammock from them?

I got a rope hanging kit from Pier 1, but it’s just the rope and hook attachments, nothing else – and I remember from college that the slackliners would always put big pieces of foam between the ropes and the trees. Should I be doing that, or is it overkill?

These are two white birch trees, if that makes a difference – they are hardwood but do not have strong bark.