If you commute or use your bike for errands, you have to park your bike. In most cities, that means you have to lock it to something--a sign post, a parking meter, a telephone pole. Or a tree.
To tell you the truth, I've only hitched my wheels to a trunk once or twice in my life. No U-lock is wide enough, and you'd need a very long chain or cable. Also, in many places, it's forbidden to lock your bike to a tree.
Even where it's allowed, I prefer not to do that to a tree It's definitely not good for the tree--or, apparently, the bike:
(Images from Guy Sports.)