[lukan]

I don't have a picture at hand, but on frequently used rock climbing spots, the young trees at the bottom, where the partner with the rope stands, can be very chairlike, too. I once asked myself why are they so conveniently formed, while leaning against one, but then I got it.

[constantius]

I had to re-read your comment, even thought it was a joke I didn't get! So for others who are as tired as me: the people holding the rope kept leaning against the tree for support, but not too much, as long as it existed, so that's how the tree got shaped into a chair.