I don’t know anything about the history of the tree, but this is a picture of the Janis Joplin Tree. According to the page, it’s “next to Hippie Hill in Golden Gate Park!”
My search also turned up a thread on the BritishExpats board that mentions the tree (post #5):
Whoops. Wrote too soon.
Hippie Hill, for my & everyone’s info, is where the park deadends before Haight Street. Which, of course, makes perfect sense. So there’s the precise location…more of less.